International audienceThe effect of stress state and loading path on the ductile fracture of aluminum 2024-T351 is characterized through tension–torsion experiments on tubular specimens. The experimental program includes proportional and non-proportional loading paths leading to the onset of fracture at nearly plane stress conditions at stress triaxialities between 0 and 0.6. Stereo digital image correlation is used to measure the displacements and rotations applied to the specimen shoulders. An isotropic non-quadratic Hosford plasticity model with combined Voce–Swift hardening is used to obtain estimates of the local stress and strain fields within the specimen gage section. The hybrid experimental–numerical results indicate a higher strain to fracture for pure shear than for uniaxial tension. The calibration of a Hosford–Coulomb fracture initiation model suggests that the ductility of aluminum 2024-T351 decreases monotonically as a function of the stress triaxiality, whereas it is a non-symmetric convex function of the Lode angle parameter. It is shown that a simple non-linear damage accumulation rule can describe the effect of non-proportional loading on the strain to fracture

No abstract

International audienceThe concept of ratchetting strain as a crack driving force in controlling crack growth has previously been explored at Portsmouth using numerical approaches for nickel-based superalloys. In this paper, we report the first quantitative experimental evidence of near-tip strain ratchetting with cycles, as captured in situ by digital image correlation (DIC) technique on a compact tension specimen of stainless steel 316L, using both Stereo and SEM systems. The evolution of the near-tip strains with loading cycles was monitored whilst the crack tip was kept stationary. The strains normal to the crack plane were examined over selected distances from 6 to 57 lm to the crack tip for a number of cycles. The results show that strain ratchetting occurs with loading cycles, and is particularly evident close to the crack tip and under higher loads. 3D finite element models have also been developed to simulate the experiments and the results from the simulation are compared with those from the DIC measurements. This is the first time that near-tip strain ratchetting has been captured in situ at the peak loads during cyclic loading

International audienc

No abstract

No abstract

International audienceFatigue crack growth tests in mixed-mode II + III were performed on maraging steel and Ti-6Al- 4V. The 3D evolutions of the crack fronts -measured by SEM after interrupted tests- were analyzed, taking into account the reduction in effective crack driving force by the interlocking and friction of the asperities of the crack surface. Under small-scale yielding conditions, the mixed-mode crack growth rates were found to correlate best with Keff2 II + 1.2Keff2 III in maraging steel, while for Ti-6Al-4V, Keff2 II + 0.9Keff2 III appeared suitable. For extended plasticity, a crack growth prediction method is proposed and validated for Ti-6Al-4V. This method is based on elastic-plastic F.E. computations and application, ahead of each node of the crack front, of a shear-dominated fatigue criterion

International audienceUltrasonic fatigue tests (test frequency : 20kHz) and conventional tension-compression fatigue tests (10Hz) have been conducted on annealed and 10% pre-strained specimens of 0.13% carbon steel. Small holes were introduced on the specimen surface to investigate the effect of test frequency on small crack growth. The dynamic stress concentration factor and the stress intensity factor under ultrasonic fatigue tests were checked to be almost the same as those of conventional tension-compression fatigue tests. However, the fatigue properties were dependent on the test frequency. Ultrasonic fatigue tests showed longer fatigue life and lower fatigue crack growth rate for the annealed and 10% pre-strained specimens. Slip bands were scarce in the neighborhood of cracks under ultrasonic fatigue tests, while many slip bands were observed in a wide area around the crack under conventional fatigue tests. In order to explain the effect of test frequency on fatigue strength, dynamic compression tests with Split Hopkinson Bars were carried out. The stress level increases substantially with the strain rate. Thus, the increase in fatigue strength might be, to a large extent, due to a reduction in crack tip cyclic plasticity during ultrasonic fatigue tests