PSI - Issue 17

C P Okeke et al. / Procedia Structural Integrity 17 (2019) 596–601 C P Okeke et al / Structural Integrity Procedia 00 (2019) 000 – 000

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the fatigue propagation is governed by the material bulk property. It is apparent that the scatter is minimal at high stress level, this is because at high stress the effect of specimen surface imperfection to the crack nucleation is less significant as the loading is more dominant. However, as the stress amplitude decreases, the surface condition starts to play a role in the fatigue life. The vacuum cast polyurethane is prone to voids due to low pressure manufacturing process. Also, this method of manufacturing can result in inter-sample variations. Hence, the level of scatter seen at 40% load level may not be surprising. In comparison to the fatigue life scatter of conventional injection moulded Polycarbonate (PC) and Polymethyl-methacrylate (PMMA) materials of automotive lamps, Okeke et al (2018), at 40% fatigue load, the maximum life is 11 times of the minimum life for vacuum cast PU while for PC and PMMA is 1.9 and 2.7 respectively. Also, across three fatigue load levels, the fatigue life of vacuum cast PU is much lower than that of conventional polymers.

Figure 4: Fatigue life curve of vacuum cast polyurethane (a) a-N curve, (b) S-N curve

4.3. Microscopic fractography

a

b

Propagation Initiation

c

Fast fracture

Figure 5: Fractured surface micrograph, 75x magnification – (a): 80% loading, (b): 60% loading, (c): 40% loading

Fig (5) shows the fracture surface micrograph of the vacuum cast polyurethane specimens at three different load levels. At each load level, a fracture surface of one of five specimens tested is shown. The image was obtained with 75x magnification. The three stages involved in the mechanical fatigue failure are highlighted on the fracture surface. The first stage known as crack nucleation or initiation is the site where crack originates. The second stage is propagation; the crack grows resulting in final failure of the specimen. It can be seen from the fracture surface of the three load levels that crack propagation site increases in size as the load level decreases. The crack nucleation originates at one edge of the notch area and orientated 45° angle in relation to the applied load and then follows perpendicular to the load direction as crack propagates. This corresponds to the observation made by Totten (2008). The circled areas of the 80% and 60% load levels specimen fracture surface micrograph show surface tear, for 80% load level, it is near the final fast fracture region while for 60% load level it is near the area of the crack initiation. This shows that vacuum cast polyurethane can be sensitive to high stress loading.