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Fracture Toughness of Engineering Material by Professor Kim Wallin

Fracture Toughness of Engineering Material by Professor Kim Wallin

Fracture Toughness of Engineering Materials - estimation and application Kim Wallin

Professor K R W Wallin is a recognised expert in the field of fracture mechanics. Indeed his work has provided a major input to the currently accepted ASTM Standard on the Master Curve Method , E1921. This method facilitates characterisation of ductile to brittle fracture for ferritic steels. The book "Fracture Toughness of Engineering Materials - Estimation and Application" was written by Kim Wallin during his five year tenure as an Academy Professor funded by the Academy of Finland,and provides an authoritative guide and practical handbook for advanced students, fracture mechanics practitioners and R&D specialists.

Fracture Toughness of Engineering Materials - Estimation and Application provides a valuable and up-to-date addition to the bookshelf of both specialist and practicing scientists and engineers wishing to increase their knowledge of how fracture toughness is estimated and fracture mechanics is applied in structural integrity assessments.

Review by Dr David Lidbury, Serco TAS

Defect assessments are conducted routinely in many industries to assess the integrity or fitness for purpose of components whose failure would have unacceptable consequences, in terms of safety and economics. An essential feature of any defect assessment is an evaluation of the stability of a known or postulated flaw, which involves a comparison of load and resistance. Thus, proximity to crack initiation or unstable crack growth is assessed by calculating the crack driving force and comparing it with the material fracture toughness. This presents a major difficulty which is only partially addressed by current fracture toughness test standards. While fracture mechanics procedures for calculating the crack driving are relatively well developed, the material fracture toughness appropriate to a particular assessment can be much more difficult to estimate. Often this is because only an indirect guide as to the correct value to use is available, e.g. via knowledge of a transition temperature or impact energy. Even if fracture toughness data are available, there is the complication that the mechanisms of ductile and brittle fracture are quite different and that they respond differently to parameters such as temperature, loading rate, structural size, and geometry and load configuration (constraint). Moreover, variations in fracture toughness values from heat to heat or batch to batch can be considerable, requiring different statistical considerations for brittle and ductile fracture. This is further compounded if fracture toughness has to be evaluated over a range of temperatures for which a transition from ductile to brittle behaviour is possible.

The basic outline of the book is as follows:

The aims and objectives are set out clearly in the Introduction, where it is noted that the text is primarily concerned with the fracture toughness of metals, particularly structural ferritic steels. The following chapter introduces the major elastic and elastic-plastic crack driving force parameters and the corresponding definitions of fracture toughness, classified according to the extent of ductile tearing which precedes macroscopic failure. Chapter 3 covers the testing of bend specimens, and reviews some standard expressions used in the elastic and elastic-plastic regimes for the estimation of stress intensity factor (KI) and J-integral values. Added-value advice is given on adjustments which may be considered, as required, to extend the range of applicability of the standard expressions. Chapter 4 will be of particular interest to many practitioners, since it comprises valuable background to, and a very detailed description of, Wallin's acknowledged Master Curve description of brittle fracture, covering the fracture toughness of both homogeneous and inhomogeneous materials. Chapter 5 deals with ductile fracture toughness and, like the preceding chapter, includes coverage of specimen measuring capacity, data scatter, temperature dependence, effects of specimen side-grooving, and size and constraint effects. The following three chapters respectively consider: loading rate effects on brittle fracture and ductile tearing, and crack arrest; engineering interpretation of the Charpy impact test; and indirect estimations of fracture toughness - covering various ductile-brittle transition temperature concepts, as well as estimation of ductile tearing properties from Charpy-V tests and other impact tests, such as the drop-weight tear test (DWTT). (The exposition in Chapter 7 is particularly thorough and detailed.) Against the background provided by the first eight chapters, the final four chapters address a number of important application areas. Chapter 9 briefly introduces the methodology of structural integrity assessment and outlines the European SINTAP/FITNET procedures. Chapter 10 covers a number of issues affecting the transferability of fracture toughness test results to the assessment of the integrity of components and structures, specifically: the relevance of J-based fracture toughness measurements to the assessment of ductile tearing and cleavage in large-scale tests or real components; constraint effects, based on a two-parameter description of fracture behaviour; and the effects of warm prestressing (WPS) on effective cleavage fracture toughness. Chapter 11 provides a brief overview of statistical methods in fracture toughness estimation, with a focus on the measurement of cleavage initiation fracture toughness. The concluding chapter presents a number of examples (including the analysis of thermal shock and pressurised thermal shock tests) based on the methods presented in the previous chapters, again with emphasis on fracture toughness at temperatures in the brittle failure regime.

The overall aim of the book is stated as to provide advice on how best to estimate a material's fracture toughness (advising on test procedure and suitable parameter or providing relationships between available parameters) and how to apply the result in a structural integrity assessment. The emphasis is on application of relatively simple, often innovative, analytical models, rather than reliance on heavy-duty computational methods. On the author's admission, some parts of the book, due to their novelty, may appear outside commonly accepted views and therefore controversial. Although demanding more than a casual familiarity with fracture mechanics concepts, the book contains a wealth of data and illustrative analyses, detail and fresh insights on both familiar and less familiar themes which will be of great value to its intended readership: indeed, the author states that approximately 80-85% of the material presented in just over 500 pages has not been previously published.

A summary at the end of each chapter would have provided the reader with an overview of the various complex issues discussed within it. Moreover, a concluding chapter, drawing together (at least to the extent currently possible) the various threads forming the unified view of fracture toughness estimation that the author set out to explore in the Introduction, would have been useful. It would certainly have enhanced the reader's appreciation of the extent to which the book had met its original aims. But these are relatively minor points, and might be addressed in a subsequent edition. Overall, "Fracture Toughness of Engineering Materials - Estimation and Application" can be recommended as providing a valuable and up-to-date addition to the bookshelf of any specialist wishing to increase their knowledge of how fracture toughness is estimated in structural integrity assessments.

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