Deconsolidation in glass mat thermoplastic composites : Analysis of the mechanisms

During reheating and post-processing of thermoplastic-based composites, deconsolidation is often observed: the volume fraction fibre decreases and the void content increases. In this article, the phenomena leading to deconsolidation are investigated, with particular emphasis on the elastic release of stress in the preform, also called springback effect. A model is proposed to simulate the evolution of the specimen thickness with time. A comparison with model experiments consisting in relaxation of glass mats in polyethylene-glycol is provided. This result, together with reheating experiments of Glass Mat reinforced Thermoplastics GMT parts, showed that deconsolidation is mainly governed by the elastic behaviour of the fibre preform. It is also observed that the air initially dissolved in the matrix tends to coalesce during reheating due to diffusion, but also to tensile forces induced by the springback effect

Graded glass mat reinforced polypropylene

In the impregnation process, when pressure is applied, the dry fiber preform is first compressed and then unloads as the matrix flows within the pores. It has been shown earlier that for glass mat-reinforced thermoplastic (GMT) materials, the time to provide complete infiltration of the matrix is significantly shorter than the unloading time of the reinforcement. This effect is exploited here to control the processing time in order to provide full infiltration but limited relaxation of the fiber bed, thereby producing a graded fiber content structure. Symmetric glass fiber/polypropylene (PP) composites were impregnated for different processing times to produce parts with a higher fiber content on the surface, decreasing towards the center. The elastic modulus in bending of the GMT parts was measured by a three-point flexure test while the distribution of fibers in the matrix was quantified using optical microscopy combined with image analysis. The flexural modulus of GMT was found to decrease with the impregnation time, in good agreement with the prediction from infiltration and mechanics theory. Controlled processing could therefore be used to maximize the bending stiffness of GMT

Deconsolidation in glass mat thermoplastics: influence of the initial fibre/matrix configuration

The influence of the initial configuration of glass mat and polypropylene on the deconsolidation during post-processing of the resulting composite is investigated. Deconsolidation experiments of a classic type of glass mat reinforced thermoplastic (GMT) produced by infiltration and a new type of commingled mat produced by consolidation are carried out, followed by void content measurements. A model predicting the kinetics of deconsolidation and final void content after reheating is applied to the two different systems. The numerical results completed by compressive tests carried out on both initial dry glass preforms show that the difference in fibre bundle arrangements leads to different deconsolidation kinetics and final state

Unsaturated flow in compressible fibre preforms

The isothermal infiltration of compressible fibre preforms was modelled taking into account the gradual saturation of the porous medium and the fibre tows. Similarities between this and drainage/imbibition processes, led to the adaptation of a finite-element code originally developed for soil mechanics. The dual scale of the porous medium was accounted for by considering delayed radial flow into the fibre tows. The chosen case study materials were polypropylene transversally infiltrating glass fibre mats. Local preform strain and stress profiles, as well as micro and macro-saturation, and local matrix pressure profiles were obtained. The influence of numerical and material parameters is discussed in relation to experimentally observed phenomena