Suzanne Degallaix

Cyclic plasticity of a duplex stainless steel under non-proportional loading

The low-cycle fatigue behavior of a duplex stainless steel, 60 % ferrite - 40 % austenite, is studied under tension-compression/torsion loading at room temperature. The influences of loading direction and loading path are analyzed. It is shown that the duplex stainless steel has an isotropic behavior under tension-torsion loading in monotonic as well as in cyclic conditions. The loading path induces an over-hardening on cyclic hardening of duplex stainless steel, but lower than the one on austenitic stainless steels. The effect of loading history is studied in terms of strain amplitude, mean strain and loading path. It is shown that only histories in strain amplitude and loading path have a small effect on the stabilized stress.

Analysis of the hysteresis loop in stainless steels. I. Austenitic and ferritic steels

In constant amplitude cyclic straining the hysteresis loops of an austenitic and a ferritic steel were recorded and their shape was analysed. The generalised statistical theory of the hysteresis loop based on the Masing hypothesis that considers the effective stress and the distribution of the critical internal stresses of the elementary volumes was employed. The second derivative of the hysteresis half-loop yields information on the effective stress and the probability density function of the critical internal stresses and its evolution in cyclic straining and in dependence on the strain amplitude. The changes of the effective modulus are also reported. The total cyclic stress can be separated into the effective and internal stress components. The high effective stress in ferritic steel is connected with the difficult motion of screw dislocations in the b.c.c. structure. The evolution of the probability density function of the austenitic and ferritic steels during cyclic straining is discussed in terms of changes of the internal dislocation arrangement.

Microstructure in 316LN stainless steel fatigued at low temperature

Abstract The internal structure of AISI 316LN austenitic stainless steel cyclically strained at liquid nitrogen temperature has been studied using transmission electron microscopy and electron diffraction. High amplitude cyclic straining promotes the transformation of austenite with face centred cubic (f.c.c.) structure into e-martensite with hexagonal close packed (h.c.p.) structure and α′-martensite with distorted base centred cubic (b.c.c.) structure. Thin plates containing e-martensite were identified in all grains. α′-martensite nucleates at the intersection of the plates in grains with two or more systems of plates and can grow in the bands. The orientation of transformed phases follows the Shoji–Nichiyama and Kurdjumov–Sachs relations. Mechanisms of low temperature cyclic straining are discussed.

Effective and internal stresses in cyclic straining of 316 stainless steel

The hysteresis loop of a 316 austenitic stainless steel was recorded and analysed in order to separate the contributions of the effective and internal stresses. The difficulties of application of the Kuhlmann-Wilsdorf-Laird method are discussed and a new method based on the statistical approach to analysis of the hysteresis loop (SAP method) is proposed. In contrast to the Kuhlmann-Wilsdorf-Laird method, this method allows evaluation of the effective elastic moduli and the effective stresses in tension and in compression without ambiguity. The SAP method was applied to cyclic loading of 316 austenitic stainless steel and the effective elastic moduli and effective and internal stress components of the cyclic stress in cyclic loading and their dependence on the number of cycles and on the applied strain amplitude was studied. The effective elastic moduli decrease appreciably with the strain amplitude. The effective stress represents an important fraction of the total stress (approximately 30%) and for most of the life it is constant. The hardening-softening curves and the cyclic stress-strain curve are determined mostly by the internal stress.

Analysis of the hysteresis loop in stainless steels II. Austenitic–ferritic duplex steel and the effect of nitrogen

Abstract The hysteresis loop of austenitic and austenitic–ferritic duplex stainless steels, both with different levels of nitrogen, was analyzed using a generalised statistical theory. The plot of the second derivative yields information on the average effective stress and the probability density function of the critical internal stresses, its evolution in cyclic straining and its dependence on the strain amplitude. The probability density function in duplex steels has two peaks, which correspond to the two-phase structure. The position of these peaks plotted versus the fictive stress corresponds very well to the position of the peaks in the individual phases. The changes of the effective moduli and the evolution of the effective and internal components of the cyclic stress are reported. Nitrogen takes a strong effect on the probability density function of the critical internal stresses in austenitic steel. The suppression of the second peak formed in high amplitude straining is discussed in terms of the appearance of planar dislocation structures, which develop due to the presence of nitrogen.

Fatigue of glass/epoxy composite in three-point-bending with predominant shearing

Unidirectional and multilayer composites are usually subjected to bending, and often fail in fatigue by interlaminar shearing. This work studies new experimental conditions in fatigue, using a three-point-bending test with predominant shearing, while tensile/compressive stresses are significant. The experimental device geometry is validated under monotonic loading by comparison between finite element results and experimental observations. The deflection amplitude vs. fatigue life curve is used as a pseudo-Wohler curve. An approximate drawing of maximum deformation vs. fatigue life curve is proposed using Fourniers model. Experimental results in fatigue are well accounted for by this model.

Experimental and numerical study of the low-cycle fatigue behaviour of a cast metal matrix composite Al–SiCp

The low-cycle fatigue behaviour of a cast metal matrix composite has been studied experimentally and numerically. Tension–compression cyclic tests have been carried out in situ in a scanning electron microscope to observe damage mechanisms of a AS10U3NG alloy based composite reinforced with 10 vol.% SiCp. Micro-cracks always initiate on the specimen surface, on the biggest casting defects. Then the small cracks mainly propagate in the eutectic zone by precipitate fractures or by either particle/dendrite or particle/precipitate debondings. One of these cracks becomes the main crack. When the main crack reaches a ‘critical length’, it tends to grow in mode I through the dendrites and by particle fracture in front of the crack. As far as the numerical study is concerned, a finite element analysis models the composite behaviour under tension–compression cyclic loading, before crack initiation, until a stabilised cycle is reached. Numerical and experimental stress–strain hysteresis loops are compared. Numerical and experimental cyclic stress–strain curves are in a good agreement.

On the analysis of the hysteresis loop of ferritic steel in cyclic straining

Abstract Stress–strain hysteresis loops of a ferritic stainless steel during cyclic straining with increasing strain amplitudes were recorded and the shapes of the hysteresis loops were analyzed. An extra strain around zero stress as a result of magnetoelastic effect was identified. Its effect on the determination of the effective and internal stresses in cyclic straining is discussed.

Cyclic behaviour of a duplex stainless steel under multiaxial loading: Experiments and modelling

Abstract The low-cycle fatigue behaviour of a duplex stainless steel, 60 % ferrite — 40 % austenite, is studied under tension-compression/torsion loading at room temperature. It is shown that the duplex stainless steel has an isotropic behaviour under cyclic proportional loading. The non-proportional loading paths induce an extra-hardening, but lower on duplex stainless steel than on austenitic stainless steels. Three models able to account for the extra-hardening are identified and tested on the experimental data base. Two of them give accurate predictions.

Internal and effective stress analysis in stainless steels using the statistical approach method

Abstract The statistical approach method was applied to the analysis of the hysteresis loops in low-cycle fatigue of stainless steels with different crystalline structures. This method, based on Masing hypotheses, allows to determine the effective stress, the effective elastic modulus and the ‘probability density function’ of the internal critical stresses. The dependence of these quantities on the applied strain amplitude was investigated in austenitic, ferritic and duplex stainless steels. The ‘probability density function’ in austenitic stainless steel exhibits a single peak. It changes appreciably with strain amplitude. This dependence on the strain amplitude was less important in the ferritic stainless steel. A double peak form was found on the ‘probability density function’ in duplex stainless steel, which is believed to correspond to the plastic deformations of the two phases, γ and α, respectively.