SHAPE DISTORTION SIMULATIONS OFCARBON/EPOXY COMPOSITES USING A SIMPLIFIEDMECHANICAL CONSTITUTIVE MODEL

摘要

During the last decade a lot of effort has been spent on development of tools forprediction of manufacturing induced residual stresses and shape distortions. The motivations forthis are the high costs and lead-time associated with redesign and tool modifications that oftenare necessary before the specifications of high quality composite parts are met. The mechanicalconstitutive models used in these simulation tools ranges from linear or incremental elasticity tothe most advanced and presumably most accurate models utilising thermo-chemo-rheologicallycomplex viscoelasticity.In a previous paper1 the authors investigated the influence of the cure schedule on shapedistortions of glass/epoxy angle brackets manufactured by resin transfer moulding (RTM). Animportant observation was that the spring-in did not change for a range of cure conditions andtherefore we concluded that for cure conditions typical for RTM and autoclave consolidation ofprepregs it may be possible to use a simpler mechanical constitutive relation thanviscoelasticity. The constitutive model must however be able to treat both thermal effects andchemical shrinkage as well as frozen-in deformations caused by the mechanical boundaryconditions during in-mould cure. Linear or incremental elastic models do not properly describethe frozen-in deformations and therefore a new, but relatively simple, mechanical constitutiverelation was developed. The details of the model development and FE-implementation aredescribed elsewhere2.In the present paper we use the new mechanical constitutive model in predictions ofshape distortion for resin transfer moulded carbon fibre/high temperature epoxy U-beams andcompare the predictions with experimental results. Of particular interest is the ability to predictthe influence of fibre content and beam radius and therefore we study a through-thicknesshomogeneous fibre lay-up ([(0/90)n] plain weave). The predictions are obtained by computersimulation of residual stress development during the two-step curing process (in-mould curefollowed by a free-standing post cure) including mechanical interaction between tooling andcomposite. The properties of the curing composite material are calculated from resin and fibreproperties using micro-mechanics. Predicted spring-in is compared to measured values for twobeam radii and two fibre contents. In the computer simulations the mechanical interaction withthe tooling results in a small difference in spring-in between the small and large radius U-beamsafter demoulding. Increasing the fibre volume content from 46% to 52% leads to a decrease ofthe predicted spring-in angle of 0.1o. The overall agreement between predicted andexperimentally determined spring-in is good which support the validity of the simulationapproach and mechanical constitutive relation. The results also show the importance of usingappropriate mechanical boundary conditions during cure and residual stress/shape distortionsimulations.