Issue 30

M. N. James, Frattura ed Integrità Strutturale, 30 (2014) 293-303; DOI: 10.3221/IGF-ESIS.30.36 296 Figure 3 : Confocal laser scanning microscope image of a sharp crack-like MnS inclusion in puddled steel. Figure 4 : Example of brittle intergranular (rock candy) fracture during fatigue loading in cast steel. The particular example shown is due to aluminium nitride embrittlement. F RACTURE CONTROL AND FATIGUE DESIGN STRATEGIES atigue is a process of crack initiation and growth by microplasticity in metallic alloys or, more generally, a nonlinear constitutive stress-strain (σ-ε) response leading to hysteresis in the cyclic σ-ε curve and an associated input of energy into the material via damage mechanisms (in the case of metallic alloys this is the movement and interaction of dislocations in the crystal lattice). Fracture is sudden, catastrophic collapse under either an applied static load (certain cases), a steadily increasing load or as the final stage in the fatigue process. The defining characteristics of these two phenomena are often that:  Applied loads are much less than general yield (i.e. plasticity is highly localised to cracks or sharp defects)  There is little prior evidence of imminent failure, i.e. the overall structure is still behaving in a linear elastic fashion. In this context, however, it should be noted that there are now standardised processes for structural design which explicitly consider the possibility of failure by the conjoint mechanisms of plastic collapse and fast fracture [e.g. 8]. This two-parameter approach to fracture control uses a Failure Assessment Diagram (FAD), and has been driven by the use of high toughness metallic alloys often operating at high temperatures. It has been made possible through a detailed F