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Engineering design of fibre-reinforced polymer (FRP) composite components requires reliable methods for measuring out-of-plane mechanical properties in the through-thickness (T-T) material direction. Within this work, existing indirect and direct test methods and geometries for measuring T-T tensile properties have been evaluated through experimental testing and finite element analysis (FEA). Experimental testing showed variations, particularly in failure properties, for both indirect (failure strengths from 10–94 MPa) and direct (failure strengths from 48–62 MPa) geometries. Results were shown to be in good agreement with FEA, which also confirmed stress concentration factors. A linear relationship between the magnitude of stress concentration factors and experimentally determined T-T tensile failure strengths was observed for all but one of the direct geometries evaluated. Improved knowledge of stress concentration factors from this work should help instil confidence for industry to use T-T tensile properties determined from these methods.

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.

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