materials testing guide
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Materials Testing eBookA Compilation of Technical Tips, Questions and Answers, and Customer Features from the TechNotes Newsletter
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Tensile Testing 4 Green Plastic: The Garbage Dump Killer? 5
Study Shows Differences in Mesh Materials for Hernia Repair 7
Why Alignment is Important in Tensile Testing 8
Testing of High Strength Rebar 9
Hidden Sensors Provide Extra Safety 10
The Bridge to Safety 11
What's Inside Your Arteries? Testing Could Reveal Your Risk of Stroke 12
Q: We take great care to ensure our test setup is consistent and our test equipment is as good as it can
be, but our Poissons ratio values still show too much variability. Is there anything else we can do? 13
Multi-Purpose Grip Shields 14
Q: Can I trust my strain figures when they are derived from crosshead position rather than from an
extensometer? 15
Q: When testing some specimens, the strain values appear to go backwards when the specimen is
yielding. Could extensometer slippage be causing this effect? 16
Q: Which grips are best for testing thin metal specimens? 17
Can clip-on extensometers affect my strain results when testing thermoplastics? 18
Do Your Test Results Change When Your Operators Change? 19
Study Reveals Benefits of Video Extensometers 20
Protect Your Investment with Proper Use of Grip Accessories 21
Tightening Your Wedge Grips 22
Q: How can I get better r-value results when using clip-on extensometers? 23
Q: What style of extensometer do I need? 24
Faster, More Consistent Testing With Pneumatic Grips 25
Q: How do I select an extensometer when determining a yield stress? 26
Worn Grip Faces? 27
Choosing the Right Grips 28
Why Am I Not Seeing Upper Yield? 29
What to Consider When Measuring Plastics 30
How can I improve the accuracy and repeatability of my Poisson's Ratio results? 31
The Best Solution for Gripping Low-Force Specimens 32
Why do I see a negative load after clamping my tensile specimen? 33
Indicating the Correct Gauge Length for Your Specimen 34
Grip Attachment Techniques 35
Q: Why does the speed of tensile testing after yield vary from material specification to material
specification? In your opinion, is there a significant difference in results? 36
Q: Why do I see a negative load value when I grip my specimen? 37
Q: Why am I getting low modulus values from my test machine? 38
Protecting Our Environment: Reducing Waste in Landfills 39
The Invisible Rebar: Microscopic Nanotubes Dramatically Increase Material Strength 40
Compression Testing 41 The Tower of Babel: Testing the Possibilities 42
A New Hip Material 43
Recent Testing Uncovers Titanics Mystery 44
From CO2 to Solid Rock 45
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Hardness Testing 46 Increasing Efficiency in Knoop and Vickers Testing 47
Best Practices: Which Rockwell Scale to Use 48
Q: How far apart should I space each Rockwell hardness test 49
Hardness Testing on Cylindrical Specimens 50
The Difference between a Knoop and a Vickers Test 52
Q: What is the difference between a Knoop and a Vickers test? 52
How GR&R Helps Your Rockwell Testing Process 53
Q: What is a Jominy test? 54
How Can Testing Strengthen Your Smile? 55
ASTM E18-07: New Changes will Affect Your Rockwell Hardness Indenters 56
Hardness Testers: Closed-Loop or Deadweight? 57
Different Rubber Hardness Scales for Your Testing Needs 58
Select Jaw Faces Based on the Hardness of Your Specimens 59
Q: How do I know when my hardness test block is no longer useful? 60
Impact Testing 61 Why Should I Instrument My Impact Tests? 62
Why Instrumented Impact Testing is Becoming More Popular 63
Q: What Causes the First Peak in the Load Curve of My Impact Test Data? 65
Q: How Much Energy Should I Use for My Impact Test? 66
Damaged Tups Change Results 67
Fatigue Testing 68 Q: I want to perform cyclic testing on my static testing machine. How fast can I go? 69
3M Ensures Quality under Different Test Conditions 70
Lab-grown Tissue 71
MacGyver-style Leg Brace May Reduce Amputations 72
Volvo Meets the Challenges of High Strain Rate Testing 73
Characterizing Spinal Range of Motion for Development of Improved Devices 74
Patients Own Tissue Repairs Torn Ligaments 75
Simulating Physiological Conditions of Implants 76
Bend Testing 77 Q: What is the difference between a single-point and a 4-point flexure test? 78
The Impenetrable Ship 79
Torsion Testing 80 Q: How can I measure the torsional properties of a pipe or cylinder? 81
Environmental Testing 82 Testing at High or Low Temperatures 83
Using Grips in a Low Temperature Chamber 84
Component Testing 85
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Challenges in Testing Biomedical Components 85
Q: How does side loading and specimen/component misalignment of varying geometries of medical
devices and implants affect my test results? How should I best address these challenges? 87
How does the mechanical testing of solar cells contribute to the "green energy" initiative? 88
Software Tips 89 Q: When writing a procedure in Bluehill Software, how do I deal with "Toe Compensation" (as described
in ASTM D882) when testing the secant modulus (1%) of a thin film (1-5 mils)? Should I add a preload?
And how much is appropriate? 90
Capturing Testing in Action 91
Q: The way we currently test for N-value is cumbersome. We are looking for a way to improve productivity.
Is there a way that we can get the program to automatically assign the uniform elongation at the end of
the calculation, instead of having to do it manually? 92
Q: What happens if power is suddenly lost during a test? Will I lose all my data in Bluehill? 93
Q: How can I be certain my extensometer is ready to use? 94
Correcting for Compliance 95
Benefits of the Preload Feature in Bluehill Software 96
Q: Do I Need to Enter Dimensions for Each Specimen? 97
Service and Calibration Tips 98 Are You Always "Investigation-Ready?" 99
Q: Can you give me a letter certifying that my test is in accordance with a specific ASTM or ISO standard?
101
Q: What does accreditation mean and how does it affect testing standards? 102
How do you move a 250,000 pound deadweight stack, while maintaining its integrity and accuracy? 103
Errors in Testing 104 Are You Receiving the Highest Quality Test Results? 105
What is Data Rate? 106
Q: What is the Relationship between Accuracy and Resolution? 107
When You Shouldn't Balance the Load Cell 108
Test Specimen Cutting and Stamping 109
Testing Standards 110 Q: What types of international testing standards are used in the medical device industry? 111
Q: What is 21 CFR Part 11 and how does it affect me? 112
Q: I've been testing to ASTM test standards and now I've been asked to do the ISO equivalent. What is the
difference between ASTM and ISO? Can I use my existing test fixtures? 113
Q: What testing standards serve as guidelines and requirements for the development and manufacture of
hip implants? 114
Customer Stories 115 Research Institute Partners with Private Steel Company 116
Materials Science for Young Minds 118
The Sound of Quality 119
The Science Behind Superhuman Strength 120
Formula 1 Racer Gears Up With Carbon Fiber 121
Materials Testing Explored in High School 122
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Tensile Testing
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A tensile test, also known as tension test, is probably the most fundamental type of mechanical test you can
perform on material. Tensile tests are simple, relatively inexpensive, and fully standardized. By pulling on
something, you will very quickly determine how the material will react to forces being applied in tension. As the
material is being pulled, you will find its strength along with how much it will elongate.
Tensile Testing
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Tensile Testing
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Green Plastic: The Garbage Dump Killer?
The Great Pacific Garbage Patch is a colossal floating garbage dump in the northern Pacific Ocean. Roughly the
size of Texas, it lies between Hawaii and San Francisco. It contains about 3.5 million tons of trash, much of it
plastic--shoes, toys, bags, pacifiers, wrappers, toothbrushes, and bottles are only part of what can be found in
this dump. A similar dump exists in the Atlantic Ocean.
The global buildup of plastic, both in the sea and along every shoreline, is an environmental nightmare. Most
commercial plastics are produced from petroleum. These plastics degrade into small pieces so plastic waste
builds up and can exist for many years. A great deal of research has taken place to develop biodegradable
plastics that break down with exposure to sunlight, water or dampness, bacteria, enzymes, and so on. Instron
customer Metabolix, Inc. has been researching for two decades to develop a commercially viable
biodegradable plastic from corn sugar and has recently made the leap from research to commercial production
with their product Mirel.
Plastics produced from plant material are not new; they have been around for more than 150 years. First
produced in 1845, polylactic acid (PLA), a thermoplastic polyester, was made by fermenting various
agricultural products such as cornstarch. Dow Chemical revived PLA production in the 1950s, but high
production costs precluded its widespread use. In the 1980s, the British chemical company Imperial Chemical
Industries (ICI) developed Biopol, a bioplastic produced through bacterial action. Polyhydroxyalkanoate (PHA)
polymers are produced by most species of bacteria from food sources such as plant sugars and oils. One of
these PHAs, known as polyhydroxybutyrate (PHB), has properties similar to those of polypropylene. But once
more, ICI was unable to produce Biopol cheaply enough to compete with conventional plastics. Monsanto
purchased Biopol from ICI in 1996. In 1998, Monsanto discontinued its bioplastics operations due to high
costs and limited commercial opportunities. It sold its interests to the U.S. bioscience company Metabolix that
began researching and developing a cost-effective process for manufacturing PHB-based plastics. In 2006,
Metabolix formed a joint venture called Telles with the agricultural giant Archer Daniels Midland to
commercialize a bioplastic under the name Mirel.
Mirel is designed as a suite of products, each of which can
withstand heat and cold, is capable of containing food products,
and biodegrades in natural soil and marine environments, home
composting and industrial composting facilities, where these
facilities are available. However, like nearly all bioplastics and
organic matter, Mirel is not designed to biodegrade in
conventional landfills. The rate and extent of Mirels biodegradability depends on the size and shape of the articles
made from it. As with any new material, its testing requirements
have been extensive. Its product data sheet gives mechanical
test specifications for tensile strength?, elongation at break,
flexural modulus, flexural strength, notched IZOD impact values,
and melt flow figures, using ASTM and ISO standards.
Photo courtesy of Mirel
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After initially producing Mirel bioplastics in a pilot plant, Telles opened a new production plant in Clinton, Iowa,
USA, in December 2009 with a 50,000 ton/year capacity.
One of the first Mirel products is the injection molding grade used to make 60% of the pen components for the
$1.25 Biodegradable Paper Mate pen made by Newell Rubbermaid. The pen costs more to manufacture, but
Paper Mate forecasts a strong demand. Other potential applications are cups, food containers, beverage
cartons, razor handles, brushes, applicators, cell phones, erosion control netting, plant pots, and plant clips.
The success of the venture is partially linked to consumers continued and increasing demand for green products, though businesses also use the material as a cost savings measure, in applications where
biodegradation saves time and labor. The market appears confident that the demand is there, with Metabolix
more than doubling its share price since February despite a $38 million loss last year. It remains to be seen if
the current enthusiasm to take care of the environment can eventually have the effect of shrinking or even
eliminating the ocean garbage dumps.
What is Tensile Strength?
Ultimate strength of a material subjected to tensile loading. It is the maximum stress developed
in a material in a tensile test.
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Study Shows Differences in Mesh Materials for Hernia Repair
Twenty years ago a patient undergoing hernia surgery would be marked by a noticeable scar, endure a long
recovery time, and according to a medical study, up to 20% of these patients would experience a recurrent
hernia. Due to medical advancements, hernia surgery is now less invasive, has a quicker recovery time, and
decreased risk of recurrence (less than 1%). What is this magical medical advancement? Laparoscopic
surgery.
According to Dr. Corey Deeken,
Director of the Biomedical
Engineering and Biomaterials
Laboratory at Washington
Universitys School of Medicine, it is important for surgeons to choose an
appropriate prosthetic mesh material
when performing laparoscopic hernia
repair.
In the world of hernia repair, there are so many materials and pre-
formed sizes available for surgeons
to choose from, Deeken said. The mesh that is right for a particular
patient and type of repair may not be
the best choice for the next patient.
Deeken, a biomedical engineer,
wants to give surgeons more
standardized information to compare
when choosing what is best for their
patients. This includes a recent project to characterize the properties of a variety of mesh materials available
for hernia repair applications. During this project, Deeken and her team used a tensile testing system to
measure the biomechanical properties of more than 25 different hernia repair materials using techniques such
as suture retention and tear testing, as well as standard uniaxial and mesh strength testing. Deeken hopes to
present the data from this study at an upcoming surgical conference to make surgeons aware of differences in
the biomechanical properties of hernia repair materials.
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Tensile Testing
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Why Alignment is Important in Tensile Testing
Laboratories performing low-cycle fatigue tests know how important it is to have good alignment of the test
specimen relative to the principle stress axis. There is an increasing awareness of the role alignment can play
in the accuracy of tensile testing results.
Organizations, such as NADCAP and ASTM, are addressing this in the form of laboratory accreditation and
methodology for measuring alignment. For example, a NADCAP audit checklist for a composite materials
testing lab will now include an alignment check of the testing instrument and refer to ASTM E1012 - Standard
Practice for Verification of Test Frame and Specimen Alignment under Tensile and Compressive Axial Force
Application as the method of checking alignment. This process ensures the testing instrument is capable of performing tensile tests that produce less than 10% bending for non-brittle materials and less than 5%
bending for brittle materials.
To meet the bending requirements noted above, the testing instrument must be designed and built to a high
standard and the alignment of the loading frame, load cell and grips must be measured to determine the
percent bending. This is typically done using an alignment specimen having a total of 12 strain gauges; four at
the upper gauge length area, four in the center, and four at the lower gauge length area. The outputs of the 12
strain gauges are used to calculate concentricity error and angularity error. Our AlignPro Software is available
to perform the calculations for percent bending and provide a guide to the adjustments needed to correct for
bending that exceeds acceptable limits.
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Tensile Testing
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Testing of High Strength Rebar
Many standards govern rebar testing including: ASTM A 370, ASTM A 615, ASTM A 996, BS 4449 and EN
10002-1. The mechanical tests these standards outline can be demanding on operators and testing
equipment. So, when testing large rebar samples #14 (all grades) we suggest using a single test space load frame in lieu of traditional dual test space styles. For this test we used a 1500 KN model, which has a
capacity of 1500 kN (337,500 lbf) and accommodates rebar specimens ranging in length from 400 mm to
700 mm.
This load frame features a top-mounted hydraulic actuator which places the
loading area at ground level. This significantly reduced our lifting requirements
for loading the heavy rebar specimens. Additionally, we were able to perform
both tension and bend tests on the rebar sample by adding compression
adapters to the tension grips. This saved change-over time because we didnt need to use the overhead crane to remove the large, heavy tension grips. Adding
the compression adapter and bend fixture took only a few minutes and involved
tightening a few screws.
For the tension test we used hydraulic wedge grips because the initial clamping
force reduced grip slippage on the uneven surface of the rebar. These hydraulic
wedge grips accept rebar specimens from 10 mm (0.39 in) to 70 mm (2.75 in) in diameter. The grip jaws are
vee-shaped with a custom-cut groove to accept the ribs found on rebar.
Finally, we used an automatic extensometer to capture strain. The model we selected, an M300B, has an
adjustable gauge length from 10 mm to 300 mm (required for most rebar applications). It automatically
clamps to the ribs of the rebar surface when a test is started and unclamps at a specified point during the test.
The strain data can be used for required modulus? and yield? calculations.
What is Modulus?
Rate of change of strain as a function of stress. The slope of the straight line portion of a stress-
strain diagram. Tangent modulus of elasticity is the slope of the stress-strain diagram at any point.
Secant modulus of elasticity is stress divided by strain at any given value of stress or strain. It also is
called stress-strain ratio. learn more >>
What is Yield?
Indication of maximum stress that can be developed in a material without causing plastic
deformation. It is the stress at which a material exhibits a specified permanent deformation and is a
practical approximation of elastic limit. learn more >>
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Hidden Sensors Provide Extra Safety
When someone says "fiber optics", you most likely think of telecommunications and not aircrafts or stadiums.
However, fiber optics can be embedded in structures to continuously monitor mechanical strain and
temperature changes, a technological breakthrough in the sensing industry.
Historically, such measurements were captured using electrical-type sensing devices, but in extreme
environments, such technology can be vulnerable. On the other hand, optical fiber sensors are rugged,
efficient, and extremely light, making them particularly interesting for the aerospace industry.
Fiber Optic Sensors & Sensing Systems (FOS&S), a
Belgium-based company, turns optic fiber into a sensor
by exploiting a physics law known as the Bragg condition.
Simply put, through exposing the core of a fiber to intense
ultraviolet light, the reflective properties can be used to
measure temperature and strain.
FOS&S is currently working on two notable projects. The
first is monitoring the structural health of the Athens
Olympic Velodrome roof structure (designed by the
famous architect Santiago Calatrava). The second project
is the in-flight structural health monitoring of aircraft
structures for Airbus. FOS&S uses an Instron testing
system for the calibration of its strain sensors and
performs tensile tests on composite samples embedded
with Fiber Optic Sensors.
"Placing fiber optic sensors in structural elements of an
airplane enables continuous monitoring of the actual
distribution of mechanical strain and temperature data
within these structures," says Mark Voet, CEO of FOS&S.
"This way, it immediately alerts operators of abnormal load situations like excessive vibration and internal
damage, allowing them to take the appropriate remedial action."
Photo courtesy FOS&S
Photo courtesy of FOS&S
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The Bridge to Safety
Eighty percent of all earthquakes occur along the edge of the Pacific Coast. So far in 2007, there have been
nearly 15,000 detected earthquakes worldwide. Depending on its force, some buildings, roadways or bridges
could collapse.
When sitting in stopped traffic on a bridge, do you
wonder how it holds hundreds of tons without
collapsing? California Department of
Transportation (Caltrans) ensures California is
getting the highest quality materials for bridge
and highway projects throughout the state by
testing materials, from concrete to reinforcing
bars to structural steel components and couplers.
With a daily volume of nearly 300,000 vehicles,
one of the busiest bridges in the USA is the 71
year old west-coast San Francisco-Oakland Bay
Bridge (SFOBB). This 4.5 mile (7.2 km) long
bridge consists of two major spans. Once
deemed impossible to build, Caltrans designated
the SFOBB as the emergency lifeline route to use
in disaster response activities. This requires the
bridge to be secure, fully functional, and
earthquake-resistant. In 1989, the bridge closed
for more than a month due to repairs needed
after the Loma Prieta earthquake. In response,
the eastern span between Oakland and Yerba
Buena Island is now being replaced by an entirely
new crossing making the bridge less susceptible to damage during an earthquake. This is known as the East Span Seismic Safety Project.
"We are using Instron's testing system to tensile test large diameter steel
bars (#14 and #18) to ASTM A 615, ASTM A 706 and ASTM A 722
specifications," said Rosme Aguilar, the Structural Materials Testing Lab
Branch Chief. "This custom built 2 million pound (8,896 kN) capacity
system has replaced our existing testing system because its 1 million
pound (4,448 kN) capacity could no longer handle materials of larger
diameter and strength that require a higher capacity."
The system, which stands more than 26 feet (8 meters) high, is located at
the Structural Materials Testing Lab in Sacramento, CA. As California's
only state transportation testing lab accredited by the American
Association for Laboratory Accreditation (A2LA), it quickly responded to a
recent bridge collapse due to a tanker truck explosion. The lab had the
responsibilities of assisting with the damage assessment to determine if
the material properties of the steel girders and bent caps had been
compromised due to the heat from the tanker truck fire. Remarkably, the
damaged bridge was fully functional in 18 days.
Photo courtesy of CalTrans
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What's Inside Your Arteries? Testing Could
Reveal Your Risk of Stroke
Strokes are the second most commonly feared condition; in
fact 2 out of 3 people know someone who has suffered a
stroke. In order to better understand prevention and
treatment, many researchers are studying the causes of
strokes, including the Department of Engineering at The
University of Cambridge under the direction of Dr. Michael
Sutcliffe.
Together with his colleagues, Dr. Sutcliffe is studying plaque
(a material that is deposited on the walls of the arteries)
and the hardening of the carotid artery, which can lead to a
stroke. The aim of their research is to develop better
methods for estimating a persons risk of having a stroke and to improve therapy selection.
Dr. Sutcliffe is testing plaque-filled arteries using an Instron
3367 30kN Universal Testing Machine to understand the
modulus and strength changes associated with plaque
growth. These results will be used in models of the carotid
artery to predict rupture of the artery and fluid flow
patterns.
"In the future, we will be working with others to link the way
cells change their shape under stress with tissue-level
testing and clinical experimental work. Our aim is to
understand how the stresses these plaques experience in
the arteries affect the way they grow," said Dr. Sutcliffe.
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Q: We take great care to ensure our test setup is consistent and our test
equipment is as good as it can be, but our Poissons ratio values still show too
much variability. Is there anything else we can do?
A. Poissons Ratio is defined by the division of transverse strain by axial
strain. Instron has carried out extensive
reproducibility studies to investigate
inconsistent results between labs, as
well as within individual labs. Difficulty
in calculating the ratio relates directly to
the measurement of transverse and
axial strain at very small strain ranges.
As you indicate, a consistent setup with
accurate equipment is vital. For most
plastics, the recommended
extensometer is a high-resolution
biaxial extensometer. It is equally
important to use the appropriate grips.
Pneumatic side acting grips are
preferred since they are self-aligning
and offer adjustable clamping
pressures, which allows for consistent
clamping forces on the specimen from
one to the next.
You should try setting up a small preload value on all your future test methods for plastics.
When specimens are initially placed into grips, they can be subjected to small compressive forces. These
forces can cause specimens to bend imperceptibly, causing inaccurate and inconsistent results. We have
shown that establishing a small preload as a part of the test method eliminates those compressive forces on
specimens and improves the repeatability of results.
What is Preload?
A test segment where the crosshead moves to load the specimen to a specified value before a test
starts. Data is not captured during the preload segment.
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Multi-Purpose Grip Shields
The new Instron pneumatic side-acting grips are supplied with adjustable
jaw face shields on either side of the grip. You can adjust the position of
the shields so that you can insert a specimen between the jaw faces, but
the shields help to prevent you from inadvertently placing a finger between
the jaws.
Many people dont realize that the shields also provide useful guidance for specimen centering. There are two centering guides on the shields, one for
round specimens and the other for flat specimens.
A notch in the shield arms is aligned with the center of the grip jaws. This
notch is useful when mounting a round or a thin specimen such as wire or
thread. When the shields are correctly installed and aligned for the
specimen size, you insert the specimen between the shields and hold it
against the notch while you close the grip jaws. You are then assured that
the specimen is centered.
For flat specimens, there are marks engraved at intervals on the shield
arms equidistant from the center. When inserting a flat specimen, you use
the marks as a guide to accurately locating the specimen in the center of
the grip jaws.
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Q: Can I trust my strain figures when they are derived from crosshead position
rather than from an extensometer?
A. Crosshead movement is measured using a high-resolution encoder. When you move the crosshead with no
specimen installed, the reported measurement of that movement is often more accurate than for many
extensometers.
However, when you install a specimen and apply a tensile or compressive
load, the accuracy of the measurement of crosshead movement becomes
dependent upon the system compliance.
Compliance refers to the tendency of the various components of a test
system to deflect under load. Consider every component in a test system as
equivalent to a very stiff spring. When you apply a load to that component,
even a major item such as a crosshead, it will deflect, either bending,
stretching, or compressing. If it is a very stiff spring the deflection is tiny,
but still measurable. Compliance is the inverse of stiffness; the stiffer, the
less compliant.
There are three sources of compliance in a system: the load frame, the load string components, and the
specimen itself.
The load frame is designed with a very high stiffness. Instron measures the stiffness at a particular load and
publishes that figure as part of the specifications of the load frame.
Load string compliance is usually not known. There may be few or many components in a load string; grips or
fixtures, couplings, one or more load cells, and so on. Many components do not have published stiffness
values.
The specimen compliance is usually what you are trying to measure.
As a rule of thumb, if the compliance of your specimen is around 100 times greater than the compliance of the
load frame and the load string components, you can assume that the reported crosshead movement is
equivalent to the deflection experienced by the specimen. However, if you are testing a very stiff specimen, you
should always use an extensometer to measure specimen deflection.
If using an extensometer is not possible, then you should evaluate the system compliance before the test.
Either install an extremely stiff specimen and apply a tensile force, or install compression platens and apply a
compressive force with the platens touching each other. The resulting deflection measurement gives a close
indication of the system compliance. When you test the specimen, you can remove this value from the result.
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Q: When testing some specimens, the strain values appear to go backwards
when the specimen is yielding. Could extensometer slippage be causing this
effect?
A. Yes it could be, but if you are aware of it and you have mounted
the extensometer correctly its unlikely. Its more probable that the strain is really going backwards.
Many metal alloys have a non-homogenous structure with grains of
different sizes and orientation, and they also contain various
impurities. Under loads that are sufficient to cause the material to
yield, bands of localized plastic deformation, known as Luders bands, can form in the otherwise unyielding portion of the material.
These bands of dislocations are the main contributor to the
discontinuous yielding portion of the stress/strain curve. They can
occur both inside and outside of the gauge length of the specimen, moving along the length of the specimen as
the load increases.
Your extensometer is probably reacting to yields that are occurring both inside and outside of the gauge length,
which can create this phenomenon of backwards strain.
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Q: Which grips are best for testing thin metal
specimens?
A: Screw side-action grips open the door for specimen slippage,
high standard deviation, and low throughput. We recommend self-
tightening wedge grips for metal applications. They offer
improvement in all of these areas, do not require any tools, and are
easy to use.
Once the specimen is inserted between the jaw faces, manually
turn the lever to close the wedged faces and apply only a slight
amount of clamping force. This is sufficient enough for the jaw
faces to pull on the specimen once the test is started. The clamping
force increases as the specimen is pulled, eliminating jaw breaks
that are normally caused by high initial clamping force. The exact
model of grips and faces often requires a discussion about the
material youre testing.
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Can clip-on extensometers affect my strain results when testing thermoplastics?
There are a variety of attributes used to describe
thermoplastics since properties are dependent on the
polymer, as well as additives. In some instances,
thermoplastics are relatively soft, so knife edges on
traditional clip-on style extensometers may cause premature
failures. This occurs when high stress points are created
where the knife edges contact the specimen. In other
instances, thermoplastics may be rigid, if glass or talc is
added. For these materials, significant energy releases may
occur at failure, possibly damaging the clip-on extensometer
since they are in direct contact with the specimen.
Non-contacting video extensometers overcome both issues by
providing a means to measure specimen strain without
having direct physical contact with the specimen. A high
resolution digital camera and real-time image processing
allows the device to acquire accurate strain data without
interfering with the specimen.
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Do Your Test Results Change When Your Operators Change?
During a tensile test the specimen is subjected to a force purely in the tensile direction along a single axis. If
the specimen is not aligned properly, it will be pulled along multiple axes, which can cause premature
specimen failure and adversely affect any measurements captured during testing. Also, operators could load
the specimen differently or inaccurately causing erroneous end results.
To ensure proper alignment and accurate results amongst multiple operators, we suggest using a specimen
alignment device. These specimen centering devices (which are shaped like an "L") are attached to the grip
and help center the specimen within the grip faces. The operator can adjust the centering device making sure the specimen is centered and not over-inserted. Once the operator has the centering device set properly,
each subsequent specimen will be in the same location in the grips, allowing for accurate, repeatable results.
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Study Reveals Benefits of Video Extensometers
We conducted a study to better understand the extensometry needs
of our customers. We found that many of our customers
experienced common issues while testing strain with clip-on
extensometers. Included are a few solutions from that study, along
with information about an alternative to clip-on extensometers.
From those surveyed, our study revealed:
Problem: 77% of those testing fragile, expensive, or delicate
specimens (including tendons and sutures) struggled to capture
strain without damaging their sample. These customers reported
that the weight of a clip-on extensometer influenced the sample's
behavior under test.
Solution: Since video extensometers do not come in contact with the
specimen, it makes them less damaging to the samples. They can
also be used with in vivo testing of biomedical samples.
Problem: Customers testing specimens that break violently were
unable to use a clip-on extensometer through failure. They also
reported problems with broken extensometers and felt uneasy
about lab operators removing a clip-on device while the specimen
was under load.
Solution: Video extensometers offer lab operators the convenience of capturing strain through failure since
they do not need to be removed.
Problem: Many of our customers testing at high and low temperatures struggled to find traditional
extensometry solutions that worked well with chambers.
Solution: 86% of those who used chambers preferred video extensometers over traditional clip-on styles.
In conclusion: we discovered that, of our customers who have used both traditional-style and video
extensometers, 77% preferred the video technology. Learn more about non-contact video extensometry, or
contact an applications specialist.
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Protect Your Investment with Proper Use of Grip Accessories
Do you notice your internal crosshead grip jaws extending outside of the system's crosshead while running
tests? If you do, this may cause extensive damage to the machine. This style of grip is most commonly used in
our SATEC Series, a product line designed to deliver high-capacity tensile forces up to 3,000 kN. Since this force is so high, it can deform the machine's crosshead if the proper accessories are not used. This damage
may be irreparable and require replacement of several costly components, not to mention cause downtime.
To prevent this, we suggest using grip spacers (also called filler plates) to accommodate different sizes of
specimens while keeping the jaws inside the crosshead.
View an animation on In-head Grip Parts & Grip Accessories
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Tightening Your Wedge Grips
A common mistake many customers make to troubleshoot specimen slippage
when using their mechanical wedge style tensile grips is over tightening them.
Over tightening a wedge grip can damage the grip and exert unwanted load on
the specimen. The mechanical design of a wedge grip works in the following
way:
1. A tension force is applied to the specimen 2. This tension force causes the specimen to pull downward on the jaw
faces (provided there is good bite between the jaw faces and the
specimen)
3. The faces slide through the grip body along the wedge path 4. The faces then squeeze the specimen
This entire process is self-tightening the higher the tensile load, the harder the jaw faces squeeze in on the specimen. While over-tightening isn't an
effective way to improve slippage, customers can minimize specimen slippage
by improving the bite through the use of proper jaw faces and ensuring the
specimen contacts at least 2/3 of the grip faces.
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Q: How can I get better r-value results when using clip-on extensometers?
A: Determining r-value for ASTM E 517 requires precise measurement of axial and transverse strain. When
using clip-on extensometers, make sure you are practicing
the following techniques:
1. Set gauge lengths
2. Align instruments on the specimen
3. Zero instruments with no load on the specimen
4. Check that the knife edges do not deform the specimen
5. Be certain that the specimen is not bent
6. Ensure specimen markings havent deformed the specimen
7. Be certain specimens have smooth edges and meet
ASTM E 517
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Q: What style of extensometer do I need?
A: There are two main styles of extensometers contacting and non-contacting. Contacting extensometers are widely used and provide accurate strain measurement. However, some applications (like biological tissue or
thin film) demand a device that won't damage the specimen or affect test results. Non-contacting
extensometers provide an ideal solution for delicate specimens, for specimens that break violently, for tests
conducted in a chamber, and for specimens of varying lengths and elongations. An Instron Applications
Engineer can recommend the correct instrument for your testing.
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Faster, More Consistent Testing With Pneumatic Grips
While screw side action grips are appropriate for certain applications, you may experience long setup times,
premature specimen breaking at the jaws (due to over-tightening) or specimen slippage (due to under-
tightening). In addition, you will always need one hand to tighten the grip while the other hand holds the
specimen, which is not always convenient and may result in a misaligned specimen.
Upgrading to pneumatic side action grips could make your testing easier and faster.
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Q: How do I select an extensometer when
determining a yield stress?
A: Extensometers are available from 1% to 3000%+ full
scale travel, but using the longer travel is not always the
best solution. When testing stiff specimens, such as steel,
an extensometer with 10% or less travel is recommended to
ensure adequate resolution for the determination of yield.
On the other hand, materials such as plastics commonly
yield at greater strain values, and therefore an instrument
with 50% travel is recommended. Long travel instruments
(100% or more) should be
reserved for high-elongation
specimens, such as rubber. An
Instron Application Engineer can
recommend the correct
instrument for your specimen
type.
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Worn Grip Faces?
Efficient gripping of your test specimen is important for reliable, trouble-free testing. Like any tool, you need to
keep your jaw faces in good condition for optimum performance.
Chipped, worn or clogged teeth on jaw faces can produce slippage and with
it, the temptation to use excessive force, increasing the likelihood of jaw
breaks.
Unevenly worn faces can also produce undesirable bending effects.
Rubber-coated faces can gradually degrade over time in your shop
environment, particularly in higher temperature conditions.
Cord and yarn grips rely on a smooth, polished surface for optimum
resistance to jaw breaks. The original surfaces can wear with heavy use.
The best way to restore lost gripping efficiency is with a new set of jaw
faces.
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Choosing the Right Grips
Successful gripping solutions require the specimen to be held in a way that
prevents slippage and jaw breaks and ensures axiality of the applied force. In
some cases the gripping requirements are very specific and a purpose-
designed grip or fixture is necessary to meet a particular testing standard.
However, in most cases, you can use general purpose accessories. General
purpose grips and fixtures have the advantage of being able to grip a wide
variety of specimen types and materials using a range of options such as
different jaw faces, alignment fixtures, etc.
The most important step in successful gripping is to choose the best set of
grips for your specimen type. To learn more about different grips and fixtures,
browse our online Accessories Catalog.
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Why Am I Not Seeing Upper Yield?
Are you testing for upper yield strength, but not seeing a "dip" in your stress/strain curve? This is often the
result of using improper test control parameters. During yielding, the strain rate needs to be as constant as
possible. This is best achieved by using crosshead position or strain control*.
For example, if you run a test in stress control at the onset of yielding, the testing machine will accelerate to
maintain the desired stress rate. Incorrectly running in load control causes unwanted acceleration. This
prevents the stress from dropping relative to the increase in strain. As a result, the upper yield strength
calculation will fail because it cant find the dip in the stress-strain curve (a zero or negative slope).
To correct this situation, set up the test to use stress control during the first half of the elastic portion. Prior to
the onset of yielding, switch to either position or strain control. We have software packages that are designed
to allow for control transition.
* Refer to test standards such as ASTM E8, ISO 6892 or EN10002 for the allowable test rates
during yielding.
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What to Consider When Measuring Plastics
When injection molding plastics, the outer surface of the specimen may collapse inward, causing it to form a
concave shape. Commonly referred to as "sink", it may cause variations in the specimen's thickness as a result
of the concave surface.
When a specimen exhibits sink, it is important to understand the affect it may
have on stress-based calculations, such as yield stress or modulus. Sink will
often cause the measured cross-sectional area to appear larger than what it
actually is and this can result in lower modulus, yield stress, and other stress-
based calculations.
Methods to measure cross-sectional area may vary depending on the standard
you are testing to.
Standards may specify different procedural requirements, as well as different
requirements for the measuring device itself. In some cases, it may not be possible to account for sink. For
potential solutions, view our animation.
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How can I improve the accuracy and repeatability of my Poisson's Ratio results?
Poisson's Ratio? is the ratio of transverse strain divided by axial strain in
the elastic region of a uniaxial tensile test. It is a measure of how much of
a material contracts under tensile conditions, and is typically on the order
of 1/3 (0.3). Since the displacement associated with transverse strain can
be 10 to 12 times smaller than the displacement for axial strain (~4 times
smaller gauge length multiplied by ~3 times smaller displacement), the
accuracy of Poisson's Ratio is often limited by the accuracy of the
transverse extensometer.
Improving the accuracy and repeatability is best achieved by using a high-
resolution biaxial extensometer designed specifically for this purpose. View
our complete solution for testing Poisson's Ratio.
What is Poissons Ratio?
Ratio of lateral strain to axial strain in an axial loaded specimen. It is the constant that relates
modulus of rigidity to Young's Modulus in the equation:
E = 2G(r + 1)
Where E is Young's Modulus; G, modulus of rigidity; and r, Poisson's ratio. The formula is valid only
within the elastic limit of a material. A method for determining Poisson's ratio is given in ASTM E-
132.
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The Best Solution for Gripping Low-Force Specimens
Low-force biomedical testing applications vary widely, and include specimens
such as native tissue, bio-engineered tissues, hydrogels, and contact lenses. In
most cases, these specimens are tested in a heated, fluid environment that
simulates physiological conditions; in other cases, the specimens are hydrated
for several hours before testing. Generally, most customers assume that rubber-
coated or serrated faces provide the ideal gripping
solution. But do they?
Rubber-coated faces tend to cause specimen
slippage, while serrated faces cause premature
failure.
A study conducted by the Instron Application Lab proved the best gripping
solution to be sandpaper or a grip surface called SurfAlloy, a surface that
resembles sandpaper. This slightly roughened surface provides enough friction to
prevent slipping, and not too much grit that could cause premature failure.
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Why do I see a negative load after clamping my tensile specimen?
This is due to the fact that material is being forced out of the grip as a result of the squeezing, which can cause
a compressive load on the specimen, even with the best grip in particular for softer materials such as elastomers.
When the sample is clamped in one grip, there is no apparent load on the sample since it still has a free end.
However, when it is squeezed by the second grip, the material flows out of the grip, causing the specimen to be
in compression. This will show up as a
negative load on the readout before the test has begun.
If this is the case, you should NOT balance
out the load because the load you see is
real; balancing it would introduce error into
your test results. If you are experiencing
this, you need to move the machine's
crosshead to remove the compressive load.
There are two ways to do this:
1. Manually adjust the crosshead, for
example with a thumbwheel; or
2. Through software features, like the
preload function
Alternatively, we suggest using the load
protect feature, which limits the maximum
force applied to your specimen by
automatically ensuring the force on your
specimen remains within the pre-set
bounds. It removes the possibility of the
crosshead going into compression in real
time.
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Indicating the Correct Gauge Length for Your Specimen
Understanding how specimen dimensions differ is important when setting up your calculations for a tensile
test. Most calculations are based off stress and strain, and since both are dimension dependent, it is
important to specify the correct values.
For specimens that have the same cross-sectional area from end to end (tubes, rods, rectangles and fibers),
the gauge length is determined by simply measuring the distance between the grip faces (refer to image).
However, the most common shape is the "dog bone" specimen (refer to image). Unlike the specimens
mentioned above, its non-uniform shape often causes mistakes in identifying the gauge length. When a "dog
bone" specimen is tested, most of the stretching occurs within the narrow region and not in the tabs because
they have a larger cross-sectional area. Since most of the stretching occurs within the narrow region, that
length should be used as the gauge length.*
*Note: There is a small amount of stretch within the tabs of the specimen. In order to get the most accurate
strain results, we suggest using an extensometer.
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Grip Attachment Techniques
Do you have a loose connection between your
grips and the testing frame? If so, you could have
slack in your load train that can cause blips in
your curves, skew modulus data or alter
extension results. There are two solutions to
make sure you have a rigid connection. If your
grips are equipped with a check nut, make sure
it's tightened against the adapter, away from the
grip. We use the check nut consistently in our lab
because it's simple and
convenient. If you do not have
a check nut, an effective and
inexpensive solution is to place
a spring below the lower grip
inside the clevis adapter.
These techniques will remove
the slack in your load train and
ensure you're measuring true
extension and not the
movement of your grips.
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Q: Why does the speed of tensile testing after yield vary from material
specification to material specification? In your opinion, is there a significant
difference in results?
A: In tensile testing, most materials are sensitive to the rate at which they are stretched, meaning some of their
properties are dependent on the rate of straining during the test. This effect is most noticeable after plastic
flow occurs, although some properties can be affected while in the elastic region. There is no intrinsically
correct strain rate for a given material, but to allow comparison of test results it is important for all tests to be
done within a range of rates. Test
standards define the range over
which results will be consistent
and therefore, comparable.
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Q: Why do I see a negative load value
when I grip my specimen?
A: The closing action of wedge action grip jaws often
applies a compressive load to the specimen. If your
indicator is set to auto-zero at the start of the test, you
may see lower load values. Remove the auto-zero
function for the load channel to correct the low reading.
Another way to reduce negative load caused by wedge
action grips is to use the specimen protect feature
available on most newer Instron control systems.
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Q: Why am I getting low modulus values from my test machine?
A: Elastic modulus values are affected
mainly by how strain is measured. Small
errors in strain measurement can result in
large errors in modulus values. If
crosshead extension is used for strain, it
will include small deflections of the
testing machine and the grips under load,
which are added to the specimen
elongation. This results in artificially large
strain values and lower-than-expected
modulus values. The most accurate
approach for modulus measurement is to
measure strain by applying an
extensometer directly to the gage length
of the sample, thereby eliminating errors
from other sources of deformation.
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Protecting Our Environment: Reducing Waste in Landfills
More than 60% of the refuse going to local landfills is business/industrial waste. Much of the plastic from this
waste could actually be recycled. However, in order to recycle plastics, the materials must be recovered from
the items they are part of; and the many plastic types must be separated from non-plastic materials and from
each other.
MBA Polymers, Inc. is leading the
way with research, development
and even large-scale commercial
efforts in plastic recycling. MBA
Polymers, and its newly opened
manufacturing plants in China and
Austria, recover high-value plastics
from popular electronics such as
computers, televisions and even
automobiles. Using a proprietary
separation process developed over
the past 12 years by R&D Manager
Brian Riise and several colleagues,
MBA is able to remove non-plastic materials from complex durable
goods and recover purified streams of ABS and high-impact polystyrene
flakes. These flakes are then extruded into pellets; a process that
requires less than 10% of the energy needed to manufacture virgin
plastics. MBA then measures several common mechanical properties,
including tensile properties, using an Instron universal testing system.
"After the MBA separation process, we are able to create products with
mechanical properties similar to what you would find in a virgin plastic.
We tend to sell these pellets to customers who normally use virgin
plastics, including some very demanding electronic equipment
manufacturers," says Riise.
Photo courtesy of MBA Polymers, Inc.
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The Invisible Rebar: Microscopic Nanotubes Dramatically Increase Material
Strength
One of the exciting new building blocks for very small systems is carbon nanotubes (CNTs). These single- or
multi-walled cylinders, made up of carbon atoms, are about 1/100th of the diameter of one piece of human
hair.
What makes CNTs attractive is that they are light (about 1/6 the weight of steel), strong (about 100 times
stronger than steel), electrically conductive (more conductive than Copper), thermally conductive and UV
absorbing.
A promising application for CNTs is nanocomposites, where
tubes are combined with another material (either an epoxy or
polymer). The CNTs behave much like fibers in wood or rebar
in concrete. The fibers are strong and make up most of the
strength, whereas the matrix holds the fibers in place and
makes it a usable material.
In 2004, Nanocomposites, Inc. licensed the Rice University
patented process for functionalizing CNTs, a process which
affects the surface of the nanotubes and makes them more
suitable for mixing with polymers.
The process dramatically reduces the CNTs tendency to stick
together, thereby allowing them to mix and bond with the
matrix, significantly improving mechanical properties. For
example, by adding treated CNTs to a rubber compound, Nanocomposites,
Inc. measured a 35% increase in ultimate tensile strength.
Additionally, 90% of the material's strength is retained at temperatures up to 400F (204C).
Nanocomposites, Inc. used an Instron electromechanical testing machine equipped with a video extensometer
to measure the mechanical properties of their materials at ambient and elevated temperatures.
Photo courtesy of Michael Strck
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Compression Testing
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A compression test determines behavior of materials under crushing loads. The specimen is compressed and
deformation at various loads is recorded. Compressive stress and strain are calculated and plotted as a stress-
strain diagram which is used to determine elastic limit, proportional limit, yield point, yield strength and, for
some materials, compressive strength.
Compression Testing
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The Tower of Babel: Testing the Possibilities
The story of the Tower of Babel has fascinated scholars for centuries. The goal of the builders was to reach the
heavens. An ancient document called the Book of Jubilees mentions the tower's height as being 5433 cubits
and 2 palms, which is almost 2.5 kilometers (about 1.55 miles). That is certainly higher than any man-made
structure today, but is that possible? The building materials of the time
were simply bricks of mud and straw. So just how tall could the tower
have been?
Instron had the opportunity recently to meet with Professor Linn Hobbs
of the Department of Materials Science and Engineering at
Massachusetts Institute of Technology (MIT). Dr. Hobbs along with two
colleagues teaches the course Materials in the Human Environment that investigates the development of materials and technologies
through human history. As evidenced by the name, the range of class
teachings and projects is wide and our discussions included research
into brick and mortar construction, natural-fiber rope bridges in the
Andes, and whether the Egyptian pyramid blocks were cast in place
rather than quarried and then lifted into place. The well-appointed MIT
laboratories have several Instron test instruments that enable the
students to evaluate the capabilities of the materials that they produce
during the class.
To estimate the possible theoretical height of a brick-built tower, Dr.
Hobbs has his students manufacture bricks using clay, sand, and straw
folded together. Some bricks are sun-dried while others are fired in a
furnace, as they would have been as building technology advanced. A
series of empirical compressive tests on the bricks using an Instron
electromechanical testing system evaluates their individual strength
and from these values they can calculate the possibilities.
The sun-dried bricks withstand compressive loads up to 4000 lb/sq in.
A pyramidal structure built with these bricks and with a wide base to
spread the weight of the structure could reach around 1500 ft., or
around a quarter of a mile. However, a new technology had developed
that imparted much greater strength to the bricks; they were baked in
wood-fired furnaces. When baked bricks are compressed, they can
withstand 20,000 lb/sq in., which equates to a possible height for the
tower of 10,500 ft or almost two miles high. Thats around four times as high as the worlds tallest building, the Burj Khalifa in Dubai. Its also high enough to have given altitude sickness to any Mesopotamians
strong enough to reach the top!
The aim of this fascinating inter-disciplinary course is to teach
innovative thinking to our future materials scientists, civil and
construction engineers, archeologists, architects, and so on, through an
understanding of how materials and their uses and physical properties
have developed over time. It doesnt hurt that building walls, pyramids, and plant-fiber bridges is great fun as well.
Photo courtesy of MIT
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A New Hip Material
The first surgery to replace a damaged hip joint with an artificial joint was performed just 50 years ago. Today
more than 190,000 hip replacement surgeries are performed in the USA
alone.
During this time, there have been many improvements to the surgical
techniques and to the technologies and materials of the replacement
joints but inherent problems remain. One of these is the slow
deterioration of bone tissue around the prosthetic material due in part to
uneven load distribution between the prosthetic and the bone itself.
Dr. Afsaneh Rabiei, a professor of mechanical, aerospace, and biomedical
engineering at North Carolina State University has recently developed a
new composite metal foam material that offers, among many other
possibilities, the development of new hip joint prostheses that may
overcome this problem.
Artificial hip joints are usually manufactured using solid titanium, which is
many times stiffer than the bone into which it is secured. The implant
therefore assumes the majority of the loads exerted by walking and
running. Regular load-bearing exercise is an important factor in good
bone health. The bone around the implant, being now deprived of much
of the load, loses density and strength, a phenomenon known as stress
shielding. In time this deterioration, together with other changes due to
biological reactions with the cement used to secure the implants to the
bone, can cause the implant to loosen, resulting in the need for further
surgery to reseat or replace the joint.
Metal foams have been around since the late 1940s. Most are developed by introducing gases into molten
metal, which cools to form a matrix of thin-walled metal. However, the cellular structure is difficult to control,
leading to variations in cell wall thickness and random cell shapes and sizes. The resulting mechanical
properties of the material are unpredictable and inconsistent.
Dr. Rabieis composite metal foam material uses preformed hollow metal spheres. These are packed together randomly, and the spaces between
the spheres are filled with metal powder. The whole is then sintered to
form a sturdy composite structure. The foam displays superior
compressive strength and energy absorption capabilities as compared to
existing metal foams, while exceeding strength to density ratios.
The ability to control the size, the wall thickness, and the percentage of
spheres added to the matrix allows close control of the stiffness and
durability of the metal foam. The foam can therefore be manufactured to
closely match the stiffness of bone, thus eliminating stress shielding.
Other benefits of the new material are energy absorption, so they
cushion the shock of each step. The composites pores also provide places where natural bone can grow and anchor the implant in place.
The combination and predictability of these properties offers promise for use in other applications where light
weight, high stiffness and energy absorbsion capabilities are important, such as automobile crumple zones,
and structural members in air, naval, and space craft.
Photo courtesy of Arthritis Research & Therapy
Photo courtesy of Dr. Afsaneh Rabiei
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Recent Testing Uncovers Titanics Mystery
The Unsinkable Ship sank in less than 3 hours back in 1912. Did the Titanic sink simply due to the impact of
an iceberg and the speed of the ship or was it a malfunction in the mechanical property of a key material
holding the ship together?
A recent study, conducted by Tim Foecke of the National Institute of Standards and Technology (NIST), and his
colleagues, tested the rivets of the ship's hull; rivets that were made of wrought iron, not steel like the rest of
the ship's rivets. The one big difference: wrought iron tends to soften at lower temperatures.
Using a SATEC Series universal testing system, Foecke and colleagues simulated the ship's design with 2
pieces of 1-inch thick steel plates held together with wrought-iron rivets. Through a compression test, they
were able to simulate the force on the rivets and found that the rivet heads broke off, proving their
substandard quality. As the rivet heads popped, the steel plates separated, allowing water to pour into the
ship's hull at a very fast rate.
"If the wrought iron rivets were up to standards, they would have been fine," says Foecke. "But since there was
no method for quality checking, the rivets used on the Titanic were not up to standards, which caused them to
fail prematurely."
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From CO2 to Solid Rock
Did you know that each day we pump 70 million tons of CO2 into
the Earth's atmosphere?
Suzanne Hangx, M.Sc. of Utrecht University and her Dutch
colleagues at CATO are using Instron equipment in their research
to remove this greenhouse gas from the atmosphere. They are
studying CO2 Capture and Storage (CCS), a technology that may
provide 100 years of CO2 storage beneath the
Earth's surface. So how does CCS work? Below
the Earth's surface there is a vast volume of
storage space available through unminable coal
beds, depleted oil and gas reservoirs and aquifers.
CO2 is captured at power plants and pumped
underground into these storage spaces. As the
CO2 spreads through the reservoir or aquifer it will
partially dissolve into the present pore water,
which results in the formation of an acid fluid. This
fluid interacts with the porous rocks and causes the carbon to settle out through mineralization, resulting in a
stable, solid rock.
In addition, there are several organizations around the world performing CCS research, ensuring it doesn't lead
to undesirable sinking of the Earth's surface. In order to understand and quantify the effects of CCS, Hangx
performed constrained compression tests on granular CO2-injected rock samples using an Instron static
testing system and a special compaction vessel
"Our results show that geomechanical processes, like grain cracking, are significantly inhibited in CO2-injected
samples and geochemical effects are negligible on short time scales. Our testing is proving that CCS is a viable
and safe way to reduce greenhouse gas emissions," says Hangx.
Currently, there are a handful of CCS test sites around the world. With expanded implementation, CCS may
allow precious time to work on improving energy efficiency and using renewable energy sources.
Photo courtesy of Michael Strck
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Hardness Testing
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Simply stated, hardness is the resistance of a material to permanent indentation. It is important to recognize
that hardness is an empirical test and therefore hardness is not a material property. This is because there are
several different hardness tests that will each determine a different hardness value for the same piece of
material. Therefore, hardness is test method dependent and every test result has to have a label identifying
the test method used.
Hardness Testing
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Increasing Efficiency in Knoop and Vickers Testing
Two of the most common hardness tests
are Knoop and Vickers that are used in
micro and macro testing. These tests
determine the material hardness based
on measuring the size of a diamond-
shaped impression from an application of
a force. The nature of the test process
typically dictates a relatively light force
application, resulting in extremely small
impressions that must be manually
measured. Traditional techniques involve
the use of microscopes with objective
lenses to manually measure through an
eyepiece. This is a time-consuming,
subjective, and potentially error-filled
process. Its not uncommon for a technician to manually produce and
measure hundreds of indentations during
a day which means that operator fatigue could compromise the measurements.
During the past several years, automated processes have become a more popular technique. What used to
take 25 minutes to test manually now takes 5 minutes to test with an automatic tester. Newer technology
eliminates much of the hardware that created operational challenges and cluttered workspaces. It typically
consists of:
Automatic rotating turret
Actuation in the Z axis for applying the indentation and for automatic focusing of the specimen
Automatic XY traversing motorized stage and USB video camera integrated to the test frame
Stage movement through a virtual joystick or stage controllers
Together, these produce a fully-automated hardness testing system. When loaded with samples and a stored
program, it can be left alone to automatically make, measure, and report on an almost a limitless amount of
indentation traverses.
"Using an automated tester is very useful. The biggest benefit to our lab is the amount of time it
saves us. What used to take us 1.5 weeks to test now takes us 2.5 days. Automated testing allows
for less human error and frees up time for the operator to do other jobs. Plus, it saves us money."
Dipak Patel, Prudential Steel
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Best Practices: Which Rockwell Scale to Use
Rockwell hardness values are a combination of a hardness number and a scale symbol representing the
indenter and the minor and major loads. The symbol HR and the scale designation represent the hardness
number. The combination of indenter and test force make up the Rockwell scale. These various combinations
make up 30 different scales and are expressed as the actual hardness number followed by the letters HR and
then the respective scale. A recorded hardness number of HRC 63 signifies a hardness of 63 on the Rockwell
C Scale. Higher values and
Rockwell scales indicate harder
materials, such as hardened steel
or tungsten carbide.
The majority of applications are
covered by the Rockwell B and C
Scales for testing steel, brass, and
other metals. However, the
increasing use of other materials
requires a basic knowledge of the
factors that must be considered in
choosing the correct scale. The
choice is between the regular and
superficial hardness tests (a
lighter, 3 kg minor load test), and
the diamond and various carbide
ball indenters
The operator relies on the engineering specifications that are established at the material design. If no
specification exists or there is doubt about the suitability of a predetermined scale, refer to our Best Practices
article published in Industrial Heating Magazine.
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Q: How far apart should I space each
Rockwell hardness test
A: Indent spacing is a common concern during
specimen testing or coupon block verification. The
purpose for these distances is to ensure that any
new indentation is not influenced by work hardening
of the materials edge or material around a previous indentation. The accepted criteria are that the
distance from the center of any indentation should
be at least three times the diameter of the
indentation. The distance from material edge to the
center of any indentation should be at least two and
one-half times the diameter of the indentation. Also,
the edge distance requirement ensures that the
indentation's area of contact permits proper
support.
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Hardness Testing on Cylindrical Specimens
When performing hardness tests on cylindrical, convex, or concave surfaces, the
operator should understand that the actual results may be inaccurate due to the
curvature of the material. In most cases, these inaccurate results should be
accounted and adjusted for when reporting actual material hardness. Due to the
material curvature, several important factors may contribute to the invalid reading
including the actual material hardness, the applied force, the size and
shape of the indentation, and the diameter or radius of the test
piece.
However, there are many techniques operators should consider to minimize
errors.
Correction Factors
If the curved sample is used for material control purposes only, there may
be sufficient information and comparative data generated that allows operators to benchmark values and
processes. To make correction factors necessary, as indicated by ASTM, it is advisable to compare the
hardness of the rounded material with the hardness value of a flat piece. In a convex (curvature that extends
outward) or cylindrical piece, the reduction in lateral support will result in the indenter penetrating further into
the material which translates to apparent lower hardness readings. In this case a correction factor must be
added to the generated result. In
contrast to convex surfaces,
concave surfaces will provide
higher material support due to the
curvature towards the indenter
and result in apparently harder
material due to production of a
shallower indent. In this case, a
correction factor must be
subtracted. If the diameter of the
material is greater than 25 mm
the surface will provide suitable
surface structure for testing and
corrections are not required.
Lower diameter materials will
need the correction factor added
to the test result.
Proper Test Type
If the diameter of the material is smaller than 3.175 mm, Rockwell testing is not recommended. Instead,
operators should use a Knoop or Vickers test, which can accommodate lower diameters down to thin gage
wires. Most digital Rockwell testers provide the means to input the diameter of the curve. This input
automatically adds or subtracts a correct factor from the test results. In manual dial gage testers, ASTM
correction tables should be referenced to determine the correction factor. All corrections produce approximate
results and should not be expected to meet exact specification.
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Hardness Accessories
Another important factor in testing curved material is
proper specimen support. The supporting anvil should
be selected to match the specimen geometry and to
ensure exact alignment of the indenter to the radius. It
should be rigid and provide full support to prevent
deformation.
ASTM E18 is a good reference for anvil selection. The
anvil must position the test specimen perpendicular to
the indenter. A V style anvil is ideal for supporting cylindrical parts. A cylindron anvil is suitable for larger
diameter parts. Elongated parts that extend beyond the
frame should be supported with a Vari-rest type fixture
to prevent part tilt or movement. Specialized anvils can
accommodate varying geometries and radiuses.
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The Difference between a Knoop and a Vickers Test
Knoop and Vickers tests are used in micro and macro hardness testing to determine material hardness. It is
based on measuring the impression from an application of a force.
The Knoop test uses a diamond indenter ground to pyramidal form that produces a diamond shaped
indentation with an approximate ratio between long and short diagonals of 7:1. The depth of indentation is
about 1/30 of its length. When measuring the Knoop hardness, only the longest diagonal of the indentation is
measured. Originally the Knoop Hardness Number (KHN) was calculated by using this length and load in a
formula. Then, look-up tables became a popular source to find the KHN. Currently, most KHN results are
generated by digital measurement that automatically calculates the hardness number.
The Vickers test uses a ground squared pyramid. The depth of the indentation is about 1/7 of the diagonal
length. Unlike the Knoop test, when calculating the Vickers Diamond Pyramid hardness number, both
diagonals of the indentation are measured. The mean of these values used in a formula with the load
determines the Hardness Vickers value (HV). Similar to the Knoop test, tables of these values are available,
and the most current techniques utilize electronic or imaging measurements.
When choosing a test type you need to review the material, surface finish, geometry, thickness, uniformity and
other characteristics.
Q: What is the difference between a Knoop and a Vickers test?
A: Knoop and Vickers hardness scales are used for determining the hardness of a range of samples, including
thin materials or wires, coatings and small precision parts. In both cases, the hardness value is determined by
measuring the size of the indent and the test forces range from 1 g 100 kgf. They're defined by ASTM test methods E 384 and E 92. The Vickers indenter is a pointed, square-based diamond and the Knoop indenter is
a rhombic-based diamond.
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How GR&R Helps Your Rockwell Testing Process
A GR&R study determines how much of the tolerance in your testing process comes from the variation in the
equipment and the operators. When operator error or equipment error becomes a significant portion of the
tolerance, it's hard to determine if the results are accurately measured.
Performing GR&R reveals a lot about how well your system is reading Rockwell hardness; provides insight to
potential problems; and determines if you need
additional testing, such as direct verification.
A study conducted on 30 testers used daily
showed that 90% failed a direct verification
even though they passed an indirect verification
using test blocks. These testers consume most
of the allowable tolerances.
Adjustments using test block verification do not
accurately characterize an instrument's
performance.
A full GR&R study involves multiple operators
performing 90 tests using Rockwell test blocks. The calculated results reveal the inaccuracy of the tester.
Acceptable GR&R values vary depending on the tester type (analog, digital, closed loop), as well as the quality,
condition, and calibration status of the tester.
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Q: What is a Jominy test?
A: A Jominy test is a method for determining the hardenability of
steel. A test piece that typically measures 25 mm x 100 mm is
heated to a pre-determined temperature and quenched by a jet of
water sprayed onto one end. When the specimen is cold,
hardness measurements using the Rockwell HRC scale (10 kg
minor and 150 Kg major forces) are made at specific intervals
along the test piece from the quenched end. Test results are then
plotted on a standard chart. Hardness values are the highest at
the quenched end of the specimen. You should find that the
values decrease proportionally as you move to the other end.
We have found that using holding fixtures improves the accuracy
of our results. We recommend using automatic software and
stages to increase throughput. This setup accommodates from
one to several bars at once, and performs the tests at pre-
programmed intervals, while automatically plotting the data.
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How Can Testing Strengthen Your Smile?
There are an abundance of commercials luring us to buy different toothpastes each promising a different outcome: teeth whitening, cavity fighting, or breath freshening. Although having that bright white smile is
appealing to most, without the strength of enamel even the whitest of teeth will still decay.
Established in the 1940s, the Indiana University School of Dentistry has been researching enamel strength for
nearly 70 years. One of the school's many groundbreaking findings includes the first successful stannous
fluoride formula, the active decay-preventing agent in Crest toothpaste. Since 1999, Dr. Domenick Zero,
Professor and Department Chair and Director, Oral Health Research
Institute, and his staff have been studying various oral treatments and
preventative methods to understand their effect on the hardness of tooth
enamel. Without strong enamel, teeth become soft and prone to decay.
"Conducting hardness tests on tooth enamel allows us to measure how
much demineralization, or breakdown, of the tooth enamel has occurred,
based on changes in the size of the indentation," says Dr. Zero. "The
larger the indent the more demineralization of the enamel."
Reversing this damage isn't an easy task. However, after more than
10,000 hardness tests, the group has proven that the breakdown of
minerals in enamel can be repaired by remineralization a process which is enhanced by fluoride and helps to
harden the enamel. Zero's
studies show that when the
tooth is remineralized, the
indents get smaller.
For his research, Dr. Zero
conducts baseline
microhardness tests on the
tooth enamel. Then, he places
the tooth specimens on
dentures (or a similar
appliance), which is placed in
the mouth, and worn throughout
the study. The specimen is
eventually removed, tested again
for microhardness, and compared
to the baseline test results.
"We're able to get answers much quicker and with much less expense than if
we've had to run a full clinical trial measuring tooth decay," says Zero. "It's a way of getting clinically-relevant
info without taking years to do the study."
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ASTM E18-07: New Changes will Affect Your Rockwell Hardness Indenters
The latest changes to the ASTM E18 standard require suppliers to verify the geometry of indenters to meet
E18-07 compliance. This new requirement ensures that every Rockwell diamond indenter tip is verified for
correct cone angle and radius.
Why is this important?
The new standard ensures improved
performance of the indenters
throughout the testing range of
applicable Rockwell scales.
Old indenters verified to previous
revisions are not compliant to the
new standard and cannot be used
when testing to ASTM E18-07
unless they are verified and re-
certified to the new standard.
It's easy to verify if your indenters
are compliant to the revised
standard just view a copy of your indenter calibration certificate.
According to E18- 07, it is required
for the manufacturer to be ISO/IEC
17025 accredited by an
accreditation agency recognized by
the ILAC agreement. Examples of
such approved accrediting bodies
are NVLAP, UKAS and A2LA. The
compliance of your indenters can be
verifi