The interaction of fracture and stresswaves in brittle material ? An experiment to verify cleaving waves

Abstract

The pioneering contribution of Vary [i] in establishing the correlation between ultrasonic propagation factors and fracture toughness is of considerable practical interest.The correlation found in his experimental studies was between the ultrasonic wave velocity and attenuation characteristics of different structural materials and their fracture toughness.Though Vary provided a heuristic theoretical basis for the observed correlation, the underlying physical mechanism has remained obscure.In a recent review of literature [2], an attempt has been made to interrelate the results of acoustic emission technique, ultrasonic studies, and fracture mechanics, and thus provide a possible explanation of the physical process responsible for the above correlation.Central to this explanation is a type of stresswave identified apparently for the first time in the above review.Consider an unflawed material subject to a static stress level ~, in which a crack of length 2a and unit width is formed spontaneously. Due to the formation of the crack, there is a sudden unloading of the crack surfaces and consequent reduction in strain energy given by U = -~2 a 2 ( I -w2)/E (i) a where w and E are Poisson's ratio and YoungVs modulus, respectively, and a is the semicrack length.Here plane strain conditions with fixed grips are considered.Part of the energy released due to the reduction in strain energy is consumed in the process of creating new crack surfaces, which is given by(2) S where y is the unit surface energy.Assuming that there is no other form of energy dissipation, rest of the energy is converted into stresswaves which propagate through the medium.These stresswaves are usually termed acoustic emissions and the energy content in them would be given byThe stresswaves generated by the crack formation travel outwards from the site of crack nucleation, in all directions in the medium° It is suggested [2] that a component of the stresswave travels along the Int Journ of Fracture 24 (1984)