[en] The damage mechanisms of a hypoeutectic Al-Si-Cu foundry alloy, obtained through die casting, have been studied. Observations were made during tensile tests using a tensile test machine that was inserted into an scanning electron microscope. The microstructure features that are critical for damage were determined. The cleavage fracture of intermetallic particles was found to be the main process that limited the alloy ductility. Shrinkage cavities mainly play a role in the final fracture stage; the reduction in cross-section area induced by shrinkage cavities was assumed to be the main contribution of these defects to the fracture of the specimens. Attempts were made to determine the cleavage stress that initiated the fracture of the particles. Three-dimensional finite element computations were performed considering particular locations at the free surface of the observed fractured particles. The maximum principal stress was calculated for different locations of the particle with reference to the free surface. Different shapes and different elastic behaviour were studied. It was shown that the influence of a free surface on the stress level in a particle depends on its shape. The results were compared with those found in literature concerning the fracture strength of particles in Al alloys

[en] An experimental study was carried out at 320 deg. C in order to determine the damage kinetics of a thermal aged duplex stainless steel. Compression and tensile tests were performed at 320 deg. C and at 20 deg. C for comparison. Typical steps of damage for this material were found at room temperature. At 320 deg. C, no bulk damage was observed, but strain and damage were localized. The experimental mechanical results revealed the existence of dynamic strain aging. The absence of bulk damage is linked to this phenomenon: as soon as damage appears in ferrite, there is localization and fracture follows

[en] Models allowing the prediction of the failure of structures by crack propagation were first introduced in the 50's using linear fracture mechanics whose principles were first proposed by Griffith (1920). This approach was extended to non linear cases (plasticity and visco-plasticity) in the 70's based on the work of Rice (J or C* integrals); it has been largely adopted by the industry. However this so called global approach cannot deal with all practical cases and cannot explain all experimental observations as, for instance, the warm pre-stress effect (WPS). The local approach to fracture, which relies on a fine analysis of strains, stresses and damage of highly solicited regions (cracks, notches...) of structures is an alternative which allows to solve problems encountered while applying the global approach. It has been developed since the 80's in particular in France. Important research efforts are currently undertaken in this field in Europe (France, Germany), United States and Japan. This book presents several aspects of the local approach to fracture: damage mechanisms, experimental techniques, damage evolution law and failure criteria, modelling of damage, numerical simulation. This work is the result of a collective work carried out by the best french specialists (Ecole des Mines de Paris, Ecole Centrale Paris, ENS Cachan, Universite de Louvain, INSA Lyon, ONERA, EDF). (authors)

[en] We propose an adaptive numerical strategy that aims at developing reliable and efficient numerical tools to model dynamic crack propagation and crack arrest. We use the cohesive zone theory as behavior of interface-type elements to model crack. Since the crack path is generally unknown beforehand, adaptive meshing is proposed to model the dynamic crack propagation. The dynamic study requires the development of specific solvers for time integration. As both geometry and finite element mesh of the studied structure evolve in time during transient analysis, the stability behavior of dynamic solver becomes a major concern. For this purpose, we use the space-time discontinuous Galerkin finite element method, well-known to provide a natural framework to manage meshes that evolve in time. As an important result, we prove that the space-time discontinuous Galerkin solver is unconditionally stable, when the dynamic crack propagation is modeled by the cohesive zone theory, which is highly non-linear. (authors)

[en] A comparative experimental and FEM study has been carried out, in order to investigate dynamic and constraint aspects of the Charpy test. Standard V-notch Charpy specimens were tested under dynamic and static loading conditions. 2-D plane strain and 3-D models were employed in numerical analysis. In order to incorporate strain-rate effects, an elastic-viscoplastic constitutive equation has been applied, based on actual test data obtained for a low-alloy structural steel. Fully dynamic analysis clearly indicated inertial effects. Modal analysis enabled the confirmation of the origin of the oscillations on the load-displacement curve as beam vibration of the specimen resulting from interaction with the elastic striker. (orig.)

[en] The study of the fracture of a low alloy steel in the domain of brittle-ductile transition has permitted us to highlight the increasing presence (along with the stress temperature) of clusters of second phase inclusion on fracture areas. It is shown, through finite element simulations, that these clusters play a secondary role in the initiation of the cleavage process but their influence on the propagation of the ductility fracture appears to be important. The anisotropy of the strength impact can be explained by taking into account the geometry of inclusions and their spatial distribution. This study is based on simulations and on a series of impact testing performed on 16MND5 (A508-Cl.3) steel samples, this steel is used to design reactor vessels. (A.C.)

[en] In order to understand the toughness of nodular cast iron, the flow stress have been studied numerically by using Gurson's model considering that nodular cast iron can be regarded as a porous material. The damaged zone ahead of the crack tip is studied by SEM observations on the pre-polished surface of a CT 25 specimen, before and after ductile tearing. It is shown that the damaged zone is very large in the GGG 40 nodular cast iron (through almost the whole ligament ahead of the crack), so the linear elastic fracture mechanics is not valid in nodular cast iron for small specimens. The force versus crack opening displacement curves calculated in 2D for plane strain and plane stress conditions give respectively an upper and a lower limit as compared with the experimental curve. Only 3D calculations lead to a good fit with the experimental curve. This result shows that the stress state ahead of the crack tip of a nodular cast iron is intermediate between plane stress and plane strain conditions, due to the high density of voids inside the damaged material. (orig.)

[en] Charpy V-notch impact testing is widely used in the toughness assessment of large forged components, e.g. the pressure vessel for pressurised water reactors (PWR). At low temperature, A508 Cl.3 nuclear pressure vessel steel fails by cleavage fracture. The results reported here are part of both an experimental program and numerical investigations which aim at the establishment of a non-empirical relationship between the lower shelf Charpy V-notch energy, CVN, and the fracture toughness, K_Ic, of this material. Here, the applicability of the Beremin cleavage fracture model to the Charpy specimen is demonstrated. (orig.)