A DISCRETE APPROACH FOR PREDICTING VOID FORMATION DURING VACUUM CONSOLIDATION OF AUTOCLAVE COMPOSITES

摘要

Porosity/voids are common manufacturing defects in laminated composite components and structures. The susceptibility to manufacturing defects greatly increases in contoured autoclave composite sections. In particular, composite helicopter rotor components are typically thick and often have areas with a tight radius of curvature, which make them especially prone to process-induced porosity defects. Tight radii inherently result in non-uniform consolidation pressure, causing the formation of large voids, or pockets of entrapped air, at ply interface. Such defects may significantly affect structural integrity and increase rejection rates in the production of rotor composite components. Recent advances in high-fidelity non-destructive inspection of composites, such as X-ray Computed Tomography (CT), have shown that reduction of strength and fatigue performance of laminates can be strongly related to shape, size, and location of individual critical voids at ply interface. However, none of the existing tools available for process modelling are able to predict the formation of such individual defects, including their geometry and position in the composite parts.The Office of Naval Research (ONR) project “Physics-Based Composite Process Simulation” seeks to fill the gaps in understanding the underlying physical principles governing the formation and evolution of manufacturing defects. In particular, understanding and modelling defect formation at the early stages of the manufacturing process might be the missing link to enable the development of practical engineering solutions allowing for better control of the manufacturing process. Debulking, or vacuum consolidation, has been a common practice extensively used by rotorcraft manufacturers to reduce the amount of “bulk” or air entrapped during the lay-up of curved and thick composite rotor components prior to curing. The debulking process typically spans several hours and requires intensive manual interventions. Yet, parameters controlling the debulking process, including frequency, temperature and pressure, are determined empirically with no guaranty that they are optimum or applicable when new materials and new geometries are introduced.This work presents the latest developments of a new approach introduced by the authors that relies on finite element modeling (FEM) and discrete representation of the critical entrapped air pockets at ply interface for simulation of air removal during vacuum consolidation of autoclave prepreg composites. The approach uses cohesive elements enriched with pore pressure degree of freedom inserted at ply interface. In particular, this work includes the development of an experimental procedure for measurement of the cohesive properties associated with the tackiness of uncured autoclave prepregs. Experimental results are also compared with FEM simulation for validation of the discrete modeling approach.