Fracture initiation revealed by variations in the fatigue fracture morphologies of PA 66 and PET fibers

Abstract

The tensile and fatigue fracture morphologies of polyamide 66 (PA 66) and polystyrene (PET) fibers are well known [1]. Tensile, and also creep, failure in both fibers is similar and generally involves crack initiation at or near the fiber surface, followed by slow crack propagation, as shown in Fig. 1 [2]. Tensile failure occurs when the remaining cross section can no longer support the induced stress and rapid failure occurs, giving the type of failure morphology shown in Fig. 2. Crack propagation under conditions which induce fatigue in these fibers is markedly different from tensile failure. In both fibers, crack initiation is, usually, as in monotonic tension, at or near the surface. However, under fatigue conditions the crack propagation is deviated so as to run at a slight angle to the fiber axis, gradually reducing the load bearing cross section [3]. Complementary broken ends of a PA 66 fiber broken in fatigue are shown in Fig. 3. It can be seen from Fig. 3 that the one broken end consists of a long tongue of material, the free end of which is the point of crack initiation. It can be noted that the tongue is always curved inwards towards the fiber axis, suggesting a residual compressive stress in the surface of the formally intact fiber. A closer inspection of the tongue shows that the fatigue fracture surface is concave, with respect to the fiber surface and not convex as would be expected if a pealing mechanism was driving crack propagation [4]. The complimentary end reveals the reduction of the load bearing section and both areas of final failure show that the final stage of fracture is by the tensile process shown above. Fatigue failure in PET fibers occurs under similar cyclic conditions to those which produce fatigue failure in PA 66, but the angle of crack penetration is smaller so that longer tongues are generally seen and final failure occurs often behind the fatigue crack tip by a creep process. In both tensile and fatigue failure the process of crack initiation is poorly understood, though some fracture morphologies seem to indicate the role of a thin surface layer on some fibers or the presence of an irregularity at the point of initiation. The present study has identified several unusual fracture morphologies, which have been induced under conditions which were known to provoke fatigue failure. The slower crack propagation which occurs during the fatigue process has allowed the causes of the initiation of some cracks to be determined. It is thought that these observations may be of relevance in the understanding of crack initiation in both tensile and fatigue failure. Figure 1 Slow crack growth from the surface of a PA 66 fiber under monotonic tensile loading.