Issue 30

M. N. James, Frattura ed Integrità Strutturale, 30 (2014) 293-303; DOI: 10.3221/IGF-ESIS.30.36 299  This procedure requires fracture mechanics based crack growth rate data and a K-calibration for the region of interest in the structure or component, plus knowledge of an appropriate fracture toughness parameter. FEA may be necessary to establish the stresses present in the region of interest  Extensive full-scale structural testing as well as specimen testing may be necessary to assure structural integrity assessment procedures This type of assessment may represent around 5-12% of the capital cost of a sophisticated structure, which is likely to be cost-effective when considering that the percentage of failures associated with crack or fracture problems remains rather high in both engineering structures and in aircraft [17]. Table 1 presents information from reference 17 on frequency of occurrence of these failure mechanisms. Percentage of Failures Structures Aircraft Corrosion 29 16 Fatigue 25 55 Brittle fracture 16 - Overload 11 14 High temperature corrosion 7 2 SCC/corrosion fatigue/HE 6 7 Creep 3 - Wear/abrasion/erosion 3 6 Table 1 : Frequency of failure mechanisms in structures and aircraft [17]. In this table, HE represents hydrogen embrittlement and SCC stands for stress corrosion cracking. The complexity of structural testing on a modern airliner is indicated in data provided in the Boeing Airliner magazine [18] which indicates that one hundred hydraulic actuators simultaneously apply flight spectrum loads, measured via 4,300 strain gauges, to a complete 777 airframe. The effectiveness of defect-tolerant design, coupled with this type of testing programme, in reducing the maintenance costs of aircraft has been discussed in a paper by Goranson [19] in terms of labour hours expended on maintenance as a function of aircraft type and manufacturing production line position. ‘Maintenance labour hours per aircraft’ is a measure of the effectiveness of design improvements resulting from the development and application of durability standards [19]. Reference 19 demonstrates significant order of magnitude improvements in maintenance labour costs between first and second generation wide and standard body aircraft. L EARNING FROM HISTORY : SUCCESS AND FAILURE n enormous amount of literature has been published on fatigue design, the application of fracture mechanics to structural design and on failure analysis, and the reliability achieved in the key industrial sectors of transport, energy extraction and generation, and pressure vessels is truly outstanding. The 2011 edition of Injury Facts published by the US National Safety Council [20] indicates the average death rate over the ten year period from 1999 to 2008 was 0.01 per 100,000,000 passenger miles for scheduled airlines and 0.05 for rail and buses. In other industrial sectors, particularly where structures are fabricated by welding, there remains a greater potential for cracking problems to occur through fatigue or fracture. Reasons for this higher incidence of cracking-related problems include:  Insufficient attention to environmental influences  Lack of awareness of the importance of detail design to the fatigue performance of major structures (geometry and fabrication practice)  Inadequate communication between the various parties involved in major design projects (end user, fabricator, design team and process owner)  Insufficient knowledge of the requirements of, and limitations inherent in, current fatigue design codes The remainder of this paper will briefly present some case study examples of the type of problems that occur and their underlying root causes which, very often, include lack of communication and/or inadequate knowledge of the interactions A