Towards a Planar Cruciform Specimen for Biaxial Characterisation of Polymer Matrix Composites

Abstract

This paper details work undertaken towards the development of a standard test method for the biaxial response of planar cruciform specimens manufactured from carbon fibre-reinforced plastic (CFRP) laminates and subject to tension-tension loading. Achieving true biaxial failure in a cruciform specimen without the need for the inclusion of a stress raiser, such as a hole, in the gauge-section, is a subject attracting much research globally and is by no means a trivial exercise. Coupon designs were modelled using finite element analysis (FEA) in order to predict the stress and strain distributions in the central region of the specimen. An Instron biaxial strong-floor test machine was used to test the specimens. Strain gauges were used to measure the strain in the specimen arms and to assess the degree of bending. Digital image correlation (DIC) was used to measure the full-field strain distribution in the central gauge-section of the specimen and this was compared to values measured using strain gauges. The strain readings obtained from strain gauges, DIC and FEA predictions were in good agreement and showed that the strain distribution was uniform in the central gauge-section, but that strain concentrations existed around the tapered thickness zone. These regions of strain concentration resulted in interlaminar failure and delamination of the laminate propagating into the specimen arms.

A novel fixture for determining the tension/compression-shear failure envelope of multidirectional composite laminates

“…However, tubular specimens are not representative of typical laminated composite materials coupon specimens or structures which are flat/planar or display a large curvature radius compared with the laminate thickness, and where the arrangement of reinforcement fibres is ply-wise discretely discontinuous. For that reason, cruciform specimens often have been chosen [4][5][6][7][8][9] to characterise the biaxial mechanical performance of composite laminates for biaxial tensiontension, tension-compression and compression-compression loading. Successful testing of cruciform specimens requires extensive and careful machining of the specimens to create a reduced gauge section thickness and a corner fillet to prevent premature failure outside the gauge section.…”

Correlation image system 3D DANTEC as a modern system for measuring deformation

“…In strength calculations, patterns based on assumptions are commonly used, although they are rarely practically met. More general bending descriptions are known that do not depart from these assumptions, but the benefits of their practical use are generally small compared to the complexity of the calculations they cause [6][7][8]. Flexural strength is a conventional quantity.…”

Cruciform Specimen Design for Biaxial Tensile Testing of SMC

“…Makris et al [14] and Makinde et al [4] optimized the specimen shape by a numerical optimization technique with a parametric finite element model. Lamkanfi et al [15] and Gower and Shaw [16] showed that geometrical discontinuities like the transition zone to the tapered thickness area have a major influence on the strain distribution leading to premature failure. Serna Moreno et al [17] compared specimens with arms of different widths for chopped glass-reinforced polyester.…”