A Study on the Applicability of Dynamic Linear Fracture Mechanics and Its Extension

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The researches on the mechanics of dynamic fracture have been reviewed and it has been pointed out that there are many reports on peculiar behaviors of impact fracture toughness K_<Id> and fast crack propagating toughness K_<ID>, and that these peculiar behaviors prevent the development of the linear dynamic fracture mechanics. The importance of developing a reliable method for measuring the dynamic fracture toughnesses K_<Id> and K_<ID> and the necessity for clarifying the extent of applicability of the liner dynamic fracture mechanics have been also pointed out.In this research, firstly the accuracy of the finite element calculation of the dynamic stress intensity factor K (t) has been studied and a simple formula for determining K (t) of an impacted one-point bend specimen has been proposed based on the Euler-Bernoulli beam theory. By using the one-point-bend impact tester and the above simple formula, an impact fracture toughness K_<Id> test has been carried out on silicon carbide, … More sialon, PSZ, alumina and alumina-zirconia composite at temperatures up to 1200^。C. The existence of the contact force acting on the crack surfaces has been confirmed and its effect on the measured K_<Id> value has been discussed in relation to test conditions such as loading rate, specimen size and time-to-fracture.The impact fracture toughness K_<Id> of alumina-powder/aluminum composite and alumina-fiber/aluminum composite has also been measured at temperatures up to 200^。C by using a one-point bend impact tester. Young's modulus, Poisson's ratio and yield stress were measured at testing temperatures, and the dynamic stress intensity factor has been calculated by applying the plastic zone correction. The experimental results has been qualitatively explained by taking the creep deformation of the matrix into consideration.A finite element simulation of measuring K_<Id> by the caustics method in the range of short time-to-fracture has been performed. It has been found that there exists a delay time to detect crack initiation because of the slowness of the development of the stress high enough to have a significant effect on caustics. Some of the peculiar experimental data of K_<Id> have been explained by taking the delay time of detection of crack extension by the caustics method into consideration. Less