Metal Failures: Mechanisms, Analysis, Prevention
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Metal Failures: Mechanisms, Analysis, Prevention
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| ISBN-13: | 9781118421161 |
|---|---|
| Publisher: | Wiley |
| Publication date: | 08/26/2013 |
| Sold by: | JOHN WILEY & SONS |
| Format: | eBook |
| Pages: | 496 |
| File size: | 35 MB |
| Note: | This product may take a few minutes to download. |
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Metal Failures
Mechanisms, Analysis, PreventionBy Arthur J. McEvily
John Wiley & Sons
ISBN: 0-471-41436-0Chapter One
Failure AnalysisI. INTRODUCTION
Despite the great strides forward that have been made in technology, failures continue to occur, often accompanied by great human and economic loss. This text is intended to provide an introduction to the subject of failure analysis. It cannot deal specifically with each and every failure that may be encountered, as new situations are continually arising, but the general methodologies involved in carrying out an analysis are illustrated by a number of case studies. Failure analysis can be an absorbing subject to those involved in investigating the cause of an accident, but the capable investigator must have a thorough understanding of the mode of operation of the components of the system involved, as well as a knowledge of the possible failure modes, if a correct conclusion is to be reached. Since the investigator may be called upon to present and defend opinions before highly critical bodies, it is essential that opinions be based upon a sound factual basis and reflect a thorough grasp of the subject. A properly carried out investigation should lead to a rational scenario of the sequence of events involved in the failure as well as to an assignment of responsibility, either to the operator, the manufacturer, or the maintenance and inspection organization involved. A successfulinvestigation may also result in improvements in design, manufacturing, and inspection procedures, improvements that preclude a recurrence of a particular type of failure.
The analysis of mechanical and structural failures might initially seem to be a relatively recent area of investigation, but upon reflection, it is clear the topic has been an active one for millenia. Since prehistoric times, failures have often resulted in taking one step back and two steps forward, but often with severe consequences for the designers and builders. For example, according to the Code of Hammurabi, which was written in about 2250 BC (1):
If a builder build a house for a man and do not make its construction firm, and the house which he has built collapse and cause the death of the owner of the house, that builder shall be put to death. If it cause the death of a son of the owner of the house, they shall put to death a son of that builder. If it destroy property, he shall restore what ever it destroyed, and because he did not make the house which he built firm and it collapsed, he shall rebuild the house which collapsed at his own expense.
The failure of bridges, viaducts, cathedrals, and so on, resulted in better designs, better materials, and better construction procedures. Mechanical devices, such as wheels and axles, were improved through empirical insights gained through experience, and these improvements often worked out quite well. For example, a recent program in India was directed at improving the design of wheels for bullock-drawn carts. However, after much study, it was found that improvements in the design over that which had evolved over a long period of time were not economically feasible.
An example of an evolved design that did not work out well is related to the earthquake that struck Kobe, Japan, in 1995. That area of Japan had been free of damaging earthquakes for some time, but had been visited frequently by typhoons. To stabilize homes against the ravages of typhoons, the local building practice was to use a rather heavy roof structure. Unfortunately, when the earthquake struck, the collapse of these heavy roofs caused considerable loss of life as well as property damage. The current design codes for this area have been revised to reflect a concern for both typhoons and earthquakes.
The designs of commonplace products have often evolved rapidly to make them safer. For example, consider the carbonated soft-drink bottle cap. At one time, a metal cap was firmly crimped to a glass bottle, requiring a bottle opener for removal. Then came the easy-opening, twist-off metal cap. These caps were made of a thin, circular piece of aluminum that was shaped by a tool at the bottling plant to conform to the threads of the glass bottle. If the threads were worn, or if the shaping tool did not maintain proper alignment, then the connection between cap and bottle would be weak and the cap might spontaneously blow off the bottle, for example, on the supermarket shelf. Worse than that, there were a number of cases where, during the twisting-off process, the expanding gas suddenly propelled a weakly attached cap from the bottle and caused eye damage. To guard against this danger, the metal caps were redesigned to have a series of closely spaced perforations along the upper side of the cap, so that as the seal between the cap and bottle was broken at the start of the twisting action, the gas pressure was vented, and the possibility of causing an eye injury was minimized. The next stage in the evolution of bottle cap design has been to use plastic bottles and plastic caps. In a current design, the threads on the plastic bottle are slotted, so that, as in the case of the perforated metal cap, as the cap is twisted the C[O.sub.2] gas is vented, and the danger of causing eye damage is reduced.
Stress analysis plays an important role both in design and in failure analysis. Ever since the advent of the industrial revolution, concern about the safety of structures has resulted in significant advances in stress analysis. The concepts of stress and strain developed from the work of Hooke in 1678, and were firmly established by Cauchy and Saint-Venant early in the nineteenth century. Since then, the field of stress analysis has grown to encompass strength of materials, and the theories of elasticity, viscoelasticity, and plasticity. The advent of the high-speed computer has led to further rapid advances in the use of numerical methods of stress analysis by means of the finite element method (FEM), and improved knowledge of material behavior has led to advances in development of constitutive relations based upon dislocation theory, plasticity, and mechanisms of fracture. Design philosophies such as safe-life and fail-safe have also been developed, particularly in the aerospace field.
In a safe-life design, a structure is designed as a statically determinant structure that is intended to last without failure for the design lifetime of the structure. To guard against premature failure, the component should be inspected at intervals during its in-service lifetime.
In the fail-safe approach, the structure is designed such that if one member of the structure were to fail, there would be enough redundancy built into the structure that an alternate load path would be available to support the loads, at least until the time of the next inspection. (The use of both suspenders and a belt to support trousers is an example of a fail-safe, redundant approach.) Consideration must also be given to the spectrum of loading that a structure will be called upon to withstand in relation to the scatter in the ability of materials to sustain these loads. As indicated in Fig. 1-1, danger of failure is present when these two distributions overlap.
In addition, new fields such as fracture mechanics, fatigue research, corrosion science, and nondestructive testing have emerged. Important advances have also been made in improving the resistance of materials to fracture. In the metallurgical field, these advances have been brought about through improvements in alloy design, better control of alloy chemistry, and improvements in metal processing and heat treatment. The failure analyst often has to determine the nature of a failure; for example, was it due to fatigue or to an overload? In many cases, a simple visual examination may suffice to provide the answer. In other cases, however, the examination of a fracture surface (fractography) may be more involved and may require the use of laboratory instruments such as the light microscope, the transmission electron microscope, and the scanning electron microscope.
Many of today's investigations are quite costly and complex, and require a broad range of expertise as well as the use of sophisticated laboratory equipment. In some instances, the investigations are carried out by federal investigators, as in the case of the TWA Flight 800 disaster (center fuel tank explosion), where both the Federal Bureau of Investigation (FBI) and the National Transportation Safety Board (NTSB) had to determine if the cause of the failure was due to a missile attack, sabotage, mechanical failure, or an electrical-spark-ignited fuel tank explosion. The case of the Three Mile Island accident (faulty valve) involved the Nuclear Regulatory Commission (NRC), and the Challenger space shuttle disaster (O-ring) involved the National Aeronautics and Space Administration (NASA). Many investigations are also carried out by manufacturers to ensure that their products perform reliably. In addition, a number of companies now exist for the purpose of carrying out failure analyses to assist manufacturers and power plant owners, as well as to aid in litigation. The results of many of these investigations are made public, and thus provide useful information as to the nature and cause of failures. Unfortunately, the results of some investigations are sealed as part of a pretrial settlement to litigation, and the general public is deprived of an opportunity to learn that certain products may have dangers associated with them. A company may decide on the basis of costs versus benefits that is cheaper to settle a number of claims rather than to issue a recall. This policy can sometimes be disastrous, as in the case of the recent rash of tire failures. Another example involved a brand of cigarette lighter that repeatedly malfunctioned and caused serious burn injuries. It was only after some fifty of these events had occurred and the cases had been settled that the dangers associated with this item were brought to light in a public trial.
An important outcome of failure analyses has been the development of building codes and specifications governing materials [the American Society for Testing and Materials (ASTM)], manufacturing procedures [the Occupational Safety and Health Administration (OHSA)], design [the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Codes, the Federal Aviation Administration (FAA), NASA, American Petroleum Institute (API)], construction (state and municipal codes), and operating codes (NASA, NRC, FAA). These codes and standards have often been developed to prevent a repetition of past failures, as well as to guard against potentially new types of failure, as in the case of nuclear reactors. Advances in steel making, nondestructive examination, and analytical procedures have led to a reduction of the material design factor (safety factor) for power boilers and pressure vessels from 4 to 3.5 (2). (Allowable stress values based upon the tensile strength are obtained by dividing the tensile strength by the material design factor.) Today, the reliability of engineered products and structures is at an all-time high, but this reliability often comes with a high cost. In fact, in the nuclear industry, compliance with regulations intended to maximize safety may be so costly as to warrant the taking of a reactor out of service. It is also important for manufacturers to be aware of the state of the art as well as the latest standards. The number of manufacturers of small planes has dwindled because of product liability losses incurred when it was shown that their manufacturing procedures did not meet the current state-of-the-art safety standards. To guard against product failures, a number of firms now are organized in such a way that failure analysis is a line function rather than a staff function, and a member of the failure analysis group has to sign off on all new designs before they enter the manufacturing stage.
II. EXAMPLES OF CASE STUDIES IN FAILURE ANALYSIS
A. Problems with Loads and Design
1. Problems with Wind Loadings The Tay Bridge was a 10,300 foot long single track railroad bridge built in 1878 to span the Firth of Tay in Scotland (3). A portion of the bridge consisted of 13 wrought iron spans, each 240 feet in length and 88 feet above the water, which were supported by cast iron piers. On the fateful day of December 28, 1879, a gale developed with wind speeds up to 75 mph. That evening a passenger train, while making a scheduled crossing, plunged into the Firth, together with the 13 center spans, and 75 passengers and crew members lost their lives.
The subsequent investigation revealed that a major cause of the disaster was that the gale force winds produced lateral forces on the passenger cars that were transmitted to the bridge structure and led to its collapse. Such wind loading had not been properly taken into account in the design stage. This disaster underscored the obvious fact that all potential loading conditions must be considered in order to design safe and reliable structures.
Today, we are much more aware of the importance of wind loading in structural design. Nevertheless, from time to time, problems still arise. For example, the Citicorp Tower in New York City was built in 1977 in accord with the building code, which required calculations for winds perpendicular to the building faces. However, this was a unique structure in that a church occupies one corner of the building site, and the CiticorpTower is built over and around it. In 1978, it was discovered that the building was unstable in the presence of gale-force quartering winds, that is, winds that come in at a 45° angle and hit two sides of the building simultaneously. The building was quickly reinforced to insure its safety in the event of all types of wind loading, and a potential disaster was averted.
An instance where wind loading did result in a spectacular failure was that of the Tacoma Narrows suspension bridge, which failed in 1940 after only four months of service. The bridge, which connected the Olympic peninsula with the mainland of Washington, had a narrow, two-lane center span over a half mile in length. The design was unusual in that a stiffened-girder, which caught the wind, was used, rather than a deep open truss, which would have allowed the wind to pass through. The design resulted in low torsional stiffness and so much flexibility in the wind that the bridge was known as "Galloping Gertie." As the wind's intensity increased to 42 mph, the bridge's rolling, corkscrewing motion also increased, until it finally tore the bridge apart. The ultimate cause of the failure was the violent oscillations, which were attributed to forced vibrations excited by the random action of turbulent winds as well as to the formation and shedding of vortices created as the wind passed by the bridge.
2.
Continues...
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Table of Contents
Preface xv1. Failure Analysis 1
I. Introduction 1
II. Examples of Case Studies Involving Structural Failures 6
III. Summary 25
References 25
Problems 26
2. Elements of Elastic Deformation 27
I. Introduction 27
II. Stress 27
III. Strain 32
IV. Elastic Constitutive Relationships 35
V. State of Stress Ahead of a Notch 44
VI. Summary 46
References 46
Appendix 2-1: Mohr Circle Equations for a Plane Problem 46
Appendix 2-2: Three-Dimensional Stress Analysis 49
Appendix 2-3: Stress Formulas Under Simple Loading Conditions 54
Problems 57
3. Elements of Plastic Deformation 59
I. Introduction 59
II. Theoretical Shear Strength 59
III. Dislocations 61
IV. Yield Criteria for Multiaxial Stress 68
V. State of Stress in the Plastic Zone Ahead of a Notch in Plane-Strain Deformation 70
VI. Summary 74
For Further Reading 75
Appendix 3-1: The von Mises Yield Criterion 75
Problems 76
4. Elements of Fracture Mechanics 80
I. Introduction 80
II. Griffith’s Analysis of the Critical Stress for Brittle Fracture 80
III. Alternative Derivation of the Griffith Equation 83
IV. Orowan-Irwin Modification of the Griffith Equation 84
V. Stress Intensity Factors 85
VI. The Three Loading Modes 88
VII. Determination of the Plastic Zone Size 88
VIII. Effect of Thickness on Fracture Toughness 89
IX. The R-Curve 91
X. Short Crack Limitation 92
XI. Case Studies 92
XII. The Plane-Strain Crack Arrest Fracture Toughness, K I a, of Ferritic Steels 95
XIII. Elastic-plastic Fracture Mechanics 96
XIV. Failure Assessment Diagrams 98
XV. Summary 101
References 101
Problems 102
5. Alloys and Coatings 105
I. Introduction 105
II. Alloying Elements 106
III. Periodic Table 107
IV. Phase Diagrams 108
V. Coatings 126
VI. Summary 130
References 130
Problems 130
6. Examination and Reporting Procedures 132
I. Introduction 132
II. Tools for Examinations in the Field 132
III. Preparation of Fracture Surfaces for Examination 133
IV. Visual Examination 133
V. Case Study: Failure of a Steering Column Component 134
VI. Optical Examination 135
VII. Case Study: Failure of a Helicopter Tail Rotor 136
VIII. The Transmission Electron Microscope (TEM) 136
IX. The Scanning Electron Microscope (SEM) 138
X. Replicas 142
XI. Spectrographic and Other Types of Chemical Analysis 143
XII. Case Study: Failure of a Zinc Die Casting 144
XIII. Specialized Analytical Techniques 145
XIV. Stress Measurement by X-Rays 146
XV. Case Study: Residual Stress in a Train Wheel 149
XVI. The Technical Report 150
XVII. Record Keeping and Testimony 151
XVIII. Summary 154
References 155
Problem 155
7. Brittle and Ductile Fractures 156
I. Introduction 156
II. Brittle Fracture 156
III. Some Examples of Brittle Fracture in Steel 159
IV. Ductile-Brittle Behavior of Steel 161
V. Case Study: The Nuclear Pressure Vessel Design Code 168
VI. Case Study: Examination of Samples from the Royal Mail Ship (RMS) Titanic 172
VII. Ductile Fracture 177
VIII. Ductile Tensile Failure, Necking 177
IX. Fractographic Features Associated with Ductile Rupture 183
X. Failure in Torsion 185
XI. Case Study: Failure of a Helicopter Bolt 185
XII. Summary 188
References 191
Problems 191
8. Thermal and Residual Stresses 196
I. Introduction 196
II. Thermal Stresses, Thermal Strain, and Thermal Shock 196
III. Residual Stresses Caused by Nonuniform Plastic Deformation 200
IV. Residual Stresses Due to Quenching 204
V. Residual Stress Toughening 207
VI. Residual Stresses Resulting from Carburizing, Nitriding, and Induction Hardening 207
VII. Residual Stresses Developed in Welding 209
VIII. Measurement of Residual Stresses 211
IX. Summary 211
References 211
Appendix 8-1: Case Study of a Fracture Due to Thermal Stress 212
Problems 213
9. Creep 216
I. Introduction 216
II. Background 216
III. Characteristics of Creep 217
IV. Creep Parameters 220
V. Creep Fracture Mechanisms 222
VI. Fracture Mechanism Maps 224
VII. Case Studies 225
VIII. Residual Life Assessment 230
IX. Stress Relaxation 232
X. Elastic Follow-up 233
XI. Summary 234
References 234
Problems 234
10. Fatigue 237
I. Introduction 237
II. Background 237
III. Design Considerations 240
IV. Mechanisms of Fatigue 246
V. Factors Affecting Fatigue Crack Initiation 254
VI. Factors Affecting Fatigue Crack Growth 257
VII. Analysis of the Rate of Fatigue Crack Propagation 261
VIII. Fatigue Failure Analysis 273
IX. Case Studies 276
X. Thermal-Mechanical Fatigue 285
XI. Cavitation 285
XII. Composite Materials 286
XIII. Summary 287
References 287
For Further Reading 290
Problems 290
11. Statistical Distributions 293
I. Introduction 293
II. Distribution Functions 293
III. The Normal Distribution 294
IV. Statistics of Fatigue; Statistical Distributions 296
V. The Weibull Distribution 298
VI. The Gumbel Distribution 302
VII. The Staircase Method 307
VIII. Summary 310
References 310
Appendix 11-1: Method of Linear Least Squares (C. F. Gauss, 1794) 311
Problems 314
12. Defects 316
I. Introduction 316
II. Weld Defects 316
III. Case Study: Welding Defect 321
IV. Casting Defects 328
V. Case Study: Corner Cracking during Continuous Casting 329
VI. Forming Defects 329
VII. Case Studies: Forging Defects 330
VIII. Case Study: Counterfeit Part 332
IX. The Use of the Wrong Alloys; Errors in Heat Treatment, etc. 333
X. Summary 334
References 334
Problems 334
13. Environmental Effects 336
I. Introduction 336
II. Definitions 336
III. Fundamentals of Corrosion Processes 337
IV. Environmentally Assisted Cracking Processes 342
V. Case Studies 348
VI. Cracking in Oil and Gas Pipelines 350
VII. Crack Arrestors and Pipeline Reinforcement 352
VIII. Plating Problems 353
IX. Case Studies 353
X. Pitting Corrosion of Household Copper Tubing 356
XI. Problems with Hydrogen at Elevated Temperatures 356
XII. Hot Corrosion (Sulfidation) 358
XIII. Summary 358
References 358
Problems 359
14. Flaw Detection 360
I. Introduction 360
II. Inspectability 360
III. Visual Examination (VE) 364
IV. Penetrant Testing (PT) 364
V. Case Study: Sioux City DC-10 Aircraft 367
VI. Case Study: MD-88 Engine Failure 374
VII. Magnetic Particle Testing (MT) 375
VIII. Case Study: Failure of an Aircraft Crankshaft 378
IX. Eddy Current Testing (ET) 382
X. Case Study: Aloha Airlines 384
XI. Ultrasonic Testing (UT) 384
XII. Case Study: B747 389
XIII. Radiographic Testing (RT) 389
XIV. Acoustic Emission Testing (AET) 391
XV. Cost of Inspections 393
XVI. Summary 393
References 394
Problems 394
15. Wear 396
I. Wear 396
II. The Coefficient of Friction 397
III. The Archard Equation 398
IV. An Example of Adhesive Wear 399
V. Fretting Fatigue 399
VI. Case Study: Friction and Wear; Bushing Failure 403
VII. Roller Bearings 404
VIII. Case Study: Failure of a Railroad Car Axle 410
IX. Gear Failures 410
X. Summary 414
References 414
Problems 415
Concluding Remarks 417
Solutions to Problems 419
Name Index 469
Subject Index 473