Microstructural effects on fracture toughness in AA7010 plate (original) (raw)
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FUNDAMENTALS OF FRACTURE MECHANICS
FUNDAMENTALS OF FRACTURE MECHANICS, 2019
Fracture is a problem that society has faced for as long as there have been man-made structures. The problem may actually be worse today than in previous centuries, because more can go wrong in our complex technological society. Major airline crashes, for instance, would not be possible without modern aerospace technology. Fortunately, advances in the field of fracture mechanics have helped to offset some of the potential dangers posed by increasing technological complexity. Our understanding of how materials fail and our ability to prevent such failures have increased considerably since World War II. Much remains to be learned, however, and existing knowledge of fracture mechanics is not always applied when appropriate. While catastrophic failures provide income for attorneys and consulting engineers, such events are detrimental to the economy as a whole. An economic study [1] and [2] estimated the annual cost of fracture in the U.S. in 1978 at 119billionwhichwasabout4119 billion which was about 4% of the gross national product. Furthermore, this study estimated that the annual cost could be reduced by 119billionwhichwasabout435 billion if current technology were applied, and that further fracture mechanics research could reduce this figure by an additional $28 billion. This research paper will introduce several important means of understanding and dealing with fracture in stressed materials. Keywords: Creep, energy balance, stress intensity, fatigue, fracture 1. Atomistic of Creep Rupture Creep rupture is a conceptually simple mode of failure in which a specimen is subjected to a constant uniaxial stress at constant temperature and humidity, and the time to fracture recorded. The fact that rupture can occur later and perhaps much later than the time of application of stress implies that fracture is a time dependent process in which damage takes place within the specimen and accumulates until the specimen no longer has sufficient strength to prevent total rupture. As a very simple approach to the damage accumulation process, a first-order mechanism might be proposed in which the number of unbroken bonds decreases at a rate proportional to the number of unbroken bonds remaining: where n is the fraction of unbroken bonds remaining and K is a rate constant for the process. In such a process the number of unbroken bonds goes to zero only at t→∞, and clearly fracture will occur well before that. Perhaps a reasonable scaling law would take the creep-rupture lifetime t f to scale with the average time t for a bond scission, which can be computed as: Following another approach which describes yield as a thermally activated stress aided rate process, the bond scission process is viewed similarly and the rate constant K is written as: where E * and V * are an activation energy and volume, and ψ is the stress on the bond. Determining ψ is nontrivial, as ψ obviously varies over the distribution of bonds and is dependent on the material microstructure. But as another approximation, the atomic stress might be taken to scale with the externally applied stress, giving equation 1 below:
International Journal of Fracture 100: 55--83, 1999
The mechanisms of fatigue-crack propagation are examined with particular emphasis on the similarities and differences between cyclic crack growth in ductile materials, such as metals, and corresponding behavior in brittle materials, such as intermetallics and ceramics. This is achieved by considering the process of fatiguecrack growth as a mutual competition between intrinsic mechanisms of crack advance ahead of the crack tip (e.g., alternating crack-tip blunting and resharpening), which promote crack growth, and extrinsic mechanisms of crack-tip shielding behind the tip (e.g., crack closure and bridging), which impede it. The widely differing nature of these mechanisms in ductile and brittle materials and their specific dependence upon the alternating and maximum driving forces (e.g., K and K max ) provide a useful distinction of the process of fatigue-crack propagation in different classes of materials; moreover, it provides a rationalization for the effect of such factors as load ratio and crack size. Finally, the differing susceptibility of ductile and brittle materials to cyclic degradation has broad implications for their potential structural application; this is briefly discussed with reference to lifetime prediction.
On the Fracture Toughness of Advanced Materials A R C H
2009
S E On the Fracture Toughness of Advanced Materials A R C H By Maximilien E. Launey, and Robert O. Ritchie* N E W S Few engineering materials are limited by their strength; rather they are limited by their resistance to fracture or fracture toughness. It is not by accident that most critical structures, such as bridges, ships, nuclear pressure vessels and so forth, are manufactured from materials that are comparatively low in strength but high in toughness. Indeed, in many classes of materials, strength and toughness are almost mutually exclusive. From a fracture-mechanics perspective, the ability of a microstructure to develop toughening mechanisms acting either ahead or behind the crack tip can result in resistance-curve (R-curve) behavior where the fracture resistance actually increases with crack extension; the implication here is that toughness is often developed primarily during crack growth and not for crack initiation. Biological materials are perfect examples of this; moreo...
Metallurgical Transactions A, 1982
The effects of microstructure and strength on the fracture toughness of ultra high strength aluminum alloys have been investigated. For this study three ultra high purity compositions were chosen and fabricated into 1.60 mm (0.063 inches) sheet in a T6 temper providing a range of yield strengths from 496 MPa (72 ksi) to 614 MPa (89 ksi). These alloys differ only in the volume fraction of the fine matrix strengthening precipitates (G.P. ordered + "q'). Fracture toughness data were generated using Kahn-type tear tests, as well as R-curve and Jc analyses performed on data from 102 mm wide center cracked tension panel, tests. Consistent with previous studies, it has been demonstrated that the toughness decreases as the yield strength is increased by increasing the solute content. Concomitant with this decrease in toughness, a transition in fracture mode was observed from predominantly transgranular dimpled rupture to predominantly intergranular dimpled rupture. Both quantitative fractography and X-ray microanalysis clearly demonstrate that fracture initiation for the two fracture modes occurred by void formation at the Cr-dispersoids (E-phase). In the case of intergranular fracture, void coalescence was facilitated by the grain boundary "q precipitates. The difference in fracture toughness behavior of these alloys has been shown to be dependent on the coarseness of matrix slip and the strength differential between the matrix and precipitate free zone (crM-trpFz). A new fracture mechanism has been proposed to explain the development of the large amounts of intergranular fracture observed in the low toughness alloys. *The term "matrix microstructure" will be used to refer to the grain interior microstructure and includes the continuous, aluminum-rich a phase and the identity, volume fraction, and size of the strengthening precipitates exclusive of the dispersoids.
On the Fracture Toughness of Advanced Materials
Advanced Materials, 2009
Few engineering materials are limited by their strength; rather they are limited by their resistance to fracture or fracture toughness. It is not by accident that most critical structures, such as bridges, ships, nuclear pressure vessels and so forth, are manufactured from materials that are comparatively low in strength but high in toughness. Indeed, in many classes of materials, strength and toughness are almost mutually exclusive. In the first instance, such resistance to fracture is a function of bonding and crystal structure (or lack thereof), but can be developed through the design of appropriate nano/microstructures. However, the creation of tough microstructures in structural materials, i.e., metals, polymers, ceramics and their composites, is invariably a compromise between resistance to intrinsic damage mechanisms ahead of the tip of a crack (intrinsic toughening) and the formation of crack-tip shielding mechanisms which principally act behind the tip to reduce the effective "crack-driving force" (extrinsic toughening).
Mechanical Engineering Series, 2011
The use of general descriptive names, registered names, trademarks, etc. in this publication does not or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Preface This book has evolved from lectures on fracture mechanics and micromechanics which we held for students of engineering and natural sciences over the years. It is primarily meant as an aid for students learning the foundations of these subjects. At the same time this book may also serve as an introduction into these fields for researchers and practitioners in industry and to provide the theoretical background for solving respective problems. The book covers the most important areas of fracture mechanics and gives an introduction into micromechanics. Our major concern was the presentation of principal concepts and methods in a clear and sound manner as a basis for a deeper entry into the matter. The presentation mainly focuses on the mechanical description of fracture processes; yet, material specific aspects are also discussed. To keep the text self-contained, continuum mechanical and phenomenological foundations are recapitulated first. They are followed by a brief survey of classical fracture and failure hypotheses. A major part of the book is devoted to linear fracture mechanics and elastic-plastic fracture mechanics. Further chapters deal with creep fracture and dynamic fracture mechanics. An extensive chapter treats foundations of micromechanics and homogenization. Finally, elements of damage mechanics and probabilistic fracture mechanics are presented. Suggestions for further reading are listed at the end of each chapter. The first edition was well accepted by the readers making a new edition necessary. We have used this chance to incorporate a number of extensions which partly are influenced by new developments in the field of fracture mechanics. Discussed are, among others, the crack initiation at notches, cohesive zone models, the peel test, fragmentation, and strain localization due to damage and material softening. Furthermore, following suggestions from many students, supplementary examples have been added as problems at the end of some chapters. The authors are indebted to all who have contributed to this book. This particularly includes those from whom we have learned or, as Roda Roda has put it ironically: "Copying from four books yields a fifth profound book". Special thanks go to Mrs. Dipl.-Ing. H. Herbst who has prepared most of the figures. Finally, the pleasant cooperation with the publisher is gratefully acknowledged.
A critical review of fracture mechanics as a tool for multiaxial fatigue life p.PDF
Plastics belong to the most complex and probably least understood engineering materials of today. Combining the best aspects of design, mechanical properties and manufacturing, the structural integrity of plastics is on par with aluminium and can in some cases even rival those of steels. One of the most important aspects of plastics is the ability to tailor-drive their material properties for a specific purpose or towards a specific strength value. The morphology of plastics is directly dependent on the manufacturing process, e.g. injection moulding, extruding and casting. Plastics contain multiple phases (crystalline, amorphous, oriented), and are in no sense at all isotropic, although integrally deduced mechanical properties may appear to claim the opposite. As such, it becomes obvious that attempting to analyse such materials using conventional material models and explanations of mechanics is an inherently complex task. The static situation alone requires concepts such as creep, relaxation and rate effects to be incorporated on a numerical level. If the load situation changes, such that cyclic loading is acting on the continuum, with the morphology taken into account (without considering the actual geometrical shape), then the result is that of a complex multiaxial fatigue case. Classical theories used for treating fatigue such as SN or eN analysis have proven much less successful for plastics than they have for metals. Fatigue crack propagation using fracture mechanics has seen some success in application, although appropriate crack initiation criteria still need to be established. The physical facts are more than intriguing. For injection moulded parts (being the most common manufacturing process in place), fracture is in most cases seen to initiate from inside the material, unless the surface has been mechanically compromised. This appears to hold true regardless of the load case. In this review, we have scrutinised physically useful methods of crack initiation, as well as the use of fracture mechanics for multiaxial fatigue life prediction of injectionmoulded plastics. Numerical tools have been utilised alongside experimental experience and public domain data to offer what we hope will be a contemporary overview, and offer an outlook for future research into the matter.
WIT transactions on modelling and simulation, 2001
In this work, the use of the Weibull stress as a measure of the failure probability of cracked body is tested. Fracture of large engineering structures and conventional safety assessment and integrity of components and structures still remains a top field of research in the experimental and the theoretical fracture mechanics. Weibull stress seems to be a parameter for prediction of cleavage failure of cracked bodies and the study is focused to assess the effects of constraint loss on cleavage fracture toughness (J,). T o quantify the relative effects of constraint variation on the cleavage fracture toughness the form of the toughness-scaling model based on the Weibull stress o, is investigated. Local material parameters have been calculated arising from Beremin approach. It is based on weakest link assumption and incremental fracture probability, which depends not only on the maximum principal stress, but also on the equivalent plastic strain. Accepting this approach to the analysis of local criteria for cleavage fracture the location G,, and shape parameters m were calculated using FEM for notched tensile bars having various type of geometry. The aim of the paper can be seen in fracture toughness correction from various specimen geometries to small scale yielding (SSY). The fracture resistance has been assessed using data from static tests of the bend specimens and from the axisymmetric notched tensile specimens. The standard finite element method package Abaqus was applied and the manganese cast steel considered for storage and transport container for spent nuclear fuel (SKODA) was selected as a n experimental material.