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In this book the authors focus on the description of the physical nature of cleavage fracture to offer scientists, engineers and students a comprehensive physical model which vividly describes the cleavage microcracking processes operating on the local (microscopic) scale ahead of a defect. The descriptions of the critical event and the criteria for cleavage fracture will instruct readers in how to control the cleavage processes and optimize microstructure to improve fracture toughness of metallic materials. Physical (mechanical) processes of cleavage fracture operating on the local (microscopic) scale, with the focus on the crack nucleation and crack propagation across the particle/grain and grain/grain boundaries Critical event, i.e., the stage of greatest difficulty in forming the microcrack, which controls the cleavage fracture Criteria triggering the cleavage microcracking with incorporation of the actions of macroscopic loading environment into the physical model Effects of microstructure on the cleavage fracture, including the effects of grain size, second phase particles and boundary Comprehensive description of the brittle fracture emerging in TiAl alloys and TiNi memory alloys
A complete understanding of the fracture mechanisms of steel in the ductile/brittle transition region requires analysis not only of crack initiation, but also of crack propagation. This paper reviews micrographic and fractographic experiments that give insight into both phenomena, and suggests a frame-work through which both may be related. Unstable cleavage crack initiation can occur after some blunting of the original fatigue precrack or after some stable crack growth. In either event, instability appears to be triggered by the fracture of a brittle micro-constituent ahead of the precrack. The large scatter in reported KIc values within the transition region reflects the size distribution and relative scarcity of these trigger particles. While a large number of models have attempted to correlate toughness in the ductile/brittle transition regime to events occurring ahead of the crack tip, surprisingly little attention has been paid to events occurring behind the crack front. Fractographic evidence as well as metallographic sectioning of arrested cracks show that the mechanism of rapid crack propagation by cleavage is affected strongly by partial crack-plane deflection which leaves unbroken ligaments in its wake. The tearing of these ligaments by dimple-rupture is the dominant energy-absorbing mechanism. Etch-pit experiments using an Fe-Si alloy show that the crack-tip stress intensity based on plastic zone size is extremely low. Keywords: HSLA(High Strength Low Alloy), Ductile fracture, Shear fracture, Fracture toughness.
This symposium, held during Materials Week '97 in Indianapolis, Indiana, September 14-18, 1997, was dedicated to Dr. George R. Erwin, Professor Emeritus of the University of Maryland. The symposium proceedings is a collection of 29 papers and 10 abstracts on the subjects of fracture mechanics; dislocation theory of fracture; atomistic fracture; micromechanical and microstructural modeling of cleavage; cleavage mechanisms, fractographic characterization; experimental techniques; and cleavage studies in metals, welds, intermetallics, ceramics, and minerals. The diverse topics reflect the wide range of disciplines that Professor Irwin has impacted through his pioneering work on fracture mechanics.
Micromechanisms of Fracture and Fatigue forms the culmination of 20 years of research in the field of fatigue and fracture. It discusses a range of topics and comments on the state of the art for each. The first part is devoted to models of deformation and fracture of perfect crystals. Using various atomistic methods, the theoretical strength of solids under simple and complex loading is calculated for a wide range of elements and compounds, and compared with experimental data. The connection between the onset of local plasticity in nanoindentation tests and the ideal shear strength is analysed using a multi-scale approach. Moreover, the nature of intrinsic brittleness or ductility of perfect crystal lattices is demonstrated by the coupling of atomistic and mesoscopic approaches, and compared with brittle/ductile behaviour of engineering materials. The second part addresses extrinsic sources of fracture toughness of engineering materials, related to their microstructure and microstructurally-induced crack tortuosity. Micromechanisms of ductile fracture are also described, in relation to the fracture strain of materials. Results of multilevel modelling, including statistical aspects of microstructure, are used to explain remarkable phenomena discovered in experiments. In the third part of the book, basic micromechanisms of fatigue cracks propagation under uniaxial and multiaxial loading are discussed on the basis of the unified mesoscopic model of crack tip shielding and closure, taking both microstructure and statistical effects into account. Applications to failure analysis are also outlined, and an attempt is made to distinguish intrinsic and extrinsic sources of materials resistance to fracture. Micromechanisms of Fracture and Fatigue provides scientists, researchers and postgraduate students with not only a deep insight into basic micromechanisms of fracture behaviour of materials, but also a number of engineering applications.
The initiation and propagation of cleavage microcracks in coarse-grained vacuum-melted ferrite, containing 0.035 and 0.007 per cent carbon, were studied by means of tensile tests carried out between room temperature and -196 C, and by special metallographic procedures. Cleavage microcracks develop in ferrite during the strain-hardening portion of the stress-strain curve at low temperatures, and are initiated mainly by cracks which form in the carbides. Twinning does not play an important role in crack initiation over the entire temperature range studied. Carbide cracks during plastic deformation at all temperatures investigated, but they lead to microcracks in the ferrite only when the applied stress is high enough to permit the carbide cracks to act as Griffith cracks. Carbide cracks also lead to the formation of large voids during the necking of specimens tested in the ductile and transition temperature regions. Pre-existing twins provide strong barriers to microcrack propagation. Twinning also causes the disappearance of the discontinuous-yield phenomenon at low temperatures. A model for microcrack initiation by carbide cracking is proposed, and the conditions leading to brittle fracture are discussed. (Author).
This textbook provides a comprehensive guide to fracture mechanics and its applications, providing an in-depth discussion of linear elastic fracture mechanics and a brief introduction to nonlinear fracture mechanics. It is an essential companion to the study of several disciplines such as aerospace, biomedical, civil, materials and mechanical engineering. This interdisciplinary textbook is also useful for professionals in several industries dealing with design and manufacturing of engineering materials and structures. Beginning with four foundational chapters, discussing the theory in depth, the book also presents specific aspects of how fracture mechanics is used to address fatigue crack growth, environment assisted cracking, and creep and creep-fatigue crack growth. Other topics include mixed-mode fracture and materials testing and selection for damage tolerant design, alongside in-depth discussions of ensuring structural integrity of components through real-world examples. There is a strong focus throughout the book on the practical applications of fracture mechanics. It provides a clear description of the theoretical aspects of fracture mechanics and also its limitations. Appendices provide additional background to ensure a comprehensive understanding and every chapter includes solved example problems and unsolved end of chapter problems. Additional instructor support materials are also available.