me 215 – engineering materials i dr. oğuzhan yilmaz assistant professor mechanical engineering...
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ME 215 – Engineering Materials I
Dr. Oğuzhan YILMAZAssistant Professor
Mechanical EngineeringUniversity of Gaziantep
Chapter 7
Brittle Fracture and Impact Properties
2
Introduction
A great deal of attention was directed to the brittle failure of welded ships and tankers.
Failures occured during winter months and when the are in heavy seas and anchored at dock.
This fact focussed on that normally ductile mild steel can become brittle under certain conditions.
Therefore, researches aimed to understand the mechanism of brittle fracture and fracture in general.
While the brittle failure of ships concentrated great attention to brittle failure in mild steel.
Brittle failures in tanks, pressure vessels, pipelines, and bridges have been noticed.
3
Introduction
There are five kinds of fracture in metals based on the nature of process:
I. Ductile,
II. Brittle,
III. Adiabatic shear,
IV. Creep,
V. Fatigue fracture.
Three basics factors contribute to a brittle-cleavage type of fracture:
(1) triaxial state of stress(2) Low temperature (3) High strain rate or rapid rate of loading
4
Introduction
There are circumstances under which certain ductile materials behave
as brittle.
Two important cases of this type of failure (i.e. brittle failure of ductile
materials) are:
1.Fatigue failure (which was studied previously)
2.Brittle fracture (which is going to be treated here).
5
Introduction
Common examples of catastrophic failures of structures caused by
brittle fracture are:
- Welded ships & tankers made of mild steel (during World War II)
- Rails of railways during cold winter periods.
6
Introduction
Brittle fractures in steel structures usually occur without visible or audible
warnings at stresses less than nominal Sy value.
Such fractures usually initiate at sharp notches and crack-like defects,
and may subsequently propagate through a complete structure at faster
than speed of sound.
Load-time history for an instrumented Charpy test (Dieter, 1988)
7
Brittle Fracture
Radiating pattern of markings is important as they point back towards the
origin of fracture, allowing the point of crack initiation to be traced (Fig. 1).
origin of fracture
radiating markings
Figure 1
8
Brittle Fracture
Fig. 2 shows the crack initiation and propagation with herringbone type
surface markings. The direction of crack propagation is the opposite to
the direction of crack initiation.
direction of crack propagation
direction of crack initiation
Figure 2
9
Impact Properties
Many engineering components are subjected to suddenly applied loads
and they are expected to transmit or absorb this impact load.
The energy of impact load can be absorbed by part as elastic or plastic
deformation.
In design stage, it is aimed that this energy of impact load is absorbed as
elastic deformation.
After load is passed, this elastic strain is released or transmitted, and the
structure does not suffer permanent deformation.
10
Impact Properties
However, the elastic range may be exceeded due to unexpected service
conditions or faulty design. In such cases, most ductile metals exhibit
some plastic deformation in two ways:
(1)it can redistribute the stress (thus, reducing harmful effects
(2)the visible appearance of plastic deformation itself can be a warning
for taking further precautions.
In a brittle metal structure, no noticeable deformation is observed and
fracture happens without warning.
Due to this fact, necessary cautions must be taken when using brittle
metals (e.g. using large safety factors).
11
Impact Properties
However, serious problems can arise when a ductile metal fractures in a
brittle manner without any prior plastic deformation.
Many metals which show a ductile behaviour in static tensile tests
exhibit a brittle behaviour under impact loading at low temperatures.
Thereby, the information from tensile tests is not enough to predict the
behaviour in such cases.
12
Impact Properties
The property of a material relating to work required to cause rupture is
toughness, which depends on the ductility and ultimate strength.
It is known that a high-rate of loading results in an increase in strength,
but a reduction in ductility. When forces are applied suddenly for very
short time intervals, another effect of such forces is to produce stress
waves.
13
Impact Properties
Not all materials respond in the same way to variations in strain rate.
For instance, a slowly applied point load shatters the glass while a high-
speed bullet punctures a fairly clean hole.
Similarly, sealing wax behaves in a ductile manner at low strain rate, but
snaps into two under a sharp blow.
14
Impact Velocity
The toughness of a material does not vary greatly over a considerable
range in striking velocity. However, above some critical speed (varying
from material-to-material), the energy required for rupture of a material
appears to decrease rapidly. This critical velocity is associated with rate
of propagation of plastic strain and is effected by the specimen length.
ρEVp E : Young’s modulus, MPa
ρ : mass density, kg/mm3
Velocity of stress wave (Vp) should
be distinguished from the velocity
of particles in stressed zone (Vx):
EVS x
In elastic region, velocity of
plastic wave propagation in
a cylindrical bar (Vp) is:
Following equation shows that stress (S) depends
on particle velocity (Vx) in addition to E and ρ:
e
epx dVV0
e : plastic strain
corresponding to Vx
15
Specimen Shape
The specimen shape also has a marked effect upon its capacity to resist
impact loads. A plain ductile bar will not fracture under an impact load at
normal temperatures. If the specimen is notched, fracture can happen
under a single blow.
Figure 3
Many different notch
configurations used
in impact tests are
suggested in ASTM
E23 & DIN 50115.
However, Charpy
and Izod are the two
standard classes of
specimens used for
notched-bar impact
testing (Fig. 3).
16
Impact Testing (Pendulum Type)
In pendulum type impact testing, the
impact load is produced by swinging
of an impact weight (W = m * g) from
initial height (h0) through the arc of a
circle, thus striking and fracturing the
notched specimen (Fig. 4). After that,
the weight reaches maximum height
(h1). Neclecting frictional losses, the
energy used to fracture the
specimen (U) is then approximately
defined as:
Figure 4
Absorbed Energy
(energy to rupture)
= Initial Potential Energy
(energy before rupture)
– Final Potential Energy
(energy after rupture)U = m * g * (h0 – h1)
The absorbed energy (U), indicated on the scale of tester, is expressed
in joule (i.e. N*m) or kg*m in metric system and in inch-pounds in British
system. This energy value is sometimes called “impact toughness”.
17
Charpy and Izod Type Impact Tests
The Charpy specimen is supported at the ends
and struck in the middle (Fig. 5).
However, the Izod specimen is a cantilever
beam with a notch on the tension side to ensure
fracture when the impact load is applied (Fig. 6).
Figure 5
Figure 6
Charpy and Izod type impact tests bring out the
notch behavior (Brittleness vs Ductility) by
applying a single overload of stress.
The notch behavior in an individual test applies
to specimen size, notch geometry and testing
conditions. Thus, such a behavior cannot be
generalized to the other specimen sizes or
conditions.
Charpy
Izod
18
Impact Fracture
brittle fractureductile fracture
19
Factors Affecting Impact Properties
Impact toughness values are greatly influenced by the testing conditions.
-The most pronounced is the effect of temperature on notch behaviour of
material.
- Tangential striking velocity should not be less than 3 m/s nor more than
6 m/s.
- Rigidity of testing machine and its parts are important since some
energy is absorbed by the machine itself.
20
Factors Affecting Impact Properties1
.
Temperature:
The notched-bar impact test has the greatest
importance in determining “ductile-to-brittle
transition” of a metal.
This transition occurs at a temperature below
which the material is brittle and fractures with a low
energy absorption & low ductility, and above which
it is ductile.
The transition actually covers a range of temperatures in which degree of
brittleness increases gradually as temperature falls.
It is very difficult to make a universal definition of transition temperature as
two different materials having the same transition temperature may have
different failures.
Therefore, there are many definitions of this temperature.
21
Factors Affecting Impact Properties
Fig. 7
Ta: The average temp. corresponding to minimum impact strength (15 ft/lb).
Tb: The lowest temp. (Fracture Transition Plastic - FTP) at which the specimen exhibits 100% shear fracture.
Tc: The temp. (Fracture Appearance Transition Temperature - FATT) at which 50% of fracture is ductile.
Td: The average temp. between ductile and brittle fracture, i.e. (Tb+Tf)/2.
Te: Like Ta, it is a special temp. (Ductility Transition Temperature - DTT) based on an arbitrary low-impact energy toughness.
Tf: The temp. (Nil Ductility Temperature - NDT) for 100% brittle fracture.
The definitions of transition temperature (Fig. 7) are as follows:
Figure 8
22
Factors Affecting Impact Properties
2. Composition: In metals, the impact testing is mostly applied to steel,
testing of nonferrous materials is seldom.
- The main factors influencing brittleness of steels are:
composition, heat treatment and section size.
- The greater is the hardness,
the higher is the transition temperature.
Considering the effect of composition in
steels, carbon content plays important
role (Fig. 8).
The optimum combination of properties
in quenched and tempered low alloy
steel occurs for 0.3 - 0.4 % C.
The effect of other elements on
impact properties can be found
in the textbook.
23
Factors Affecting Impact Properties
4
.
Microstructure: The shape of carbide
precipitates in steel has a great effect on
impact toughness.
A tempered martensitic structure has the
best combination of strength and fracture
toughness. Tensile properties of such
structures of the same carbon content
and the same hardness are alike, but
great variations in their impact toughness
with temperature.
3. Grain Size: As the grain size increases, transition temperature
increases and fracture stress decreases. Thereby, it is possible to
improve ductility and toughness of steel by obtaining ultrafine grain size.
24
Factors Affecting Impact Properties
5. Orientation: The orientation of test bar in a formed product affects both
the impact energy and the value of Fracture Appearance Transition
Temperature,FATT, as well as the tensile ductility. For rolled products,
orientation does not have a great influence on FATT.
Effect of specimen orientation of Charpy transition-temperature
curves (Dieter, 1988)
25
Ductile-to-Brittle Transition (Embrittlement)
1. Hydrogen Embrittlement: Hydrogen produces severe embrittlement in
many metals. Even very small amount of hydrogen can cause cracking
in steel and titanium. It may be introduced during melting and entrapped
during solidification, or it may be picked up during heat treatment, acid
pickling, electroplating or welding.
2. Temper Embrittlement: Tempering some steels within 450 - 590 °C
results in temper brittlement, which is manifested by increase in impact
transition temperature. It is due to segregation of certain elements to
grain boundaries, giving local hardening to fracture.
3. Blue Brittleness: Low-carbon steels exhibit two types of aging which
causes an increase in transition temperature: quench aging & strain
aging. Strain aging is the slow increase in hardness in steels finished
by cold work (mainly cold rolling). Blue brittleness is attributed to strain
aging caused by heating cold worked steel to around 205 °C.
26
Alternative Impact Tests
It is not possible to obtain realistic results by conventional tests if specimens
are of thicknesses greater than Charpy and Izod specimens. So, there are
two alternative tests of practical importance:
1. Drop Weight Test: This test (ASTM E208) is
employed to determine NDT of ferritic steels
of 15.9 mm or thicker. A simple rectangular
specimen is subjected to a single impact load
(free-falling weight with energy of 340-1630
J) at selected temperatures to determine
max. temp. at which specimen breaks (Fig.
9).
Figure 9
A crack-starter weld (63.5 mm long & 12.7 mm
wide) is deposited on tension side of
specimen. An artificial notch is cut at the centre
of weld bead length to start crack, and the
specimens are tested to determine the NDT.
27
Alternative Impact Tests
2. Dynamic Tear Test: This test (ASTM E604)
is used to determine resistance of a material
to rapid progressive fracturing. Single-edge
notched beam is impact loaded in 3-point
bending (Fig. 10). The notch is machined to
start a crack. The impact energy is imparted
by a swinging pendulum or a drop-weight of
highly sufficient capacity at a test velocity of
4.9 - 8.5 m/s. Hence, the total energy loss
during separation is recorded.
Figure 10
Figure 11
At temperatures below NDT, the fracture is
flat and completely brittle without any shear
lips (Fig. 11). Above NDT, absorbed energy
increases, surface begins to develop shear
lips becoming progressively more dominant.
At FTE and above, fracture is fully ductile.
28
Impact Testing of Plastics
The impact tests for plastics can be divided into two groups:
a) using instruments where energy is imparted by a swinging pendulum.
b) using free-falling weights or other impactors to impart energy.
The pendulum type machines are similar to those
used for testing metals, but smaller in size and
capacity to comply with low-energy requirements of
plastics.
Charpy and Izod type plastic specimens were
given in Fig.4.
As a standardized type, ISO Charpy specimen is:
a rectangular bar (120 mm long, 15 mm wide, 10
mm thick), which can be tested without or with
rectangular notch (2 mm wide and 3.3 mm high).
29
Impact Testing of Plastics
It is more difficult to interpret the results of impact testing for plastics:
1. The test may be too severe (may cause brittle behavior unrealistically).
2. The test result may be dependent more on crack propagation resistance
than ability to resist crack initiation.
3. Test conditions may give misleading results even on a comparative basis.
4. Test conditions may probably be unrelated to service conditions.
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