Issue 30

M. N. James, Frattura ed Integrità Strutturale, 30 (2014) 293-303; DOI: 10.3221/IGF-ESIS.30.36 293 Focussed on: Fracture and Structural Integrity related Issues Fracture-Safe and Fatigue-Reliable Structures M. N. James University of Plymouth, School of Marine Science & Engineering, Plymouth, PL4 8AA, England Nelson Mandela Metropolitan University, Department of Mechanical Engineering, Port Elizabeth, South Africa mjames@plymouth.ac.uk A BSTRACT . Learning from history is, by popular account, something at which human beings are not particularly good; George Bernard Shaw having stated that “we learn from history that we learn nothing from history”, while the Spanish philosopher George Santayana apparently claimed that “those who cannot learn from history are doomed to repeat it”. 1 This is certainly true in the field of structural integrity where, some 150 years after the first full-scale structural fatigue tests were carried out, fracture-safe and fatigue-reliable design can be achieved to a statistical probability in complex and sophisticated structures, such as aircraft. Alongside this, however, failures of large, and expensive, welded structures can still occur from such simple causes as inadequate communication, and lack of awareness of the importance of the design of structural details to the overall fatigue life and failure. This paper considers several examples of such difficulties in the context of the development of fatigue design philosophies and the success or otherwise of learning from the history of failures. K EYWORDS . Structural failure; Fatigue-reliable; Fracture-safe; Fatigue design; Design failure. I NTRODUCTION ome 170 years after the term fatigue 2 was first coined and 150 years after the first full-scale structural fatigue tests were carried out, many large and expensive structures still experience very early fracture compared with their design fatigue lives. This paper will explore some of the reasons why this can occur with nominally well-designed and fabricated structures and draws some conclusions regarding the lessons to be learnt and how these might be disseminated to working engineers. This will be done using, as the main example, two large rotating shells with a replacement capital cost of some $20M, which should have been designed to provide a service fatigue life of 20 years, and which experienced severe shell fracture within 5-7 months of commencing service. This example is instructive, as at first sight it seems a fairly straightforward case of inadequate fatigue design, while on closer inspection it became clear that a number of interacting factors were involved which made a replace/repair decision 1 At least according to the web site age-of-the-sage.org, see the web page - http://www.age-of-the-sage.org/philosophy/history/learning_from_history.html 2 The origin of the term ‘fatigue’ is uncertain; some papers have ascribed the first use of the term fatigue to J. V. Poncelet in his book Introduction à la mécanique industrielle: physique ou expérimentale published in Paris in 1839 (or to lectures given by him around that time). This volume appears to rather discuss the work and fatigue of ‘living motors’, i.e. horses and human beings, rather than mechanical components under cyclic loads. Neither do the seminal papers by C. Hood [1] or W. J. M. Rankine [2] appear to use the term. S