Jean Petit

Pic de relaxation associe aux interstitiels intrinseques dans le zirconium deforme a basse temperature

Resume Des mesures de frottement interieur effectuees a trois frequences suffisamment distinctes (environ 1, 16 et 350 Hz) ont permis de preciser les caracteristiques dun pic de relaxation observe sur le zirconium deforme a basse temperature et non porte au-dela de −68°C. Les valeurs obtenues pour lenergie dactivation et le facteur de frequence (respectivement 0,27 ± 0,02 eV et 10 13±0,8 sec −1 ), la faible largeur du pic permettant de le definir a laide de deux simples temps de relaxation, et le fait que la disparition de ce pic a −68°C ne puisse etre directement reliee a un mecanisme depinglage des dislocations, conduisent a proposer un mecanisme de relaxation faisant intervenir des defauts ponctuels, du type paire oxygene-auto-interstitiel, formes au cours de la deformation a basse temperature.

Near-threshold fatigue propagation of physically through-thickness short and long cracks in a low alloy steel

In this paper, the near-threshold fatigue behavior of physically through-thickness short cracks and of long cracks in a low alloy steel is investigated by experiments in ambient air.xa0Physically through-thickness short fatigue cracks are created by gradually removing the plastic wake of long cracks in compact tension specimens. The crack closure is systematically measured using the compliance variation technique with numerical data acquisition and filtering for accurate detection of the stress intensity factor (SIF) at the crack opening. Based on the experimental results, the nominal threshold SIF range is shown to be dependent on the crack length and the characteristic of the crack wake which is strongly dependent on the loading history. The effective threshold SIF range and the relation between the crack propagation rate and the effective SIF range after the crack closure correction are shown to be independent on crack length and loading history. The shielding effect of the crack closure is shown to be related to the wake length and load history. The effective threshold SIF range and the relationship between the crack growth rate and the effective SIF range appear to be unique for this material in ambient air. These properties can be considered as specific fatigue properties of the couple material/ambient air environment.

Fatigue Crack Propagation in Thin Wires of Ultra-High Strength Steel

Fatigue crack propagation is studied in thin wires of about 1 mm in diameter of an ultrahigh strength steel (σmax> 2400MPa) in ambient air and in air with controlled residual humidity. A specific equipment is developed based on an electro-dynamic testing machine equipped with an environmental chamber for air humidity control. Threshold tests are run using a load shedding procedure specifically adapted to the specimen size. The relation between load ratio and crack closure is evaluated from constant Kmax tests. The results are discussed on the basis of fracture surface observations and of existing modelling for environmentally assisted fatigue crack propagation.

Near threshold fatigue crack growth in ultrafinegrained copper

The near threshold fatigue crack growth in ultrafine-grained (UFG) copper at room temperature was studied in comparison to conventional coarse-grained (CG) copper. The fatigue crack growth rates da/dN in UFG copper were enhanced at ΔK ≤ 7 MPa√m compared to the CG material. The crack closure shielding, as evaluated using the compliance variation technique, was shown to explain these differences. The effective stress intensity factor amplitude AKeff appears to be the same driving force in both materials. Tests performed in high vacuum on UFG copper demonstrate the existence of a huge effect of environment with growth rates higher of about two orders of magnitude in air compared to high vacuum. This environmental effect on the crack path and the related microstructure is discussed on the basis of fractography observations performed using scanning electron microscope and completed with field emission scanning electron microscope combined with the focused ion beam technique.

3D Numerical Study on how the Local Effective Stress Intensity Factor Range Can Explain the Fatigue Crack Front Shape

The plasticity-induced crack closure of through-thickness cracks, artificially obtained from short cracks grown in CT specimens of 304L austenitic stainless steel, is numerically simulated using finite elements. Crack advance is incremented step by step, by applying constant ΔK amplitude so as to limit the loading history influence to that of crack length and crack wake. The calculation of the effective stress intensity factor range, ΔKeff, along curved shaped crack fronts simulating real crack fronts, are compared to calculation previously performed for through-thickness straight cracks. The results for the curved crack fronts support that the front curvature is associated to constant ΔKeff amplitude, thus assumed to be the propagation driving force of the crack all along its front.

Influence of crack front shape on 3D numerical modelling of plasticity- induced closure of short and long fatigue cracks

The simultaneous effect of crack length and crack front shape on plasticity-induced crack closure (PICC) for a 304L austenitic stainless steel is simulated through 3D numerical modelling using finite element software Abaqus for through-thickness cracks with different curved crack fronts in CT specimens in comparison with bidimensional through crack with a straight front. The influence of possible loading history effect is avoided by applying constant K amplitude. The local stress intensity factor range for crack opening Kop is evaluated from the simulation of the loss of the last local contact between the crack lips near the crack tip. The pertinence of the different crack front shapes is discussed in term of the effective stress intensity factor range Keff and in comparison with the experimental crack front observations.