Strain field measurements by digital image correlation today offer new possibilities for analysing the mechanical behaviour of materials in situ during mechanical tests. The originality of the present study is to use this technique on the micro-structural scale, in order to understand and to obtain quantitative values of the fatigue surface damage in a two-phased alloy. In this paper, low-cycle fatigue damage micromechanisms in an austenitic-ferritic stainless steel are studied. Surface damage is observed in real time, with an in situ microscopic device, during a low-cycle fatigue test performed at room temperature. Surface displacement and strain fields are calculated using digital image correlation from images taken during cycling. A detailed analysis of optical images and strain fields measured enables us to follow precisely the evolution of surface strain fields and the damage micromechanisms. Firstly, strain heterogeneities are observed in austenitic grains. Initially, the austenitic phase accommodates the cyclic plastic strain and is then followed by the ferritic phase. Microcrack initiation takes place at the ferrite/ferrite grain boundaries. Microcracks propagate to the neighbouring austenitic grains following the slip markings. Displacement and strain gradients indicate probable microcrack initiation sites

This first part aims to study the correlation between the cyclic softening observed during low-cycle fatigue in a 2507 duplex stainless steel (DSS) alloyed with 0.17% N and the dislocation structure development. Tension-compression and tension-compression- torsion fatigue tests were carried out under total strain control at Δεteq/2 = 0.5%. The dislocation structures were characterized by transmission electron microscopy during cycling with interrupted tests. The austenite showed planar arrangements of dislocations during all the fatigue life, the ferrite showed evolution from a homogeneous distribution of dislocations to a structure of dislocation bundles in tension-compression, and to a refined wall structure in tension-compression-torsion. It is proposed that the evolution of the dislocation structure in the ferrite plays an important role during the cyclic softening in the DSS.

No abstract