Co-authors​ :

 María Del Carmen SERNA MORENO (SPAIN), Sergio HORTA MUÑOZ (SPAIN) 

In recent years, the use of cruciform specimens has allowed for the monitoring of material response under biaxial stresses. While traditional assessments focused on positive load ratios, there has been a recent shift towards their application in load cases involving at least one compressed direction. This study specifically investigates the geometrical instability of a ±45º symmetric Carbon Fiber Reinforced Polymer (CFRP) laminate using a cruciform specimen [1]. The region tested under equal bi-compression is made up of a [±45]S laminate, which exhibits significant flexural-torsional coupling, and has a shape and boundary conditions similar to a clamped square plate. The compression-compression test (CC test) is designed to provide insights into the fundamental aspects of the bifurcation phenomenon, while avoiding instability and non-linear effects in the arms of the cruciform specimen. Full-field displacements and strains are registered by Digital Image Correlation (DIC). Deflection is detected from the beginning of the test due to the growth of geometrical imperfections, which could be influenced by non-symmetrical loading at the initial stages of the test, leading to some data scatter. Then, the bifurcation stress is found to differ from the theoretical value but is close to the critical stress obtained according to actual regulations. However, due to the complex response of laminated composites, further work is needed to estimate the relation of the global instability with apparent material non-linearities and final failure. The complexity of interpreting multiaxial responses in anisotropic materials is increased by the intricate experimental analysis required for evaluating the out-of-plane displacement of a laminate that exhibits strong coupling and local effects. Before the strain bifurcation, the apparent biaxial stiffness aligns with the prediction of the Classical Laminated Plate Theory. To contribute to the advancement of the current state of the art, this research explores whether the out-of-plane displacement can provide valuable information for quantifying the bending and twist moments experienced by the laminate at the onset of bifurcation. The orientation of the DIC deflection surface is influenced by the different stress states at the ply-level, which are evidenced in the local strain gauge measurements. The research concludes that the CC test with cruciform specimens could be a viable option for analyzing the elastic and buckling characteristics of a CFRP laminate in the presence of multiaxial compression loads.