A study on the longitudinal compression strength of fibre reinforced composites under uniaxial and off-axis loads using cross-ply laminate specimens

Abstract

Longitudinal compression testing of unidirectional FRP laminates remains a challenge due to the difficulty in applying high compressive loads without stress concentrations and boundary effects leading to premature failure. This work aims to critically evaluate different specimen designs and laminate configurations, cross-ply in particular, for the determination of longitudinal compression properties of unidirectional plies. To this end, a comprehensive experimental campaign has been carried out, comparing strength, stiffness, and failure modes across different specimen designs and laminate configurations. The investigated cross-ply specimens produced comparable results without many of the issues observed in the testing unidirectional material and, therefore, are strongly recommended for the determination of longitudinal compressive strength. Finally, the cross-ply material was tested under off-axis compression to study the effects of shear on the longitudinal compression strength using a series of compression specimens cut at different angles between 0 and 15° to the direction of the laminate.

Strain-rate and off-axis loading effects on the fibre compression strength of CFRP laminates: Experiments and constitutive modelling

“…Following the quasi-static study in [5], a series of dynamic off-axis compression tests have been performed using a similar cross-ply specimen configuration and data reduction methods to extract the longitudinal compression strength from the axial measurements.…”

“…The specimens were tested at high strain-rates using a Split-Hopkinson Pressure Bar (SHPB) system, described in section 2.2, and longitudinal ply failure data was extracted from the specimen strengths using Classical Lamination Theory (CLT), following the methodology described in [5,8].…”

“…Based on the previous work in [5], cross-ply material was selected over thick unidirectional (UD) material, however, some minor modifications to the quasi-static specimen design were necessary.…”

“…Leading fibre kinking theories, such as those by Budiansky and Fleck [1,2] and Pinho [3,4], can accurately predict failure under uniaxial compression given the initial misalignment or waviness of the fibres and the nonlinear shear response of the laminate. However, under multi-axial loading, different models start to give diverging predictions based on how they address the evolution of fibre misalignment under transverse stresses [5]. In addition, even some of the aspects where all models agree have yet to be fully verified experimentally, such as the effects of strain-rate or temperature.…”

“…Based on recent work by the authors [5], who proposed an improved testing methodology to measure the uniaxial and off-axis compressive strength of CFRP laminates using cross-ply (CP) specimens, and previous work by Ploeckl et al [7], who showed a roughly 40% increase in the uniaxial compression strength of a quasi-isotropic (QI) laminate from quasi-static to high strain-rate testing, the work presented here aims to investigate the effects of strain-rate on the fibre kinking failure envelope by testing the same IM7/8552 CFRP from [5] under high-rate off-axis compression.…”