Effet de l'ecrouissage sur la propagation d'une fissure a partir d'un trou charge dans des alliages d'aluminium 2024-T351 et 7475-T7351

The cold working process applied to a hole of a joint assembly is a simple and low cost technique currently used in aerospace industry to delay the crack initiation emanating from the hole and to slow down its propagation. In this study, the influence of this process on the fatigue propagation life of a hole transmitting a full load in double shear is investigated. Plate specimens of two aluminium alloys (Al 2024-T351 and Al 7475-T7351) containing a hole with an edge margin (ratio between the distance from the center of the hole to the edge of the plate and the hole diameter) of 2.4 are considered. The level of cold working applied to the hole is 3.58%.; This study is composed of three parts: experimental program for investigating the crack extension under cyclic loading, numerical analysis for determining the residual stresses due to cold working process and fatigue propagation life calculations using NASGRO software.; For the first part, a fatigue propagation test program has been conducted on specimens having a non cold-worked hole or a cold-worked hole. For each specimen, an initial corner flaw was done at the hole edge before starting the test and tests were carried out under a cyclic load with a constant amplitude and a load ratio Rsigma (minimum cyclic load/maximum cyclic load) of 0.05. Three different loads were considered, resulting in three nominal stress levels: 87, 116 and 145 MPa (12.6, 16.8 and 21.0 ksi). During a fatigue test, the load transfer was achieved at the hole edge and the crack length was monitored by means of a video camera system. Concerning the second part, a finite element analysis was performed to establish the residual stress fields resulting from the cold working and, then, the stress fields of the cold-worked hole subjected to a cyclic loading. These results were then used as a data input for the software intended for calculating the fatigue propagation life. Finally, for the last part, a crack propagation software (NASGRO 5.0) incorporating several known models in the field of material fatigue behaviour was used to compute the crack propagation lives taking into account the stress fields obtained from the finite elements analysis.; The test results have shown that, for a given nominal cyclic stress, the Al 2024-T351 aluminium alloy specimens have a propagation life larger than those of Al 7475-T7351 aluminium alloy, for both non cold worked and cold worked holes. For both alloys, the life improvement factor (ratio between the life of specimen with a cold worked hole and that with a non cold worked hole) resulting from cold working effect depends upon the applied stress; it decreases with an increase of the stress level. The life improvement factor for the first alloy ranges between 2.5 and 14.9 in comparison to 2.7 to 17.4 for the second alloy. In terms of the normalized stress (ratio between the nominal stress and the ultimate tensile stress), the life improvement factor for both alloys may be reasonably represented by a single equation.; Concerning the characteristics of the ruptured surface of the specimens, in all cases the crack extension is of the diametral type (two crack fronts in the ligaments on both sides of the hole). The cracked surfaces have shown that, in all cases, final failure occurred first by cleavage (brittle fracture) at one of the crack fronts, and then followed by ductile fracture (tension and bending) of the other crack front.; The results obtained from finite elements analysis have indicated that a cold working process of 3.58% applied to a hole generates a compressive residual stress near the hole edge; its intensity (in absolute value) reaches a maximum value (typically, 427 MPa for Al 2024-T351 and 445 MPa for Al 7475-T7351) at a location about 1.0 mm from the hole edge at the mid-thickness plane of the specimen. This stress has its intensity decreased farther from the hole edge and becomes in tension at a location of about 3.0 mm (from the hole edge), reaches a ma...