Ichihito Narita

Fracture toughness tests on ENF specimens using a three-point bending test were conducted under six conditions based on maximum load and maximum load displacement. After rapid load removal from peak displacement, each specimen was reloaded to investigate compliance changes and observe the behavior of the adhesive layer near the crack tip during testing. Furthermore, the fracture surfaces of the specimens after testing were observed after immersion in fluorescent penetrant. An attempt was made to compare the actual crack length with the effective crack length calculated using the CBBM method based on the R-curve. The difference between the calculated crack length based on compliance at load removal and the observed crack length was estimated to correlate with the extent of damage progression accompanying crack growth. Comparing plots calculated by CCM with the corresponding R-curves calculated by CBBM confirmed that, for each plot, the maximum effective crack length and energy release rate from the CBBM-calculated R-curve were smaller than the observed crack length and energy release rate. Furthermore, in the crack propagation zone under Mode II loading, a fracture surface morphology with large, overlapping irregularities was observed because the crack propagated by flexing the bonded surface longitudinally. In contrast, photographs of fractures forced by tensile testing showed a fracture surface morphology with small irregularities or a flat surface.