Compressive failure strain of unidirectional carbon fibre composites from bending tests

One of the major difficulties in establishing material properties for composites is to obtain reliable compressive strengths either from the current direct compressive testing methods due to the stress concentration or from the indirect flexural tests due to the localised stress concentration and strain gradient effect. In the present paper, unconventional flexural tests on carbon fibre/wood sandwich and monolithic composite beams have been developed and compressive failure strains of unidirectional carbon fibre composites have been obtained with high values and low variability. The compressive strain of 1.36 % obtained for the IM7/8552 material is 14 % higher than the highest published value found and the variability is only 3 %. Strain gradient effects on the compressive failure strain from measurements with the same stressed volume have also been quantified for the first time using the proposed carbon fibre/wood sandwich beam with various wood core depths. The increase in failure strain due to the strain gradient effect reduced with the increase of total beam thickness and approached a constant value when the beam was thicker than 20 mm.