Roger Fougères

Characterization of internal damage in a MMCp using X-ray synchrotron phase contrast microtomography

The initiation and development of damage inside a 6061 Al alloy reinforced with SiC particles has been studied during in situ mechanical tests using high resolution synchrotron X-ray tomography. The high coherence of the X-ray beam used improves the detection of reinforcements in the matrix as well as the detection of cracks. Qualitatively, the same damage mechanisms are observed at the surface and in the bulk of the sample, the rupture of the SiC particles being the dominant mechanism for the early stages of plastic deformation. Quantitatively, however, it is found that the geometrical characteristics of surface SiC particles differ from those of bulk particles and that the damage growth rate is larger inside the sample. This result can be understood in terms of elastic energy and normal stress levels in the SiC particles as calculated by finite element method (FEM) analysis.

Experimental study of porosity and its relation to fatigue mechanisms of model Al–Si7–Mg0.3 cast Al alloys

The microstructure and fatigue properties of three model AS7G03 cast aluminium alloys containing artificial pores have been studied. Synchrotron X-ray tomography has been used to characterise in three dimensions the pore population in the alloys. The development of fatigue cracks in relation with local crystallography has been studied by means of electron back scattered diffraction (EBSD). Both the average number of cycles to failure and the lifetime scatter depend on the pore content specially at high stress level. The mechanism leading to the initiation of a crack from a pore has been identified. The crack propagation at high stress level appears to be quite insensitive to microstructural barriers and can be reasonably well described by a Paris type law. At low stresses, however, short cracks are often observed to be stopped at grain boundaries and the fatigue life is no longer predicted by a simple propagation law.

Characterization by X-ray computed tomography of decohesion, porosity growth and coalescence in model metal matrix composites

Abstract Damage mechanisms of model materials have been studied using in situ tensile tests coupled with high resolution X-ray tomography. This non destructive technique revealed that 50% of the particles were pre-damaged by the extrusion. The initiation and growth phases of the damage process were quantified using the three dimensional images. The growth phase, measured both locally (on isolated particles) and globally (in the entire block) was compared with the Rice and Tracey prediction which was shown to overestimate the global prediction and to give a reasonable agreement of the local growth rate. Discrepancies between prediction and experiments could be partly quantified by introducing the effect of the growth threshold in the Rice and Tracey analysis. The scatter in the measured thresholds and growth rates were attributed to local crystallography and to local spatial arrangement effects.

Study of fatigue damage in 7010 aluminum alloy

Microscopic mechanisms controlling the fatigue damage in 7010 aluminum alloy are analysed. Cracks are initiated by the fracture of second-phase particles. Particles located in grains with a twisted cubic texture have been observed as preferential damage sites. Cracks grow either along intergranular or transgranular paths both in recrystallized and unrecrystallized regions. The final rupture mechanism depends on the stress level: at low and intermediate fatigue stress amplitudes the failure occurs by the unstable propagation of single cracks; at very large stresses multiple crack coalescence is observed. A model for the fatigue damage accumulation is presented. First a crack deviation model based on linear elasticity is described. It enables one to predict if crack deviation is to occur when encountering a grain or subgrain boundary. Finally, a fatigue life time model is developed in order to predict the cycle number to rupture and the qualitative influence of microstructural parameters. Good agreement is observed between experimental and predicted results at intermediate fatigue stress amplitudes.

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.

A dislocation model for internal damping due to the thermal expansion mismatch between matrix and particles in microheterogeneous materials

Abstract A transient low frequency internal damping, called δ T , has been observed in various aluminium-based microheterogeneous materials (Al-12.8wt.%Si alloy containing large silicon particles, Al/12 vol.%SiC, Al-7075 alloy/15 vol.%SiC). On cooling δ T is characterized by a low temperature broad poorly fined maximum. Moreover, δ T increases with increasing the cooling rate and decreasing the frequency. It is shown that the observed phenomena are linked with the internal stresses owing to the thermal expansion mismatch between particles and matrix. Finally, a model based on the movements of dislocations in the vicinity of the interface particle-matrix is suggested to explain the experimental features of δ T .

A model for damage in a clustered particulate composite

Abstract Previous models for the deformation of two-phase materials containing hard, brittle particles have been extended to allow the simultaneous treatment of damage and particle clustering. The model is based on a self-consistent analysis and uses an incremental, tangent modulus approach. Two problems are tackled. In the first we treat the effect of a distribution of particle sizes on damage development. We show that once the effect of the size distribution on the overall level of damage has been calculated a single damage parameter can be used to determine the stress–strain behavior with good precision. This validates the approach taken in our previous work. In the second problem we incorporate damage (in the form of particle cracking) into an elastic–plastic deformation model for a material containing a heterogeneous distribution of particles. The level of damage, as well as the local stress and strain, is allowed to vary between regions with different particle densities. The results indicate that clustering has a small but significant weakening effect on two-phase materials. This is in contrast with the effect of clustering in the absence of damage when a strong strengthening effect results. The results can be rationalized in terms of load redistribution between regions during the course of deformation.

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.