Stress-Intensity Factors and fatigue growth for surface cracks

The structural safety of metallic components, such as bars, plates, pipes and shells, should be assessed by also taking into account the influence of defects, which can greatly affect the reliability of these components, particularly when they are subject to cyclic loading. During last decades, the problem of cracked solids has been extensively analised by the Authors, by considering: a)round bars, such as bolts, screws, pins, reinforcements, tendons for prestressed concrete structures, supports in suspension and stayed bridges; b) double-curvature shells (having to distinct principal curvature radii) such as pressure vessels, pipe elbows and fuel tanks. As has experimentally been observed, elliptical-arc surface cracks may grow through the crosssections or the thicknesses of the above components, reducing their strength and provoking catastrophic failures. Therefore, accurate stress analyses have to be performed. Assuming a linear elastic behaviour of the material, the stress field near the crack front can be represented by the stress-intensity factor (SIF). The calculation of the SIF along the front of both almond and sickle-shaped surface cracks can be carried out through three-dimensional finite element analyses together with the one-quarter point displacement method. Different Mode I loading conditions are analysed: tension, bending, rotary bending, eccentric axial loading. Further, elementary stress distributions acting on the crack faces have been analysed, and the related results have been used to approximate evaluations of the SIF under complex stresses (for instance thermal and residual stresses), by also employing the superposition principle and the power series expansion of such complex stresses. The above structural components frequently present geometrical discontinuities (as changes in crosssection sizes, fillets, ribbers, etc) or are obtained by assembling different parts jointed together through welding processes. Such geometrical variations, often modelled by means of notches, are preferential sites for crack nucleation, due to the stress concentration. Since the stress field can be quite different from that found out in the unnotched structural components previously introduced with an identical surface flaw, the SIF is computed by taking into account the notch effect, for both semi-circular and V-shaped notches. An elliptical-arc part through crack is then assumed to be located at the notch root. The problem of a flawed solid becomes more complex in the case of simultaneous presence of more than one crack: as a matter of fact, the cracks could interact and give rise to SIF values higher than those experienced by the body with only one crack. Such a case has been studied by the Authors for a plate containing twin coplanar flaws. Finally, the fatigue behaviour is evaluated using the above SIF results. In particular, crack paths, crack growth rate curves and crack front evolutions are determined through a numerical procedure, based a two-parameter theoretical model proposed by the first Author. The results determined by the present authors are compared with those available in the literature.