Fatigue Properties of Highly Oriented Polypropylene Tapes and All-polypropylene Composites

This study describes the fatigue behaviour of newly developed all-polypropylene (all-PP) tapes and woven composites [1], with reference to the composite processing conditions, testing temperature and comparison to commercial alternatives. The possible damage development mechanisms are also part of the present paper. All-PP tapes used in this study are highly oriented (90 wt% oriented phase) and their failure behaviour follows that of other highly oriented polymers. The degree of macromolecular orientation, the level of crystallinity, the structure and morphology of the sample, as well as the testing temperature are decisive parameters for their fatigue life. The fatigue response of PP tapes (S-N curve) is shown in Figure 1. Stage A of fatigue is usually associated with orientation and structural changes in the amorphous phase of the fibres [2], which is absent here. Due to their highly fibrillar character Stage A and B are not separable and an approximately linear decrease with the logarithm of N appears from the beginning of the fatigue test. The fatigue limit of PP tapes is observed to be at stresses as high as 400MPa, which represents around 65% of the UTS of the material and is quite high compared to values obtained in case of other highly oriented polymer fibres. Fig. 1: Fatigue response of PP tapes at RT represented by S-N curves (arrows signify specimens that did not fail after 10 cycles) The fatigue properties of all-PP composites are tested on the example of belt-pressed (BP) and vacuum bagged (VB) all-PP specimens (see Figure 2) [1]. Both specimens behave similarly under fatigue since the endurance life is found to be at about 65% of the UTS of each 10 10 10 10 10 10 10 10 10 0 100 200 300 400 500 600 700