Abstract
Abstract— Fracture surfaces of both service and laboratory fatigue fractures frequently show dark tongue-shaped marks. In fatigue tests on 7075–T6 specimens such tongues were produced by high peak loads. Measurements indicated that a tongue is not formed during a single burst of crack extension but is the result of a number of successive pop-ins requiring an increasing load. Therefore tongue formation is a quasi-stable phenomenon. The tunnelling fracture in the centre of plate thickness is accompanied by unfailed ligaments at the plate surfaces which reduce the stress intensity at the crack tip. The effect of different material conditions and loading direction on tongue forming was studied. A new model was developed to describe the growth of a tongue. The model was in good agreement with the various test results. The analysis of the problem has some relevance to validity requirements for K1c. A formula pertaining to tongues proposed by Forsyth was slightly modified and found to be approximately correct for the present results.
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