Michael A. Zampaloni

Non-orthogonal constitutive equation for woven fabric reinforced thermoplastic composites

One of the ultimate objectives of this study was to investigate the feasibility of shaping preconsolidated woven FRT (fabric reinforced thermoplastics) using stamp thermo-hydroforming, a new forming method for composite manufacturing. A new constitutive model has been developed based on a homogenization method by considering the microstructures of composites including both the mechanical and structural properties of fabric reinforcement. In particular, the current model aims to account for the effect of the fiber strength difference and orientation on anisotropy and also to simulate shear deformation without significant length change, common in FRT composite forming. For validation purposes, the model was implemented in an explicit dynamic finite element code and tested for in-plane simple shear, pure shear, uniaxial tension, and draping behavior of woven composites.

Stamp Thermo-Hydroforming: A New Method for Processing Fiber-Reinforced Thermoplastic Composite Sheets:

The stamp thermo-hydroforming process involves supporting the thermoplastic sheet with a bed of heated viscous fluid that applies a hydrostatic pressure across the part throughout the forming process. This hydrostatic pressure produces a through-thickness compressive stress that delays the onset of delamination, reduces the formation of wrinkles due to frictional traction forces, and results in better-formed parts. The overall goal of this research is to verify, through experimentation and numerical modeling, that the stamp thermo-hydroforming process provides a suitable alternative to conventional thermoforming methods, such as bag molding and stamp forming, as a means for processing glass-mat fiber-reinforced thermoplastic materials. The goal of this paper is to solely address whether the use of a counteracting pressure can be beneficial during the forming of fiber-reinforced thermoplastic sheets more specifically; this paper addresses the experimental setup that was developed and built to conduct small-scale stamp thermo-hydroforming experiments with a hemispherical punch, and discusses some results obtained using this setup. Experiments were performed on a random oriented continuous glass-mat fiber-reinforced polypropylene with hydrostatic pressure applied locally on the top surface of the composite sheet, as it was being stretched by a hemispherical punch. An increase of up to 10% in draw depth was achieved by applying local hydrostatic pressures up to 207 kPa (30 psi). In addition to the draw depth improvements, the application of the counteracting hydrostatic pressure also delayed the onset of delamination.

Sheet forming analysis of woven FRT composites using the picture-frame shear test and the nonorthogonal constitutive equation

This study investigates the validity of a nonorthogonal constitutive equation that has been developed for simulating the deformation behaviour of woven fabric thermoplastic (FRT) composites. The model incorporates shear material properties measured from picture-frame shear testing into the constitutive model. Two different types of single layer materials were investigated; a commingled woven fabric (preform) and a consolidated form of the preform. Picture-frame testing at elevated temperatures was conducted on the consolidated samples and experimental test data was used to determine the relationship between shear stress and shear strain. Experimental validation of the model was conducted utilising a stamp thermo-hydroforming press without the use of the counteracting fluid. To address an important issue in utilising picture-frame shear testing, a virtual test was performed with experimentally determined data in an implicit finite element solver. The stamping forming simulation was also performed by the constitutive equation with the shear material properties, showing good agreement with experimental results.