Domenico Gentile

Descemetic DALK and Predescemetic DALK: Outcomes in 236 Cases of Keratoconus

Purpose: To report the outcomes of our experience with deep anterior lamellar keratoplasty (DALK) in patients with keratoconus. Methods: A retrospective evaluation of 236 eyes of 198 patients that have undergone DALK between 2000 and 2006 using the Tsubota, Sugita, Melles, or Anwar technique. We analyzed the frequency of true Descemet membrane exposure, which we termed dDALK, and the number of eyes in which a predescemetic plane was achieved, which we termed pdDALK. Pre- and postoperative visual acuity, endothelial cell count, and central corneal thickness were evaluated on 120 eyes followed in our department. Results: A total of 139 of 236 (59%) eyes were classified as dDALK, with the Anwar technique showing the highest incidence of exposure of Descemets membrane (127 of 164, 77%). Descemet ruptures occurred in 25 of 236 cases (10.5%). Three ruptures were converted to penetrating keratoplasty (PK). There was no difference in visual acuity between the pdDALK and dDALK groups at an average follow-up of 30.4 months, although the eyes in the dDALK group seemed to have faster visual recovery. Best spectacle corrected visual acuity postoperatively was at least 20/30 in 80-85% of eyes at the patients last visit. Endothelial cell loss was 11-13%, with most of the loss occurring in the first 6 months. Conclusions: Performing DALK, we had the greatest likelihood of reaching Descemets 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.

Identification of the parameters of a non-linear continuum damage mechanics model for ductile failure in metals

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.

Crack Initiation and Growth in Bimetallic Girth Welds

Bimetallic girth welds are characteristics of clad pipe technology. When dealing with propagation issues, fracture mechanics concepts usually are no longer applicable as a result of the extensive and non-homogeneous plastic deformation along bi-material interface that occur at the crack tip even below design allowables. In this study, ductile crack initiation and propagation in bi-material girth welds was investigated using a Continuum Damage Mechanics (CDM) model proposed by Bonora [1]. For the base, weld and clad metal, ductile damage model parameters have been determined by means of inverse calibration technique using fracture data obtained on smooth and round notched tensile bar specimens. Firstly, the damage model was validated predicting ductile crack growth occurring in single end notch (SEN(T)) geometry sample comparing the applied load vs crack mouth opening displacement with experimental measurements. Successively, the model was used to investigate ductile crack initiation and propagation for under clad circumferential weld crack under remote tension.

Experimental modeling of strain-dependent cyclic plasticity for prediction of hysteresis curve

Although attempts have been devoted to consider the strain range effect in the material models, identification of material constants for accurate modeling the material response under cyclic loading within a wide range of strain amplitude is still a challenge. The experiments show that the cyclic stress–strain curves are severely dependent on the strain range for ductile metals. In most of the cyclic material models, only the stabilized cycle is considered to compute the constants of the models. Considering this strategy in the simulation of ductile metals subjected to cyclic loading may lead to erroneous results particularly for the initial cycles of the loading. In this study, strain-controlled tests were conducted to study the cyclic behavior of oxygen-free high thermal conductivity pure copper at different strain ranges. Each cycle of the hysteresis curve was divided into a tensile and a compressive half cycle. The yield stress and the constants of the four-rule Chaboche kinematic hardening model were computed for each half cycle using an automated program developed based on the genetic algorithm optimization. The results indicated that the constants of Chaboche model were dependent on the strain range and the accumulated plastic strain. Therefore, new strain range–dependent relations for isotropic and kinematic hardening conditions were proposed and the constants of the relations were computed. The proposed model could accurately simulate the stress–strain curve of the hysteresis loop from monotonic loading to the stabilized cycle.

A Revised Approach to Damage Measurement Based on Stiffness Loss Technique

In the last decades damage mechanics has received a lot of attention and has proved to be a powerful approach to describe the occurrence of failure in materials. In order to became an effective tool of practical application in the industrial design world, clear, reliable, and — when possible — simple procedures and practices, aimed to perform damage measures and to identified damage model parameters, are needed. In the literature, only a limited number of papers addresses the issue of the damage measurements. Among the possible different techniques, the measure of the progressive loss of stiffness as a function of the increasing inelastic strain level, is the most commonly used and referred. Even though this technique is conceptually simple, it can be affected by a number of error that can invalidated the measures. In this paper a clear and effective procedure based on the use of finite element simulation and experiments is presented. The proposed procedure allows one to correctly account for the geometry changes occurring in the sample even in the post necking regime where stresses and strain are no longer uniform along the section and the gauge length. The proposed methodology has been used to measure damage in 99.99% pure copper.Copyright

Dynamic Crack Tip Opening Displacement (DCTOD) as governing parameters for material fragmentation

Fragmentation in metals can be approached either by Mott statistical or by Energy-based fragmentation theory. Recently, Grady showed that the two theories can be reconciled showing that the material parameter that drives tendency to fragmentation and fragment size is the dynamic fracture toughness. Experimental data do not completely agree with these conclusions. In this paper, the dynamic crack tip opening displacement (CTOD) is proposed as fracture parameter which can account for plastic deformation occurring prior fracture. Here, an experimental procedure for determining the critical dynamic CTOD is presented. The circumferential crack bar tension (CCB(T)) was investigated for its use with tensile Hopkinson bar testing equipment. The calibration function in the dynamic range was determined via finite element analysis (FEA). The critical dynamic CTOD was measured using both high speed video recording, with digital image correlation (DIC) technique, and clip gauge at the crack mouth. The proposed procedure has been used to investigate dynamic fracture resistance of high purity copper (99.98%) and to correlate it with available fragmentation data.

COD OF OFF-CENTERED CRACKS IN PIPES UNDER BENDING: EXPERIMENTAL MEASURES AND MODEL VALIDATION

The leak area of circumferential part-through crack in pipe under bending depends on the location of the crack with respect to the bending plane. In LBB analysis, taking the crack symmetrically placed with respect to the bending plane no necessarily is a conservative assumption. In this work the COD distribution of circumferential cracks in pipes, off-centered placed with respect to the bending plane, was investigated experimentally. Thick and thin pipe geometries have been analyzed examining different crack lengths and off-axis angle values. The COD distributions have been measured by means of digital image correlation (DIC) technique and used to validate an analytical model (HCM Hodograph Cone Method) proposed by the authors.Copyright

Failure Mode of Pipes Containing Circumferential Cracks Under Bending and Its Consequence on the Application of NSCM Criterion

The results of some 60 tests performed in Italy on 2”, 4”, 6” and 8” pipes of A 106 B and 304 stainless steel, carrying circumferential through-wall cracks of various size under four point bending conditions (FPB), at room temperature and 300° C, have been analyzed using the Net Section Collapse Moment Criterion (NSCM) and the dimensionless plastic zone parameter (DPZP). Most of the test results have shown that the NSCM applies even though the DPZP is lower than unity. This apparent inconsistency is due to the fact that cracked pipes under bending fail by plastic hinge formation of the type occurring in FPB specimens carrying notches, like the Charpy VN ones, as predicted by the slip line theory. Under these conditions, two half circle plastic zones develop at both sides of the notch while the plastic zone straight ahead the notch tip is almost negligible. FE Calculations have confirmed this behavior: the plastic zone underneath the crack tip has not yet reached the neutral axis when the plastic hinge is formed on the sides of the piping, making the NSCM applicable. This, actually, implies that the DPZP as presently used in the screening criteria is not precisely the proper parameter to adopt in the assessment of the NSCM criterion applicability.Copyright

Numerical simulation of self-piercing riveting process (SRP) using continuum damage mechanics modelling

The extended Bonora damage model was used to investigate joinability of materials in self-piercing riveting process. This updated model formulation accounts for void nucleation and growth process and shear-controlled damage which is critical for shear fracture sensitive materials. Potential joint configurations with dissimilar materials have been investigated computationally. In particular the possible combination of DP600 steel, which is widely used in the automotive industry, with AL2024-T351, which is known to show shear fracture sensitivity, and oxygen-free pure copper, which is known to fail by void nucleation and growth, have been investigated. Preliminary numerical simulation results indicate that the damage modelling is capable to discriminate potential criticalities occurring in the SPR joining process opening the possibility for process parameters optimization and screening of candidate materials for optimum joint.

Modification of the Bonora Damage Model for shear failure

The Bonora damage model was extended to account for shear-controlled damage. To this purpose, a new definition for the damage dissipation potential in which an explicit dependence on the third invariant of deviatoric stress was proposed. This expression leads to damage rate equation in which two contributions, the first for void nucleation and growth damage process the latter for shear fracture, can be recognized. For the JIII controlled damage contribution, only two additional material parameters are necessary of easy experimental identification The extended model formulation was verified predicting the failure locus for AL 2024-T351 alloy. Finally, the failure locus for stress state combinations, where the minimum material ductility is expected, was determined.