Crack Formation and Propagation
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n from thèse articles is that the load history, microstructure, and external environment significantly affect the crack's reaction to an applied load. Size, shape, and orientation are not the only parameters that describe the full stâte of the crack; the state of the material surrouriding the crack tip should also be determined. In the first article, W.L. Morris and M.R. J a m e s tackle t h e q u e s t i o n of fatigue crack formation or initiation. In most structural materials, cracks initiate at microstructuràl inhomogerieities such as i n c l u s i o n s , i n t e r f a c e s , or g r a i n boundaries. They grow erratically with a wide distribution of sizes. Some become d o r m a n t as others grow, until one becomes dominant—this is the one we hâve to worry about. Initiation is a statistical process we must understand in order to predict a structure's life. P.K. Liaw focuses on microstructural effects on the propagation rate of long cracks. Particularly interesting microstructures are fouhd in metal-metal matrix composites. Duplex microstructures, grain size effects, and the dislocation density, expressed in terms of the overall strength of the material, are other ëxamples considered in this article. They can influence "crack closure" — the physical contact between the fracture surfaces which partly shieids the crack from the external applied stress and thus affects the crack propagation rate. D.B. Kasul and L.A. Heldt discuss environmehtal effects on long cracks. In fatigue, a chemical environment can enhance the crack propagation rate by orders of magnitude ovèr that in vacuum or an inert environment. Depending on the conditions, the crack mode for the same material can be différent from one environment to another. Even under static loads a crack can grow very rapidly, a process called stress corrosion cracking. The authors review the important quantifies (including stress, stress intensity, the material's microstructure,
strain rate, electrochemical potential, and pH) that affect this type of cracking, and they attempt to explain its characteristics by mechanisms proposed in the literature. The last article by O. Buck is a short review of récent advances in characterizing cracks in various stages of development. Techniques hâve been developed and applied in the laboratory and in industry to provide information on the characteristics of the crack, such as size, shape, and orientation. Some of t h è s e t e c h n i q u e s can p r o v i d e s u c h information as load is applied to the crack. It is suggested that, in thèse cases, statements on the crack dynamics a n d t h u s on t h e m o m e n t a r y crack propagation rates can be made. In spite of ail the efforts since Griffith,1 accidents do and definitely will occur. Our goal, at présent, is to reduce the probability of failure. This could be done by replacing critical structures more frequently, but this is an expensive proposition. The alternative is to inspect structures more systematically and at appropriate intervais. Such philosophies are now becomi
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