numero25

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 25 (2013) 138-144; DOI: 10.3221/IGF-ESIS.25.20 138 Special Issue: Characterization of Crack Tip Stress Field Initiation and growth behavior of very-long microstructurally short fatigue cracks P. Lorenzino, A. Navarro Escuela Superior de Ingenieros, Universidad de Sevilla. Camino de los Descubrimientos s/n, 41092, Seville, Spain. A BSTRACT . The present paper describes a novel experimental technique recently presented that allows one to study interactions between the crack and microstructural barriers with an unprecedented level of ease and detail. The method consists in increasing the grain size of Al1050 Aluminium alloy until the centimetre scale by applying a series of mechanical and heat treatments. Once the thermo-mechanical treatment is completed and the desired microstructure obtained, a circular notch is machined on each specimen, and the samples are subjected to push-pull fatigue loading. Several combinations of notch and microstructural sizes have been tested. This method provides an easy way to record and analyse the effect of the microstructure upon crack growth rate. It was observed that the space between successive crack-tip arrests correlates well with the material grain size. Another interesting observation is that in the majority of the cases studied the cracks did not initiate at the point of maximum stress concentration. This is surprising since the classical methods of notched fatigue limit analysis clearly indicate the horizontal symmetry axis as the initiation and propagation direction for push- pull loading. K EYWORDS . Short crack; Microstructural barrier; Grain size; Notch; Fatigue crack initiation; Fatigue limit; Crack growth rate. I NTRODUCTION he topic of this conference is the characterization of the crack tip stress and strain fields. The inauspicious beginnings of the characterization of fatigue crack growth rate of long cracks by the stress intensity factor range as described in the Paris law is a well-known episode in fracture mechanics lore. There, a single parameter embodies the physics of the problem and crack growth rate can be expressed as a unique function of ∆K I irrespective of the geometry of the component or the type of load applied. But Paris law breaks down for small cracks. Microstructurally short fatigue crack growth is a prime example of a situation where a single parameter characterization of the crack stress and strain fields does not seem to be entirely appropriate. It is a very important problem from the practical point of view, for there are many situations where the life of the component is “decided” when the crack length is of the order of the grain diameter. This is particularly the case in long-cycle fatigue problems when loading close to the fatigue limit. It is now well established that the fatigue limit in metallic materials is really a threshold condition for the propagation of very small cracks. Experimental evidence has shown that, in plain specimens subject to uniaxial loads, cracks form on persistent slip bands (PSBs) and grow along them. At stresses below the fatigue limit, cracks start growing fast but then they stop and become non-propagating. At stresses just above the fatigue limit, cracks decelerate and may temporarily halt a number of times, but they do not stop growing altogether. Later on they accelerate and finally reach a regime of apparent continuous propagation. Microscopic observations have identified the locations of minimum crack growth as microstructural barriers to slip propagation such as grain or phase boundaries. The crack decelerates on T