Understanding How Components Fail Wulpi Pdf Writer

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Understanding How Components Fail Wulpi Pdf Writer

ABSTRACT Forensic metallurgists are asked to address failures across a wide range of materials, length-scales, and applications. This requires in-depth knowledge of metallurgical principles, manufacturing, and engineering fields.

The metallurgist will be asked to determine whether or not the appropriate engineering or quality standards have been followed – and this may be the Standards that were in place at the time of manufacture, not those currently in place – and whether the failure results from use or abuse. The paper reviews how these skills have been applied to a range of historical and contemporary cases involving failure and discusses some of the issues that are important for determining the root cause of a problem. Some difficulties in current approaches are also presented.

Introduction Forensic metallurgy is part of the field of forensic engineering which applies engineering science to issues that relate the investigation of unforeseen failures, crashes, disasters, or other incidents. The term ‘forensic’ strictly means in application to a court of law and implies that there may be a criminal aspect to the issue at hand but could also imply negligence or breach of contract [ Fraser J, Williams R.

Handbook of forensic science. Devon, UK: Willan Publishing; 2009. The result of a forensic investigation typically involves the preparation of technical engineering reports, and may require giving testimony and providing advice to assist in the resolution of disputes affecting life or property.

Often, the outcome of forensic investigations is agreed before resolution in court is required. In this paper, and indeed in the literature more broadly, ‘forensic’ is (mis)interpreted as the application of forensic techniques to the investigation of materials, products, structures, and components that have either failed in service or have failed to perform as intended. So, for example, the work of Lewis et al. [ Lewis PR, Reynolds K, Gagg CR. Forensic materials engineering: case studies. CRC Press; 2003. ] often contains case studies where the analyses were used as evidence in legal proceedings, but references to ‘forensic’ engineering often refer simply to a failure analysis that may never be exposed in legal proceedings.

A forensic investigation in an industrial context may involve understanding how products or components could be redesigned to eliminate future failures. Forensic metallurgy is used to determine how materials fail across many different length scales, from understanding why a connect has failed in a microelectronic device using high-resolution transmission electron microscopy or atomic force microscopy, [ Ross RJ, editor. Microelectronics failure analysis desk reference sixth edition. Materials Park ( OH): ASM International; 2011. ] to determining whether coins are counterfeit, [ Gagg CR, Lewis PR.

Counterfeit coin of the realm – review and case study analysis. Eng Fail Anal. 2007;14: 1144– 1152.

Doi: 10.1016/j.engfailanal.2006.11.063,, Hida M, Mitsui T, Minami Y. Forensic investigation of counterfeit coins. Forensic Sci Int. 1997;89: 21– 26. Doi: 10.1016/S0379-070-4, ] to understanding how the composition of the steel in the Titanic may have influenced the failure [ Foecke T. Metallurgy of the RMS Titanic NISTIR 6118; 1997., Foecke T, Hooper-McCarty JJ. Quantitative metallography and microanalytical analysis of particles in iron rivets recovered from the wreck of the RMS titanic.

Microsc Microanal. 2009;15: 524– 525. Doi: 10.1017/S099127,, ] or why the World Trade Centre collapsed on the 9th September 2001 [ Banovic SW, Foecke T, Luecke WE, et al. The role of metallurgy in the NIST investigation of the world trade center towers collapse.

2007;59: 22– 30. Doi: 10.1007/s11837-007-0136-y, ]. Forensic metallurgy is also a routine tool used by manufacturers to understand how metals are performing in service and for life extension purposes in for example steam or nuclear power plant.

The metallurgical analysis may often be combined with additional analysis of the expected stresses and loading on any component [ Cook RD, Malkus DS, Plesha ME, et al. Concepts and applications of finite element analysis. New York: John Wiley & Sons Inc; 2001., Hattingh DG, James MN, Newby M, et al. Damage assessment and refurbishment of steam turbine blade/rotor attachment holes. Theor Appl Fract Mech. 2016;83: 125– 134.

Doi: 10.1016/j.tafmec.2015.11.001, ]. A range of questions can be asked during a typical metallurgical failure analysis such as: • At the point of first use of the product did it meet the design specification?

Did the mechanical properties of the material meet the design specification? Were the chemical and microstructural properties of the material as anticipated? • Were there flaws in the product from the original manufacture?

• Were there issues with the original design of the product/component/structure? • Were the designed properties for the material sufficient for the loads/temperatures/environments that the product experienced in service? • Is there evidence that correct maintenance had been carried out during the lifetime of the component? • Is there evidence that there was incorrect operation of the component either deliberately or accidently (over-torqueing for example)? Had the specified design life been exceeded? • Had the component been adequately serviced or inspected?

Further details of failure mechanisms and specific issues related to failure analysis can be found in e.g. [ Lewis PR, Reynolds K, Gagg CR. Forensic materials engineering: case studies. CRC Press; 2003. ], ASM [ ASM handbook volume 12: fractography. Materials Park ( OH): ASM International; 1987. ], Wulpi [ Wulpi DH.

Understanding how components fail. Materials Park ( OH): ASM International; 1999. ], Hainsworth and Fitzpatrick [ Hainsworth SV, Fitzpatrick ME. Forensic engineering: applying materials and mechanics principles to the investigation of product failures. Forensic Sci Med Pathol. 2007;3: 81– 92. Doi: 10.1007/s12024-007-0013-6, ], and Ross [ Ross RB.

Investigating mechanical failures: the metallurgist’s approach. Springer; 1995. The purpose of this article is to assess the current state-of-the-art in forensic metallurgy and highlight challenges for future progress in this field. One of the difficulties in any forensic investigation for a particular specialist is that the root cause of the failure may not be linked to their specialism. The metallurgy may be irrelevant, for example, if the component has been used inappropriately or overloaded: in which case a metallurgical assessment simply shows that the product was manufactured to specification. For example, the author has personally conducted forensic assessment of many ladder failures, and in all cases the failure was from abusive loading rather than any problems with the metallurgy of the ladders.

Occam’s razor, where ‘Among competing hypotheses, the one with the fewest assumptions should be selected’, is often a good starting point for any analysis. Metallurgists need to think carefully before embarking on expensive testing if the result is not likely to be of practical benefit. Dr Ken Reynolds of The Open University used to tell the anecdote of an incident early in his career working in a pipe mill in the West Midlands. There was an intermittent problem with pipes cracking during the final drawing stage, that had baffled many eminent metallurgists, whose best diagnosis was segregation of alloying elements under gravity while the pipes were stored vertically between processes. The real reason became apparent when Reynolds took a short cut through the annealing plant to the canteen one rainy evening, and came across an operative cooking bacon on a shovel: he had opened the furnace door to do so and hence the pipe ends were not at temperature.

So the problem was intermittent because this would only happen when that particular operative was on the night shift and had bacon for his supper! Initial inspection An initial examination of a failed metallic component will often indicate the mode of failure. A visual inspection will look at the general shape and colour of the component and whether or not wear, corrosion or pitting is apparent, any obvious features such as large inclusions or porosity, or evidence of gross damage or abuse of the specimen. After the initial analysis, additional higher magnification lenses or loupes, stereo microscopes, or scanning electron microscopes will be used to examine the fine detail of the fracture surfaces of the failed component. It is important to properly preserve any fracture surfaces for detailed examination [ Zipp RD, Dahlberg EP. Preparation and preservation of fracture specimens in ASM handbook volume 12: fractography.

Materials Park ( OH): ASM International; 1987. The initial examination will help to determine the root cause of the failure be it overload, creep, fatigue, corrosion, or wear [ ASM handbook volume 12: fractography. Materials Park ( OH): ASM International; 1987., Gagg CR, Lewis PR. In-service fatigue failure of engineered products and structures – case study review. Eng Fail Anal. 2009;16: 1775– 1793. Doi: 10.1016/j.engfailanal.2008.08.008, ] although additional investigation of the composition, microstructure, and mechanical properties may be required to definitively define the mechanism.

Analysing the microstructure Many of the desired mechanical properties of materials are achieved by applying specific thermomechanical treatments to components. An examination of the microstructure can determine whether these treatments have been appropriately applied.

Grain-sizes and shapes can be determined by metallographic analysis to see whether a specific average grain size was achieved. The analysis of grain size and shape can be determined by traditional linear-intercept methods such as those defined in the ASTM standards [ Schulz U, Braue W. Degradation of La2Zr2O7 and other novel EB-PVD thermal barrier coatings by CMAS (CaO–MgO–Al 2O 3–SiO 2) and volcanic ash deposits.

Surf Coat Technol. 2013;235: 165– 173. Doi: 10.1016/j.surfcoat.2013.07.029, ] or by techniques such as Electron-Back-Scattered diffraction [ ASTM E112-13. Standard test methods for determining average grain size. West Conshohocken ( PA): ASTM International; 2013. The shape of grains will help to determine whether or not the appropriate cold-working processes were applied (flat pancake shape grains indicating cold-rolling for example) and whether or not the material was cast or forged [ Ross RJ, editor.

Microelectronics failure analysis desk reference sixth edition. Materials Park ( OH): ASM International; 2011. Precipitate distributions can also be examined. Typical questions in this area might be: are the precipitates dispersed throughout the grains or collected at grain boundaries?; and are they the optimum size and shape that would be expected? Precipitate chemistry and shape can be useful in indicating the thermal history of a specimen whether or not that is a temperature that has been exposed to high temperatures for long times in e.g. Alloys used in steam power plant [ Humphries FJ.

2001;36: 3833– 3854. Doi: 10.1023/A:592, ] or whether an aluminium alloy used in an aerospace application has been optimally aged [ Krishna R, Atkinson HV, Hainsworth SV, et al. Gamma prime precipitation, dislocation densities, and TiN in creep-exposed inconel 617 alloy. Metall Mater Trans A.

2016;47: 178– 193. Doi: 10.1007/s11661-015-3193-9, ]. Physical properties In order to understand whether or not a material failed because it was not sufficiently robust for the application, it is necessary to conduct an assessment of the mechanical properties of the material. This can be challenging if the fragment of material that is retrieved is small. The key mechanical properties are usually strength and toughness. In order to determine the strength of the component, the first tool that is often used in failure analysis is hardness testing, where indentations are made into the material using an indenter of known geometry and a known applied load. The hardness of a material can be related to its strength by a simple empirical relationship (for example for Vicker’s indentations, H□3 σ y where H is the hardness and σ y the yield stress) [ Chen Y, Weyland M, Hutchinson CR.

The effect of interrupted aging on the yield strength and uniform elongation of precipitation-hardened Al alloys. 2013;61: 5877– 5894. Doi: 10.1016/j.actamat.2013.06.036, ]. The hardness test may subsequently be followed up by more accurate testing if the hardness is not as anticipated. A material’s toughness is more difficult to determine from small samples.

The toughness of a material is a measure of its resistance to crack growth and there is often a conflict between developing materials that are both high strength and tough [ Tabor D. J Inst Metals. 1951;79: 1., Ritchie RO.

The conflicts between strength and toughness. 2011;10: 817– 822. Doi: 10.1038/nmat3115,, ]. Toughness is also difficult to ascertain from an inspection of a failed component as whether or not a material may exhibit a ductile or a brittle fracture can be affected by the geometry of the component, with plane strain conditions in large samples leading to brittle failures in materials that would otherwise be considered as ‘ductile’ [ Launey ME, Ritchie RO. On the fracture toughness of advanced materials. 2009;21: 2103– 2110.

Doi: 10.1002/adma.200803322, ]. Sometimes more detailed knowledge of mechanical properties such as fatigue life are required. A major failure of railway track occurred at Hatfield in 2000 [ Demaid A. Milton Keynes, UK: The Open University; 2006. [cited 2017 Feb 20].

Available from:. Railway fatigue failures: an overview of a long standing problem. Materwiss Werksttech. 2005;36: 697– 705.

Doi: 10.1002/mawe.200500939 Train derailment at Hatfield: a final report by the independent investigation board office of rail regulation; [cited 2006 July]. Available from: ]. The cause of the failure was gauge-corner cracking caused by rolling-contact fatigue and the track broke into over 300 pieces over a distance of approximately 35 m. Inspection of the wider UK rail network showed that there were an additional 2000 sites with potentially dangerous cracks. This incident demonstrated that regular inspection, and more importantly reacting to the information obtained from inspection, is critical in preventing failure. Many companies use rigorous non-destructive testing techniques to look for defects, such as dye penetrant, radiographic, eddy current, or ultrasound inspection. In the aerospace industry for example, immersion ultrasonic inspection is widely used for investigating whether or not flight-critical parts contain defects [ Olver AV.

The mechanism of rolling contact fatigue: an update. Proc IMechE Part J: J Eng Tribol. 2005;219: 313– 330. Doi: 10.1205X9808, ]. The inspection looks at wall thickness, surface and internal defects and discontinuities and determines whether a part is suitable for use. Rigorous inspection is important for eliminating failure ahead of time.

Failure from environmental factors Many metallic materials that are subjected to high temperatures and/or wet and moist environments will suffer from oxidation and corrosion. Corrosion/oxidation failures can be prevented by good design and regular maintenance. One particular area of corrosion failure that can lead to unanticipated failures is stress-corrosion cracking.

In order for stress-corrosion cracking to occur there must be a specific chemical environment and a tensile stress present. One of the earliest examples of stress-corrosion cracking (or season cracking) was in the failure of British Forces brass cartridges stored in stables in monsoon season in India. The cracking was found to be caused by ammonia from horse urine and residual stresses left over from the cold-drawing process used to form the cartridges [ Fahr A, Wallace W. Aeronautical applications of non-destructive testing.

Lancaster ( PA): DEStech Publications Inc. Torrent Microsoft Office 2011 Mac Free. ; 2014. A more recent example where stress-corrosion cracking was found to be an issue is in aluminium aircraft landing gear [ Moore H, Beckinsale S, Mallinson CE. The season cracking of brass and other copper alloys. J Inst Metals.

1921;XXV: 59– 125. The necessary stress can arise from residual stresses introduced during manufacture of the component (from e.g. Hole drilling and reaming), cyclic differential expansion and contraction of the landing gear components, or the repeated application of mechanical force on landing. Aluminium alloys are susceptible to stress-corrosion cracking by chlorides which can be present from moisture from flying over a marine environment. Another type of environmentally assisted cracking is hydrogen embrittlement. In 2014, a number of bolts failed on the Leadenhall ‘Cheesegrater’ Building in London.

Laing O’Rourke and Arup engineers determined that the failure mechanism was hydrogen cracking [ Witherell CE. Aircraft landing gear fracture in handbook of case histories in failure analysis. 1, ASM International; 1992.

The Cheesegrater has a novel design that comprises a tapering, perimeter-braced structure with office floors connected to a support core. The structure uses over 16,000 tonnes of steel. In order to connect different aspects of the building design, 5 inch (12.7 cm) diameter steel ‘megabolts’ are used [ [cited 2017 Mar 13]. Available from: -views/press-releases/2015/ ].

Five of these bolts failed over a period of time. Steels with hardnesses greater than 380VHN are particularly susceptible to hydrogen embrittlement, but this can be mitigated by using appropriate ‘baking’ procedures to drive off the hydrogen and this procedure is well-known for fasteners [ [cited 2017 Mar 13]. Available from: ]. In order for hydrogen-induced cracking to occur it requires (i) a steel susceptible to hydrogen-induced cracking (ii) stress (residual or applied), and (iii) atomic hydrogen to be present. A typical fracture surface on a component that has failed by a hydrogen-induced cracking mechanism is characterised by a brittle intergranular morphology. Sometimes a fracture surface will exhibit a brittle intergranular fracture around the source of the crack (e.g. A thread root) followed by a ductile fast fracture in the rest of the component that failed once a critical crack size is reached.

This transition in fracture surface morphology however is not unique to hydrogen-induced cracking: it can occur from other mechanisms such as fatigue or overload. The difficulty therefore for a failure investigation is determining whether or not the initial microcrack was initiated by hydrogen-induced cracking. The main issue is determining the source of hydrogen which can either be ‘internal’ from processes such as electroplating or ‘external’ from processes such as corrosion. Louth [ Louth MR.

Hydrogen embrittlement of metals: a primer for the failure analyst. J Fail Anal Prevent.

2008;8: 289– 307. Doi: 10.1007/s11668-008-9133-x ] notes that the presence of intergranular cracking in a material that normally fails by ductile fracture is not sufficient to confirm that a component failed from hydrogen embrittlement (for example, temper embrittlement would also give intergranular cracking). There is no unique fracture mode that characterises hydrogen embrittlement and thus the susceptibility of the material to hydrogen embrittlement, the hydrogen content and operating environment, temperature, load, strain rate, and specimen history must all be considered. Detecting atomic hydrogen on metallic fracture surfaces is currently not possible and thus while it may be tempting to attribute a failure to hydrogen embrittlement, care must be taken in confirming the mechanism definitively. Design factors, statistical variability issues and the human factor One of the key issues for any forensic metallurgical analysis is to understand how design factors can contribute to the failure mechanisms and modes. For example, stress concentrations are known to be a significant factor in failures of structures [ 14 Code of Federal Regulations [CFR] 26 Subpart C, 2011. [cited 2017 Feb 20].

Available from:., Swift T. Damage tolerance in pressurized fuselages. Magic Quadrant For Integrated Workplace Management Systems Pdf. In: Simpson DL, editor. 11th plantema memorial lecture, new materials and fatigue resistant aircraft design. Warley, UK: Engineering Materials Advisory Services Ltd.; 1987. Notch sensitivity is a particular issue in iron-based alloys and was an important factor in the failure of the Liberty Ships during and after Second World War [ Gagg CR.

Failure of components and products by ‘engineered-in’ defects: case studies. Eng Fail Anal. 2005;12: 1000– 1026.

Doi: 10.1016/j.engfailanal.2004.12.008, ] which were also influenced by the fact the steel they were made from underwent a ductile to brittle transition at temperatures to which they were exposed during service [ Williams M. Failures in welded ships, an investigation of the causes of structural failures. NBS Tech News Bull. Any metallurgical analysis of failure therefore needs to consider the impact of the particular failure modes in relation to that material. Large engineering structures such as aeroplanes, ships, bridges, and buildings are assembled from many component parts.

Components may be sourced from a single manufacturer and assembled into the final product or the product may be assembled from components originating from several different suppliers. This can influence issues such as variability in mechanical properties and microstructures or variability in product size (e.g.

Rolled plates might be of variable thickness and microstructure [ Tipper CF. The brittle fracture story. Cambridge: Cambridge University Press; 1962. Some industries operate total quality management processes with ‘zero defects’ in order to ensure safety or give themselves the competitive edge [ Savage G. Dealing with crisis – solving engineering failures in formula 1 motor racing. Eng Fail Anal. 2010;17: 760– 770.

Doi: 10.1016/j.engfailanal.2009.08.009, ]. Finally, there may be either intentional or unintentional human aspects to the failure, either by oversight at the design stage or in subsequent inspection or repair.

An example of unintentional damage was that caused to fuselage skin lap joints in aircraft by the use of sharp tools during paint and sealant removal which led to scribe marks that could cause cracks or fatigue damage [ FAA Flight Standards Information Bulletin for Airworthiness – FSAW 03-10B, dated November 20, 2003: “Fuselage Skin “Scribe Mark” Damage on Boeing 737 Aircraft.” ()., Khan MK, Fitzpatrick ME, Hainsworth SV, et al. 2011;59: 7508– 7520. Doi: 10.1016/j.actamat.2011.08.034, ]. Summary Safety in engineering improves through our experiences of failure. Manufacturers have ever-tighter controls over metal chemistry and microstructure through heat treatment processes. Companies (particularly those working in safety-critical areas) operate stringent total quality management processes with full traceability of raw materials combined with rigorous inspection and acceptance processes. Designers have better tools and insights into effects such as stress concentration.

Engineering standards for application of materials (and designs) in different sectors (e.g. Nuclear, aerospace, automotive) are ever more stringent and challenging, and have built-in safety factors to try to anticipate issues; and the development and application of these standards is critically important in preventing fatal accidents. Tools and techniques for inspecting materials before use have progressed considerably to try to engineer out unanticipated failures. However, the best design, manufacturing, and assembly will never eliminate issues from human interaction with the final product. Additionally, leaner design, use of cheaper materials with less stringent standards, and new materials with as-yet unanticipated susceptibilities will keep leading to unforeseen failures. The challenge for the forensic metallurgist is in ensuring all aspects of the failure are understood from examination of the specimen history and in retrieving the relevant samples that reveal the root cause of the accident/incident or issue.

Metallurgical failure analysis is complex. There are challenges that have to be overcome, some of which are technical and others, such as financial or time constraints, that may limit the information that can be obtained during an investigation. Issues in the technical analysis of engineering failures include: • Can representative samples of the failure be retrieved? • Have the samples been properly preserved so that the fracture surface reveals information relevant to the fracture rather than issues from subsequent handling? • Are the analytical techniques available to the investigator that will give the necessary information? • Does more research need to be conducted to understand the root cause of the failure: either ‘pure’ research to further understand a particular, novel, failure mechanism; or applied research using existing knowledge to explain the particular issue at hand.

In many legal disputes, there are often conflicting expert opinions that are tabled. For example, the report by Errichello et al. [ Errichello R, Sheng S, Keller J, et al. Wind turbine tribology seminar: a recap, US DOE Report DOE/GO-1 [cited 2017 Mar 14]. Available from:.

] showed that four different hypotheses were presented for the mechanisms of bearing failures in wind turbines. Two different bearing manufacturers identified environmental mechanisms (hydrogen-enhanced local plasticity; and brittle fracture followed by crack propagation due to corrosion fatigue cracking respectively. Two turbine manufacturers, by contrast, identified component failure (from adiabatic shear bands and severe plastic deformation, respectively.

In that case that the appointment of experts with no vested interest in the outcome may have been preferable. In the UK, for a case below a cost level of £X, the court will appoint a single expert who takes information from all parties and provides a joint report with the aim of keeping the cost down. Above this financial level, or in particularly complex cases, multiple experts will be appointed. In legal disputes, experts’ reports are prepared for the court and each expert has to make a declaration that their report is not influenced by who pays. However, it is not always easy to demonstrate whether or not this is the case. Experts may be biased – consciously or unconsciously – by the briefing given to them by their instructing solicitor; and/or a solicitor may seek out and select and expert who is inclined towards a particular interpretation of a failure.

Any investigation will also be potentially limited by financial constraints that impact on the level of analysis available and often leave the investigator in a difficult position in terms of determining the cause of the failure.

Weld repair—Analyze the failure before attempting the repair November 9, 2004 By: When something breaks, you acknowledge the shock, scratch your head, take stock of the situation, and look for the fastest way to repair the item and put it back into operation. The pressure to repair quickly is understandable, but common sense suggests stopping for a moment and trying to understand what caused the break before attempting the repair. Failure Analysis Almost anything can fracture. The science investigating the origins of fractures is called failure analysis, and it is used to establish responsibilities for fractures and to determine preventive measures for avoiding future occurrences.

An introductory, interesting book on this subject was written by Donald J. Wulpi and is titled Understanding How Components Fail.

1 Service Failures This article discusses only in-service weldment breakage, also described as service failures. Weldments are assemblies with parts joined by welding.

Failures occurring during or immediately following welding are easier to deal with, because all conditions are known. So if the item was welded originally, it should be weldable again for repair, right? Yes, but only if you know the materials and their conditions and whether they still are exactly as they were at fabrication time—no heat treatment or other surface conditioning has been introduced.

You also must know the precise process and welding procedure that were used in the first place, which usually isn't the case. What Caused the Break? Before attempting any repair, you must determine why the break occurred. If you restore the item exactly to its original condition, chances are another breakage will occur.

(At that time it may well be none of your concern, but you must operate professionally at all times). While a fully implemented professional investigation by an experienced metallurgist would be the best recourse, this usually is justifiable only in selected cases—for example, for presenting claims to the manufacturer or to the insurance company. It is mandatory, however, if injury to persons or property loss was or could have been involved.

Left to involved people, the human urge to clear oneself and to find fault in someone else's actions or inactions can interfere with an investigation. The main reasons that weldments fail are: • Inadequate material or properties. • Poor design. • Poor workmanship. • Excessive unanticipated service conditions.

The Investigation Process Even if the person in charge is not specifically trained as a failure analyst, a few investigative steps should always be taken: • First take care that nothing be moved, manipulated, reassembled, or fixed. • Document the condition of the weldment when the breakage was found. • Write down all that is known, and question all who were present.

• Note the ambient temperature at the time of occurrence. • Take pictures, both general and close-ups. • Protect the place from rain and other environmental disturbing factors Firmly resist the pressing urge (your own and of those around you) to supply a theory for the breakage, especially before having assembled all the information. A description of the weld profile as visible under low-power magnification should include such details as dimensions and fit-up as much as they can be determined visually. If possible, these details should be compared with design requirements. When the structure operates normally at elevated temperature, it is probably under some code legislation that may request an official investigation.

A weld breakage usually is a crack or a fracture. Much information can be drawn from an exact description of the failure. A crack should be characterized by its dimensions, by its orientation (longitudinal, along the weld bead, or transversal), and by its position relative to the weld itself (on the weld bead or on its sides, in the heat-affected zone, or in the base metal). If the fracture is open, do not reassemble the mating parts.

Doing so can obliterate important clues. Inspect the fractured surfaces with a low-power lens or microscope that can show internal defects like gas holes or pores, nonmetallic inclusions, or indications of fatigue failure in the form of concentric beach marks.

The presence of macroscopic deformations and the fibrous or glassy aspect of the surfaces should be assessed to reveal if the failure was ductile (with deformation) or brittle (without deformation). Specific colors on the surfaces should be remarked; they might be clues about local heating and oxidation. The extent of corrosion, if present, has to be determined and documented. The presence of arc strikes on the surface, improper starting conditions, or accidental contact may be at the origin of considerable damage. Hardness testing is a very informative, simple, nondestructive test. However, selecting the proper locations, especially if the weldment must be sectioned for testing, may be beyond what can be expected from a technician not specifically trained for this kind of investigation.

The materials involved should be known and their properties checked for conformance to specifications. No weld repair should be attempted without this essential knowledge. Having this information allows you to select the proper repair procedures and filler metal. If materials are not known, an effort should be made to provide at least qualitative information. This information can be obtained by X-ray fluorescence, a nondestructive test readily available from many metal-related services.

Having assembled and organized all the facts, you now should be able to formulate an educated guess as to the possible causes of the failure. Was faulty workmanship the culprit? A professional investigation service uses metallographic examinations of weld sections to look for weld defects in the original weldment. Obviously, faulty welds should have been detected by inspection after manufacturing, but nobody is perfect.

If the original weld was faulty, a repair weld performed with utmost care should improve the future in-service performance of the repaired item. A design change is not normally applicable for repair. However, if it is clear a faulty design caused the failure, an improvement might be introduced. But you should be aware that adding stiffness may make the matter worse, by increasing internal stresses and paving the way for the next fatigue fracture. If the breakdown was sudden but caused by a progressively deteriorating condition of certain components (as in the case of fatigue fractures or corrosion), a corrective action program should be initiated. The plan has to incorporate periodic examination of the parts involved, after the structure is repaired, to detect dangerously spreading cracks before much damage occurs again. Cracks must be removed completely by careful grinding before rewelding.

If the base metal is in acceptable condition, weld repair may be attempted with suitable ductile filler metal or low-hydrogen electrodes. The process selected should introduce minimum heat and residual stresses, and possibly should be followed by light hammer peening. Preheat and/or postheat, if necessary. If separation has occurred, then a proper joint has to be designed and prepared, possibly by introducing a transition element to make up for the volume of metal to be discarded. Experience and common sense always are important, and even more so when dealing with weld repairs.

1 Understanding How Components Failby Donald J. Wulpi is available from Click on Bookstore, then on Failure Analysis, and then on Understanding How Components Fail.