The role of binder architecture and cooling rate on residual stresses and meso-scale damage in orthogonal 3D woven composites

UoM administered thesis: Doctor of Engineering

  • Authors:
  • Sarvesh Singh Dhiman

Abstract

Composite materials have been shown to provide significant advantages over traditional materials such as metals and alloys due to their enhanced specific strength and stiffness properties, as well as the ability to reinforce specific areas according to performance requirements. However, conventional composites are limited by their poor out-of-plane performance and arduous manufacturing processes. Three-dimensional woven composites (3DWCs) are a relatively new category of materials which overcome this limitation by facilitating arrangement of fibres in all three principal directions. With the improved performance of 3DWCs, however, novel challenges arise. The addition of through-thickness reinforcements can lead to increased residual stresses during the manufacturing process. These stresses, which develop predominantly due to the mismatch in thermal expansion between the constituent fibre and matrix materials, can lead to unintentional matrix microcracking and delamination. As a result, there must be an understanding of the effects of three-dimensional woven architecture and manufacturing processes on the development of residual stresses in order to attain optimal performance. This research investigated the effect of weave architecture (specifically binder tow size and binder tension) and post-cure cooling rate on residual stress formation in 3DWCs, through experimental investigations and finite element analyses. Experimentally, the development of residual strains at two cooling rates were quantified for each architecture using embedded optical fibre sensors (OFS). The effect of cooling rate on tensile properties was also quantified. Multi-scale finite element models were developed to predict the progression of residual stresses within the 3DWCs during manufacture. A micro-scale model was established to compute temperature-dependent, thermo-mechanical properties of the reinforcing tows, and temperature-dependent viscoelastic behaviour was considered for the matrix. Meso-scale models were developed to simulate transient heat transfer and subsequent residual stress development during cooling. Predicted damage locations resulting from residual stresses corresponded extremely well with actual regions of damage observed in experiments. A continuum damage mechanics-based model was developed to simulate the effects of the manufacture-induced residual stresses on tensile properties, which was also validated with experimental data. This research showed that increased binder tow size and binder tension during weaving both increase the magnitude of residual stresses, however the cooling rate has the most significant effect, which was evident from the OFS measurements, microscopy and numerical modelling. Increased micro-cracks resulting from the faster cooling rate was detrimental to tensile performance. Numerical modelling indicated localised thermal gradients within the composite during cooling which were more prominent due to the binder tow path traversing the thickness of the material. Parametric studies were conducted using the validated model to propose methods to decrease residual stresses, including process optimisation, modification of weave architecture and modification of constituent materials.

Details

Original languageEnglish
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Award date1 Aug 2020