MECHANICAL PROPERTIES5

MECHANICAL PROPERTIES

Group No. 1

Marzo, Shintaro D.

Abaya, Princess Monica B.Briones, Minette A.Dinglasan, Alyssa R.Lasi, Frances Doreen B.Real, Mark Joseph M.

Introduction and Terminology for Mechanical PropertiesReporter: Marzo, Shintaro D.The mechanical properties of a material describe how it will react to physical forces. Mechanical properties occur as a result of the physical properties inherent to each material, and are determined through a series of standardized mechanical tests.Having knowledge about the mechanical properties of a material is essential in material selection. Some materials can become brittle when temperatures are low and/or strain rates are high. The special chemistry of the steel used on the Titanic and the stresses associated with the fabrication and embrittlement of this steel when subjected to lower temperatures have been identified as factors contributing to the failure of the ships hull.There are some commonly used terminologies used for mechanical properties: Stress: Force per unit area over which the force is acting. Strain: Elongation per unit length. Modulus of Elasticity (E): Youngs modulus, or the slope of the linear part of the stressstrain curve in the elastic region. It is a measure of the stiffness of the bonds of a material and is not strongly dependent upon microstructure. Plastic deformation or strain: Permanent deformation of a material when a load is applied, then removed. Elastomers: Natural or synthetic plastics that are composed of molecules with spring-like coils that lead to large elastic deformations (e.g., natural rubber, silicones). Viscous material: A viscous material is one in which the strain develops over a period of time and the material does not return to its original shape after the stress is removed. Anelastic (viscoelastic) material: A material in which the total strain developed has elastic and viscous components. Part of the total strain recovers similar to elastic strain. Some part, though, recovers over a period of time. Examples of viscoelastic materials include polymer melts and many polymers. Typically, the term anelastic is used for metallic materials. Stress relaxation: Decrease in stress for a material held under constant strain as a function of time, which is observed in viscoelastic materials. Stress relaxation is different from time dependent recovery of strain. Viscosity (): Measure of the resistance to flow, defined as the ratio of shear stress to shear strain rate (units Poise or Pa-s). Newtonian Materials: in which the shear stress and shear strain rate are linearly related (e.g., light oil or water). Non-Newtonian: Materials in which the shear stress and shear strain rate are not linearly related; these materials are shear thinning or shear thickening (e.g., polymer melts, slurries, paints, etc.). Shear thinning (pseudoplastics): Materials in which the apparent viscosity decreases with increasing rate of shear. Shear thickening (dilatant): Materials in which the apparent viscosity increases with increasing rate of shear. Thixotropic behavior: Materials that show shear thinning and also an apparent viscosity that at a constant rate of shear decreases with time. Rheopectic behavior: Materials that show shear thickening and also an apparent viscosity that at a constant rate of shear increases with time.References:http://www.engineershandbook.com/Materials/mechanical.htmhttps://www.youtube.com/watch?v=FSGeskFzE0sThe Science and Engineering of Materials, 6e, Askeland, Fulay, Wright

Hardness of Materials, Strain Rate Effects and Impact BehaviorReporter: Dinglasan, Alyssa R.

HARDNESS OF MATERIALSHardness the relative capacity or resistance of a material to scratching or indentation.Measurements of Hardness: Scratch Hardness - the measure of how resistant a sample is to fracture or permanent plastic deformation due to friction from a sharp object.

Rebound hardness - also known as dynamic hardness, measures the height of the "bounce" of a diamond-tipped hammer dropped from a fixed height onto a material.

Indentation hardness - measures the resistance of a sample to material deformation due to a constant compression load from a sharp object.Hardness TestsRockwell Hardness Test uses a small-diameter steel ball for soft materials and a diamond cone which is called a Brale, for harder materials. The testing machine automatically measures the depth of penetration of the indenter and converts it to the Rockwell hardness number (HR). There are two variations of this test, the Rockwell C (HRC) test which is used for hard steels and the Rockwell F (HRF) which is more suited for aluminium.Brinell Hardness Test uses a hard steel sphere (usually 10mm in diameter) which is forced into the surface of the material. The diameter of the impression is measured and the Brinell hardness number (HB) is calculated.Vickers hardness test similar to Brinell hardness test but usually uses a diamond pyramid indentor.Knoop hardness test a microhardness test, forming such small indentations that a microscope is required to obtain the measurement. The load t apply should be less than 2N.

STRAIN RATE EFFECTS AND IMPACT BEHAVIORWhen a material is subjected to a sudden, intense blow, in which the strain rate is extremely rapid, it may behave in a much more brittle manner than it is observed in the tensile stress.An impact test is used to evaluate the brittleness of a material under these conditions. Note that the strain rates in this test are much higher.In the Izod test (usually for plastics), a heavy pendulum, starting at an elevation h0, swings through its arc, strikes and breaks the specimen, and reaches a lower final elevation hf. Now, if we know the initial and final elevations of the pendulum, we can calculate the difference in potential energy. This difference is the impact energy absorbed by the specimen during failure.Impact toughness is the ability of a material to withstand an impact blow.Tensile toughness is the area under the true or engineering stress-strain curve.In both cases, we are measuring the energy needed to fracture a material. The difference is that, in tensile tests, the strain rates are much smaller compared to those used in an impact test. Another difference is that in an impact test we usually deal with materials that have a notch.Fracture toughness is the ability of the material containing flaws to withstand applied load.Properties Obtained from Impact Test Ductile to Brittle Transition Temperature (DBTT) the temperature at which a material changes from ductile to brittle fracture. Notch Sensitivity this cause concentrating stress and reducing toughness of materials. The absorbed energies are much lower in notched specimens if the material is notch-sensitive.References:https://en.wikipedia.org/wiki/Hardness http://en.wikipedia.org/wiki/Charpy_impact_test https://www.youtube.com/watch?v=tpGhqQvftAohttps://www.youtube.com/watch?v=RJXJpeH78iUhttps://www.youtube.com/watch?v=G2JGNlIvNC4https://www.youtube.com/watch?v=5HcsPgkusZ0http://realitypod.com/2011/08/top-10-hardest-materials/The Science and Engineering of Materials, 6e, Askeland, Fulay, Wright

Fracture MechanicsReporter: Marzo, Shintaro D.Fracture mechanics is the discipline concerned with the behavior of materials containing cracks or other small flaws. The term flaw refers to such features as small pores (holes), inclusions, or microcracks. Fracture toughness measures the ability of a material containing a flaw to withstand an applied load.From investigating fallen structures, engineers found that most failure began with cracks. These cracks may be caused by material defects (dislocation, impurities...), discontinuities in assembly and/or design (sharp corners, grooves, nicks, voids...), harsh environments (thermal stress, corrosion...) and damages in service (impact, fatigue, unexpected loads...). Most microscopic cracks are arrested inside the material but it takes one run-away crack to destroy the whole structure.To analyze the relationship among stresses, cracks, and fracture toughness, Fracture Mechanics was introduced. The first milestone was set by A. A. Griffith in his famous 1920 paper that quantitatively relates the flaw size to the fracture stresses. However, Griffith's approach is too primitive for engineering applications and is only good for brittle materials.Fracture mechanics is significant for its approach allows us to design and select materials while taking into account the inevitable presence of flaws.1. Selection of a Material: If we know the maximum size of flaws in the material and the magnitude of the applied stress, we can select a material that has a fracture toughness KC or K1C large enough to prevent the flaw from growing.2. Design of a Component: If we know the maximum size of any flaw and the material (and therefore its KC or K1C has already been selected), we can calculate the maximum stress that the component can withstand. Then we can size the part appropriately to ensure that the maximum stress is not exceeded.3. Design of a Manufacturing or Testing Method: If the material has been selected, the applied stress is known, and the size of the component is fixed, we can calculate the maximum size of a flaw that can be tolerated. A nondestructive testing technique that detects any flaw greater than this critical size can help ensure that the part will function safely. In addition, we find that, by selecting the correct manufacturing process, we can produce flaws that are all smaller than this critical size.

Brittle fracture refers to any crack or imperfection limits the ability of a ceramic to withstand a tensile stress. This is because a crack (sometimes called a Griffith flaw) concentrates and magnifies the applied stress.Three ways of applying a force to enable a crack to propagate:

Mode I fracture Opening mode (a tensile stress normal to the plane of the crack)

Mode II fracture Sliding mode (a shear stress acting parallel to the plane of the crack and perpendicular to the crack front)

Mode III fracture Tearing mode (a shear stress acting parallel to the plane of the crack and parallel to the crack front)

References:http://www.efunda.com/formulae/solid_mechanics/fracture_mechanics/fm_intro.cfmhttp://en.wikipedia.org/wiki/Fracture_mechanicsThe Science and Engineering of Materials, 6e, Askeland, Fulay, Wrighthttps://www.youtube.com/watch?v=o1r84NWV710

Microstructural Features of Fracture in Metallic Materials & Microstructural Features of Fracture in Ceramics, Glasses, and CompositesReporter: Abaya, Princess Monica B.

Microstructural Features of Fracture in Metallic MaterialsFracture is the separation of a body into two or more pieces in response to an imposed stress that is static and at temperatures that are low relative to the melting temperature of the material. Failure in structures leads to lost of properties and sometimes lost of human lives. Failure in metallic materials can be divided into two main categories; Ductile and Brittle Fracture. Ductile fracture normally occurs in a transgranular manner in metals that have good ductility and toughness and are usually caused by simple overloads, or by applying too high a stress to the material. It involves a large amount of plastic deformation and can be detected beforehand. Microvoids are formed when a high stress causes separation of the metal at grain boundaries or interfaces between the metal and small impurity particles.Brittle Fracture occurs in high-strength metals and alloys or metals and alloys with poor ductility and toughness. It is frequently observed when impact, rather than overload causes failure. Brittle fracture is more catastrophic and has been intensively studied. In some cases, the crack may take an intergranular path particularly when segregation or inclusions weaken the grain boundaries. Chevron Pattern is a common fracture feature produced by separate crack fronts propagating at different levels in the material. It is visible with the naked eye or a magnifying glass and helps us identify both the brittle nature of the failure process as well as the origin of the failure.

Microstructural Features of Fracture in Ceramics, Glasses, and CompositesIn Ceramic Materials, Ionic or covalent bonds permit little or no slip. Consequently, failure is a result of brittle fracture. Most crystalline ceramics fail by cleavage along widely spaced, closely packed planes. Fracture surface is typically smooth and frequently no characteristic surface features point to the origin of the fracture. Glass is also a fracture in a brittle manner. Conchoidal fracture is a fracture surface containing a very smooth mirror zone near the origin of the fracture, with tear lines comprising the remainder of the surface.Polymers fail by either a ductile or brittle mechanism. Composites are fracture in fiber-reinforced composite materials is more complex. These composites contain strong, brittle fibers surrounded by a soft, ductile matrix, as in boron- reinforced aluminum.

References:http://neon.mems.cmu.edu/rollett/27301/L6A_GriffithEq-15Oct07.pdfwww.ttu.ee/public/s/Sustainable.../materials/..._/L13-14_Fracture.pptxhttp://books.google.com.ph/books?id=ZbBWT6r4VG4C&pg=PA198&lpg=PA198&dq=Microstructure+Features+of+fracture+in+Ceramics,+Glasses+and+Composites&source=bl&ots=18m5qWtUPq&sig=gz0kb3BnGCEvuEjKtDAhP6QBJfk&hl=en&sa=X&ei=LfveU7TMCNO6uASI-YLwDg&ved=0CB4Q6AEwAQ#v=onepage&q=Microstructure%20Features%20of%20fracture%20in%20Ceramics%2C%20Glasses%20and%20Composites&f=falsehttps://www.youtube.com/watch?v=6apSK893p7EThe Science and Engineering of Materials, 6e, Askeland, Fulay, Wright

FatigueReporter: Lasi,Frances Doreen B.

Fatigue is the structural damage that results from repeated or otherwise varying stress which never reaches a level sufficient to cause failure in a single application. Fatigue is also the initiation and growth of a crack, or growth from a pre-existing defect, which progresses until a critical size is reached. Some terminologies used for fatigueness of a material:Fatigue: is the lowering of strength or failure of a material due to cyclic loading.Stages of Fatigue FailureInitiation: is the most complex stage of fatigue fracture.The initiation site of a given fatigue fracture is very small, never extending for more than two to five grains around the origin. Propagation: is a stage where fatigue causes the microcrack to change direction and grow perpendicular to the tensile stress. Rapid Fracture Crack: As the propagation of the fatigue crack continues, gradually reducing the cross-sectional area of the part or test specimen, it eventually weakens the part so greatly that final, complete fracture can occur with only one more load application. Fatigue Life:The number of cycles permitted at a particular stress before a material fails by fatigue.Fatigue Test: Measures the resistance of a material to failure when the stress below the yield strength is repeatedly applied.Fatigue Strength: the stress required to cause failure by fatigue in a given number of cycles, such as 500 million cycles.S-N Curve (Whler Curve) : a graph showing stress as a function of number of cycles in fatigue.Endurance Ratio: Endurance limit divided by the tensile strength of the material. The ratio is about 0.5 for many ferrous metals.Endurance limit: An older concept that defined a stress below which a material will not fail in a fatigue test.Tensile strength (TS) or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before failing or breaking.

Factors that affect Fatigue Failure: Mean Stress: increasing the mean stress level leads to a decrease in fatigue life.Surface Effects: For many common loading situations, the maximum stress within a component or structure occurs at its surface. Consequently, most cracks leading to fatigue failure originate at surface positions, specifically at stress amplification sites. Design Factors: The design of a component can have a significant influence on its fatigue characteristics. Any notch or geometrical discontinuity can act as a stress raiser and fatigue crack initiation site; these design features include grooves, holes, keyways, threads, and so on. Surface Treatments: During machining operations, small scratches and grooves are invariably introduced into the work piece surface by cutting tool action. These surface markings can limit the fatigue life.

Figure (a), represents a revolving shaft with sharp corner Figure (b) is a revolving shaft with fillet in corners for fatigue life improvement.

Case Hardening: is a technique by which both surface hardness and fatigue life are enhanced for steel alloys. This is accomplished by a carburizing or nitriding process whereby a component is exposed to a carbonaceous or nitrogenous atmosphere at an elevated temperature.Environmental EffectsThermal fatigue: is normally induced at elevated temperatures by fluctuating thermal stresses; mechanical stresses from an external source need not be present. Corrosion fatigue: is the failure that occurs by the simultaneous action of a cyclic stress and chemical attack.References:The Science and Engineering of Materials, 6e, Askeland, Fulay, WrightCalister, William D. Jr. ; Rethwisch, David G. Materials Science and Engineering, 8th edition John Wiley & Sons, Inc. , 2010https://www.youtube.com/watch?v=6apSK893p7Ehttp://en.m.wikipedia.org/wiki/Fatigue_(material)

CreepReporter: Minette A. Briones

Creep which sometimes called cold flow is a time dependent permanent deformation at high temperatures, occurring at constant load or constant stress and the tendency of solid material to move slowly or deform permanently under the influence of mechanical stresses. It is more severe in materials that are subjected to heat like metals for long periods, and generally increases as they near their melting point.Creep test will help to determine the creep characteristic of a metal, wherein it measures the resistance of a material to deformation and failure when subjected to a static load below the yield strength at an elevated temperature.

Stages of Creep

In the first stage or the primary stage of creep of metals, many dislocations climb away from obstacles equals the rate at which the dislocations are blocked by other imperfections. The second state which is known as steady-state creep is the most understood stage wherein the steady-state portion of the creep curve is the creep rate. Creep rate =

During the third stage or the tertiary creep, necking begins, the stress increases, and the specimen deforms at an accelerated rate until failure occurs.

References:en.wikipedia.org/wiki/Creep_(deformation)https://www.youtube.com/watch?v=hUk2_Y34WRIThe Science and Engineering of Materials, 6e, Askeland, Fulay, Wright

Stress Rupture and Stress CorrosionReporter: Mark Joseph M. Real

Stress Rupture is the sudden and complete failure of a material held under a definite constant load for a given period of time at a specific temperature. In stress rupture testing, loads may be applied by tensile bending, flexural, biaxial or hydrostatic methods. Ductile stress-rupture failures occur at high creep rates and relatively low exposure temperatures and have short rupture times. Brittle stress-rupture failures show only little necking and occur more often at smaller creep rates and high temperatures. And theres a way to measure the stress rupture by means of Stress Rupture Test (SRT) determines the tendencies of materials that may break under an overload. During the stress rupture test, material is subjected to a constant load at a constant temperature while the time to rupture is measured. The reported results are very useful in the selection of materials where dimensional tolerances are not critical, but rupture cannot be allowed.

Stress Corrosion is a phenomenon in which materials react with corrosive chemicals in the environment. This lead to formation of cracks and lowering of strength. Stress Corrosion is evidenced when the metal strength loss resulting from the combined stress and corrosion is greater than the effects of stress and corrosion acting separately. The magnitude of the combined effect is a measure of the susceptibility of the material to stress corrosion. SCC is the conjoint action of stress and a corrosive environment which leads to the formation of a crack which would not have developed by the action of the stress or environment alone. Why is it a problem? Because, it can happen unexpectedly and rapidly after a period of satisfactory service leading to catastrophic failure of structures or leaks in pipe work. Where does it occur? Typical SCC failures are seen in pressure vessels, pipework, highly stressed components and in systems when an excursion from normal operating conditions or the environment occurs.

Stress corrosion cracking(SCC) is the growth of crack formation in acorrosiveenvironment. It can lead to unexpected sudden failure of normallyductilemetals subjected to atensile stress, especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal otherwise. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals.

Stress corrosion cracking (SCC) results from the combined action of three factors:1. Tensile stresses in the material1. A corrosive medium - especially chloride-bearing or hydrogen- sulphide (H2S) media. Chloride-induced SCC normally occurs above60C (140F).1. The use of material susceptible to stress corrosion cracking (SCC)

Methods of minimizing stress corrosion

By selecting a material that is not susceptible By controlling stresses through careful design and minimizing stress By keeping concentrations below the critical value By reducing stresses through heat treatments and careful design for manufacturing By using corrosion inhibitors during cleaning operations By coating the material and effectively isolating the material from the environment

References: http://www.labtesting.com/services/materials-testing/mechanical-testing/stress-rupture/ http://corrosion-doctors.org/Forms-SCC/scc.htm http://www.smt.sandvik.com/en/materials-center/corrosion/wet-corrosion/stress-corrosion-cracking-scc/ The Science and Engineering of Materials, 6e, Askeland, Fulay, Wrighthttps://www.youtube.com/watch?v=z0bmZGcQOu8https://www.youtube.com/watch?v=4As4csiS4bg