Towards stable End Notched Flexure (ENF) tests

Mode II fracture toughness of interfaces in laminated structures is usually assessed through standardized tests. Standards are based on samples featuring regular shapes and uniform cross-sections, in which mode II propagation happens to be unstable. We explore here, via a semi-analytical approach, the potential of more complex geometry and shapes for stabilizing the crack propagation. Results demonstrate that an end-notch flexure (ENF) sample with increasing width along the propagation direction possesses a more stable fracture compared to the classical configuration. This leads to the conceptualization of a width-tapered ENF (WTENF) that can address the instability issue encountered by the classical ENF samples. The closed-form solution of WTENF is derived, including the compliance and energy release rate of the system, based on which, the stability status diagram of WTENF has been provided. A systematic validation is performed by numerical and physical experiments, confirming the validity and the accuracy of the associated data reduction model. The WTENF can be a robust method with enhanced stability for measuring the mode II delamination toughness. Beyond solving the WTENF, the derived equations hold significant potential for other applications, such as probing the length-scale effect for delamination of fiber-reinforced composites and guiding the design of toughening strategies for interfaces.