ICSID 2017

ESIS Summer School

Dubrovnik, Croatia, August 14 - 18, 2017

Some New Ideas in Fracture Toughness Evaluation

John Landes, Professor, University of Tennessee
and Juan R. Donoso

Fracture toughness testing for brittle materials is relatively easy because the fracture point is well defined. For ductile materials fracture toughness is not a specific point but a process that is often characterized by a curve based on crack extension. Nevertheless most applications of fracture toughness for structural integrity evaluation prefer a single value of fracture toughness. What is then required is the evaluation of the crack extension during a test as well as the choice of a point to give the single fracture toughness value.

The concept of load separation, that is the separation during the loading of a cracked body into a materials deformation function and a geometry function, has given new ways of determining fracture toughness both as a process and as a single point. The geometry term in the load separated function does not depend at all on the material deformation behavior. This suggests that any deformation process can be modelled by the separated function, so long as it occurs over each occurs over the same part of the structure or specimen. Making this analogy for both plastic and elastic deformation allows the traditional parameters based on crack length or crack aspect ratio to be better determined based on an uncracked ligament length. This can result in much simpler terms for determining traditional fracture mechanics parameters like crack-tip stress intensity factor and load versus displacement compliance.

The load separation concept has also suggested ways of determining crack length during the loading process. This has shown that the materials plastic deformation properties can be used in a test method as well as the elastic properties to measure ductile crack advance. Methods related to common and concise formats will be introduced as well as variations on the method of normalization. The load intercept method is one example. The ideas presented in this lecture will suggest better and easier methods for determining traditional fracture toughness properties from what is often regarded as a difficult and expensive test.