Crack Characterization
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ack Characterization Otto Buck Introduction Although cracks in structural components are, in gênerai, highly undesirable, not ail of them are detrimental to the life of the structure. To détermine precisely how detrimental the crack actually is, one must characterize several crack parameters, particularly the location, size, shape, and orientation with respect to the applied stress. With thèse parameters and the magnitude of the applied stress, the driving force on the crack can be calculated by determining the stress intensity factor K or the stress intensity range AK. Most inspections today are performed in a scanning mode. If indications of some anomaly appear, the inspection is concentrated on thèse locations; and, if the response vérifies the présence of a flaw, the size, shape, orientation of the flaw and the potential for future growth of the flaws are determined. This process is now termed "quantitative nondestructive évaluation" (QNDE). This short review is devoted to récent advances in the characterization of cracks in various stages of development. The increase in sophistication in instrumentation and the use of computers for data analysis and interprétation has been dramatic and will certainly continue. In the laboratory, compliance gauges and electrical potential drop methods are now extensively used for crack length and shape measurements. Acoustic methods are being widely used in industry as nondestructive testing and évaluation tools because they are relatively easy to apply to real structures. A wealth of information on acoustic methods can be found in Références 1 and 2. In addition to acoustic methods, the high frequency variation of the electric potential drop method, the eddy current probe, is also used in modem industrial inspection Systems3 to detect cracks. For more détails on applying thèse three methods to fracture mechanics testing, the reader is referred to two récent books.45 44
As pointed out by Kobayashi 6 in a récent review, two- and threedimensional photoelastic and various interferometric studies for stressintensity factor détermination, for example, are now standard tools for crack characterization in the laboratory. For more détails, the reader is referred to a séries of articles.7,8 Optical methods (Moiré interferometry, the interferometric displacement gauge, caustic and laser speckle techniques), stereoimaging, and acousto-elastic measurements are also being used. It is interesting to note that in combination with numerical analyses, any of the above expérimental techniques may provide additional information.9 Such a combination of methods is called a "hybrid experimentalnumerical analysis." Crack Length Measurements Accurate détermination of crack length, a, is a primary prerequisite for fracture toughness, XIC, and fatigue
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Compliance Gauge Methods The use of strain gauges for crack length measurements is based on the idea that the introduction of a crack or the increase in the crack length makes the spécimen, containing the crack, more compilant than it had been previo
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