A two scale damage concept applied to fatigue

The ductile type of damage is a phenomenon now well understood. Once the fully coupled set of constitutive equations is identified, Damage Mechanics is a powerful tool to predict failure. Brittle materials do not exhibit such a damageable macroscopic behavior. Nevertheless, they still fail. On the idea that damage is localized at the microscopic scale, a scale smaller than the mesoscopic one of the Representative Volume Element (RVE), we propose a three-dimensional failure modeling for monotonic as well as for fatigue loading. We develop a two scale model of what we shall call brittle damage: at the microscopic scale, micro-cracks or micro-voids exhibit a damageable plastic-like behavior with no effect on the global (mesoscopic) elastic behavior. Microscopic failure is assumed to coincide with the RVE failure. This model turns out to represent quite well physical phenomena related to high cycle fatigue such as the mean stress effect, the nonlinear accumulation of damage, initial strain hardening or damage effect and the nonproportional loading effect for bi-axial fatigue. The model has been implemented as a post-processor computer code. A simplified identification procedure for the determination of the material properties is given. This revised version was published online in July 2006 with corrections to the Cover Date.     

Low-damage dry-etched grating on an MQW active layer anddislocation-free InP regrowth for 1.55-μm complex-coupled DFB lasersfabrication

A grating dry-etched through the upper wells of a multiquantum-well active layer has been characterized before and after regrowth. TEM observation and carrier lifetime measurements have shown growth-free defects of the epitaxial layers. As a consequence, quasi-100% monomode oscillation on λ+1 mode has been achieved on complex-coupled distributed-feedback GaInAsP QW lasers fabricated with such grating     

Comparison of Auger recombination in GaInAs-AlInAs multiple quantum well structure and in bulk GaInAs

Carrier lifetime has been measured in GaInAs-AlInAs multiple quantum well structures and in thick GaInAs samples for local carrier densities between2 times 10^{17}and5 times 10^{19}cm^-3. Carrier lifetime and Auger recombination are found to be very close (±20 percent) in the two structures. The Auger limited T0values calculated for DH and MQW lasers are found to be, respectively, 93 and 170 K. Optimization of the quantum well laser as a function of the number of wells in the active region is discussed.     

Multiaxial creep–fatigue under anisothermal conditions

Because creep–fatigue is mainly studied in uniaxial tension, it is shown here how to proceed to perform both experiments and calculations under multiaxial loading and when the temperature varies both in time and space. The constitutive equations used are those of elasto‐visco‐plasticity coupled or not, to damage, with isotropic and kinematic hardening. It is shown that the unified damage law first proposed for ductile failure and then for fatigue may also be applied to multiaxial creep–fatigue interactions with a new expression for the damage threshold. The procedure for the identification of material parameters is described in detail. Finally, it is shown that the uncoupled calculation procedure, where damage is calculated as a post‐processing of an elasto‐visco‐plastic computation, gives satisfactory results in comparison to the fully coupled analysis; the latter being more accurate but very expensive in computer time.     

One damage law for different mechanisms

We consider here a general three-dimensional kinetic damage law. It uses the thermodynamic of irreversible processes formalism and the phenomenological aspects of isotropic damage. It gives the damage rate as a function of its associated variable, the strain energy density release rate and the accumulated plastic strain rate. Associated with different plastic constitutive equations, this damage law takes into account brittle damage, ductile damage, low and high cycle fatigue and creep damage. In this paper we mainly focus on creep-fatigue interaction and high cycle fatigue. Associated to a viscoplastic constitutive equation having kinematic hardening, the damage law gives the non linear creep-fatigue interaction. The agreement with experiments is good. Associated to plastic constitutive equations also having kinematic hardening but introduced in a micromechanical two scale model based on the self-consistent scheme, it models the non linear accumulation of damage induced by a succession of sequences of different amplitudes as well as the effect of the mean stress and the influence of non proportional loading.     

Non-linear creep-fatigue interaction based on a viscoplastic law coupled with damage

The purpose of this work is to develop an approach in crack initiation assessment in components operating at high temperature. Components of fossil power plants, such as rotors, casings or headers, have to face the combination of highly damaging processes due to the operating temperatures and the high numbers of start-ups and shut-downs. The processes involved are indeed creep, thermal fatigue and creep-fatigue interaction. Our approach is based on viscoplastic constitutive equations coupled with a three dimensional law of continuum damage evolution. These laws use a formalism based on the thermodynamic of irreversible processes. Our goal is to represent the crack initiation of structures loaded with low cycles thermal loads more accurately. To identify the different constitutive coefficients, we have chosen to study a P22 (2,25CrMo) steel from a retired header of one of our coal fired plants. The methodology of identification is given and the influence of each variable is discussed. We focus our interest on the creep-fatigue interaction which is a major parameter in the estimation of residual life of low used fossil power plant. A numerical simulation is given, which shows the necessity to use viscoplastic constitutive equations including a kinematic hardening, to take into account the non-linear aspect of the creep-fatigue interaction. Then, we detail an example of a finite elements calculation. The hypothesis and the particularities of this calculation are explained. The results are analysed and linked to experimental datas. We expect to apply soon those laws to various industrial materials and structures such as turbines (casings and rotors) or header to improve the residual life estimation of our plants.     

Hot exciton effects and relaxation in coupled quantum wells

By studying photoluminescence and photoluminescence excitation in coupled quantum wells, we have studied the influence of the tunneling time on the relaxation of the photoexcited carriers. The ability to vary the coupling strength by the barrier thickness, allowed us to evidence hot exciton relaxation effects as well as resonant Raman scattering.