Materials Science and Engineering for the 1990s Maintaining Competitiveness in the Age of Materials (1989) / Chapter Skim
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Appendix C: Performance
Appendix C: Performance
Pages 242-254
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From page 242... ...
The increasing power of computers, coupled with new theoretical developments, now makes it possible to develop quantitative methods for solving some of the major problems in this field. In particular, accurate codes may become available to aid in the microstructural design of complex materials such as composites, or for making reliable predictions of the lifetimes of structural materials in service.
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From page 243... ...
These macroscopic structures include grains, preexisting cracks or pores, precipitates and inclusions that may contribute to strengthening but may also introduce cracks or voids, fibers embedded in matrices, and surface films or coatings. Thus the relevant background for performance research includes not only the quantum mechanical concepts essential to understand matter at the atomic scale, and the thermodynamic, chemical, and kinetic concepts needed for understanding structural transformations, but also the more macroscopic concepts of deformation and transport that are relevant to processes that occur on the larger than atomic scales of materials microstructure.
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From page 244... ...
We see therefore that the scope of performance research is unusually broad. Few of its currently active researchers and advanced practitioners are involved in the entire spectrum, and fewer yet have received formal education in that breadth.
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From page 245... ...
They will find much productive work at the interfaces. SCIENTIFIC AND TECHNICAL ISSUES The basic scientific issues addressed by research in the performance of structural materials are strength, deformability, chemical degradation, and fracture.
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From page 246... ...
Such calculations necessarily involve departures from rigorous quantum mechanical principles, for example, the use of pair potentials or a modern improvement like the "embedded atom method." Examples of processes for which dynamic modeling is currently being attempted include fracture at a crack tip, chemical bond formation during separation of fracture surfaces, the motion of dislocations, interaction of glide and grain-boundary dislocations, nucleation of martensitic transformations, and entanglements of long-chain molecules during deformation and crazing of polymeric materials.
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From page 247... ...
Here, too, there are problems in explaining observed behavior in that stresses predicted for nucleation according to the standard model of vacancy condensation generally exceed stress levels inferred experimentally. Once nucleated, the growth to linkage of microcracks or cavities, causing final fracture, is usually an extremely complex process in multiphase alloys and composite materials.
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From page 248... ...
For example, increased strength causes increased stress ahead of a crack or a sharp notch. This stress acts to nucleate running cracks at brittle phases, such as carbides in steel, and also makes it possible for the crack to continue in a brittle cleavage mode across the adjoining metallic grains.
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From page 249... ...
Also, while fracture by ductile void growth to coalescence can lead to substantial macroscopic strain at fracture, the actual strain at fracture is often sharply reduced by localization. For example, it is common that well before voids generated from large impurity inclusions grow to coalescence with one another, localized shear bands develop between them and abruptly terminate the process by triggering profuse voidage from families of smaller precipitates within the band.
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From page 250... ...
Neither the microstructural alterations caused by wear nor the basis for environmental sensitivity of these processes is well understood at present. Progress in this area should lead not only to better design of wear-resistant structural materials but also to better understanding of wear inhibition by surface coating or ion implantation.
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From page 251... ...
The basic mechanism of fatigue crack growth is an irreversible process in which the crack tip opens under increasing load but does not return fully to its original configuration when the load is released. It turns out that the extent of crack tip opening and the degree to which it can be reversed by closure slip and rewelding are sensitive to the environment.
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From page 252... ...
They include the hydration of silicon-oxygen bonds of silica glasses in moist environments; the successive formation and rupture of oxide films at crack tips, usually along grain boundaries, in many metallic systems; and, as in the high-strength steels, the evolution of hydrogen in aqueous surface reactions, which promotes hydrogen embrittlement. Nearly all service applications of materials in hostile environments involve large fluctuations of loading.
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From page 253... ...
Distributed Damage While macroscopic fracture mechanics has developed according to the concept of a single dominant crack, there are circumstances when a more realistic picture is that of a broadly distributed damage zone. Here, damage refers to a multitude of microcracks or cavities such as occurs, for example, in creep rupture by broadly distributed cavitation.
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From page 254... ...
The complexity of these problems dictates that such empirical procedures will remain prominently in use, but an achievable goal for research is the provision of a more enlightened basis for them. For example, research in macroscopic fracture mechanics in the nonlinear range has identified parameters that, in certain defined circumstances, characterize the severity of deformation near the crack tip and hence serve as the loading variable in terms of which crack growth rate should be characterized in empirical studies.
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