Load-blocks and specimen deformation effects on the mixed-mode bending specimen for mixed-mode I + II interlaminar fracture of composites

• a new beam model was developed for the mixed-mode bending (MMB) specimen. • the beam model addresses load-blocks and specimen deformation effects. • despite the geometrically non-linear formulation, a single iteration suffices. • closed-form expressions for the strain-energy release rate were derived. • model predictions agreed well with finite element and experimental results. The mixed-mode bending (MMB) specimen has been standardised for measuring the mixed-mode I + II interlaminar fracture toughness of unidirectional fibre-reinforced polymers. Data analysis is based on well-known beam theory based equations derived for the mode I and mode II components. Furthermore, guidelines concerning fixture design and specimen attachments are available for avoiding significant geometric non-linearity effects. However, the latter may become relevant for the tough materials that are increasingly employed in structural applications. While existing standards for mode I and mode II tests provide corrections for geometric non-linearity, this is presently not the case of the MMB test. This paper presents an elastic beam model that takes into account load-blocks and specimen deformation effects. Despite the model geometrically non-linear formulation, a single iteration sufficed to obtain accurate specimen compliance, strain-energy release rate and mode-mix results. Closed-form expressions for the strain-energy release rate could even be obtained upon additional assumptions that are realistic for most materials. Comparison with finite element analyses showed good accuracy of the present model. Application to experimental results indicated that, even mild geometric non-linearity in early loading stages introduces additional ambiguity on the common non-linearity crack initiation criterion. The present beam model could therefore be a useful experimental data analysis tool.