Mixed-mode brittle fracture test of polymethylmethacrylate with a new specimen

Significance 

Brittle and quasi-brittle materials find a wide range of applications in engineering and related fields. Examples of quasi-brittle materials include graphite widely used in manufacturing electrical discharge machining electrodes and concrete and rocks used to construct various structures and components. Generally, brittle fracture is a common failure type associated with structures and components made of brittle materials. Due to its negative impacts, it is important to devise effective strategies for testing and predicting these failures. This requires a thorough understanding of their mixed-mode failure mechanisms and measurement of their fracture toughness. In most cases, small-scale test specimens are widely used in experimental fracture studies because carrying out such experiments in real components is often difficult and expensive.

Typically, a suitable specimen for in-depth study of I/II-mixed-mode fracture should have characteristics such as cost-effective preparation process, simple configuration, convenience and the ability to produce a complete mixed-mode spectrum. Nevertheless, despite the availability of different specimen types for mixed-mode I/II fracture experiments, most fail to meet these characteristics due to various disadvantages, limiting their practical applications. Besides experimental measuring of fracture resistance, effective fracture performance-related studies also require suitable mixed-mode fracture criteria for predicting mixed-mode fracture properties. However, a large discrepancy between experimental results and brittle fracture predictions of numerous conventional mixed-mode criteria has been reported. This can be attributed to the fact that most conventional methods ignore the effects of the T-stress that is often associated with the fracture properties of materials. Therefore, developing a new test specimen accounting for all these limitations is highly desirable.

On this account, Mr. Yifan Li, Professor Martyn Pavier and Dr. Harry Coules from the University of Bristol proposed a new specimen known as Holed-Cracked Square Plate (HCSP) to study the mixed-mode I/II fracture of brittle and quasi-brittle materials. The new test configuration comprised a square-shaped plate with a central hole as well as radial cracks emerging from its circumference. The I/II modes stress intensity factors as well as T-stress solutions for different crack lengths and central hole diameters were determined via the finite element method (FEM). In particular, the authors aimed to establish the ability of the proposed specimen to determine a complete mode mixities. Several fracture tests were performed on polymethylmethacrylate (PMMA) to investigate the applicability of the proposed specimen. The original research article is now published in the journal, Fatigue and Fracture Engineering Materials and Structures.

The research team showed that the HCSP specimen could achieve complete mode mixities by simply altering the radial crack orientation angle while maintaining a simple and suitable loading requirement and specimen geometry. Other advantages of the new configuration include its insensitive to the effects of the T-stress and small errors associated with crack orientation angles than most conventional methods. It is also simple to manufacture as it requires simple loading configurations. The fracture tests on the PMMA produced fracture resistance, fracture toughness values fracture initiation angles that were in good agreement with other similar experimental results and mixed-mode fracture theories.

In summary, a new test configuration with simple geometry and loading requirement for I/II mixed-mode fracture experiments on brittle and quasi-brittle materials have been successfully developed. The fracture tests performed on the PMMA subjected to various mixed-mode conditions validated the feasibility of the HCSP specimen in conducting mixed-mode fracture for brittle and quasi-brittle materials. In a statement to Advances in Engineering, they explained that the study findings will advance research on the property of brittle materials.

Reference

Li, Y., Pavier, M., & Coules, H. (2021). Mixed‐mode brittle fracture test of polymethylmethacrylate with a new specimenFatigue & Fracture of Engineering Materials & Structures, 44(4), 1027-1040.

Go To Fatigue & Fracture of Engineering Materials & Structures

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