Andrew Ruggiero

The Pathogenesis of Retinal Damage in Blunt Eye Trauma: Finite Element Modeling

PURPOSE To test the hypothesis that blunt trauma shockwave propagation may cause macular and peripheral retinal lesions, regardless of the presence of vitreous. The study was prompted by the observation of macular hole after an inadvertent BB shot in a previously vitrectomized eye. METHODS The computational model was generated from generic eye geometry. Numeric simulations were performed with explicit finite element code. Simple constitutive modeling for soft tissues was used, and model parameters were calibrated on available experimental data by means of a reverse-engineering approach. Pressure, strain, and strain rates were calculated in vitreous- and aqueous-filled eyes. The paired t-test was used for statistical analysis with a 0.05 significance level. RESULTS Pressure at the retinal surface ranged between -1 and +1.8 MPa at the macula. Vitreous-filled eyes showed significantly lower pressures at the macula during the compression phase (P < 0.0001) and at the vitreous base during the rebound phase (P = 0.04). Multiaxial strain reached 20% and 25% at the macula and vitreous base, whereas the strain rate reached 40,000 and 50,000 seconds(-1), respectively. Both strain and strain rates at the macula, vitreous base, and equator reached lower values in the vitreous- compared with the aqueous-filled eyes (P < 0.001). Calculated pressures, strain, and strain rate levels were several orders of magnitude higher than the retina tensile strength and load-carrying capability reported in the literature. CONCLUSIONS Vitreous traction may not be responsible for blunt trauma-associated retinal lesions and can actually damp shockwaves significantly. Negative pressures associated with multiaxial strain and high strain rates can tear and detach the retina. Differential retinal elasticity may explain the higher tendency toward tearing the macula and vitreous base.

Long-term graft survival in deep anterior lamellar keratoplasty.

Purpose: To determine corneal graft survival rates up to 10 years in a large consecutive series of deep anterior lamellar keratoplasties (DALKs). Methods: A retrospective, consecutive, noncomparative cases series of DALK procedures in a total of 806 eyes of 711 patients with stromal diseases and healthy endothelium performed between 2000 and 2009. Inclusion criterion was surgery performed by a single surgeon (660 eyes), with at least 6 months of follow-up. Graft survival was analyzed using the Kaplan–Meier method. Endothelial loss was analyzed with the Gaussian distribution and the &khgr;2 methods. Follow-up time, and preoperative and postoperative endothelial cell density (ECD) were considered in the analyses. Results: Six hundred sixty eyes of 502 patients met the entry criteria. Mean length of follow-up was 4.5 years (range, 0.5–10 years). We report an average graft survival rate of 99.3% (range, 98.5%–100%); 3 eyes (0.45%) experienced graft failure and 1 eye (0.15%) developed late endothelial failure because of an intraoperative complication. Predominant indications for DALK in this series were keratoconus (74%), postherpetic keratitis scarring (15%), and corneal stromal opacities of different etiology (11%). Endothelial loss from preoperative levels averaged 11% (range, 10%–13%) at 6 months through 10 years after DALK. ECD was unchanged between 6 months postoperatively and the last follow-up visits. Conclusions: DALK is a successful form of transplantation in stromal corneal disorders with healthy endothelium, with higher long-term graft survival rates and stable ECD 6 months postoperatively. DALK survival rates do not vary significantly over time.

Deformation and texture evolution of OFHC copper during dynamic tensile extrusion

During dynamic tensile extrusion (DTE) the material is subjected to a complex deformation history, including high strain rates, large strains and elevated temperatures. This technique provides unique means to explore material performance under extreme conditions. In this work, the microstructural evolution of 99.98% commercially pure copper during the DTE test was investigated by means of electron backscatter diffraction (EBSD). The investigation was focused on the segment of the extruded jet that remained in the die, since numerical simulation showed that material points along the longitudinal axis of such segment correspond to different stages of a common temperature compensated deformation history. Therefore, post mortem microstructure information extracted at different locations along the center line is equivalent to in situ real-time measurement during the deformation process. EBSD investigations along the center line showed a progressive elongation of the grains, and an accompanying development of a strong 〈0 0 1〉 + 〈1 1 1〉 dual fiber texture. Meta-dynamic discontinuous dynamic recrystallization (DRX) occurred at larger strains, and it was demonstrated that nucleation occurred during straining, while subsequent grain growth took place during post-deformation cooling in the die. According to strain energy minimization arguments, the recrystallization resulted in an increased 〈0 0 1〉 texture component. The critical strain for recrystallization was well predicted from a power-law dependence on the Zener–Hollomon parameter, including grain size dependence and a temperature dependent activation energy. In addition, it was shown that 〈0 0 1〉 and 〈1 1 1〉oriented grains develop different dislocation substructures during straining, exhibiting elongated cells/micro-bands and typical cell structures, respectively. The present results also confirm that dynamic tensile ductility increases with decreasing initial grain size as a result of grain refinement and lowering of dislocation and twin densities during DRX.

Ductile damage evolution in high purity copper taylor impact test

Recently, the continuum damage mechanics model proposed by Bonora (Eng. Fract. Mech. 58, 1997) has been updated to account for stress triaxiality effect on model parameters, (Bonora et al., AIP Conf. Proc. 1195, 2009). This model enhancement allows to predict ductile damage initiation under varying stress states (uniaxial stress, uniaxial strain, and complex load paths) and dynamic loading conditions. In this work, the model has been used to investigate ductile damage developments in Taylor anvil and symmetric Taylor impact (rod-on-rod) configuration. Although the two configurations are equivalent for right scaled impact velocities, experimental evidences show that when ductile damage occurs in rod-on-rod not necessarily also develops in Taylor anvil impact. It has been found that, in the two impact configurations, the stress triaxiality builds up differently with plastic strain leading to different conditions for ductile damage initiation. Taylor impact tests have been designed and performed with the gas-gun facility at the University of Cassino. Damage investigation results obtained on recovered samples have been compared with rod-on-rod data reported in the literature and used to validate the proposed model predictions.Recently, the continuum damage mechanics model proposed by Bonora (Eng. Fract. Mech. 58, 1997) has been updated to account for stress triaxiality effect on model parameters, (Bonora et al., AIP Conf. Proc. 1195, 2009). This model enhancement allows to predict ductile damage initiation under varying stress states (uniaxial stress, uniaxial strain, and complex load paths) and dynamic loading conditions. In this work, the model has been used to investigate ductile damage developments in Taylor anvil and symmetric Taylor impact (rod-on-rod) configuration. Although the two configurations are equivalent for right scaled impact velocities, experimental evidences show that when ductile damage occurs in rod-on-rod not necessarily also develops in Taylor anvil impact. It has been found that, in the two impact configurations, the stress triaxiality builds up differently with plastic strain leading to different conditions for ductile damage initiation. Taylor impact tests have been designed and performed with the gas...

On the Role of Material Post‐Necking Stress‐Strain Curve in the Simulation of Dynamic Impact

Accurate material constitutive modeling is critical for the quality and reliability of the simulation of dynamic impact processes. Under dynamic loading ductile structural components can experience very large deformation that, in terms of equivalent plastic strain, can be well beyond the strain at which necking would occur in uniaxial tensile tests. Unfortunately, the material flow curve for ductile metals can only be measured over a limited strain range while at large strain it is either extrapolated or corrected in some way. In this paper the role of the approximation of the material flow law in the post‐necking range is investigated with particular reference to the Taylor test. Result seems to indicate that the flow law governs the shape of the mushroom region while temperature and strain rate have a more important role in the resulting deformed shape of the intermediate and rear portion of the specimen geometry. In particular, the temperature effect exponent in the J&C expression can be calibrated bas...

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.

Simplified Approach for Fracture Integrity Assessment of Bimetallic Girth Weld Joint

Recent extensive use of Corrosion Resistance Alloy (CRA) as internal protection layer of standard carbon-steel pipes (clad and lined pipe) in the oil and gas industry requires an intensive use of bimetallic welds.Since some degree of defects in welds is inevitable, and in codes and standards (such as BS7910) the case of bi-metallic joint is usually not considered, some R&D’s activities are ongoing to define specific design guidance for an Engineering Critical Assessment (ECA) aimed at determining flaw acceptance criteria for fabrication of bimetallic joints.Based on the limited guidance in the literature, proposed procedures for ECA on CRA welds seem not cover the root/hot pass weld region, for which the requirement of “zero defect” became mandatory. As direct consequence, it penalizes the weld fabrication rate, particularly if “J-lay” or “S-Lay” methods are adopted. Furthermore, they are investigating on cases where weld material is overmatching the base metal or for a limited partial overmatching, despite for CRA welds, such conditions, seem quite difficult to be fully met, if current consumable materials present in the marked are selected.Aim of present paper is to describe how any standard ECA procedure (ordinarily used to assess carbon-steel welds) may be alternatively adopted to assess CRA welds for clad & lined pipe material, if specific conditions are respected. For this purpose a few number of elastic-plastic Finite Element Analysis (FEA) is required to identify and/or extends the validity limits which have to be met in order to be conservative in the use of selected standard procedure. Outer, inner and under clad flaws, located along the weld fusion line, were investigated. Such approach, certainly leads to a quite conservatism, but gives the advantage to provide a safe flaws acceptance criterion in root/hot pass weld, and it may be also applied for any level of weld partial overmatching condition.Despite proposed simplified approach is suitable until moderate plastic straining, it may be appropriated for any ECA on CRA pipe when “J-lay” or “S-lay” installation method is adopted, and/or for many riser’s configuration, and/or for several flowline routing also if exposed to post-buckling condition. It is demonstrated that the proposed simplified approach, when applied under moderate plastic strain conditions, provides accurate J-integral solutions compared to the complex method as proposed by current R&D.Copyright

Modeling ductile metals under large strain, pressure and high strain rate incorporating damage and microstructure evolution

In this work, a constitutive modeling that couples plasticity, grain size evolution (due to plastic deformation and dynamic recrystallization) and ductile damage has been developed. The effect of grain size on the material yield stress (Hall-Petch) and on the melting temperature has been considered. The model has been used to investigate computationally the behavior of high purity copper in dynamic tensile extrusion test (DTE). An extensive numerical simulation work, using implicit finite element code with direct integration, has been performed and the results have been compared with available experimental data. The major finding is that the proposed model is capable to predict most of the observed features such as the increase of material ductility with the decreasing average grain size, the overall number and size of fragments and the average grain size distribution in the fragment trapped into the dime.

Ductile damage in Taylor-anvil and rod-on-rod impact experiment

At equivalent impact velocity, pressure in Taylor and ROR impact experiment is not the same and this reflects in the resulting condition for ductile damage development. In this work, finite element parametric simulation was performed to investigate pressure wave development as a function of material and target work hardening curve. Using the Bonora damage model, the impact velocity necessary for generating ductile damage in high purity copper was assessed. Taylor and ROR experiments were performed at different equivalent impact velocities and metallographic investigation were performed on impacted samples in order to validate damage model predictions. Results seems to indicate that ROR configuration is more appropriate for 2damage model validation while the Taylor anvil is more suitable for strength model assessment.

DAMAGE DEVELOPMENT IN HIGH PURITY COPPER UNDER VARYING DYNAMIC CONDITIONS AND MICROSTRUCTURAL STATES USING CONTINUUM DAMAGE MECHANICS

In the framework of CDM, the evolution of the plastic strain, at which damage processes initiates, as a function of stress triaxiality, making the assumption of constant damage dissipated work is derived. Based on this, the CDM model proposed by Bonora (Eng. Frac. Mech., 58, 1‐2, 1997) has been used to predict the occurrence of ductile failure at different stress triaxiality conditions, under both quasi‐static and dynamic loading conditions. This solution allows the possibility to correctly predict the conditions for which ductile damage can initiate under uniaxial strain (such as that for the flyer plate impact test) and mixed conditions such as those in the Taylor impact test. The solution offers the possibility to correlate continuum damage model parameters to micro structural features such as grain size and purity grade.