Impact fatigue properties of glass fiber-reinforced thermoplastics

The impact fatigue properties of three kinds of glass-fiber or glass-bead reinforced polymers were studied through uni-axial and cyclic indentation tests with acoustic emission (AE) measurements and visco-elastic properties measurements. The detailed failure mechanisms and damage development mechanisms were investigated with acoustic velocity measurements and optical microscope (OM) observations. Prior to fatigue tests, the materials were characterized by basic mechanical tests such as tensile tests, indentation tests, and drop weight tests. In performing fatigue tests, the authors paid special attention to the effect of interval times between loading on the fatigue properties of those materials. It was found that the numbers of cycles to failure were strongly dependent on the duration of the interval time. In the case of uni-axial fatigue tests, the interval times resulted in greater numbers of cycles to failure. On the other hand, in cyclic indentation tests, the interval times resulted in smaller numbers of cycles to crack initiation and ultimate failure. The results of AE measurements were in good agreement with the results of cycles to failure in fatigue tests and the test conditions. Attention was also paid to the relationship between the size of the damage zones and interval times between loading. The difference in the cycles to failure due to the different loading modes and interval times were understood by the differences in the mechanisms of damage zone development, where breakage of glass, micro-voiding, change in the orientation of glass fibers, and plastic deformation of matrix occurred. The degree of damage localization had a strong effect on the fatigue life. The difference in the damage development mechanisms were found to be caused by the difference in the elastic response of the specimens due to the different loading mode (uni-axial or indentation) and the interval time.