MechChem Africa May 2019

Why bother with failure analysis?

MechChemAfrica welcomes new columnist Tim J Carter, who will be presenting a quarterly column titled Failure and how to avoid it . Here he introduces failure analysis and why it offers value for money.

O K, so the systemis brokenandhas to be repaired to restore produc- tion. Since the spares needed are imported, it’s going to be expen- sive and, if the parts needed are not in the stores as spares, it will take time. In addition, wehave thecostoftheactual repairitselfover and above the cost of the spares, and the cost ofproductionloss,whichcaneasilyexceedthe sum total of the rest. Sowhy add an additional cost of analysing the failure? Failure analysis is not a cheap undertaking. If it is suggested that such an exercise canbe carriedout cheaply, the result- ing report probablywon’t beworth the fee, no matter how small. An on-site examination is always a good place to start when it’s time to gather evi- dence, although the evidence may be ‘well distributed’ over the surrounding scenery in some of the more spectacular failures. Failure analysis frequently requires the use of complex equipment and tech- niques, such as scanning electronmicroscopy. Electron microscopes are neither cheap to install or operate. The costs of hiring one can be over R1 000 per hour, while using an up- to-date state-of-the-art instrumentmay cost a fewmillion Euros. Similarly, determination of the material of construction of the failed part – involving chemical analysis, mechanical testing and examination of the microstructure – is not cheap either. Like the SEM, these techniques use equipment that is expensive to install and operate, and theprocedures canbevery time- consuming for highly skilled and experienced

staff. Such people are hard to find and costly to keep. Examination of the fracture surface of the broken part, a technique known as ‘fractography’, requires specialist skills. I have known supposed failure analysts who simply comparetheappear- ance of the fracture

ing a mixture of ethanol vapour and ethylene gas. The leakdrewattention to itself by catch- ing fire, fortunately without significant col- lateral damageowing toanattentiveoperator with a fire extinguisher handy. The investigation was relatively simple in this case. Dismantling the joint showed that the cone ring hadbeen installedupside down, and a futile attempt made to seal the joint by over-tightening, destroying the sealing abil- ity of the system. A leak, even in a moderate pressure line carrying any gas will generate a charge of static electricity, and when the gas is flammable, the leak becomes self-igniting. More complex was a series of failures in a beverage can seamer, where themachine op- erates at a speed that makes direct observa- tion of the system in operation very difficult. The other problem was that, on failure, the speed of the machine effectively destroyed most of the components in the seamer head, leaving little evidence for the investigator to work with. It required the use of high-speed

with published images in the appropriate reference text-book, a method that will frequently lead directly up the garden path. The analyst needs to have a thorough under- standing of fracture mechanisms in order to determine which mechanism or mechanisms were responsible and what factors were involved in the initiation of failure. Sooner or later, those practicing the ‘com- parison’ method are sure to meet someone who works from first principles and under- stands fracture mechanisms, usually on the other side of the courtroom, and be made to look incompetent and foolish. Not to men- tion probably costing their client a great deal of money. Detailed knowledge of the normal opera- tion of the system and its history are neces- sary, which can then be compared with the fracture mechanism, leading towards what abnormal operation may have occurred. Let us consider a simple case, a leaking compression fitting on a small pipeline carry-

Tim J Carter After several years in private practice as a consulting engineer specialising in defect and failure analysis and materials selection, TimCarter joined ImpLabs inBenoni at the beginning of 2019. His career began in1962 as a trainee at Kirkstall Forge in Leeds, beforemoving toBradford University in 1966 as a technician. After graduating fromBrunel University in 1972, he joined the Research Division of High Duty Alloys in the UK, where he was employed as a metallurgist specialising in defect and failure analysis – testing airframe materials for the likes of Concorde and disk materials for the Rolls Royce RB-211 engine. He has since held positions as a part-time and honorary lecturer at the University of the Witwatersrand in the Department of Chemical and Metallurgical Engineering (2000-2006 and 2008-); a failure analyst and refurbishment manager for Executive Turbine in Lanseria (2003-2004); a specialist for the CSIR in the Failure Analysis Unit of the Manufacturing and Materials Technology Division (1994-2003); and head of metallurgy for Atlas Aircraft in Kempton Park (1988-1992). He has alsoworked for AECI and Iscor in South Africa andDarchem Engineering and Phoenix Tubeman in the UK. TimJ Carter has been a professional Member of the London-based Institute ofMaterials since 1984 andwas elected as a FellowMember in 2007. He is also registered as a chartered engineer with the Engineering Council in London. q

8 ¦ MechChem Africa • May 2019