Computation of fracture toughness of giant magnetostrictive material

摘要

Magnetostrictive materials such as Terfenol-D are increasingly being considered for demanding applications such as active noise damping, sonar devices and reactive structures, due to their large strain capability. A limiting factor for the use of magnetostrictive materials lies in their inherent susceptibility to brittle fracture. The present study applies finite element technology in support of experimental investigations to assess the mode Ⅱ fracture toughness of magnetostrictive materials. In this exploratory effort, the fully coupled non-linear behavior of the material is not considered. Rather, a methodology for converting the applied magnetic field to an equivalent mechanical load, based on the material's magnetostrictive properties, is devised and applied. The DSA-VAST finite element software is employed to model the cylindrical, pre-cracked test specimen using both conventional solid elements and enriched twenty-noded solid fracture elements. Two load cases are investigated, namely one in which a mechanical load is applied to the specimen in the absence of a magnetic field, and a second case in which both a magnetic field and a mechanical load are applied to the specimen. In the absence of an applied magnetic field, the mode Ⅱ fracture toughness is found to be approximately 4.497 m~(1/MPa), a value comparable to that reported for ceramic-like materials. On the other hand, in the presence of an applied magnetic field (simulated by an equivalent compressive prestress), the mode Ⅱ fracture toughness is reduced to 2.768 m~(1/MPa), a significant reduction from the 'zero-field' value. FE results indicate significant specimen bending and an appreciable mode Ⅲ component to the fracture behavior, both of which are consistent with observed crack growth patterns in laboratory specimens.