Catherine Verdu

Damage assessment in an Al/SiC composite during monotonic tensile tests using synchrotron X-ray microtomography

Abstract High resolution X-ray tomography is used to study the evolution of damage in an Al/SiC composite during monotonie tensile tests at room temperature. Two main damage mechanisms are observed when the plastic regime is reached: (i) The cracking of the matrix on brittle oxides resulting from the processing of the material; (ii) the cracking of SiC particles. The aspect ratio of the broken particles along the tensile direction is found to be high and the damage accumulation rate is different at the surface and in the bulk of the material. Those results are discussed with respect to the resolution of the imaging technique and to the strain level reached during the tests.

Fatigue behaviour of a nickel alloyed sintered steel

Abstract The relationship between microstructure and microscopic damage mechanisms of a powder metallurgy steel submitted to cyclic stresses was studied. Samples were prepared from Distaloy AE™ (Fe–4wt.%Ni–1.5wt.%Cu–0.5wt.%Mo) mixed with natural graphite (0.8 wt.%). The materials average density is about 7.40 kg m −3 . Porosity and phases of the initial microstructure were characterised. Fatigue tests were carried out at R =0.1 both on unnotched and notched specimens. During the tests, the details of damage initiation and crack propagation were surveyed by light or scanning electron microscopy until the material failed. Slow crack growth was studied in detail. Two propagation modes were identified. First, the crack propagates in the I mode, then the crack forks off to preferentially follow the sintered bridges. The crack growth rate law of each mode was determined. The change of the crack propagation path was linked to the formation of secondary microcracks in the sintered necks during cycling. These observations and fractographic analysis show that damage mechanisms strongly depend on the microstructure. In particular, the presence of inclusions, the network of interconnected pores and the austenitic sintered bridges appear to be critical parameters.

Damage mechanisms of a nickel alloyed sintered steel during tensile tests

Abstract Microscopic damage mechanisms of a sintered steel submitted to tensile stresses were studied. Specimens were prepared from diffusion alloyed Distaloy AE ™ (Fe-4wt%Ni-1.5wt%Cu-0.5wt%Mo) mixed with natural graphite (0,8 wt%). The material average density is about 7.40. The initial microstructure was characterised both from phases (nature, quantity…) and porosity (volume fraction, size, aspect ratio, shape factor…) point of views. Monotonic in-situ tensile tests were carried out. The details of damage initiation, crack propagation and specimen failure were surveyed by light or scanning electron microscopy during the tests. Those observations and the fractographic analysis show that damage mechanisms are strongly dependant on the microstructure. Namely, the pore characteristics and their localisation in the material microstructure appear to be critical parameters.

Study of the damage mechanisms in an OSPREY™ Al alloy-SiCp composite by scanning electron microscope in situ tensile tests

Abstract Damage mechanisms in a 7049 Al alloy + 15% SiC p metal matrix composite were studied qualitatively and quantitatively by in situ tensile tests in a scanning electron microscope with gold microgrids deposited onto the surface of the specimen. The first damage mechanisms were found to be rupture of the most elongated particles and, in smaller proportion, decohesion of the particle-matrix interface. A high aspect ratio, large size and low local volume fraction of particles appeared to increase the cracking probability. An Eshelby iterative method modified to account for the elastoplastic behaviour of the matrix was used to calculate the stress field induced by the thermomechanical treatment and mechanical loading of the composite. Knowledge of the statistical characteristics of the damaged particles permitted estimation of the critical stress for the two observed damage initiation mechanisms. In the case of particle cracking this stress depends on the particle size.

Fatigue crack growth characterization and simulation of a porous steel

The cyclic mechanical behaviour of a high strength sintered steel was investigated. During the fatigue tests on notched specimens, the details of the crack propagation were surveyed by light or scanning electron microscopy. Those observations and the fractographic analysis showed that the network of pores and the nature of the sintering bridges appear to be critical parameters. Furthermore, two crack growth steps were identified: one step in mode I and an another step in which the crack forks off. The fatigue crack growth was simulated by a method based on linear elastic mechanics taking into account the microstructural features of the material. This simulation was used to analyse the possibilities of crack bifurcation. Sintered neck microcracks in front of the main crack were introduced. The influence of pores characteristics on crack growth rate was also investigated.

Location, location & size: defects close to surfaces dominate fatigue crack initiation

Metallic cast components inevitably contain defects such as shrinkage cavities which are inherent to the solidification process. Those defects are known to significantly alter the fatigue life of components. Yet very little is known, quantitatively, on the dangerosity of internal casting defects compared to surface ones. In this study, fatigue specimens containing controlled internal defects (shrinkage pores) are used to foster internal cracking. In situ fatigue tests monitored by X ray synchrotron tomography revealed that the internal nucleation and propagation of cracks was systematically overran by surface cracking initiated at castings defects up to ten times smaller than the internal ones. These findings indicate that the presence of internal defects in cast components can be tolerated to a larger extent than is allowed by nowadays standards

Influence of forging conditions on the fatigue mechanisms of low alloy steels: a 3D study

The influence of forging conditions on the propagation of physically small fatigue cracks has been studied for two high strength steels. Two surface conditions were produced after the forging process. The subsurface microstructure of the materials has been characterized by EBSD. Small samples extracted from the original specimens were used to perform in situ fatigue tests monitored by high resolution synchrotron X-ray tomography. Fatigue cracks were initiated from an artificial defect (100 ?m wide x 50 ?m deep) introduced in the forging skin by laser machining. 3D images of the initiation and growth of those physically small fatigue cracks have been obtained. It was found that the presence of a shot-blasted skin containing a hardness and microstructure gradient influences the 3D crack shape during propagation in comparison with the materials without material properties gradient. The 3D crack shapes are rationalized in terms of crack closure effects induced by the forging processes, close to the surface.