Applicability of Asymmetric Specimens for Residual Stress Evaluation in Fiber Metal Laminates

Abstract

Residual stresses in fiber metal laminates (FML) inevitably develop during the manufacturing process. The main contributor to these stresses is the difference in the coefficients of thermal expansion (CTE) between fibers and metal in combination with high process temperatures. To quantify these stresses, the use of specimens with an asymmetric layup is an easily adaptable method. The curvature that develops after the manufacturing of flat laminates with an asymmetrical layer stack is a measure of the level of residual stresses evolving during cure. However, the accuracy of the curvature evaluation is highly dependent on specimen design and other influencing parameters. This leads to deviations when compared to other methods for residual stress quantification as can be seen from the literature. Therefore, in this work a large set of FML specimens is comprehensively investigated to identify relevant influencing parameters and derive conclusions about specimen design and evaluation techniques. For certain layups and process parameters, there is a good correlation between the curvature and the stress-free temperature, which is further covered by analytical solutions for bimetals. This correlation is the basis to transfer curvature into a stress-free temperature that can consequently be used for the quantification of residual stress levels in more complex FMLs. The transfer is validated by in situ strain measurements during cure using a strain gauge technique. Based on the results, the application of asymmetric specimens for residual stress characterization in more complex laminates is presented in the form of a workflow. The work shows the basic considerations and procedures necessary to use asymmetric specimens for residual stress quantification in FML. Furthermore, the results obtained can also be transferred to other composite materials.