2007 Volume 56 Issue 5 Pages 406-413
Two kinds of cylindrical specimens of short-glass-fiber reinforced polybuthyleneterephthalate (PBT), the one with weld line at the center (W specimens) and the other with molding direction parallel to the specimen axis (MD specimens), were fatigued under axial total-strain-controlled conditions. The fatigue life of W specimens was slightly shorter than that of MD specimens when compared at the same range of total strain, although the difference was very small. When plotted against the stress amplitude or inelastic strain energy, the fatigue life was much shorter for W specimens. The tensile peak stress in stress-strain hysteresis loops decreased with the number of cycles. Unstable fracture took place at 30 to 40% reduction of peak stress in MD specimen, while in MD specimens a sharp decrease in the peak stress and the maximum in inelastic energy took place at the 40% reduction of the tensile peak stress. Unstable fracture did not occur in W specimens. The computerized tomography (CT) using synchrotron X-rays was applied to observe nondestructively the crack growth behavior in fatigued specimens. In W specimens, crack started from the surface and extended inward as a continuous long crack, while only many small cracks observed at the central region of the specimens until unstable fracture in MD specimen.