Thermal de-consolidation of thermoplastic matrix composites-II. "Migration" of voids and "re-consolidation"

"Migration" of voids and re-consolidation in thermoplastic composites during re-heating operations, such as resistance welding, were investigated. Based on experimental observations, relevant phenomena, such as "void migration" and re-compaction of fibre reinforcement, were examined and evaluated in detail. It is shown that these phenomena are primarily related to the amplitude of the applied pressure, the matrix temperature and the reduced compaction resistance of fibre reinforcement during re-heating operations. It is revealed that the so-called "migration" of voids is in fact a continuous process of void growth (de-consolidation) and void closure under the applied pressure so that re-consolidation is resumed in the region with the voids being collapsed. A thermal-mechanical model for void "migration" and a mechanistic model for void closure were established to elaborate the mechanisms, and it is shown that the predictions based on the models were correlated well with experimental observations. In addition, other mechanisms in the process of re-consolidation, such as squeezed creep flow of the matrix melt from free edges, are also discussed.