Performing DALK, we had the greatest likelihood of reaching Descemet's membrane with the Anwar Big Bubble technique. The visual outcomes are comparable to standard PK, avoiding the risk of endothelial rejection. Endothelial cell loss was low and the cell count was stable after 6 months.
The use of damage models require a number of material parameters the identification of which should be made, when possible, based on direct measures of the damage that develops in the material. Among the possible experimental techniques to measure the occurrence of ductile damage in metals, the measure of the damage through the degradation of the material Young's modulus has been indicated as one of the more effective technique and used by several authors. In this paper, this technique is critically reviewed highlighting the number of issues that may affect the measure of damage. In particular, the attention is focused in the case where damage process initiates at the onset necking and develops in the post-necking regime where stress, strain and damage are no longer uniform in the gauged section. Since geometry variations alter the reference base length for both stress and strain definition, a procedure based on the use of finite element simulation is proposed in order to account for this effect on the measure of the stiffness loss. The procedure has been applied to two class of metals: a high strength steel and high purity copper.
Damage modelling in ductile metal has received a lot of attention in the last thirty years. Since 1969, many models have been proposed in the literature even though, in most of the cases, the experimental aspects related to the damage parameter evaluation have often been neglected. In this work the procedure to evaluate the damage parameters, for the continuum damage mechanics based model proposed by Bonora, is presented. Here, the ductile damage is experimentally measured in terms of progressive elastic modulus reduction as a function of strain measured in an hourglass-shaped specimen loaded in tension. The proposed technique, which is based on performing multiple, partial unloading, is reliable and relatively simple to implement. Additional tensile testing, performed with a round notch tensile bar, provides critical data that can be used to identify unequivocally the full damage parameter set and to assure their geometry transferability. The procedure presented here has been applied to identify the damage parameters for 20MnMoNi55 steel.
The stress triaxiality effect on the strain required for void nucleation by particle-matrix debonding has been investigated by means of micromechanical modelling. A unit-cell model considering an elastic spherical particle embedded in an elastic-plastic matrix was developed to the purpose. Particle-matrix decohesion was simulated through the progressive failure of a cohesive interface. It has been shown that the parameters of matrix-particle cohesive interface are correlated with macroscopic material properties. Here, a simple relationship for the maximum cohesive opening at interface failure as a function of material fracture toughness and yield stress has been derived. Results seem to confirm that, increasing stress triaxiality, the strain at which void nucleation is predicted to occur decreases exponentially in a similar way as for fracture strain. This result has substantial implications in modelling of ductile damage because it indicates that if the stress triaxiality is high enough, ductile fracture can occur at plastic strain lower than that necessary to nucleate damage for moderate or low stress triaxiality regime.
K E Y W O R D Smicromechanics, stress triaxiality, void nucleation, α-iron
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