title: Advancing the Contour Method to Characterise Residual Stress in Polymer Composites
Creators: name: Karebasannanavar Ramachandrappa, Praveen
Publisher: name: The Open University
Year: 2024

description: The manufacturing route for polymer composites inevitably introduces residual stress. It entails cooling from high cure temperature to room temperature accompanied by constrained shrinkage. The constraints arise from the micro-scale contraction between the fibre and matrix, as well as the macro-scale contraction between adjacent plies with varying fibre orientations inducing strain dissimilarities. The resulting residual stresses are directly associated with dimensional stability such as warpage, shape distortion and the structural integrity of composite structures (e.g. matrix cracking, reduced fibre-matrix bondage and delamination toughness). The composite structures may need to be pre-loaded to get the desired assembly tolerances, creating additional internal stresses and reducing overall performance. Therefore, knowledge and accurate characterisation of residual stress is imperative for optimising the design and structural integrity of polymer composites.

Analytical and numerical methods have been developed to predict residual stresses in polymer composites, but these procedures are computationally expensive and require cure and temperature dependent physio-chemo-rheological properties. Experimental methods offer an alternative solution for developing a quantitative understanding of the sign, magnitude and distribution of residual stresses. They are essential for validation of predictive methods. However, the characterization of bulk residual stress in polymer composites remains a challenge as there is currently no experimental approach available for this purpose.

The present study investigates the viability of using the Contour Method for measuring 2-dimensional residual stress in polymer composites. Traditionally, the method has been solely applied to metallic structures. The challenge to address is to expand its applications to non-metallic structures through identifying suitable methods for cutting the material. In this work, the feasibility of five different cutting techniques such as wire EDM, abrasive waterjet machine, diamond wire machine, milling machine with end mill and slit saw tools for contour analysis is explored. The results of contour cutting on asymmetric and symmetric epoxy Carbon Fibre Reinforced Polymer (CFRP) cross ply laminates are presented and discussed. The created cut surfaces are carefully interrogated using optical microscopy, scanning electron microscopy, and high-resolution surface topological scanning methods. The quality of contour cuts and the corresponding optimal conditions were evaluated based on three surface features criteria, such as cutting artefacts, subsurface damage and surface roughness coefficient. The deformations observed on the cut surface created by the diamond wire method, as a consequence of the relief of residual stress, clearly demonstrate the ply orientations in both asymmetric and symmetric samples. Therefore, among the five cutting techniques evaluated, the cut quality of the diamond wire machine was found to be sufficiently good for measuring deformation resulting from residual stress relaxation. The diamond wire cutting technique is evaluated by means of a thermal mapping device to ensure that the cutting temperature remains below the glass transition temperature, which is determined using Differential Scanning Calorimetry. The measured out of plane displacement data obtained from the optimum cutting process were then processed using the standard approach for contour measurement. The investigation of data analysis parameters along with the provision of guidelines to assist practitioners of the contour method measurement in selecting an appropriate surface measurement density, mask size for smoothing the deformation data, finite element mesh size for contour method data collection and analysis are reviewed. In addition, a correlation has also been discussed between the minimum resolvable residual stress length scale and the size of constituents and surface roughness induced by the diamond wire cutting process.

Finally, residual stresses through the thickness of asymmetric and symmetric samples measured by the Contour Method using diamond wire cutting are presented. The results are validated against measurements using the slitting method. In compliment, they are also compared to Classical Laminate Theory based analytical results and thermally simulated residual stress in ABAQUS and the limitations are addressed. The research marks the first instance of implementing the method on epoxy CFRP composites and showcases the capability of the contour method in determining bulk residual stress distribution across the thickness of cross-ply laminates.