INFLUENCE OF MEAN STRESS AND WELD LINES ON THE FATIGUE BEHAVIOUR OF SHORT FIBRE REINFORCED POLYAMIDE

In the present work, the influences of mean stress and weld lines on fatigue behaviour of an injection moulded 50 wt% short glass fibre reinforced (sgfr) partial aromatic polyamide were investigated. The influence of mean stress can be considered by the Haigh diagram. The shape of this diagram for sgfr thermoplastics is derived by cyclic, quasi static and creep tests. For these tests,
notched and unnotched specimens, which were processed by injection moulding, were used. At first, the material limits were determined with quasi static tests. Since plastics tend to creep under static loads, creep tests were additionally performed to investigate the reduction of sustainable mean stress.
Cyclic tests have been made at different stress ratios ranging from R = 10 to R = 0.8. Finally, for the characterization of the influence of weld lines, specimens were tested perpendicular to the weld line at several stress ratios.
The results show that stress ratio and the related mean stress have a wide influence on the life time of parts made of sgfr plastics. Caused by mean stress redistribution in notched specimens as well as in notched specimens with weld line (labelled as EMS-specimen), the drop-off in fatigue strength at high
mean stress levels is lower than for unnotched specimens. This fact has to be considered in a closed simulation chain for example by the stress redistribution according to Neuber’s rule. In general, weld lines causes a significant reduction of the fatigue strength compared to the unaffected material. An improved assessment of mean stress influence by the local stress approach is expected
by the proposed Haigh diagram for sgfr PA6T/6I - GF50.