Viscoelastically prestressed composites : towards process optimisation and application to morphing structures

This thesis covers research that focuses on facilitating the industrial application of viscoelastically prestressed polymeric matrix composites (VPPMCs). With nylon 6,6 fibre as the reinforcement and polyester resin as the matrix material, unidirectional prestressed composite samples were produced and investigated, to expand the knowledge of existing VPPMC technology, and identify the potential application of viscoelastic fibre prestressing to morphing (shape-changing) structures. To produce a VPPMC, a tensile load is applied to polymeric fibre yarns to induce viscoelastically prestress; following load removal, the yarns are cut and moulded into a matrix. Previous research has shown that by using viscoelastic fibre prestressing within a composite, mechanical properties, such as tensile strength, flexural modulus and impact toughness can be increased by up to 50%. To further understand the underlying prestress mechanisms, the viscoelastic performance of nylon 6,6 fibre was investigated in terms of creep, recovery and recovery force measurement. By using various creep loading conditions, the viscoelastic behaviour of the fibre also provided the basis for investigations into the optimisation of load-time conditions for producing prestress. This provides the first step towards facilitating the production of VPPMCs for industrial application. Since there are increasing demands for using composites within morphing technology, the application of VPPMC principles to morphing structures was studied through both experimental and numerical investigations. The viscoelastic behaviour of nylon 6,6 fibre showed approximately linear viscoelasticity under 24 h creep conditions with up to 590 MPa stress. This was further verified through use of the time-stress superposition principle: instead of a nonlinear relationship as predicted by the well-known WLF equation, a linear relationship between the applied creep stress and the stress shift factor was found. By approaching the maximum creep potential of the fibre material, impact benefits from the prestressing effect were further improved by ~75% (at ~4.0% creep strain level). Charpy impact testing and recovery force measurement demonstrated that there was an optimum level of viscoelastic fibre prestressing to maximise the mechanical benefits. A viscoelastic deformation mechanism based on the three-phase microstructural model and latch-based mechanical model was then proposed. It was found that the fibre processing time for viscoelastically generated prestress could be significantly reduced from 24 h to tens of minutes. By employing the time-temperature superposition principle, the impact benefits from viscoelastically generated prestress under the standard 330 MPa, 24 h creep (at ~3.4% creep strain level), was found broadly to be the same as subjecting the yarns to 590 MPa for 37 min creep. Hence, there was no deterioration in prestress benefits from VPPMC samples produced under both creep conditions to an equivalent of 20,000 years at a constant 20˚C. Two viscoelastic creep strain levels (i.e. ~3.4% and ~4.0%) were evaluated through Charpy impact testing, the relationships between applied creep stress and the corresponding fibre processing time followed a logarithmic trend. This suggested that the fibre processing time for prestress could be reduced further, subjecting to avoiding fibre damage. The effects from increasing creep time were found to compare with increasing stress in terms of optimum VPPMC performance. Finally, the principle of viscoelastic fibre prestressing was successfully used to produce a bistable composite structure, which could snap from one stable cylindrical shape to another when subjected to external loading. The bistable structure was produced by bonding four prestressed strips to the sides of a thin, flexible resin-impregnated fibre-glass sheet. Here, bistability was achieved through pairs of strips orientated to give opposing cylindrical configurations within the sheet. Snap-through behaviour of the bistable structure was investigated through both experimental and numerical simulation; a snap-through mechanism was subsequently proposed based on these observations.