Predictive forming of advanced thermoplastic composite structures

The objective of this research is to provide an analytical methodology to model the solid-phase forming of the advanced thermoplastic composite structures. This is achieved through the modelling of the material behavior of the APC-2 thermoplastic composite under forming conditions and the development of a numerical scheme to simulate the forming process. Material responses were observed during the entire forming process. An elastic/viscoplastic constitutive model is used to describe the inelastic and rate-dependent behavior of the advanced thermoplastic composites in the forming and relaxation stages. The strong anisotropy exhibited in the plastic flow is accounted for by an one-parameter plastic potential. Because of a rapid crystallization process in the thermoplastic resin, the thermal stresses developed during the cooling are not sensitive to the cooling rate and can be predicted by a thermo/elastic relationship. The increased mismatch in fiber and matrix properties at elevated temperatures may induce a significant dependence of the composite laminate properties on the change in fiber orientation during forming. The case of a strong scissoring effect on the thermal expansion behavior of ($\pm$45) $\sb{\rm ns}$ laminate is shown. A finite element code was developed to numerically simulate the solid-phase forming process. An isoparametric four-node Mindlin plate element with assumed transverse shear strain distribution was implemented in the program. The program was implemented in an updated Lagrange scheme to model the forming of initially preconsolidated and flat composite laminates into general shell structures. Simple contact condition at the part-tool interface is assumed. In the integration of the elastic/viscoplastic constitutive equation, an implicit return-mapping algorithm was developed. This numerical integration scheme is efficient and unconditionally stable. Even though the forming of advanced thermoplastic composites appears to resemble the sheet metal forming process, the underlying deformation mechanisms involved are fundamentally different. Deformation in the former case is mainly contributed by the intraply and interply shear, instead of membrane stretch and local bending. In the finite element implementation, all the inelastic transverse shear deformation at the forming temperature is attributed to the slip in the soft resin-rich layer at the interface of adjacent plies. This assumption reflects the dominance role of interply shear and also facilitates the entire simulation process by keeping the analysis to the first order plate/shell model. The cases of forming (90/0) $\sb{\rm ns}$ and ($\pm$45) $\sb{\rm ns}$ composite laminates over cylindrical and double-curvature tools are included as illustrations