Advancing the Contour Method for the Measurement of Residual Stress in Unbalanced Asymmetric CFRP Laminate

K R, Praveen; Bouchard, P. John; Hosseinzadeh, Foroogh; Lefebvre, Fabien and Guillon, Damien (2022). Advancing the Contour Method for the Measurement of Residual Stress in Unbalanced Asymmetric CFRP Laminate. In: 11th International Conference on Residual Stresses: ICRS11, Mar 2022, France.



Residual stresses are inevitably generated in composites during the curing phase of manufacture, where cooling from high cure temperature to room temperature is accompanied by constrained shrinkage. The high cure temperature represents a stress free state owing to the viscous nature of the matrix. Due to their inherent anisotropic and heterogeneous nature, both fibre and matrix constituents contract and expand differently leading to constraint induced differences in strains between fibre and matrix at the micro-scale and between plies having different fibre orientations at the macro-scale. 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). Therefore, knowledge and accurate characterisation of residual stress is imperative for optimising the design and structural integrity of polymer composites.

Up until now, numerous analytical, computational and experimental methods have been developed to characterise residual stresses in polymer composites. However for analytical models, knowledge of anisotropic viscoelastic behaviour of materials involves extensive characterisation in order to capture complex thermo-chemo-rheological properties [1]. Numerical models get computationally expensive when internal state variables such as cure and temperature dependent viscoelasticity, mechanical properties and chemical shrinkage of resin are considered to simulate the entire complicated curing cycle. Furthermore, composites may also undergo temperature and moisture changes during and after the manufacturing process inducing hygrothermal residual stress; but these parameters cannot be readily predicted in numerical or analytical models. In addition, the reliability of the numerical and analytical solutions are highly dependent on the quality of input data. Experimental measurements offer an alternative solution for developing a quantitative understanding of the sign, magnitude and distribution of residual stresses. But there is no consensus regarding what experimental method to use for measuring bulk (through-thickness) residual stress in polymer composites. The present PhD research will investigate the viability of using the Contour Method for measuring residual stress in polymer composites.
The contour method is a powerful measurement technique that provides two-dimensional (2D) map of residual stress in engineering components [2]. It involves sectioning the component in two halves along the plane of interest, measuring the deformation of the created cut surfaces and use the measured deformations to back calculate residual stresses by an elastic stress analysis. Traditionally, the contour method has been solely applied to metallic structures. The challenge to address in this research is to expand its applications to non-metallic materials such as composites.

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