Abstract:Comprehending the in-plane biaxial behaviour of composite materials is essential for its utilisation in aeronautical, automotive, and civil engineering structures, where materials frequently encounter intricate stress states. This study examines the biaxial behaviour of glass fiber-reinforced polymer (GFRP) laminate cruciform specimens with fibre orientations of [±10°], [±20°], [±30°], [±70°], and [±90°]. The samples were fabricated using compression moulding and evaluated on a customised biaxial testing apparatus, with strain quantified using Digital Image Correlation (DIC) and strain gauges. At a constant biaxial stress ratio of 1:1, notable strain behaviours were recorded, encompassing brittle-type strain increments and strain-relieving processes. The [±10°] sample exhibited a pronounced rise in longitudinal strain (εx) and nonlinear variations in transverse strain (εy), whereas the [±20°] sample demonstrated no shear strain in the brittle zone. The [±30°] sample demonstrated an 18.22% reduction in transverse strain inside the strain-relieving zone, succeeded by a 12% increase prior to failure. Samples exhibiting [±70°] and [±90°] fibre orientations demonstrated analogous strain patterns, with longitudinal strain shifting from compression to tension before to failure. Failure modes in all samples included matrix cracking and fibre pull-out, originating in the central gauge section. The DIC results validated strain gauge measurements and finite element analysis (FEA) predictions, demonstrating uniform stress distribution and zones that alleviate strain. The findings offer significant insights into the behaviour of GFRP composites under biaxial loading, highlighting the influence of fibre orientation on stress and strain distribution, and facilitating the design of more robust composite materials for advanced technical applications.

Keywords: In-plane biaxial test; flat cruciform sample; DIC; Strain relieving, Strain gauge measurement.