Issue 30

M. N. James, Frattura ed Integrità Strutturale, 30 (2014) 293-303; DOI: 10.3221/IGF-ESIS.30.36 297 understanding of the response of materials to loading and by the development of fracture mechanics, which is the branch of applied mechanics dealing the behaviour of cracked bodies. Fracture control and fatigue design strategies have developed alongside greatly enhanced analytical, numerical and experimental techniques to understand and characterise the response and behaviour of components and structures subject to applied loads and displacements. A brief outline of the historical development of these fracture control strategies is also given, as a precursor to considering the successes and failures in “learning from history”. Safe-life design This philosophy is underpinned firstly, by experimental testing to establish the stress-life or strain-life (S-N) curve for the material under conditions that match those anticipated in service. As noted above, the first data of this type was reported by Wöhler who was the Locomotive Superintendent of the Royal Lower Silesian Railways in the second half of the 19th century [3, 4]. The second important requirement is the application of a so-called ‘factor of safety’ to ensure that the design level of stress or strain experienced by the component or structure is significantly less than the value from the S-N curve corresponding to the required cyclic fatigue life. This parameter is actually a ‘factor of uncertainty’ reflecting imprecise data on:  Material properties and condition (e.g. heat treatment, notches, surface damage)  Service environment and conditions  Applied loads and displacements Early structural testing of fatigue loaded structures often relied on simply factoring the measured yield, proof or tensile strength of the material, e.g. by loading the wings of wooden framed aircraft with bags of lead shot until fracture occurred. Interestingly, this technique is still widely applied to the wings of homebuilt aircraft, and it is worth noting that this simple approach works best with composite structural materials, such as wood, where the fatigue strength can be a high percentage (≈70-80%) of the tensile strength at lives ~10 6 cycles [9]. Ferritic alloys tend to show a ‘fatigue limit’ stress where the sustainable stress becomes asymptotic with the x -axis for lives typically greater than about 2 x 10 6 cycles. This is related to pinning of dislocations in the crystal lattice by interstitial atoms, such as carbon, nitrogen or boron. Nonferrous alloys do not exhibit this fatigue limit behaviour and it is usual to refer to an ‘endurance limit’ stress for these alloys that is applicable to a specific cyclic life. The S-N approach is still widely and successfully used on such important components as crankshafts and connecting rods. The key requirement is that the conditions used in fatigue testing closely match those of the component in service. It is now routine practice in several industries to use computer-controlled testing applying a load spectrum that replicates the measured service load spectrum. Failures can still occur when there has been:  Incorrect assessment of: o Service loads (including incorrect assessment of different load state contributions) o Environment o Vibration or resonant frequency problems  Implementation of new materials/technology with a priori unforeseen or unknowable consequences  Unanticipated changes in: o Usage (loads or type of service duty) o Environment (corrosion and temperature) o Surface condition (including inadvertent damage)  Human error o Inadequate communication amongst relevant parties o Inadequate fabrication/machining o Inadequate inspection/maintenance Fail-safe design This uses the same basic philosophy as safe-life design but takes account of the possibility of cracking of critical components. These are defined as parts whose failure would lead to catastrophic loss of the structure, e.g. wing or tail plane spars in an aircraft. Widespread manufacture and operation of aircraft with metallic structures, and a consequent relatively high number of failures was the main driving force behind this advance. The new developments included:  Redundant load paths in critical areas which can redistribute the load in the event of a partial failure of the structure and/or act as crack arrestors