Colloquia

Summary

Forming processes for fiber reinforced thermoplastic materials have steadily increased in popularity for aerospace applications. The ability to produce defect-free and first-time-right thermoplastic parts requires accurate forming simulations. Simulation software for such forming processes requires detailed and accurate behavior- and material characterization. Friction experiments that measure the resistance to sliding between plies contribute to this characterization and are commonly conducted at a constant sliding rate with varying normal pressures and temperatures. Friction curves resulting from these experiments have a transient response consisting of a stress peak followed by gradual decay towards a steady-state plateau. The exact mechanisms behind this transient response are not yet entirely understood and friction experiments conducted at constant sliding rates provide limited information towards explaining these mechanisms.

This research uses novel friction experiments with advanced sliding rate profiles in order to further elucidate the transient friction response of unidirectional thermoplastic composites. A phenomenological framework is established in order to interpret the evolving state of the ply-ply interface. Interrupted friction experiments make use of different transition points during the transient peak of the friction response to determine onset of slip. Variable dwell times for interruption are used to measure the interface recovery. Linear acceleration profiles are used to observe the effects on the behavior of the transient peak under different deformation histories.

The results of interrupted friction experiments indicate that the onset of slip occurs before the initial peak stress is reached, challenging the assumption of a no-slip condition commonly used to determine peak stress. Experiments with variable dwell times show similar recovery and healing behavior as observed in thermoplastic welding and further strengthen the notion of wall-slip as a governing mechanism due to the comparable interfaces. Linear acceleration profiles indicate that the transient response is strongly dependent on the loading history rather than on the sliding rate alone. The insights gained from these experiments pave the way for more targeted friction experiments in future work and provide the means to develop new friction models that could improve forming simulation software.