DECONSOLIDATION IN COMPOSITE PROCESSING:COMPARISON OF TWO GLASS FIBER-POLYPROPYLENESYSTEMS

摘要

In thermoplastic based composite processing, a molten matrix impregnates a dry fiber preformproducing preconsolidated blanks. These composites parts are afterwards subjected to post-processing,which consists in reheating followed by stamping or flow molding. During reheating, the materialgenerally lofts, and its void content increases. In this paper, the phenomena leading to deconsolidationare thus investigated, with particular emphasis on the elastic release of stress in the preform, alsocalled spring-back effect. A model is proposed to simulate the kinetics of the phenomenon and theevolution of the specimen thickness. A comparison with model experiments is also provided. Finally,a comparison of two different initial material structures of glass mat and polypropylene is proposed.Deconsolidation experiments of a classical type of GMT and a new type of commingled system arecarried out. It is shown that for both systems the void increase due to deconsolidation exceeds 50%. Itcan be then concluded that if the fiber volume fraction is identical and homogenous in the structure,the void increase due to deconsolidation is the same. Furthermore, the model experiments ofspringback have shown that void formation plays a major role in deconsolidation and is induced by thelofting of the fiber preform: when the fiber preform tends to recover its initial relaxed thickness,tensile forces are induced in the molten matrix which lead to void growth.