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Methodologies for failure analysis: a critical survey
Materials & Design Materials and Design 25 (2004) 117–123 www.elsevier.com/locate/matdes
Methodologies for failure analysis: a critical survey Paulo M.S.T. de Castro *, A.A. Fernandes Faculdade de Engenharia, Universidade do Porto, Rua Dr Roberto Frias, 4200-465 Porto, Portugal Received 16 June 2003; accepted 23 September 2003
Abstract Based on a critical survey of relevant literature, and own experience, a failure analysis methodology is presented, focusing on fracture and fatigue problems. Failures investigated by the authors include brittle fracture and fatigue of structures and machine elements, such as transportation equipment, structural connections, engines, etc. Further to their economic significance, some of the failures were of special concern because of safety, environmental or legal considerations. Difficulties for carrying out the analyses included circumstances not entirely known or documented, damaged fracture surfaces, or insufficient material to carry out comprehensive mechanical testing. Further to the detailed study of the available evidence, stress analyses using numerical methods such as the finite element or the boundary element methods, materials testing and scanning electron microscopy have been used as required. Suggestions for R&D efforts in the field are put forward. 2003 Elsevier Ltd. All rights reserved. Keywords: Failure analysis; Fatigue; Fracture; Structural integrity
1. Introduction Certainly many disasters are not related to structural integrity problems such as fatigue and fracture. However, many failures originated by fatigue and fracture may have tragic consequences – as for example in transportation equipment such as railways or aircraft, or in the process industries. Fitness-for-service criteria are continuously evolving, reflecting advances and new consensus in the technical and scientific communities. Although there is a wealth of literature on the topic of the present paper (for example, [1]), the continuous development of the subject justifies the need for an updated critical survey. 1.1. Liability for defective products in the European Union In the European Union (EU), Member States shall adopt and publish the laws, regulations and administrative provisions necessary to comply with EU ÔdirectivesÕ. For example, Portuguese defective product *
Corresponding author. Tel.: +351-2-205-1241; fax: +351-2-2059125. E-mail address: [email protected] (P.M.S.T. de Castro). 0261-3069/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2003.09.020
liability legislation – decree-law 131/2001 of April 24, 2001 – transposes into national Portuguese legislation the European Union directive 1999/34/CE. It defines product as anything movable, single or incorporated into a movable or stationary assembly. A product is defective if it does not offer the safety the user is entitled to expect, bearing in mind all circumstances, namely presentation, reasonable use and the time when the product was put in circulation. A product is not considered defective just because an improved product is subsequently put in circulation. The producer is responsible for damage caused by the defective product. Producer is the manufacturer of the finished product, of a component or of the material, or someone that attach his name, trademark or other distinctive sign to the product. Under Portuguese law, producer is also someone that imports into the European Union products for selling, or any other form of distribution, or any supplier of a product of which the EU producer or importer is not identified. Escapes from liability include presumption that the defect did not exist when the product was put in circulation, that the defect is compatible with mandatory regulations, or that the state-of-the-art at the time the product was put in service could not discover the defect and identify it as source of
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Nomenclature Acronyms AFCEN Association Francßaise pour les Regles de ConAPI ASME ASTM BSI CEGB ECSS EFAM EPRI ESA
ception et de Construction des Materiels des lectro-nucleaires (France) Chaudires E American Petroleum Institute (USA) American Society of Mechanical Engineers (USA) American Society for Testing and Materials (USA) British Standards Institution (UK) Central Electricity Generating Board (UK) European Cooperation for Space Standardization Engineering flaw assessment method Electric Power Research Institute (USA) European Space Agency
malfunction. As it is to be expected, applicable laws of EU countries are broadly similar. Concern with safety is present in specific EU directives. The directive for machines (98/37/CE) concerns the conditions to be met by the producer so that the machine is considered safe. Concerns new machines manufactured in the EU, new or used machines from third countries, groups of machines and complex assemblies, equipment that modify the function of a machine, and safety devices. The problem of material fatigue, for example, is mentioned in the context of protection against mechanical hazards. This directive originated the Portuguese decreelaw 320/2001 of December 12, 2001. 1.2. Effects of fracture: economic and occasionally loss of lives Although safety is the main concern of legislation, defective products which do not meet the expected performance are a cause of financial loss. Costs of failure include [2]: • direct repair or replacement cost; • loss of revenue while unavailable; • costs of finding replacement services/items during unavailability; • costs of consequential damage; • consequential costs to avoid failure on similar items; • other implications (safety, loss of confidence, image, and trade). Studies of the economic effects of fracture in the USA [3] and Europe [4] suggest that the total cost of failure in developed countries is of the order of 4% of GNP. In the European study, including contributions by the present authors, a sample of reports describing actual failures analyses was reviewed, assessing their economic relevance and attempting to identify needs for further research and/or for the establishment of codes of practice.
ETM Engineering Treatment Model EU European Union fem finite element method GDP gross domestic product HSE Health and Safety Executive (UK) IMECE International Mechanical Engineering Congress MM NBS NIST sem
and Exposition (ASME) mis-match National Bureau of Standards (now National Institute of Standards and Technology, USA) National Institute of Standards and Technology (USA) scanning electron microscopy
The use of Fracture Mechanics and failure analysis techniques may lead to important economies. A classical example is the application of Fracture Mechanics concepts to determine allowable defect sizes of the Trans Alaska oil pipeline. This is a 48 in. diameter about 1/2 in. thick X65 pipe [5], where radiographs showed defects of dimensions larger than those permitted by the DOT (Department of Transportation) Regulations 49 CFR Part 195. The Alyeska Pipeline Service Company requested waivers for these welds, based upon Fracture Mechanics analysis. The cost of the repairs to be made was enormous, namely because part of the pipe was already buried and was relatively inaccessible. DOT requested assistance from the National Bureau of Standards (now National Institute of Standards and Technology), which supported the application of Fracture Mechanics techniques to determine allowable crack sizes [6]. It was possible to demonstrate that a number of defects discovered in the girth welds as a result of post construction audit of the radiographs need not be repaired even though they failed to meet the defect acceptance criteria given in the fabrication code. Failure analyses may lead to • design, material selection or fabrication modifications; • evaluations of service conditions in the remaining life; • fitness for purpose assessments. However, the relevant techniques are not always concerned with technical/economic discussions as in the previous example. Fracture Mechanics and failure analysis knowledge is required for the study of disasters, as exemplified by the ÔAlexander L KiellandÕ oil rig in the Ekofisk field of North Sea, which broke up in March 27, 1980 due to fatigue fracture and capsized claiming 123 lives [7], the MV Estonia passenger vessel capsizing on September 28, 1994 in the Baltic sea [8] killing more than nine hundred
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passengers, or the collapse of the Ramsgate walkway on September 14, 1994 claiming six lives [9], to give just three examples of what could unfortunately be a very long list.
systematic Fracture Mechanics methodology, as was already suggested, in the field of failure analysis, by Weiss [30].
1.3. Role of fracture mechanics education
1.5. Fitness for purpose
Fracture Mechanics is a technical and scientific subject, developed mainly in the second half of the 20th century that seeks to establish quantitative relationships between • loads, • material properties (particularly toughness, or resistance that a material offers to the propagation of a crack), • size of existing crack(s). Fracture Mechanics concepts, but also failure prediction and analysis case studies [10–20], describing how concepts are applied to solve real engineering problems, should be the object of greater attention of engineers during their initial and continuing education. Also, conferences dedicated to Structural Integrity and Fracture Mechanics typically include sessions on failure analysis case studies, as exemplified by the 8th Portuguese Conference on Fracture in 2002 (see for example [21]), or are entirely dedicated to the subject [22].
Fitness for purpose procedures seeks to demonstrate the structural integrity and damage tolerance of load bearing structures. These procedures may be used at the design stage, during operation or at the end of the design life in case life extension of the equipment is under consideration. Simultaneously, these procedures are relevant for the interpretation when things go wrong and failure indeed happens. Perhaps surprisingly for the non-specialist, there is not one but a variety of structural fitness for service assessment procedures, used in different sectors of industry. Although the development of Fracture Mechanics principles in the second half of the 20th century created the scientific basis for most of the procedures, the mentioned diversity is a result of the particular circumstances of the industrial area concerned. The development of an encompassing and thoroughly validated procedure for structural failure, even restricted to the failures modes of fatigue and fracture, is certainly a worthwhile research endeavour. The application of these principles leads to benefits including • reduction of repair costs; • reduction of unavailability time; • optimization of use of materials, reducing excessive conservatism; • optimization of maintenance procedures; and ideally, avoiding failure and possible disasters. However, if failure indeed happens, these techniques may be used for understanding what went wrong, in order to do better in the future. Fitness for purpose methodologies have been developed in European Union (EU) countries, in the US and Japan. The first group includes the already mentioned ÔR6Õ [23] and BS 7910 (former BS PD6493) [24] but also procedures developed in Germany by GKSS [31], in France specifically for the nuclear power generation industry [32], or at European-wide level, for aerospace engineering by ESA [33]. In the USA EPRI [34] and ASME [35] concern the nuclear power generation industry, whereas API [36] deals with structural fitness for purpose problems of the refining and chemical process industries.
1.4. Standards and codes of practice Examples of standards and codes of practice regarding the practical application of Fracture Mechanics to real engineering problems are the well-documented ÔR6Õ procedure developed by the CEGB [23], or the British Standards Institution standard BS 7910 (and former document PD6493) [24]. However, such documents may need further development to increase their usefulness; this is illustrated, for example, in [25], where, in the context of a round-robin exercise, it is concluded that Ôit is unlikely that an engineer with only BSI PD6493 at his disposal could have reliably analysed the vesselÕs defectsÕ. An useful warning is given in [26], where it is concluded that • Fracture Mechanics based predictions of failure load may be non-conservative, because of uncertainties in the inputs, particularly fracture toughness; • large cracks and low values of the ratio ‘‘fracture toughness/yield strength’’ are of particular concern; • there has been no significant improvement in the accuracy of Fracture Mechanics analyses of failures over the 1980–1990 period. The difficulty of evaluations is aggravated by the fact that fracture toughness data is notoriously subject to scatter [27]. In a report to the Community Bureau of Reference, Baker [28] states the need for a basis on which reliability assessment could be made. This work could lead to an expert system [29] package containing a
2. Methodology 2.1. The expert and his/her work Wearne [37] indicates that there is a distinction in the incidence of failure in each class of engineering
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products, but the failures that do occur tend to be more serious. This is because of • the pursuit of economy of scale, which concentrates physical and financial risks; and • the pursuit of optimization in the design of systems and components reducing redundancy. None of the failures examined was caused by hitherto unknown physical phenomena hat occurred without prior warning: a major failure like the Tacoma Narrows bridge could be explained very quickly by von Karman, then the director of the aeronautical laboratory of CalTech, on the grounds of aerodynamic instability theory, as used in airplane design (but not, up to then, in bridge design) [38]. This illustrates the complexity of failure analysis. It involves areas of mechanics, physics, metallurgy, chemistry, corrosion, manufacturing processes, stress analysis including numerical techniques such as the finite and the boundary element methods, design analysis and fracture mechanics including environmental induced cracking, non-destructive evaluation, and probabilistic risk evaluation, among others. Of course no single person will be an expert in all those areas. The failure analyst must have the management skill to orchestrate the activities of the necessary specialist experts. The failure analysis expert typically has deep knowledge of one of the areas mentioned above and working knowledge of the others, obtained from experience, skill, training and education. In-house experts may have a bias toward his/her employerÕs viewpoint. An objective professional opinion should therefore come from someone who has no ties to and no interest related to the case. This is particularly true in court litigation, where the forensic engineer deals with the engineering aspects of legal problems [39]. Answers to be provided by the forensic expert include [40]: • what happened? • where did it happen? • how did it happen? • who caused it to happen? • who is responsible for what happened? • what are the costs to repair or replace? • what are the damages suffered by the various parties involved?
2.2. Failure analysis framework Thomas [41] suggests that the variety of problems and approaches to their solution are too great to be encompassed by a single general approach. A general structural failure analysis framework could consist of the exam of failure modes one by one (fatigue, creep, corrosion, wear, etc.); however, each of those subjects is dealt with in specialized texts, in a broader context than
failure analysis. There would be little progress in specific failure analysis knowledge proceeding that way. Alternatively, [41] synthesizes the failure analysis process considering the four more frequent factors – stress, environment, defects and material properties – and their interaction. Professional literature usually concentrates on the technical aspects of failure. However, the real reasons of failure are often procedural, involving communication deficiencies and unclear definition of responsibilities. This is true even in the most advanced technologies where formal quality and assurance procedures are in operation, as shown by the Challenger space shuttle 1986 accident [38]. Systems thinking will have therefore an important role in his field. Carper [42] lists the following causes of failure in civil structures: • site selection and development errors, • programming deficiencies, • design errors (including failure to consider some combination of loads, lack of redundancy, connection details, etc.), • construction errors, • material deficiencies, • operational errors (including change in use, inadequate maintenance, etc.). Of course, lists based on other criteria are found; Petroski [38] includes two, the first claiming to be comprehensive: • ignorance, • economy, • lapses or carelessness, • unusual occurrences (earthquakes, extreme storm, fires, etc.), whereas the second makes no claims of completeness: • limit states (overload, under-strength, movement, deterioration), • random hazards (fire, floods, explosions, earthquake, vehicle impact), • human-based errors (design error, construction error). An important concern is the question of life extension of existing equipment or facilities. As discussed in [40], elements of a systematic approach to this problem include • assessment of the materia1 conditions, • type of damage (fatigue, creep, environmental, etc.), • damage site (surface, bulk, interface), • damage extent (localized, global), • damage level (negligible, endangering the functionality, causing danger), • loading conditions, • temperature. Recent R&D work by the authors in this broad area, include a preliminary study of the use the 100 years old, riveted, Luiz I bridge over the Douro river in Porto, for use by the new metro, and on multiple site damage of airplanes (BRITE project BE95-1053 ‘‘SMAAC’’), where
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the safe operation and maintenance procedures for aging aircraft was studied in detail. 2.3. Suggested methodology Roberts [43] suggests that there should be a systematic way of performing failure analysis (a ÔmethodologyÕ, such as medical people do their autopsies. Items in a failure analysis methodology are typically [44,45]: • problem definition, • obtain background information, • investigate evidence (macroscopic and microscopic examination, including sem), • evaluate failure mechanism, sequence and causes, • perform exemplar test, • analysis and calculations, including fem and Fracture Mechanics analyses, • assess risk, • draw conclusions based on all evidence, and make recommendations. It is important to look for overall context of the failure. Concentration on only one aspect may conceal the cause. Background information, which cannot be deduced from the physical evidence, should be obtained. The field investigation should be carried out shortly after the failure. Later, laboratory examination, eventually including sem, is carried out. Fracture surfaces are extremely delicate, and contain relevant information for the failure analysis. They should be preserved carefully. Mating pieces of a fracture should not be put together, since this will damage the surfaces and will spoil the fractographic examination. By definition, destructive testing cannot be undone, and should not therefore be done prematurely. Preliminary hypotheses are made, and tested using as required exemplar experiments, and calculations. The macro and micro appearance of the failure in the exemplar test should be compared to the actual failed part. Stress analysis, eventually using fem, may be performed. Risk assessment, particularly relevant to similar items still in service and decisions to recall or not to recall, may be simply stated as risk ¼ frequency severity: Final conclusions must explain all physical evidence and verifiable facts. 2.4. Some examples The following examples taken from analyses carried out by the present authors, illustrate some of these points. Sometimes the fracture surface is difficult to interpret, even with the help of atlas of fractographs and other
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literature. This is the case of an analysis of a shaft of a maritime engine [46]. In this case, mechanical tests were carried out, with the objective of generating a number of characteristic fracture surfaces in fatigue, in fracture toughness and in impact tests, which could be compared with the actual failed part and help to identify the failure mode. The need for stress analysis of the structure, in order to predict the behaviour in the presence of defects, and to assess the necessary repair, is dealt with in [47]. Compliance with codes of practice is emphasised in [48], where recent structural failures in a fleet of urban passenger buses were analysed, and attributed to noncompliance with appropriate codes of practice for fatigue design of welded structures. Inadequate repairs may be also causes of structural failure. That was the case of the failure of forks of a heavy-duty lift truck, causing important economic damage. This failure could be explained by inadequate repair procedures [49]. Research on fatigue and fracture contributes to reduce the probability of accidents and disasters. Civil transportation is an area of great interest, due to possible impacts of accidents for example in aviation or railways. Examples that can be mentioned include the Aloha aircraft accident, explained by multiple site fatigue damage studies. This was the subject of the European Union ‘‘SMAAC’’ project already mentioned above, concerning aging aircraft and their pathology – see for example [50]. Another European Union project, with the involvement of the present authors, is the ‘‘RAILCRACK’’ project on fatigue testing of rails for railways [51], seeking to contribute to safer railways and avoidance of disasters such as the Hatfield (UK) derailment of October 17, 2000 injuring 70 and killing four people, which was caused by fatigue cracking of rails [52].
3. Concluding remarks Liability and litigation issues create a need for failure analysis. However, understanding the causes of failures is also the source of engineering knowledge, since success in structural engineering is foreseeing failure. A methodology for failure analysis was suggested, and aspects were illustrated by cases dealt with by the authors. The development of an encompassing and thoroughly validated procedure for structural fitness for purpose, even restricted to the failures modes of fatigue and fracture, is certainly a worthwhile research endeavour, as a step towards a life cycle management method for major plant and infrastructure. This work could lead to an expert system package containing a systematic Fracture Mechanics and failure analysis methodology.
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References [1] de Castro PMST, Oliveira FMF, Fernandes AA. Failure analysis: a methodology. Technology Law and Insurance 1996;1:143–50. [2] Milne I. The importance of the management of structural integrity. Engineering Failure Analysis 1994;1(3):171–81. [3] The economic effects of fracture in the United States. US Department of Commerce, NBS special publications 647-1, 6472, 1978. [4] Faria L, rapporteur. The economic effect of fracture in Europe. Final report of European Atomic Energy Community study contract no. 320105, 1991. [5] Cotton HC, Thomas DBJ. Super diameter pipes for Alaska. In: Tube and Pipe Production, the Iron and Steel Institute ISI P135, 1970. p. 37–46. [6] US Department of Commerce, National Technical Information Service, PB-260 400, 1976, ÔConsideration of fracture mechanics analysis and defect dimension measurement assessment for the trans-Alaska oil pipeline girth weldsÕ, National Bureau of Standards, prepared for the Department of Transportation, October 1976, 2 vols. [7] The ‘‘Alexander L Kielland’’ accident, Norwegian Public Reports, Oslo, 1981. [8] Final report on the MV Estonia disaster – on the capsizing on 28 September 1994 in the Baltic sea of the Ro-Ro passenger vessel, The Joint Accident Investigation Commission, Republic of Estonia, 1997. [9] Chapman JC. Collapse of the Ramsgate walkway. The Structural Engineer 1998;76(1):1–10, See also: discussion on the previous paper, The Structural Engineer 2000;78(3):22–9. [10] Broek D. The practical use of fracture mechanics. Kluwer Academic Publishers; 1988. [11] Failure analysis and prevention. In: Metals Handbook, vol. 11, 9 ed. ASM; 1986. [12] Fatigue and fracture. In: ASM Handbook, vol. 19. ASM; 1996. [13] Stevens R. Case Studies for Fatigue Education. ASTM STP 1250; 1994. [14] Fisher JW. Fatigue and fracture in steel bridges: Case studies. John Wiley; 1984. [15] Wierzbicki T, Jones N, editors. Structural failure. John Wiley; 1989. [16] Dickson JI, Abramovici E, Marchand NS, editors. Failure analysis: Techniques and applications. ASM; 1992. [17] ASM. Handbook of case histories in failure analysis, vol. 1, 1992; vol. 2, 1993. [18] Blauel JG, Burdekin FM. Case study collection on the assessment of the significance of weld imperfections. Welding in the World 1996;37(4):194–216. [19] Jones DRH. Engineering materials 3 – materials failure analysis: case studies and design implications. Pergamon Press; 1993. [20] Jones DRH, editor. Engineering failure analysis, vol. 1. Pergamon Press; 1994. [21] Jones DRH. Failure analysis in engineering structures and components. Presented at the 8th Portuguese Conference on Fracture, Vila Real, Portugal, February 20–22, 2002. [22] Bicego V, Nitta A, Price JWH, Viswanathan R, editors. Proceedings of International Conference on Case Histories on Integrity and Failures in Industry, Milano, Italy, September 28–October 1, 1999. In: Case Histories on Integrity and Failures in Industry, EMAS; 1999. [23] Assessment of the integrity of structures containing defects. British Energy Generation Report R/HR6, Revision 4, April 2001. For background information see for example: International Journal of Pressure Vessels and Piping (1988) vol. 32, theme issue: The revisions of the structural integrity assessment method CEGB/R6.
[24] British Standard BS 7910, Guide on methods for assessing the acceptability of flaws in metallic structures, 2000. For background information see also: British Standards Institution PD6493, Guidance on some methods for the derivation of acceptance levels for defects in fusion welded joints, 1980. PD6493, Guidance on methods for assessing the acceptability of flaws in fusion welded structures, 1991. [25] Hopkins P. The results of a round robin exercise on pressure vessel defect assessment. International Journal of Pressure Vessels and Piping 1990;42:333–51. [26] Rosenfield AR, Marschall CW. Fracture mechanics based failure analysis. Engineering Fracture Mechanics 1993;45(3):333–8. [27] Hudson CM, Seward SK. A compendium of sources of fracture toughness and fatigue crack growth data for metallic alloys. International Journal of Fracture 1978;14(4):R151–84, 2nd part, 1982;22(3):R59–R117. [28] Baker RG. Report on a study on metrologica1 requirements in Fracture Mechanics. Community Bureau of Reference (BCR), Commission of the European Communities, doc. XII/353/88, 1988. [29] Dym CL. Expert systems: new approaches to computer-aided engineering. Engineering with Computers 1985;1:9–25. [30] Weiss V. Towards failure analysis expert systems. ASTM Standardization News 1986:30–4. [31] Schwalbe KH, et al. EFAM ETM 97 – The ETM method for assessing the significance of crack-like defects in engineering structures, comprising the versions ETM 97/1 and ETM 97/2. GKSS report 98/E/6, 1998. See also: Schwalbe KH, et al. EFAM ETM-MM96 – The ETM Method fro assessing the significance of crack like defects in joints with mechanical heterogeneity (strength mismatch). GKSS report 97/E/9, 1997. [32] AFCEN – Association Francßaise pour les Regles de Conception et lectro-nucleaires. de Construction des Materiels des Chaudieres E RCC-M, Tome I vol.Z: Annexes techniques, 1983. [33] European Cooperation for Space Standardization (ECSS), ECSSE-30-01A, Space engineering – Fracture control, April 1999. [34] Kumar V, German MD, Shih CF. An engineering approach for elastic–plastic fracture analysis. EPRI report NP-1931, July 1981. [35] ASME. Boiler and pressure vessel code. Section XI: Rules for inservice inspection of nuclear power plant components. [36] API. Recommended practice for fitness-for-service, API 579, 1st ed. March 2000. [37] Wearne SH. A review of reports of failures. Proceedings of the Institute of Mechanical Engineers 1979;193:125–36. [38] Petroski H. To engineer is human: the role of failure in successful design. USA: Vintage Books; 1992. [39] Carper KL. What is forensic engineering? In: Carper KL, editor. Forensic engineering. Elsevier; 1989. p. 1–13. [40] Rossmanith HP. Structural failures – liability, learning from failures, technical insurance and legal consequences. Invited lecture 5th specia1 chair AIB-Vin cotte 1995, Vrije Universiteit Brussel, 21 February 1995. [41] Thomas JM. Industrial experience with structural failure. In: Wierzbicki T, Jones N, editors. Structural failure. John Wiley; 1989. p. 511–45. [42] Carper KL. Learning from failures. In: Carper KL, editor. Forensic Engineering. Elsevier; 1989. p. 14–31. [43] Roberts R. Should there be a methodology for failure analysis. In: Critical Issues in Materials and Mechanical Engineering. ASME PVP, 47. 1980. p. 171–84. [44] Robinson JN, Cox DO. Failure analysis methodology. ASME paper 83-PVP-33, 1983. [45] Wulpi DJ. Understanding how components fail. American Society for Metals; 1985. [46] Figueiredo MV, Oliveira FMF, de Castro PMST, Fernandes AA. Failure analysis of a shaft belonging to a ship engine speed
P.M.S.T. de Castro, A.A. Fernandes / Materials and Design 25 (2004) 117–123 controller. 1999 International Mechanical Engineering Congress & Exposition, November 14–19, 1999, Session DE-11A, Symposium on Failure Analysis and Prevention, 16 November, Nashville, USA. In: Feldy EC, editor. Mechanical Engineering Design Education: Issues and Case Studies, DE-vol.102. ASME; 1999. p. 97–102. [47] Novais Barbosa T, Oliveira FMF, de Castro PMST, Fernandes AA, Cracked stone crusher: Failure analysis and repair, presented at the 2000 ASME International Mechanical Engineering Congress and Exhibition, Orlando, FL, USA, 2000, 5–10 ASME paper 2000-IMECE/DE-6. [48] Fernandes AA, de Castro PMST. Fractures in a fleet of urban passenger buses: a failure analysis, submitted for presentation at IMECE 2003: 2003 ASME International Mechanical Engineering Congress and Exposition, November 16-21, 2003, Washington, DC, USA. Paper IMECE2003-41731 (draft). [49] Figueiredo MV, Oliveira FMF, Goncßalves JP, de Castro PMST, Fernandes AA. Fracture analysis of forks of an heavy duty
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lift truck. In: International Conference on Case Histories on Integrity and Failures in Industry, Milano, Italy, September 28–October 1, 1999. Proceedings: Bicego V, et al., editors. Case Histories on Integrity and Failures in Industry, EMAS 1999; p. 643–52. Also: Figueiredo MV, Oliveira FMF, Goncßalves JPM, de Castro PMST, Fernandes AA. Fracture analysis of forks of a heavy duty lift truck. Engineering Failure Analysis 2001;8(5): 411–21. [50] Silva LFM, Goncßalves JPM, Oliveira FMF, de Castro PMST. Multiple-site damage in riveted lap-joints: experimental simulation and finite element prediction. International Journal of Fatigue 2000;22(4):319–38. [51] da Silva LFM, Stewardson DJ, de Oliveira FMF, de Castro PMST. Fatigue crack growth of rails for railways. Journal of Rail and Rapid Transit, Proceedings of the Institution of Mechanical Engineers 217(Part F):89–97. [52] Health and Safety Executive, UK, Train derailment at Hatfield, 17 October 2000, Second HSE interim report, 23 January 2001.
Methodologies for failure analysis: a critical survey Paulo M.S.T. de Castro *, A.A. Fernandes Faculdade de Engenharia, Universidade do Porto, Rua Dr Roberto Frias, 4200-465 Porto, Portugal Received 16 June 2003; accepted 23 September 2003
Abstract Based on a critical survey of relevant literature, and own experience, a failure analysis methodology is presented, focusing on fracture and fatigue problems. Failures investigated by the authors include brittle fracture and fatigue of structures and machine elements, such as transportation equipment, structural connections, engines, etc. Further to their economic significance, some of the failures were of special concern because of safety, environmental or legal considerations. Difficulties for carrying out the analyses included circumstances not entirely known or documented, damaged fracture surfaces, or insufficient material to carry out comprehensive mechanical testing. Further to the detailed study of the available evidence, stress analyses using numerical methods such as the finite element or the boundary element methods, materials testing and scanning electron microscopy have been used as required. Suggestions for R&D efforts in the field are put forward. 2003 Elsevier Ltd. All rights reserved. Keywords: Failure analysis; Fatigue; Fracture; Structural integrity
1. Introduction Certainly many disasters are not related to structural integrity problems such as fatigue and fracture. However, many failures originated by fatigue and fracture may have tragic consequences – as for example in transportation equipment such as railways or aircraft, or in the process industries. Fitness-for-service criteria are continuously evolving, reflecting advances and new consensus in the technical and scientific communities. Although there is a wealth of literature on the topic of the present paper (for example, [1]), the continuous development of the subject justifies the need for an updated critical survey. 1.1. Liability for defective products in the European Union In the European Union (EU), Member States shall adopt and publish the laws, regulations and administrative provisions necessary to comply with EU ÔdirectivesÕ. For example, Portuguese defective product *
Corresponding author. Tel.: +351-2-205-1241; fax: +351-2-2059125. E-mail address: [email protected] (P.M.S.T. de Castro). 0261-3069/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2003.09.020
liability legislation – decree-law 131/2001 of April 24, 2001 – transposes into national Portuguese legislation the European Union directive 1999/34/CE. It defines product as anything movable, single or incorporated into a movable or stationary assembly. A product is defective if it does not offer the safety the user is entitled to expect, bearing in mind all circumstances, namely presentation, reasonable use and the time when the product was put in circulation. A product is not considered defective just because an improved product is subsequently put in circulation. The producer is responsible for damage caused by the defective product. Producer is the manufacturer of the finished product, of a component or of the material, or someone that attach his name, trademark or other distinctive sign to the product. Under Portuguese law, producer is also someone that imports into the European Union products for selling, or any other form of distribution, or any supplier of a product of which the EU producer or importer is not identified. Escapes from liability include presumption that the defect did not exist when the product was put in circulation, that the defect is compatible with mandatory regulations, or that the state-of-the-art at the time the product was put in service could not discover the defect and identify it as source of
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Nomenclature Acronyms AFCEN Association Francßaise pour les Regles de ConAPI ASME ASTM BSI CEGB ECSS EFAM EPRI ESA
ception et de Construction des Materiels des lectro-nucleaires (France) Chaudires E American Petroleum Institute (USA) American Society of Mechanical Engineers (USA) American Society for Testing and Materials (USA) British Standards Institution (UK) Central Electricity Generating Board (UK) European Cooperation for Space Standardization Engineering flaw assessment method Electric Power Research Institute (USA) European Space Agency
malfunction. As it is to be expected, applicable laws of EU countries are broadly similar. Concern with safety is present in specific EU directives. The directive for machines (98/37/CE) concerns the conditions to be met by the producer so that the machine is considered safe. Concerns new machines manufactured in the EU, new or used machines from third countries, groups of machines and complex assemblies, equipment that modify the function of a machine, and safety devices. The problem of material fatigue, for example, is mentioned in the context of protection against mechanical hazards. This directive originated the Portuguese decreelaw 320/2001 of December 12, 2001. 1.2. Effects of fracture: economic and occasionally loss of lives Although safety is the main concern of legislation, defective products which do not meet the expected performance are a cause of financial loss. Costs of failure include [2]: • direct repair or replacement cost; • loss of revenue while unavailable; • costs of finding replacement services/items during unavailability; • costs of consequential damage; • consequential costs to avoid failure on similar items; • other implications (safety, loss of confidence, image, and trade). Studies of the economic effects of fracture in the USA [3] and Europe [4] suggest that the total cost of failure in developed countries is of the order of 4% of GNP. In the European study, including contributions by the present authors, a sample of reports describing actual failures analyses was reviewed, assessing their economic relevance and attempting to identify needs for further research and/or for the establishment of codes of practice.
ETM Engineering Treatment Model EU European Union fem finite element method GDP gross domestic product HSE Health and Safety Executive (UK) IMECE International Mechanical Engineering Congress MM NBS NIST sem
and Exposition (ASME) mis-match National Bureau of Standards (now National Institute of Standards and Technology, USA) National Institute of Standards and Technology (USA) scanning electron microscopy
The use of Fracture Mechanics and failure analysis techniques may lead to important economies. A classical example is the application of Fracture Mechanics concepts to determine allowable defect sizes of the Trans Alaska oil pipeline. This is a 48 in. diameter about 1/2 in. thick X65 pipe [5], where radiographs showed defects of dimensions larger than those permitted by the DOT (Department of Transportation) Regulations 49 CFR Part 195. The Alyeska Pipeline Service Company requested waivers for these welds, based upon Fracture Mechanics analysis. The cost of the repairs to be made was enormous, namely because part of the pipe was already buried and was relatively inaccessible. DOT requested assistance from the National Bureau of Standards (now National Institute of Standards and Technology), which supported the application of Fracture Mechanics techniques to determine allowable crack sizes [6]. It was possible to demonstrate that a number of defects discovered in the girth welds as a result of post construction audit of the radiographs need not be repaired even though they failed to meet the defect acceptance criteria given in the fabrication code. Failure analyses may lead to • design, material selection or fabrication modifications; • evaluations of service conditions in the remaining life; • fitness for purpose assessments. However, the relevant techniques are not always concerned with technical/economic discussions as in the previous example. Fracture Mechanics and failure analysis knowledge is required for the study of disasters, as exemplified by the ÔAlexander L KiellandÕ oil rig in the Ekofisk field of North Sea, which broke up in March 27, 1980 due to fatigue fracture and capsized claiming 123 lives [7], the MV Estonia passenger vessel capsizing on September 28, 1994 in the Baltic sea [8] killing more than nine hundred
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passengers, or the collapse of the Ramsgate walkway on September 14, 1994 claiming six lives [9], to give just three examples of what could unfortunately be a very long list.
systematic Fracture Mechanics methodology, as was already suggested, in the field of failure analysis, by Weiss [30].
1.3. Role of fracture mechanics education
1.5. Fitness for purpose
Fracture Mechanics is a technical and scientific subject, developed mainly in the second half of the 20th century that seeks to establish quantitative relationships between • loads, • material properties (particularly toughness, or resistance that a material offers to the propagation of a crack), • size of existing crack(s). Fracture Mechanics concepts, but also failure prediction and analysis case studies [10–20], describing how concepts are applied to solve real engineering problems, should be the object of greater attention of engineers during their initial and continuing education. Also, conferences dedicated to Structural Integrity and Fracture Mechanics typically include sessions on failure analysis case studies, as exemplified by the 8th Portuguese Conference on Fracture in 2002 (see for example [21]), or are entirely dedicated to the subject [22].
Fitness for purpose procedures seeks to demonstrate the structural integrity and damage tolerance of load bearing structures. These procedures may be used at the design stage, during operation or at the end of the design life in case life extension of the equipment is under consideration. Simultaneously, these procedures are relevant for the interpretation when things go wrong and failure indeed happens. Perhaps surprisingly for the non-specialist, there is not one but a variety of structural fitness for service assessment procedures, used in different sectors of industry. Although the development of Fracture Mechanics principles in the second half of the 20th century created the scientific basis for most of the procedures, the mentioned diversity is a result of the particular circumstances of the industrial area concerned. The development of an encompassing and thoroughly validated procedure for structural failure, even restricted to the failures modes of fatigue and fracture, is certainly a worthwhile research endeavour. The application of these principles leads to benefits including • reduction of repair costs; • reduction of unavailability time; • optimization of use of materials, reducing excessive conservatism; • optimization of maintenance procedures; and ideally, avoiding failure and possible disasters. However, if failure indeed happens, these techniques may be used for understanding what went wrong, in order to do better in the future. Fitness for purpose methodologies have been developed in European Union (EU) countries, in the US and Japan. The first group includes the already mentioned ÔR6Õ [23] and BS 7910 (former BS PD6493) [24] but also procedures developed in Germany by GKSS [31], in France specifically for the nuclear power generation industry [32], or at European-wide level, for aerospace engineering by ESA [33]. In the USA EPRI [34] and ASME [35] concern the nuclear power generation industry, whereas API [36] deals with structural fitness for purpose problems of the refining and chemical process industries.
1.4. Standards and codes of practice Examples of standards and codes of practice regarding the practical application of Fracture Mechanics to real engineering problems are the well-documented ÔR6Õ procedure developed by the CEGB [23], or the British Standards Institution standard BS 7910 (and former document PD6493) [24]. However, such documents may need further development to increase their usefulness; this is illustrated, for example, in [25], where, in the context of a round-robin exercise, it is concluded that Ôit is unlikely that an engineer with only BSI PD6493 at his disposal could have reliably analysed the vesselÕs defectsÕ. An useful warning is given in [26], where it is concluded that • Fracture Mechanics based predictions of failure load may be non-conservative, because of uncertainties in the inputs, particularly fracture toughness; • large cracks and low values of the ratio ‘‘fracture toughness/yield strength’’ are of particular concern; • there has been no significant improvement in the accuracy of Fracture Mechanics analyses of failures over the 1980–1990 period. The difficulty of evaluations is aggravated by the fact that fracture toughness data is notoriously subject to scatter [27]. In a report to the Community Bureau of Reference, Baker [28] states the need for a basis on which reliability assessment could be made. This work could lead to an expert system [29] package containing a
2. Methodology 2.1. The expert and his/her work Wearne [37] indicates that there is a distinction in the incidence of failure in each class of engineering
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products, but the failures that do occur tend to be more serious. This is because of • the pursuit of economy of scale, which concentrates physical and financial risks; and • the pursuit of optimization in the design of systems and components reducing redundancy. None of the failures examined was caused by hitherto unknown physical phenomena hat occurred without prior warning: a major failure like the Tacoma Narrows bridge could be explained very quickly by von Karman, then the director of the aeronautical laboratory of CalTech, on the grounds of aerodynamic instability theory, as used in airplane design (but not, up to then, in bridge design) [38]. This illustrates the complexity of failure analysis. It involves areas of mechanics, physics, metallurgy, chemistry, corrosion, manufacturing processes, stress analysis including numerical techniques such as the finite and the boundary element methods, design analysis and fracture mechanics including environmental induced cracking, non-destructive evaluation, and probabilistic risk evaluation, among others. Of course no single person will be an expert in all those areas. The failure analyst must have the management skill to orchestrate the activities of the necessary specialist experts. The failure analysis expert typically has deep knowledge of one of the areas mentioned above and working knowledge of the others, obtained from experience, skill, training and education. In-house experts may have a bias toward his/her employerÕs viewpoint. An objective professional opinion should therefore come from someone who has no ties to and no interest related to the case. This is particularly true in court litigation, where the forensic engineer deals with the engineering aspects of legal problems [39]. Answers to be provided by the forensic expert include [40]: • what happened? • where did it happen? • how did it happen? • who caused it to happen? • who is responsible for what happened? • what are the costs to repair or replace? • what are the damages suffered by the various parties involved?
2.2. Failure analysis framework Thomas [41] suggests that the variety of problems and approaches to their solution are too great to be encompassed by a single general approach. A general structural failure analysis framework could consist of the exam of failure modes one by one (fatigue, creep, corrosion, wear, etc.); however, each of those subjects is dealt with in specialized texts, in a broader context than
failure analysis. There would be little progress in specific failure analysis knowledge proceeding that way. Alternatively, [41] synthesizes the failure analysis process considering the four more frequent factors – stress, environment, defects and material properties – and their interaction. Professional literature usually concentrates on the technical aspects of failure. However, the real reasons of failure are often procedural, involving communication deficiencies and unclear definition of responsibilities. This is true even in the most advanced technologies where formal quality and assurance procedures are in operation, as shown by the Challenger space shuttle 1986 accident [38]. Systems thinking will have therefore an important role in his field. Carper [42] lists the following causes of failure in civil structures: • site selection and development errors, • programming deficiencies, • design errors (including failure to consider some combination of loads, lack of redundancy, connection details, etc.), • construction errors, • material deficiencies, • operational errors (including change in use, inadequate maintenance, etc.). Of course, lists based on other criteria are found; Petroski [38] includes two, the first claiming to be comprehensive: • ignorance, • economy, • lapses or carelessness, • unusual occurrences (earthquakes, extreme storm, fires, etc.), whereas the second makes no claims of completeness: • limit states (overload, under-strength, movement, deterioration), • random hazards (fire, floods, explosions, earthquake, vehicle impact), • human-based errors (design error, construction error). An important concern is the question of life extension of existing equipment or facilities. As discussed in [40], elements of a systematic approach to this problem include • assessment of the materia1 conditions, • type of damage (fatigue, creep, environmental, etc.), • damage site (surface, bulk, interface), • damage extent (localized, global), • damage level (negligible, endangering the functionality, causing danger), • loading conditions, • temperature. Recent R&D work by the authors in this broad area, include a preliminary study of the use the 100 years old, riveted, Luiz I bridge over the Douro river in Porto, for use by the new metro, and on multiple site damage of airplanes (BRITE project BE95-1053 ‘‘SMAAC’’), where
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the safe operation and maintenance procedures for aging aircraft was studied in detail. 2.3. Suggested methodology Roberts [43] suggests that there should be a systematic way of performing failure analysis (a ÔmethodologyÕ, such as medical people do their autopsies. Items in a failure analysis methodology are typically [44,45]: • problem definition, • obtain background information, • investigate evidence (macroscopic and microscopic examination, including sem), • evaluate failure mechanism, sequence and causes, • perform exemplar test, • analysis and calculations, including fem and Fracture Mechanics analyses, • assess risk, • draw conclusions based on all evidence, and make recommendations. It is important to look for overall context of the failure. Concentration on only one aspect may conceal the cause. Background information, which cannot be deduced from the physical evidence, should be obtained. The field investigation should be carried out shortly after the failure. Later, laboratory examination, eventually including sem, is carried out. Fracture surfaces are extremely delicate, and contain relevant information for the failure analysis. They should be preserved carefully. Mating pieces of a fracture should not be put together, since this will damage the surfaces and will spoil the fractographic examination. By definition, destructive testing cannot be undone, and should not therefore be done prematurely. Preliminary hypotheses are made, and tested using as required exemplar experiments, and calculations. The macro and micro appearance of the failure in the exemplar test should be compared to the actual failed part. Stress analysis, eventually using fem, may be performed. Risk assessment, particularly relevant to similar items still in service and decisions to recall or not to recall, may be simply stated as risk ¼ frequency severity: Final conclusions must explain all physical evidence and verifiable facts. 2.4. Some examples The following examples taken from analyses carried out by the present authors, illustrate some of these points. Sometimes the fracture surface is difficult to interpret, even with the help of atlas of fractographs and other
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literature. This is the case of an analysis of a shaft of a maritime engine [46]. In this case, mechanical tests were carried out, with the objective of generating a number of characteristic fracture surfaces in fatigue, in fracture toughness and in impact tests, which could be compared with the actual failed part and help to identify the failure mode. The need for stress analysis of the structure, in order to predict the behaviour in the presence of defects, and to assess the necessary repair, is dealt with in [47]. Compliance with codes of practice is emphasised in [48], where recent structural failures in a fleet of urban passenger buses were analysed, and attributed to noncompliance with appropriate codes of practice for fatigue design of welded structures. Inadequate repairs may be also causes of structural failure. That was the case of the failure of forks of a heavy-duty lift truck, causing important economic damage. This failure could be explained by inadequate repair procedures [49]. Research on fatigue and fracture contributes to reduce the probability of accidents and disasters. Civil transportation is an area of great interest, due to possible impacts of accidents for example in aviation or railways. Examples that can be mentioned include the Aloha aircraft accident, explained by multiple site fatigue damage studies. This was the subject of the European Union ‘‘SMAAC’’ project already mentioned above, concerning aging aircraft and their pathology – see for example [50]. Another European Union project, with the involvement of the present authors, is the ‘‘RAILCRACK’’ project on fatigue testing of rails for railways [51], seeking to contribute to safer railways and avoidance of disasters such as the Hatfield (UK) derailment of October 17, 2000 injuring 70 and killing four people, which was caused by fatigue cracking of rails [52].
3. Concluding remarks Liability and litigation issues create a need for failure analysis. However, understanding the causes of failures is also the source of engineering knowledge, since success in structural engineering is foreseeing failure. A methodology for failure analysis was suggested, and aspects were illustrated by cases dealt with by the authors. The development of an encompassing and thoroughly validated procedure for structural fitness for purpose, even restricted to the failures modes of fatigue and fracture, is certainly a worthwhile research endeavour, as a step towards a life cycle management method for major plant and infrastructure. This work could lead to an expert system package containing a systematic Fracture Mechanics and failure analysis methodology.
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lift truck. In: International Conference on Case Histories on Integrity and Failures in Industry, Milano, Italy, September 28–October 1, 1999. Proceedings: Bicego V, et al., editors. Case Histories on Integrity and Failures in Industry, EMAS 1999; p. 643–52. Also: Figueiredo MV, Oliveira FMF, Goncßalves JPM, de Castro PMST, Fernandes AA. Fracture analysis of forks of a heavy duty lift truck. Engineering Failure Analysis 2001;8(5): 411–21. [50] Silva LFM, Goncßalves JPM, Oliveira FMF, de Castro PMST. Multiple-site damage in riveted lap-joints: experimental simulation and finite element prediction. International Journal of Fatigue 2000;22(4):319–38. [51] da Silva LFM, Stewardson DJ, de Oliveira FMF, de Castro PMST. Fatigue crack growth of rails for railways. Journal of Rail and Rapid Transit, Proceedings of the Institution of Mechanical Engineers 217(Part F):89–97. [52] Health and Safety Executive, UK, Train derailment at Hatfield, 17 October 2000, Second HSE interim report, 23 January 2001.