Test MethodMixed-mode fracture investigation of PMMA with initial single/double crack(s) interference using phase-shifted coherent gradient sensing method

Phase-shifted coherent gradient sensing (CGS) was used to investigate the mixed-mode fracture of brittle polymer material. A previously developed phase shifting method for reflective CGS proved effective for transmitted CGS measurement. Using this method, fracture experiments on PMMA specimens with an offset single crack under three-point bending were performed. Also, mixed-mode fracture experiments with initial double crack interference were performed. A series of CGS interferograms were used to describe the fracture characteristics around the crack tip, such as the stress intensity factor, fracture toughness, fracture mixity, crack initiation and propagation. The effects of offset distance on fracture characterization are discussed and the interference effects of double cracks were investigated through varied crack lengths and distances. The comparison between the numerical and experimental K-dominance results shows good agreement. Moreover, the crack growth directions and fracture toughness are estimated using three fracture criteria and compared to the experiment results. It is found that the maximum tangential stress (MTS) and the strain energy density (SED) criterion tally with the experiment results well when KI/KII>5. These results are useful for evaluating the strength and lifespan of PMMA materials and structures.