Indirect estimation of fiber/polymer bond strength and interfacial friction from maximum load values recorded in the microbond and pull-out tests. Part II: Critical energy release rate

A new approach to experimental data treatment in the pull-out and microbond tests has been developed. It uses the relationship between the maximum force recorded in these tests and the embedded length ('scale factor') to separately determine adhesional interfacial parameters (critical energy release rate, local bond strength) and interfacial friction in debonded regions. The new method does not require the measurement of the debond force, which corresponds to interfacial crack initiation, and is, therefore, much more convenient and simpler than 'direct' techniques involving continuous monitoring of crack growth. Using the equation for the current crack length as a function of the load applied to the fiber, based on a fracture mechanics analysis of interfacial debonding, we modeled the pull-out and microbond experiments and obtained the maximum force versus the embedded length. By varying the critical energy release rate and interfacial frictional stress to fit experimental plots, both interfacial parameters were determined for several fiber-polymer pairs. Effects of specimen geometry, residual thermal stresses, and interfacial friction on the measured values are discussed. The results are compared with those obtained with our similar stress-based approach. The energy criterion works when the embedded length is not very short, and in this range of embedded length it is better than the stress criterion. Both criteria can be complementarily used for interface characterization.