Technology & Engineering
Fracture Toughness
Fracture toughness is a measure of a material's ability to resist crack propagation and withstand fracture. It quantifies the amount of stress required to propagate a pre-existing flaw or crack within the material. Higher fracture toughness indicates greater resistance to brittle failure, making it a critical property for materials used in engineering and structural applications.
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8 Key excerpts on "Fracture Toughness"
eBook - PDF- Arun Shukla(Author)
- 2004(Publication Date)
- CRC Press(Publisher)
Test and Analysis of Fracture Toughness 115 TESTING OF Fracture Toughness Traditional design involves simplified models based on the theory of elasticity and the conventional mechanical properties of the materials. Fracture mechanics, as a recent newcomer in the engineering field, offers new approaches to fail-safe design with the objective of reducing the level of conservatism by providing a theoretical stress analysis of the cracked body. This process introduces the flaw size and geometry as additional design parameters, and it requires a good deal of knowledge of relatively new material properties such as Fracture Toughness applicable to various modes of cracking and environmental conditions. This sec-tion outlines briefly some of the practical aspects of experimental methodology that may be of interest to design engineers and other practitioners in their search for fracture-resistant materials. One of the overriding interests in this area is con-cerned with the metallic materials, including carbon and low-alloy steels, high-strength steels, and nonferrous products. “Fracture Toughness” and “fracture res-istivity” terminology is sometimes used in the literature dealing with the mech-anical and metallurgical aspects of microfracture modes. [10] It is good to keep in mind various definitions and symbols used during the earlier formative years when considering newer concepts and future developments in experimental methodology. Fracture testing methods have developed during the past 50 years out of necessity. There were very real concerns about structural failures in some of the Liberty ships, followed by the needs of the U.S. military programs in the field of missiles and rockets. Particularly annoying and costly were brittle frac-tures of high-strength materials in rocket motor cases, where conventional design methodology was inadequate to cope with the technical setbacks.
eBook - ePubMaterials
Engineering, Science, Processing and Design
- Michael F. Ashby, Hugh Shercliff, David Cebon(Authors)
- 2009(Publication Date)
- Butterworth-Heinemann(Publisher)
Chapter 8 Fracture and Fracture ToughnessChapter contents- 8.1 Introduction and synopsis 166
- 8.2 Strength and toughness 166
- 8.3 The mechanics of fracture 167
- 8.4 Material property charts for toughness 174
- 8.5 Drilling down: the origins of toughness 176
- 8.6 Manipulating properties: the strength–toughness trade-off 180
- 8.7 Summary and conclusions 183
- 8.8 Further reading 183
- 8.9 Exercises 184
- 8.10 Exploring design with CES 185
- 8.11 Exploring the science with CES Elements 185
It is easy to set a value on the engineering science that enables success, that makes things happen, but much harder to value engineering science that prevents failure, that stops things happening. One of the great triumphs of recent engineering science has been the development from the 1960s onward of a rigorous mechanics of material fracture. We have no numbers for the money and lives it has saved by preventing failures; all we know is that, by any measure, it is enormous. This chapter is about the ways in which materials fail when loaded progressively, and design methods to ensure that fracture won’t happen unless you want it to.Ductile and brittle fracture. (Image of bolt courtesy of Boltscience; www.boltscience.com )8.1 Introduction and synopsis
It is easy to set a value on the engineering science that enables success, that makes things happen, but much harder to value engineering science that prevents failure, that stops things happening. One of the great triumphs of recent engineering science has been the development from the 1960s onward of a rigorous mechanics of material fracture. We have no numbers for the money and lives it has saved by preventing failures; all we know is that, by any measure, it is enormous. This chapter is about the ways in which materials fail when loaded progressively, and design methods to ensure that fracture won’t happen unless you want it to.Sometimes, of course, you do. Aircraft engines are attached to the wing by shear-bolts, designed to fail and shed the engine if it suddenly seizes. At a more familiar level, peel-top cans, seals on food containers and many other safety devices rely on controlled tearing or fracture. And processes like machining and cutting use a combination of plasticity and fracture.
eBook - PDF- John Tien(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
Chapter VII Fracture Toughness Victor F. Zackay Earl R. Parker M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G D E P A R T M E N T C O L L E G E O F E N G I N E E R I N G , U N I V E R S I T Y O F C A L I F O R N I A B E R K E L E Y , C A L I F O R N I A I. I N T R O D U C T I O N Each chapter of this book deals with alloy design to improve a partic-ular alloy property, such as strength, ductility, or resistance to creep, fatigue, or corrosion. In a large majority of the cases studied, each de-sign is developed to avoid the ultimate material failure, i.e., fracture. Therefore, it is prudent for the alloy designer, concerned with any par-ticular alloy property discussed in this book, to pay particular attention to the importance of material resistance to fracture, i.e., Fracture Toughness. In fact, the most important mechanical property required of a struc-tural material is often resistance to sudden or catastrophic fracture. For this reason, metals are usually the materials of choice where such failure would result in loss of life or property. However, the resistance of metals to catastrophic failure is not an intrinsic and fixed quantity. On the con-trary, it changes with the amount and type of stress, the temperature, the strain rate, and environmental variables, i.e., macroscopic factors, as 213 214 Victor F. Zackay and Earl R. Parker well as with alloy composition and structure, i.e., microscopic factors. Consideration of only one set of these factors in either the design of an engineering structure or, alternatively, in the design of an alloy is in-consistent in principle and ineffective in practice. This chapter describes some of the progress made in both the macro-scopic and microscopic approaches to the problem of designing alloys with superior Fracture Toughness.
eBook - PDFHandbook of Polymer Testing
Physical Methods
- Roger Brown(Author)
- 1999(Publication Date)
- CRC Press(Publisher)
Whether a crack of given dimensions constitutes a hazard can be assessed from the material property of the critical stress-intensity factor K and the magnitude of applied stresses. The relationship between stress intensity K and Fracture Toughness Kc is similar to the relationship between stress and tensile strength. The stress intensity represents the level of stress at the tip of a crack in a specimen containing a crack, and the Fracture Toughness Kc is the highest value of stress intensity that the specimen can withstand at the given crack sharpness without fracturing, which is obviously dependent on the type of material. Three important parameters are, therefore, required for the prediction (assessment) of sudden failures: 1. Fracture Toughness, which should be determined in accordance with internationally recognized standards 2. Crack length, which can be determined by nondestructive inspection techniques 3. Levels of stress encountered in service Any of these parameters can be quantified from the values of the others using fracture mechanics relationships. There is, however, still a reluctance on the side of industry, particularly polymers-related industry, to use this concept. This soon becomes apparent if raw materials suppliers' literature or any of the current materials selection data bases are examined: there are no fracture mechanics data quoted, but there are values for static and impact strengths measured in a variety of manners. We are reminded [5] that traditional tests such as the tensile tests give the performance under the most favorable conditions, and as such cannot be used to indicate or assess the likelihood of unexpected brittle fractures in service. The Charpy and Izod impact tests were introduced to solve these problems in that they subject materials to a set of worst-case conditions in contrast to the (best-case) tensile tests. The specimens are notched and then struck at a high rate in bending with the notch in tension.
eBook - PDFMechanics of Materials
A Modern Integration of Mechanics and Materials in Structural Design
- Christopher Jenkins, Sanjeev Khanna(Authors)
- 2005(Publication Date)
- Academic Press(Publisher)
Depending upon the size of the crack and critical stress intensity of the material, the fracture stress may or may not have to exceed the yield strength. For short cracks in a ductile material such as aluminum, the fracture stress may exceed the yield strength, but will be less than or equal to the ultimate strength. 7.6.2 Fracture Mechanisms in Materials The Fracture Toughness of most engineering metals and alloys are in the range 10 to 200 MPa m p . Polymers, which have also become popular as engineering materials, have K IC values in the range 1 to 7 MPa m p . Polymer matrix reinforced composites have found very significant use as structural and lightweight materials, and have Fracture Toughness values in the range 10 to 70 MPa m p , which is comparable to some metals. Ceramics typically have low values of Fracture Toughness in the range 2 to 10 MPa m p . This can be expected from their very low ductility. Though ceramics have very high yield strength, their application in tension is limited by their susceptibility to fracture even in the presence of small cracks, due to their low TABLE 7-1 Yield strength and Fracture Toughness data for some common structural materials Yield strength, S Y K IC Material MPa ksi MPa m p ksi i p n AISI 1045 steel 269 39 50 46 4340 (500 8 F temper) steel 1570 227 57 52 4340 (800 8 F temper) steel 1405 205 85 78 A538 steel 1722 250 111 100 2024–T351 aluminum 377 55 37 34 6061–T651 aluminum 296 43 28 26 7075–T651 aluminum 534 78 29 27 Ti-6Al-4V titanium 820 119 106 96 Mechanics , Materials Link: The Fracture Test 291 Fracture Toughness. Recent efforts at developing ceramic matrix composites have improved the Fracture Toughness, though they are still not comparable to metals or polymer composites or metal matrix composites. The fracture behavior of metals, which are face-centered cubic (FCC), body-centered cubic (BCC), or hexagonal close packed (HCP), are different.
eBook - PDFFracture of Metals
An Advanced Treatise
- H. Liebowitz(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
C H A P T E R 3 FRACTURE OF HIGH-STRENGTH MATERIALS A. S. Tetelman A. J. McEvily, Jr. I. Introduction 137 II. The Effect of Plate Thickness on Fracture Toughness 146 III. Physical Basis for Fracture-Safe Design with High-Strength Materials 150 IV. Microscopic Aspects of Fracture in High-Strength Materials 156 A. Fracture of High-Strength Steels 161 B. Fracture of High-Strength Aluminum Alloys 164 C. Fracture of High-Strength Titanium Alloys 167 V. Fatigue 169 VI. Environmental Effects 174 VII. Recommended Research 177 VIII. Summary 177 Symbols 178 References 179 Abstract: This chapter presents a review of the nature of the fracture processes occurring in high-strength materials. Important parameters affecting fracture behavior of such materials are the level of stored elastic energy, component geo-metry, Fracture Toughness, and environment. Because of the occurrence of low-energy tear fracture in high-strength materials, there is no well-defined brittle-ductile transition, and temperature is therefore a less important variable than in the case of materials of lower strength. Knowledge of the characteristics of stable crack growth in high-strength materials permits a rational approach to safe design procedures to be made. Examples of design procedures are given. The metallurgical structure, particularly particle spacing and strength, together with consideration of strain hardening, are discussed in relation to Fracture Toughness of high-strength steels, aluminum, and titanium alloys. The influence of cyclic loading and environ-ment on fracture of high-strength materials is also reviewed. I. Introduction A trend in structural design is to use higher strength alloys where strength /weight ratios or component packaging are important considerations. Alloy development has been responsive to these design requirements, often 137 138 A. S. TETELMAN AND A. J. MCEVILY, JR. anticipating them, and today alloys with yield strengths in excess of 300,000 psi are available.
eBook - PDF- J A Charles, F A A Crane(Authors)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
There is, in fact, a well-developed body of knowledge of fracture in relation to the use of rubber for tyres and springs. 7.6 Materials selection for toughness It was formerly traditional in engineering practice to design initially on the basis of strength or stiffness and to disregard toughness unless it presented itself as a problem. Unfortunately, when it did this it was generally in the form of a large engineering failure: modern practice therefore is to regard toughness as having an importance at least equal to, and possibly greater than, other mechanical properties. As always, the selection problem has two aspects—that of the design as a whole and that of the material. When considering the design as a whole, it is generally true that large constructions and thick sections are less tough than small parts and thin sections. This is because of the greater plastic constraints which apply in the former cases. It may, for example, be necessary in large ships to use higher-strength steels because ordinary mild steel Materials selection for toughness 85 would require sections so thick (designed on a strength basis) that the resulting plastic constraints would be sufficient to spoil its normal toughness. But within a given class of materials there is generally an inverse relationship between strength and toughness (Fig. 7.7), so that unless the toughness is properly matched to the design the use of a high-strength material may introduce problems due to inadequate material toughness. 1000 1500 2000 Yield strength (MPa) Figure 7.7 Variation of Fracture Toughness with yield strength for various classes of high-strength steels. (From J. F. Knott.) 8 To some extent, selection procedures are easier with high-strength materials because, provided the property data are available, fracture mechanics calculations can be incorporated into the design process.
eBook - PDFFracture Mechanics
Worked Examples
- John Knott, Paul Witney(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
3. Fracture Toughness Determination The basic aim of the Fracture Toughness lest is to obtain a reproducible value of the critical toughness of the material, denoted by KIc ' for mode I opening. The object i,n this Section is to go briefly through the steps required for the experimental determination of Ktc. For more precise information and guidance, reference should be made to BS 7448 'Methods for Determination of Klc. Critical CTOD and Critical J Values of Metallic Materials'. 3.1 Outline of Test The test involves loading a test-piece which contains a pre-exlstmg crack usually developed by fatigue from a machined notch. The practical objective is to determine the applied force at which a given amount of crack extension has taken place from the pre-existing crack. This information is established from a force I displacement test record in terms of a given deviation from linearity. The plane-strain Fracture Toughness, KIc ' is then calculated using the stress analysis relationship developed specifically for the particular type of test specimen involved. 3.2 General Testing Details In order to determine the Fracture Toughness KIc value from a laboralOry test it is necessary to know the applied force, the initial crack length and the way in which the K value varies with increasing crack length, as a result of the boundary and loading conditions imposed by the particular design of test specimen - otherwise referred to as the K-COMI'UANCE or K-CAUBRATWN function. As was mentioned in Chapter 2, K-calibrations can be derived experimentally for a given specimen design (see Example 21) or mathematically, the laller procedure being the more accurate. Two standard types of testpiece are currently recommended in SS 7448 (and in ASTM E399-74T). The first is a single-edge-notched (SEN) bend testpiece and the second, a compact tension (crS) testpiece. These are illustrated in Figure 3.1 and Figure 3.2 respectively.
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