Study on Fast Fracture and Crack Arrest (The 7 th Report)

Abstract

From the viewpoint of energy balance concept, the resistance of material against fast fracture can be measured by the capacity for absorbing energy dissipated due to crack extension. With respect to brittle fracture of steel, energy absorption is associated with the formation of both flat brittle fracture surface and shear lips.
In the previous report, stress gradient type double tension tests using mild steel were carried out and it was found that shear lips were able to absorb rather large amount of energy and played an important role to arrest behavior of brittle crack. In order to evaluate the arrestability of steel, it is thus necessary to clarify the mechanism of initiation and growth of shear lips.
In this study, experiments are carried out to investigate the factors which may influence the formation of shear lips. Surface-notched double tension specimen using mild steel is adopted. Crack propagates initially in the notched region where shear lips may be restrained, and then propagates into full thickness region where crack may arrest due to fall of driving force and/or shear lip formation.
Data obtained are analyzed on the basis of dynamic fracture mechanics. Shear lips contribute very much to crack arrest phenomena and are influenced by crack velocity, temperature, constraint (stress triaxiality) and applied stress level in this case.