• Thomson, Daniel

Abstract

While the physical understanding of UD CFRPs and the processes that govern their mechanical response is continuously improving, macro-scale modelling still often relies on empirical, over-simplified constitutive laws and failure criteria. In addition, many aspects of the rate-dependency and pressure or multiaxial loading effects on the behaviour of UD CFRPs, which are crucial for applications where composite structures may be subjected to dynamic loading, are still not fully understood.The present work aimed to investigate some of these aspects and propose a more physically-based constitutive model for macro-scale applications. Since, in UD CFRPs, the complex rate and pressure effects observed in most of the failure modes come from the properties of the polymer matrix, where possible, the proposed criteria have been derived from the matrix-dominated nonlinear behaviour.The main new development in this work is the proposal of a physically-based localisation plane plasticity theory that is able to describe all of the complex pressure and strain rate dependencies observed in the matrix-dominated nonlinear behaviour as the effect of the frictional rate-dependent shear response acting on different localisation planes. However, further research is needed to establish the physical meaning behind some of the model parameters and their relation to the constituent properties, especially with regard to the effects of strain rate.Next, an in-depth study into the effects of strain rate on IFF has been performed, and the relation between this failure mode and nonlinear constitutive behaviour has been explored by comparing the yield and failure envelopes and criteria.Last, an experimental investigation into the effects of strain rate and off-axis loading on fibre compression failure has been carried out, showing a significant increase in strength under dynamic loading, which is accurately predicted by Budiansky and Fleck’s fibre kinking theory when coupled with a rate-dependent constitutive model. However, the fibre kinking theory was unable to capture the effects of shear, severely under-predicting the off-axis compression strengths.Instead, a more pragmatic approach has been proposed to predict the longitudinal compression strength under multiaxial stress states, using the damage to the shear modulus from the off-axis loads to degrade the uniaxial compression strength.The above models were coupled with a smeared damage formulation of the kind proposed by Pinho for the LaRC criteria, and implemented in an explicit FE solver to produce a ‘unified’ model for UD CFRPs, where all rate and pressure effects derive from the frictional rate-dependent shear behaviour along certain localisation planes. This model has been validated against a series of coupon-scale tests, showing its ability to accurately describe the nonlinear response and strengths across different strain rates and off-axis loading cases.Finally, to conclude the present study, a series of component-scale plate impact tests were carried out to benchmark the model’s performance at higher loading rates and length scales, from which a number of areas for future work have been identified.

Topics

• impedance spectroscopy
• polymer
• theory
• strength
• composite
• impact test
• plasticity
• shear response