fracture mechanic.pdf

Introduction to Fracture Mechanics David Roylance Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge, MA 02139 June 14, 2001 Introduction In 1983, the National Bureau of Standards (now the National Institute for Science and Tech- nology) and Battelle Memorial Institute1 estimated the costs for failure due to fracture to be $119 billion per year in 1982 dollars. The dollars are important, but the cost of many failures in human life and injury is infinitely more so. Failures have occurred for many reasons, including uncertainties in the loading or envi- ronment, defects in the materials, inadequacies in design, and deficiencies in construction or maintenance. Design against fracture has a technology of its own, and this is a very active area of current research. This module will provide an introduction to an important aspect of this field, since without an understanding of fracture the methods in stress analysis discussed previously would be of little use. We will focus on fractures due to simple tensile overstress, but the designer is cautioned again about the need to consider absolutely as many factors as possible that might lead to failure, especially when life is at risk. The Module on the Dislocation Basis of Yield (Module 21) shows how the strength of struc- tural metals – particularly steel – can be increased to very high levels by manipulating the microstructure so as to inhibit dislocation motion. Unfortunately, this renders the material in- creasingly brittle, so that cracks can form and propagate catastrophically with very little warn- ing. An unfortunate number of engineering disasters are related directly to this phenomenon, and engineers involved in structural design must be aware of the procedures now available to safeguard against brittle fracture. The central difficulty in designing against fracture in high-strength materials is that the presence of cracks can modify the local stresses to such an extent tha