Abstract: The present invention consists of a method for accurately measuring the plane strain fracture toughness of ductile and brittle materials, the method to be practiced on a specimen formed such that, when the specimen is appropriately loaded, a crack will initiate at a predetermined point or line, with such crack formed thereat being intrinsically stable such that the load executed on the specimen must be continually increased in order to further advance the crack along a predetermined path through the specimen, until, at some critical location along the predetermined crack path, the load necessary to further advance the crack reaches a maximum, thereafter decrasing as the crack advances beyond the critical location along the remainder of its predetermined path through the specimen, that critical location being essentially independent of the specimen material, it being determined by the specimen geometry alone as long as elastic plane strain conditions prevail in the specimen sufficiently close to the crack tip.

Abstract: A machine that can be used for loading of a slotted specimen for measuring the fracture toughness, fatigue crack growth resistance, or stress corrosion crack growth resistance of that specimen, the machine having a pressure bag maintained in a frame, which pressure bag can be installed within the slotted portion of the specimen and expanded to produce a desired specimen loading, the machine preferably involving a load pressure sensing device, can optionally include an arrangement for sensing volume enlargement of the specimen slot, and can also include a movable frame portion that expands with the pressure bag during inflation to discourage crimping of that pressure bag during specimen loading.