Advanced Fracture Mechanics and Structural Integrity

Ashok Saxena

• Publisher: CRC Press
• Year: 2019
• Print ISBN: 9781138544260
• eBook ISBN: 9781351004046
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Mechanics
• Index: Technology & Engineering

Advanced Fracture Mechanics and Structural Integrity is organized to cover quantitative descriptions of crack growth and fracture phenomena. The mechanics of fracture are explained, emphasizing elastic-plastic and time-dependent fracture mechanics. Applications are presented, using examples from power generation, aerospace, marine, and chemical industries, with focus on predicting the remaining life of structural components and advanced testing metods for structural materials. Numerous examples and end-of-chapter problems are provided, along with references to encourage further study.The book is written for use in an advanced graduate course on fracture mechanics or structural integrity.

1

Introduction and Review of Linear Elastic Fracture Mechanics

1.1 Why Nonlinear Fracture Mechanics

Fracture of load-bearing components is always an important consideration for engineers who design, build, operate, and maintain bridges, highways, automobiles, trains, airplanes, power plants, chemical process equipment, and numerous other large pieces of machinery. Everyone understands the catastrophic consequences of structural failure and that sometimes it happens because the factors involved in predicting it are complex and are not well understood at the time the component is designed. Since the late 1950s, the developments in fracture mechanics have contributed immensely to our understanding of fractures that emanate from cracks or crack-like defects which could potentially escape detection.

A large fraction of failures in structural components occurs due to preexisting defects or defects that initiate rapidly from clusters of nonmetallic inclusions or from other imperfections such as casting, forging, and welding defects. On the other hand, several defects also remain dormant in the components and pose no threat of fracture. In fracture mechanics, we are interested in both, the defects that can ultimately cause fracture and those that are benign.

Several fractures can be analyzed using the principles of linear elastic fracture mechanics (LEFM). These are typically brittle fractures that are accompanied by limited or small amounts of plastic deformation. When fractures are accompanied by significant amounts of plastic deformation or by creep deformation, LEFM is inadequate in those circumstances. Nonlinear fracture mechanics has been developed as a viable engineering tool since the 1970s to address specifically those classes of problems. Some examples that depend on the use of nonlinear fracture mechanics for explaining the fracture process are considered next to illustrate its need.

1.1.1 Failures in Reheat Steam Pipes

In June 1985, a 760-mm diameter welded steam pipe carrying steam at a pressure of 4.2 MPa at a temperature of 538°C suddenly ruptured and sprayed supersaturated steam on people gathered in a nearby lunchroom [1]. Several fatalities resulted and many more suffered serious injuries. The rupture occurred along the pipe’s longitudinal seam weld where a crack propagated through an axial length of 6 m. The opening of the rupture in the central region was approximately 2 m, Figure 1.1a, and several creep cracks were found at the interface of the weld and base metals as seen in Figure 1.1b. A very similar failure occurred in another power station 6 months later. In this case, the weld seam was pointed up toward the roof. The gush of steam punched a large hole in the ceiling but, unlike the previous failure, it did not result in fatalities or serious injuries. In both cases, the cause of fracture was identified to be early crack initiation in the weld region and subsequent propagation by creep damage. These mechanisms are explored quite extensively in the chapters dealing with time-dependent fracture mechanics.

FIGURE 1.1
Photograph of a steam pipe that ruptured in a power plant in June 1985 (left) and a photomicrograph of the failed region of the pipe showing the fracture and other cracks developing in the seam weld (right).

Source: https://www.bing.com/images/search?q=steam+pipe+failure+in+the+mojave+desert&qpvt=steam+pipe+failure+in+the+mojave+desert&FORM=IQFRML) [1].

1.1.2 Failure of a Steam Turbine Rotor

On June 19, 1974, a high temperature rotor of a steam turbine called Gallatin located at a power plant in Tennessee burst suddenly during a routine start-up operation [2]. A schematic diagram of the reconstructed fractured pieces of the rotor is shown in Figure 1.2. Fractographic analysis revealed two elliptical flaws in the bore region of the rotor from which the fracture initiated. These flaws initiated from clusters of manganese sulfide (MnS) inclusions that connected to form cracks under creep–fatigue conditions and subsequently grew to the critical size also due to creep–fatigue until the time when fracture occurred [3]. At the time of the sudden fracture, the rotor had experienced 106,000 h of service. This incident triggered bore-sonic inspections of rotors in similar turbines operating in power plants all over the world. It also started a flurry of research in the field of creep–fatigue crack initiation and crack growth. Several concepts described in this book are the result of research conducted on this topic in the aftermath of this failure.

FIGURE 1.2
Schematic diagram showing the fracture in the high temperature Gallatin steam turbine rot...

Table of contents

• Cover Page
• Halftitle
• Title Page
• Copyright Page
• Contents
• Preface
• Acknowledgments
• Author
• 1. Introduction and Review of Linear Elastic Fracture Mechanics
• 2. Analysis of Cracks under Elastic–Plastic Conditions
• 3. Methods of Estimating J-Integral
• 4. Crack Growth Resistance Curves and Measures of Fracture Toughness
• 5. Effects of Constraint on Fracture and Stable Crack Growth under Elastic–Plastic Loading
• 6. Microscopic Aspects of Fracture
• 7. Fatigue Crack Growth under Large-Scale Plasticity
• 8. Analysis of Cracks in Creeping Materials
• 9. Creep–Fatigue Crack Growth
• 10. Applications
• Index