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One important requirement of any fracture toughness test is the ability to detect the initiation of fracture, and it is here that detailed observations of void growth ahead of the crack tip should provide background calibration for macroscopic measurements such as compliance changes, electrical resistance changes across the notch faces, or acoustic emission output. The nature of incipient crack growth process, however, is such that the use of the term “fracture initiation” requires careful definition. The problem arises in view of the complexity of a set of microscopic-scale events occurring at a crack tip upon extension. In this paper the development of a reliable single-test method for the detection of fracture initiation is described. This method is based on the use of a double-clip-gage system attached to either a fracture toughness (KIc) or a crack opening displacement (COD) test specimen. It locates the initiation point from compliance-related changes in the double-clip-gage output signals and is successful in detecting fracture initiation, independent of specimen size, under conditions of quasi-linear elastic fracture mechanics (QLEFM), elastoplastic fractures (limbo region), and fully yielded specimens (YFM). The double-clip-gage method was calibrated by quantitative scanning electron microscopy of void growth and coalescence during a series of experiments in which ductile low-alloy steel precracked specimens were unloaded from different points on the load-displacement diagram. A three-parameter microfractographic definition of fracture initiation was established at the detectable point based on: (1) maximum lineal extent of advancing microthumbnail cracks; (2) normalized areal growth increment; and (3) the percentage of advanced crack-tip length. Following its rigorous fractographic calibration, the double-clip-gage system was used as a means of checking out other methods of fracture initiation prediction. Four methods were applied simultaneously to the same specimen: the double- (or triple) clip-gage system; an acoustic emission analyzer; a potential drop system; and the Standard Offset Procedure. There was excellent agreement between the double-clip-gage system and both acoustic emission ringdown and event count indications under valid and nonvalid fracture toughness (KIc) test conditions. By comparison, the potential-drop system exhibited appreciable and, in some materials, considerable lagging, the extent of which increased with increasing plasticity. The Standard Offset Procedure agreed well with the calibrated method only in valid KIc tests, but tended to underestimate the initiation point in substandard specimens. When used as a monitoring technique for determining various measures of fracture initiation toughness in situations involving either limited or excess plasticity, the fractographically calibrated double-clip-gage method can serve as a means of developing a unified and consistent ranking of the toughness behavior of materials under widely different conditions.
fracture toughness, fracture initiation, test methods, bend tests, crack opening displacement, calibration, fractography, crack propagation, elastic-plastic fracture
Engineering specialist, Aircraft Engine Group, General Electric Co., Cincinnati, Ohio
Reader, University of Cambridge, Cambridge,