Comparisons of Caustics and Strain Gauge Methods for Determining Mixed Mode Fracture Toughness of PMMA

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Abstract Both caustics and the strain gauge methods are considered to be effective approaches to study material mixed mode fracture toughness. However, not many studies have recorded quantitative comparisons between these two methods. In this paper, three-point-bend tests are conducted using caustics and the strain gauge method simultaneously to determine the critical mixed mode stress intensity factors of polymethyl methacrylate (PMMA) specimens. The mixed mode fracture toughness is then derived by employing both methods for a thorough comparison and analysis. Besides, the J-integral method in ABAQUS simulation software is also employed for a further illustration of these two methods. According to the results, there is a similar changing trend in toughness calculated by caustics and the strain gauge method when there is an increase in a/b ratio (a is the crack length; b is the specimen width) of PMMA specimens. The differences in terms of mixed mode fracture toughness values between these two methods are quite narrow. Besides, in comparison to KIc, the variations of KIIc obtained by both methods are shown to change drastically as the a/b ratio of the specimen increases. Additionally, by analyzing and comparing these two methods, it is found that the caustic method is more sensitive when measuring the mixed mode fracture toughness, whereas the strain gauge method can satisfy a rough and quick need for calculating key fracture parameters.