R.M. Latanision – författare
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541 kr
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For many years it has been recognized that engineering materials that are-tough and ductile can be rendered susceptible to premature fracture through their reaction with the environment. Over 100 years ago, Reynolds associated hydrogen with detrimental effects on the ductility of iron. The "season cracking" of brass has been a known problem for dec ades, but the mechanisms for this stress-corrosion process are only today being elucidated. In more recent times, the mechanical properties of most engineering materials have been shown to be adversely affected by hydrogen embrittlement or stress-corrosion cracking. Early studies of environmental effects on crack growth attempted to identify a unified theory to explain the crack growth behavior of groups of materials in a variety of environments. It is currently understood that there are numerous stress-corrosion processes some of which may be common to several materials, but that the crack growth behavior of a given material is dependent on microstructure, microchemistry, mechanics, surface chemistry, and solution chemistry. Although the mechanism by which various chemical species in the environment may cause cracks to propagate in some materials but not in others is very complex, the net result of all environmentally induced fracture is the reduction in the force and energy associated with the tensile or shear separation of atoms at the crack tip.
541 kr
Skickas inom 10-15 vardagar
John Keats, writing to Fanny Brawne [1], said "I long to believe in immortality ••••• I wish to believe in immortality - I wish to live with you forever". So much of this talk will be concerned with the ductile behaviour of crystals, plasticity in its narrower sense. We shall consider a crystal which is deforming by slip, and shall expose a surface in this crystal. We first think of the sur face as a simple mathematical cut along a low-index plane. Then we allow for the relaxation of the newly-exposed atoms, and for surface irregularities, and we consider the effect of lattice va cancies which can enter at the surface. We consider the effect of dissolving off the surface layers, either intermittently or contin uously. Then the effects of adsorption or oxidation by normal con stituents of the atmosphere must be considered, the effects of sur face alloying, and finally those of special surface-active agents. But "All/Life death does end and each day dies with sleep" [2], and plasticity in its broader sense includes the fracture ~'1hich term inates flow. Here there is a bewildering array of effects. The medium in which the crystal flows may enhance its ductility enor mously, or it may cause it to break almost without plastic deform ation, or under a load which it has already supported.