Fatigue crack propagation in short and long glass fiber reinforced injection-molded polypropylene composites

Fatigue Fracture of Neat and Short Glass Fiber Reinforced Polypropylene: Effect of Frequency and Material Orientation

“…In the first stage of the fatigue test the FCP rate decreases as already observed in previous works [2,5,10] for both neat and short glass fiber reinforced …”

“…The surface or skin layers are usually characterized by fibers aligned predominantly parallel to the mold filling direction (MFD), while in the central or core layer the fibers are preferentially oriented transversely to the MFD. For polypropylene-SGF reinforced injection molded plaques of about 3 mm thickness, Karger-Kocsis [5] reported that the core layer is much thinner than the skin layers at all volume fractions. Even if we could not perform a complete and exhaustive fiber distribution experimental analysis, a microscopic observation of the fracture surface evidenced a very limited core structure.…”

“…In particular, the fatigue properties are of paramount importance for many intended applications where components are subjected to load and environmental histories which vary in time over the period of service [2][3][4][5][6][7][8][9][10]. Fatigue damage is associated with the initiation and propagation of cracks in the matrix and/or the destruction of the bonding at the fiber/matrix interface.…”

“…Final failure of PP and its short-fiber reinforced (SFR) composites under alternating loadings, mainly occurs by fatigue crack propagation (FCP) [2,5,6,10] characterized by a stable crack acceleration range which can be reasonably well described by the Paris-Erdogan relationship [11]: (1) where da/dN is the crack growth per each cycle, A and n are constants and ∆K is the stress intensity amplitude at the crack tip under mode I (opening mode). In general, the most important testing variables affecting the FCP rate are frequency and wave form of the alternating load, temperature, environment, stress ratio and mean load [5]. Moreover material variables such as molecular weight, crystallinity, fiber content and orientation also play a major role in determining the FCP behavior [2].…”

“…Such orientation phenomena are responsible for anisotropic mechanical properties due to macromolecules partial alignment and/or, in the case of SFR composites, due to fiber preferred orientation. The effect of fiber partial alignment on the FCP behavior of SFR composites has been examined by a number of authors on several kinds of thermoplastic matrices such as polypropylene [2,5], polyamide [13][14][15][16][17][18][19], polyethylene [20], polyetheretherketone [21,22], polybutylenterephthalate [23], polyethersulphone [24], and polytetrafluoroethylene [25]. In all cases the FCP resistance results to be enhanced by the presence of fibers, particularly when they are preferentially aligned transversely to the crack growth direction.…”