Stress relaxation behaviour of glass fibre reinforced thermoplastic composites and its application to the design of interrupted in situ tensile tests for investigations by X-ray computed tomography

In situ investigations by X-ray computed tomography are widespread and have become very important for studying damage mechanisms in materials. Interrupted in situ tensile tests by X-ray computed tomography with laboratory devices have to be performed in displacement-controlled mode. In this regard, it is crucial to know at which point almost a steady state of the loaded specimen can be expected and thus an X-ray computed tomography scan at good data quality can be achieved. This study investigates the relaxation behaviour of injection moulded glass fibre reinforced polypropylene. Relaxation tests, according to a single loading and an interrupted in situ testing manner, are performed on different specimen geometries with varying fibre orientation distributions. An extension of the Kohlrausch-Williams-Watts function is used for modelling the relaxation behaviour. Two different approaches are presented in context of the proposed methodology. One approach is based on a look-up diagram and the other is the on-the-fly check during testing. The suggested methodology allows a coarse design and enables time saving optimizations of such in situ investigations.