Experimental and numerical studies on forming and failure of woven thermoplastic composites




Akhavan Zanjani, Nima

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Fuel efficiency, weight reduction, and sustainability are major global challenges fuelling research into advanced material systems. There is an urgent necessity to manufacture light weight products from highly recyclable, lightweight materials. Woven thermoplastic composites are attractive light weight candidates for the replacement of metallic parts in a wide range of industries from automotive to aerospace. They offer attractive benefits such as high specific strength, balanced thermomechanical properties, improved fatigue and wear resistance, and recyclability. However, there are two major concerns needed to be addressed properly before they can be adopted into mainstream manufacturing industries: forming and failure. This thesis investigates formability and failure behaviour of a woven self-reinforced polypropylene composite (SRPP) using a custom-built press, an open die configuration, and a real time 3D photogrammetry measurement system (ARAMIS). Specimens with novel geometries having different aspect ratios and fibre orientations were formed until catastrophic failure. Deformations and strains were measured to construct a strain-based path dependant failure envelope in a principal strain space. Optical microscopy investigations were conducted to reveal the relation between incorporated failure mechanisms and deformation modes of the composite. Then, experimental forming and failure behaviours of SRPP were benchmarked against a woven glass-fibre reinforced polypropylene composite (GRPP). Characterisation experiments were conducted on SRPP composite using a universal testing machine and a real time strain measurement system to elucidate mechanical behaviours of the composite at room temperature. The highly nonlinear behaviour of SRPP necessitated adopting an incremental deformation theory to develop constitutive stress-strain relations and construct an orthotropic material model. Material and failure models were coded in FORTRAN and implemented into a finite element analysis using the Abaqus-Implicit solver. A finite element model with a nonlinear contact condition was developed to predict formability and failure behaviours of the SRPP during stamp forming process. Comparison between experimental and finite element analysis results proved high accuracy and reliability of the developed numerical model in predicting formability and failure of a woven self-reinforced polypropylene composite. The finite element analysis predictions demonstrated the potential of the developed numerical model to accurately predict strain path, evolution of surface strains, and failure initiation in woven composites. Finally, wrinkling behaviour of the SRPP composite was investigated through a novel Modified Yoshida Buckling Test (MYBT). The inadequacy of the current wrinkling measures to predict compressive instability in woven composites was shown. A more reliable, wrinkling-sensitive criterion, based on gradient of principal strains, was proposed. An important conclusion drawn from this study indicates that proper selection of forming path, fibre orientation and specimen dimensions facilitates manufacturing complex parts from woven thermoplastic composites. The developed numerical model showed the potential to predict failure of the thermoplastic composites experiencing complicated loading conditions. This process eliminates the need to conduct expensive, time consuming trial and error manufacturing processes to achieve flawless products made of woven thermoplastic composites.






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