Issue 30

M. N. James, Frattura ed Integrità Strutturale, 30 (2014) 293-303; DOI: 10.3221/IGF-ESIS.30.36 294 rather difficult and which complicated the design of a replacement. These factors mainly derived from inadequate communication between the various parties, and included:  Imprecise specification of the operating conditions and design codes by the owner of the process technology;  Internal fixtures in high stress areas that caused high local stress concentrations and which were, in the main, eventually found to be superfluous;  Inadequate consideration of the variable amplitude and biaxial stresses arising from ovality induced in the shell during rotation;  Lack of a detailed finite element analysis of the internal fixtures;  Unauthorised alloy substitution for some parts of the internal design fixtures that led to an increased chance of weld defects, especially hydrogen cracking;  Poor weld documentation and poorly controlled preheat, which led to increased residual stresses and weld defects;  Inadequate consideration of the maintenance requirements in terms of the duty cycle of the composters, which led to excessive erosion of the shell occurring. The ultimate outcome from these interacting aspects of the failure was the instigation of legal proceedings that continued for some 5 years and cost a similar amount to the original capital cost, by which time repair of the structures had been made impossible through shell erosion. All of these issues could have been easily avoided had the various parties understood and shared more information regarding the requirements of the process for which the structures were designed, fatigue cracking, weld design and the influences of a corrosive environment. The paper will also discuss the historical background to the development of fracture-safe and fatigue-reliable structures to set in context the main tenet of the paper, namely that learning from history is extremely good in certain industrial sectors and surprisingly poor in others. H ISTORICAL BACKGROUND TO FATIGUE ailure of components and structures under varying or cyclic loading patterns has been discussed in the literature since at least 1843, when Rankine delivered a paper to the Institution of Civil Engineers in London entitled “On the causes of the unexpected breakage of the journals of railway axles; and on the means of preventing such accidents by observing the law of continuity in their construction” [2]. This was one of the first recorded papers which used the term “gradual deterioration” to describe failures of rotating or reciprocating components that occurred under cyclic loading after a period of time in service. Subsequently, in a series of papers between 1858 and 1870 Wöhler established several fundamental principles of the safe-life, or S-N, approach to fatigue design [e.g. 3, 4]. These included acquiring S-N data from full-scale tests of railway axles (reference 3, whose title translates as “Report about the experiments, which have been carried out with instruments to measure the tension and the torsion of the axles of carriages of the Koeniglichen Niederschlesischen-Maerkischen railway during travel”), and determining the effect of a notch on fatigue life (reference 4, whose title is “About experiments concerning the cohesiveness of iron and steel”). Wöhler observed the occurrence of a fatigue limit stress for steels at lives > 5 x 10 5 cycles of loading and also explored the effect of a notch on the S-N curve. The S-N philosophy is still widely used today for component fatigue design, albeit with a much more sophisticated understanding of interactions among the service environment (temperature, chemistry, loads and displacements), materials (composition, processing and microstructure), and their static and dynamic performance. Fig. 1 shows some of the fatigue data reported by Wöhler in reference [4]. Early examples of failure by fatigue and fracture were strongly influenced by the low metallurgical quality of cast iron and puddled or wrought steel [5]; the puddling furnace was an open-hearth metal-making technology used to create wrought iron or steel from the pig iron produced in a blast furnace. The major advantage of a puddling furnace was in keeping the impurities of the fuel separated from the pig iron charge. Puddled steel still contained, however, a high level of elongated manganese sulphide (MnS) inclusions (which are then crack-like defects) as shown below in Fig. 2 and 3. Early steels also exhibited low notched toughness and a high ductile-to-brittle transition temperature, meaning that failure occurred in an apparently brittle (rock candy) fashion in the presence of relatively small fatigue cracks or crack-like defects even at moderate temperatures (e.g. as illustrated in Fig. 4). This observation of apparently brittle fracture in an alloy that showed ductile tensile behaviour led to a longstanding controversy over the mechanism of fatigue cracking; the erroneous crystallinity theory advanced by Hood [1] (the concept that after a certain number of load repetitions the material became F