Comparison of caustics and the strain gage method for measuring mode I stress intensity factor of PMMA material

Abstract Both caustics and strain gage methods are shown to be effective in the determination of mode I stress intensity factor (SIF) at the crack tip. However, there is a lack of investigation with regard to the quantitative comparison of the two methods. In this paper, dynamic three-point bending tests were performed on polymethyl methacrylate (PMMA) specimens using the caustic and the strain gage methods simultaneously. The SIFs calculated by these two methods were analyzed and compared. The results show that the deviations of the SIF values between caustics and the strain gage method are slightly more significant when the orientation of the gage is obtuse rather than acute. Furthermore, this deviation increases with the gage mounted closer to the edge of the specimen when the orientation is acute. In addition, the whole cracking process could be clearly recorded by the caustic method, while the time-evolution of SIF can be roughly obtained with a limited number of strain gages during the crack propagation.