Out-of-plane multiaxial characterization and non-associative elastoplastic modeling of thick aerospace composite laminates using a modified Arcan fixture and DIC

Experimentally validated constitutive models under three-dimensional stress states are needed to support finite element analysis and virtual testing of thick composite structures used in aerospace applications. However, the nonlinear out-of-plane response of unidirectional laminates under coupled through-thickness normal and interlaminar shear stresses remains insufficiently characterized. In this study, a modified Arcan fixture combined with digital image correlation was developed to investigate the out-of-plane multiaxial behavior of thick unidirectional glass/epoxy laminates. The fixture avoids specimen drilling and incorporates anti-buckling guidance, enabling stable proportional loading from through-thickness tension–shear to pure compression and compression–shear. Full-field DIC measurements further allow the simultaneous extraction of the out-of-plane normal strain ε₃₃ and interlaminar shear strain γ ₃₁ in the gauge region, thereby providing a reliable experimental basis for biaxial constitutive parameter identification. Different from existing MAF-calibrated non-associative plasticity studies that have focused mainly on in-plane loading, the present work establishes an experimentally supported framework for out-of-plane σ ₃₃– τ ₃₁ characterization and parameter identification in thick UD laminates. The experimental results reveal pronounced tension–compression asymmetry, nonlinear deformation, and variations in the shear response induced by the through-thickness normal stress. A non-associative elastoplastic constitutive model was proposed to describe these matrix-dominated nonlinear features. The results demonstrate that the proposed MAF/DIC experimental method and non-associative elastoplastic model provide an effective route for characterizing the pre-peak nonlinear out-of-plane response of thick UD composite laminates within the investigated σ ₃₃– τ ₃₁ stress space.