Lab 5: Polymers
July 19 2000
Refer to Professor H.W. Kerr and Professor W.H.S. Lawson, “ME 215 –
Structure and Properties of Materials: Laboratory Manual”, University of
Waterloo, Waterloo, Ontario. Spring, 2000. Page 35 .
4.2 Results And Observations
The results and observations to follow include calculations and graphs that
were plotted form the data obtained during the experiment. Figure 15 and 16
shows the relationship of the energy absorbed as a function of test
temperature for steel and aluminium. The upper and lower shelf energies for
steel is 124 and 1 joules. From the graphs it is estimated that the
transition temperature for steel is -20oC. There is no transition
temperature for aluminium. Figure 17 shows the relationship between amount
ductility fracture as a function of test temperature.
1) The transition temperature is the temperature at which a material is 50%
ductile and 50% brittle. The transition temperature was measured using two
methods. The first method was done using the energy absorbed as a function
of test temperature. The transition temperature was measured approximately
at negative twenty degrees Celsius [Figure 13]. The upper shelf energy was
measured at 124 joules and the lower shelf energy was measured at 1
The second method involved the amount of ductility as a function of
temperature. The transition temperature was approximately estimated at
negative twenty degrees Celsius.(Figure 15).
2. Why do some metals not show low temperature embrittlement?
Some metals do not show low temperature embrittlement because of the crystal
structure. FCC structures have more slip planes than the BCC structures.
Thus at lower temperature the planes in a BCC crystal structure will shut
down, while those in FCC do not. Therefore FCC metals do not undergo
transition temperatures as seen in Figure 14. Therefore, FCC metals do not
show low temperature embrittlement.
1. How can steels be made less brittle at low temperatures.
Steel can be made less brittle at lower temperatures by adding additives to
the metal. Adding manganese during the manufacturing of steel, cause sulphur
to react with the manganese . This prevents the sulphur from migrating to
the grain boundaries, and decreases the transition temperature, which makes
steel less brittle at lower temperatures .
2. Discuss the accuracy of the results and possible sources of error
The results from the experiment varied from the published values for the
transition temperature of steel by 25%. Possible sources of errors include
different composition of the specimens tested, as compared to the
composition of the material on which the published value is based. Also,
discrepancies between calculated and published values could be attributed to
different temperatures at which the tests were taken. Human error is the
greatest factor taken into account in the measurement and varied
temperatures of the specimens. Human error may have occurred in the
transferring of the specimen and performing the test, since there was only a
five-second-time delay allowed.
Figure 1 shows a stress Vs number of cycles to failure for 1045 cold rolled
steel and 2024 Aluminium. The graph shows that steel has an endurance limit
and a fatigue limit, but aluminium has only an endurance limit. A fatigue
limit for a material is when the graph line levels in to a straight
horizontal line. A stress below the fatigue limit will not plastically
deform the material, regardless of the number of cycles. An endurance limit
occurs in all materials that survive at least until 106 cycle’s .
The specimen from both materials was subjected to a certain minimum and
The maximum and minimum was found to be 45 ksi and –45 ksi respectively for
1045 steel for specimen#1.
Figure 2 shows the stress-time graph for specimen#1 for 1045 steel, which
shows the tensile and the compressive stress the material undergoes with
respect to time.
Figure 3 and figure 4 are drawing of two specimens that have undergone
fracture due to fatigue. The beach line, flow lines, and initial fracture
points are all visible
1) The ultimate tensile strength of cold
rolled 1024 steel is 91 KSI. (Mpa). The ultimate tensile stress of steel
is approximately double the endurance limit . Using figure 1 the
endurance limit of steel, taken at 106 cycles, is 52 ksi. Thus the
ultimate tensile stress of steels is 104 ksi. The UTS is higher that the
published values because of imperfections on the tested specimen.
2) Figure 3 shows a solid rotating shaft that has been fractured due to
fatigue. Upon inspection, the cause of the fatigue and the way the crack
propagated can be determined. Circular beach marks start from a single
point and radiate outwards. The initial point of the beach mark
indicates the point at which the crack started. It is evident that the
crack began at the keyway of the part, since the concentration of the
beach marks is high. Certain regions have a considerable distance
between beach lines, this indicates a region of rapid fracture. The
smooth regions in the specimen represent a brittle fracture, while the
rough region represents ductile fracture. It is evident that the shaft
failed under rotation because of the peak at the centre .
Figure 4 shows another shaft that has failed due to fatigue. It has
many similar characters to the first example, such as beach marks, flow
lines, point of initial crack, and fracture boundaries. This specimen is
unique because it has multiple initiation points. Each crack has its own
set of beach lines initiating from different points. Thus initially, the
cracks grew independent of each other. It is observed that the centre of
the specimen has a distinctive beach mark, this indicates the point of
3) When designing components with cycled non-ferrous materials, such as
aluminium, extra care must be taken. Non-ferrous materials have a
fatigue limit but not an endurance limit. Thus after a certain number
of cycles the material will fail. Therefore when designing a component
the engineer must determine the number of cycles that the component will
be exposed to and the maximum stress it should withstand. The engineer
should then select a material suitable for these conditions.
4) Fatigue failures in steel components can be delayed or avoided by
many methods. A well-designed component can increase its fatigue life.
Proper design involves avoiding notches, holes, sharp edges, and welds,
which encourages stress concentrations. Treating the material’s surface
by making it harder and wear resistant, without change the properties of
the core, increases the fatigue life. Hardening the surface can be
accomplished by case hardening, nitriding, and flame and induction
hardening. Plating adds a protective layer around the material, which
increase the wear resistance. Mechanical treatments including shot
peening, cold rolling, grinding and polishing can also improve fatigue
life. Shot peening and cold rolling introduces residual compressive
stress. Grinding and polishing reduce surface roughness, stress
concentrates, and surface flaw .
5) Fatigue initiates at stress concentrations, which can be caused by
slight imperfections such as surface flaws. Furthermore the location,
the number, and the extent of the flaw imperfections on the material
determine how and when the crack will propagate. Each of the test
specimens probably had different flaws due machining and other factors.
This resulted in slightly difference in the fatigue data, hence the
Many materials fail by fatigue. Fatigue initiates at stress concentration
and propagates until failure. The magnitude of the applied stress and the
number of cycles the component is exposed to determine when the failure will
occur. Fatigue can be avoided or delayed by proper design, proper choice of
material, and by treating the materials surface.