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 Lab 5: Polymers July 19 2000

 4.1 Procedure

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 [2].

 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.

 4.3 Discussion

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 joule[Figure 13].

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[1]. 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 [1]. This prevents the sulphur from migrating to the grain boundaries, and decreases the transition temperature, which makes steel less brittle at lower temperatures [1].
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.

 Result and Observations

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 [1].

The specimen from both materials was subjected to a certain minimum and maximum stress.
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

 4.3 Discussion

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 [1]. 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 [3].

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 final fracture[3].

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[1]. 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 [2].

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 scatter [3].

 Conclusion

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.