Issue 46

Z. Lu et alii, Frattura ed Integrità Strutturale, 46 (2018) 150-157; DOI: 10.3221/IGF-ESIS.46.15 151 Soderberg 1 a m ao y S S S S   (3) SWT ( ) ao a a m S S S S   (4) Walker   1 ao a m a S S S S      (5) where S a is the stress amplitude, S m is the mean stress, S ao is the equivalent stress amplitude at zero mean stress, S u is the materials ultimate strength, S y is the materials yield strength and γ is a material constant. SWT equation is a special case of Walker equation where γ = 0.5. Mean stress was also reported to have a significant effect on fatigue behaviour of polymers [7-10] and short fibre reinforced polymer composites [10-14]. The evaluation of the mean stress correction equations were carried out by a range of researchers [7, 9-13]. Different results were reported. Zhou et al. [11] reported a good correlation using SWT on short fibre reinforced blend of polyphenylene ether ketone and polyphenylene sulphide. Mallick and Zhou [12] suggested using a modified Gerber equation to correlate a short glass fibre reinforced polyamide 6.6. Using modified Goodman, Oke et al. [13] estimated the mean stress effect of short fibre reinforced polybutylene terephthalate at stress ratio above 0.7. More recently, Mollett and Fatemi [9], Mortazavian and Fatemi [10] recommended Walker for mean stress correction after the investigation on polypropylene copolymer, polypropylene-elastomer blend, short glass fibre reinforced polybutylene terephthalate and polyamide 6. In order to examine the mean stress effect, seven materials, four thermoplastic polymers and three short glass fibre reinforced polymer composites, were tested under constant load amplitude at a range of stress ratios under room temperature with various mean stress correction equations listed above evaluated. M ATERIALS AND SPECIMENS even different materials, including four non-fibre reinforced thermoplastic polymers and three short glass fibre reinforced polymer composites, were tested. They are Polycarbonate and Acrylonitrile Butadiene Styrene (PC/ABS), Acrylonitrile Butadiene Styrene (ABS), Polypropylene (PP), Nylon and Acrylonitrile Styrene Acrylate (PA/ASA), 30%wt glass fibre reinforced Polypropylene (PP30), 20%wt glass fibre reinforced Polypropylene (PP20) and 30% glass fibre reinforced Nylon (PA6). The details are summarised in Tab. 1. The specimens were machined from plaques made of injection moulding along the flow direction. The specimen geometry was displayed in Fig. 1. Figure 1 : Specimen geometry. E XPERIMENTAL PROCEDURES he test was carried out on a 10KN INSTRON servo hydraulic test machine under constant load amplitude and 5 Hz at room temperature with a range of stress ratios from R=-1 to R=0.3. The fatigue life was defined as the final failure of the specimens with 2 million cycles as run out. S T