Preface Free

Service failures of machines and structures without warning have been experienced in many industries with varying degrees of consequential damages. Systematic analysis of such failures has generated a fund of useful information for designers, manufacturers, and users of industrial hardware. With an understanding of failures and their causes, technical remedies were incorporated at various stages and, thus, recurrence of failures prevented. National Aerospace Laboratories (NAL), Bangalore, has been involved in failure analysis and accident investigations for over four decades. Though initially the clientele was mainly from the aerospace sector, several other organizations have taken advantage of this service. The experience gained over the years has culminated in the compilation of this book, Failure Analysis of Engineering Structures: Methodology and Case Histories. The book comprises two parts. The first part is a treatment of various aspects of failure analysis with emphasis on techniques. The second part deals with failure case studies analyzed by the Failure Analysis and Accident Investigation Group in the authors’ laboratories.

The introductory chapter cites a few major industrial catastrophes encountered in the last century. The importance, philosophy, and the beneficiaries of failure analysis are covered. The failure rate in the life span of machines is comparable to the geriatric curve, high in the initial and final stages and minimum in between. The philosophy of failure analysis is to bring down the failure rate and to extend the span of minimum failure rate.

Chapter 2 covers the common causes of failures. Various failures encountered in industries could ultimately be attributed to some deficiency introduced, though inadvertently, during the various stages of manufacture and operation. These stages include design, production (which covers materials selection, processing operations, treatments, assembly, and inspection), maintenance, service abnormalities, abuses, and environmental effects. Failure cases are described with examples from various industries such as chemical, mining, aviation, power generation, and so forth to illustrate how the deficiencies ultimately led to serious failures.

The success of failure analysis depends on the use of proper analytical techniques in proper sequence. Chapter 3 describes the techniques with which every analyst should be familiar. The techniques described include collecting background information; survey of wreckage; collection and preservation of samples for further laboratory examination; tests such as chemical analysis at various levels, mechanical tests, nondestructive inspection, microscopy, fractography, simulation tests; and final analysis of results to pinpoint the cause of the failure and the sequence of events. Simulation tests often help in clinching specific issues.

Vital information about failures is provided by microscopic examination of samples at various levels of resolution and magnification. A separate chapter is thus devoted to microscopy in failure analysis. Microstructural examination with the metallurgical microscope reveals structural abnormalities, crack origin, and crack path. Fracture surface examination using optical microscope and scanning electron microscope reveals telltale marks characteristic of such failure modes as tensile overload, shear overload, rapid brittle fracture, fatigue, stress corrosion, and so forth. These are described with illustrations in Chapter 4.

Chapter 5 deals with certain advanced techniques of failure analysis. These techniques evolved in recent times, necessitated by the needs of failure analysis in sophisticated industrial systems. In thermal plants that have been in service for a long time, study of samples taken for biopsy without affecting the integrity of the plant has helped in condition monitoring of the plant and estimating its residual life. Fracture Surface Topography Analysis (FRASTA) is another advanced technique by which the evolution of the fracture process could be studied. The practical applications of this technique are described briefly. Other techniques described in this chapter are fault tree analysis (FTA), failure modes and effects analysis (FMEA), failure experience matrix (FEM), expert systems for failure analysis, study of fractals, and quantitative fractography.

Failures in machines and structures are sometimes deliberately caused by antisocial elements, using explosive devices. When metallic objects deform and fracture under explosive conditions where the strain rates are of the order of 10⁶ s⁻¹, certain distinct features can be found in the fragments produced by explosions. These features are permanent and they survive subsequent crash impact forces. These features are different from the telltale marks produced under normal static loading conditions. In Chapter 6, the signatures of explosion are catalogued under surface features, edge features, microstructural features, and fragment shapes, with illustrations. These have been extremely helpful in the investigation of aircraft accidents caused by explosive sabotage. Two such accidents are described in detail.

Major catastrophes are invariably followed by serious, prolonged, and expensive litigations in courts. Many of the failures and accidents involve a combination of component malfunction and mismanagement. While routine failure investigations aim at finding the cause of the mishap, investigations in courts attempt also to fix responsibilities and recommend punitive action. These can be extended for several years after the accident. Exact evidence with supporting data of high standard has to be presented in the courts to justify penal action. Some of the major accidents described in Chapter 2 were followed by judicial inquiries in which experts provided the required testimony. A few cases of forensic investigations are presented in Chapter 7.

The follow-up actions after failure analysis have indeed resulted in numerous advantages to industries. These advantages are described in Chapter 8. Process modification after failure analysis in a chemical industry significantly reduced the hazards encountered earlier. Choice of better and cleaner materials resulted in increased reliability and safety. Codes and standards have been modified to ensure greater safety in hazardous industries. Certain specific actions also ensured greater security in industries, especially aviation. The numerous advantages accrued after taking remedial measures, following an understanding of failures, more than justified the statement, “Many should benefit from the misfortunes of a few.”

Part 2 of the book deals with detailed description of various failure cases investigated by the Failure Analysis and Accident Investigation Group at NAL. The majority of cases deal with aviation machines, while a few cases of industrial failures are also included.

The Failure Analysis activity at NAL was nurtured by senior scientists and Directors of the Laboratory, namely, Dr. S. Ramaseshan, Dr. V.S. Arunachalam, Dr. A.K. Singh, Dr. S.R. Valluri, Dr. R. Narasimha, Dr. K.N. Raju, Dr. T.S. Prahlad, and Dr. B.R. Pai. The members of the Failure Analysis Group, Dr. T.A. Bhaskaran, Mr. C.R. Kannan, Mr. M.A. Parameswara, Mr. S. Radhakrishnan, Mr. R. Rangaraju, Mr. Dwarakanath Rao, and Mr. M.A. Venkataswamy, contributed in no small measure in the investigation of numerous cases described in Part 2.

Because failure analysis is a multidisciplinary activity, expertise from other divisions and sections of the Laboratory was drawn frequently as needed. Significant help was received from Graphic Arts, Chemical Analysis Group, X-ray Laboratory, Fatigue Laboratory, and Engineering Services. The authors are thankful to Mr. K. Venkata Ramaiah for his help in computer graphics. In compiling this book, the authors had very useful discussions with Dr. M.K. Asundi of Bhabha Atomic Research Centre, Bombay; Dr. R. Krishnan of Gas Turbine Research Establishment, Bangalore; and Dr. R. Viswanathan of Electric Power Research Institute, Palo Alto.

Designers, manufacturers, maintenance personnel, and users of machines are the beneficiaries of the results of failure analysis. It is hoped that this book would be of use to students of engineering and practicing engineers.