Kyoungsoo Park

Cohesive Zone Models: A Critical Review of Traction-Separation Relationships Across Fracture Surfaces

One of the fundamental aspects in cohesive zone modeling is the definition of the traction-separation relationship across fracture surfaces, which approximates the nonlinear fracture process. Cohesive traction-separation relationships may be classified as either nonpotential-based models or potential-based models. Potential-based models are of special interest in the present review article. Several potential-based models display limitations, especially for mixed-mode problems, because of the boundary conditions associated with cohesive fracture. In addition, this paper shows that most effective displacement-based models can be formulated under a single framework. These models lead to positive stiffness under certain separation paths, contrary to general cohesive fracture phenomena wherein the increase of separation generally results in the decrease of failure resistance across the fracture surface (i.e., negative stiffness). To this end, the constitutive relationship of mixed-mode cohesive fracture should be selected with great caution.

Serum and tear levels of nerve growth factor in diabetic retinopathy patients

PURPOSE To measure serum and tear nerve growth factor (NGF) concentrations in diabetic retinopathy (DR) patients to determine whether the NGF correlated with parameters associated with DR. DESIGN Cross-sectional comparative study. METHODS The study enrolled 254 DR patients and 71 nondiabetic controls. Patient demographic characteristics and diabetic parameters, including blood sugar levels, HbA1c, liver and renal function, were evaluated in two separate university hospitals. Serum and tear NGF concentrations and the ratio of NGF to total protein (TP) in serum and tear fluid were determined. RESULTS Serum and tear NGF levels were found to be higher in proliferative diabetic retinopathy (PDR) patients (98.7+/-12.1 ng/ml in serum, 45.6+/-6.3 ng/ml in tear) than in nondiabetic controls (18.5+/-6.1 ng/ml in serum and 8.3+/-4.7 ng/ml in tear) and nonproliferative diabetic retinopathy (NPDR) patients (26.6+/-5.1 ng/ml in serum and 8.6+/-4.2 ng/ml in tear; p<.001 for both serum and tear differences). Similarly, NGF levels were higher in PDR patients than in controls and NPDR patients after adjusting for possible confounding factors such as age, gender, serum blood urea nitrogen, creatinine, and diabetic parameters. In addition, the NGF-to-TP ratio for both serum and tear fluid was higher in the PDR group compared with the control and NPDR groups. NGF levels correlated well with diabetes duration, HbA1c, and blood sugar levels and diabetic nephropathy. CONCLUSIONS NGF concentration may be a good parameter for evaluating DR status. In addition, serum and tear NGF concentrations correlated strongly, indicating that tear fluid assays may offer an effective, accurate, and noninvasive option for NGF measurement.

Fracture behavior of functionally graded concrete materials for rigid pavements

Currently, in concrete pavements, a single concrete mixture design and structural surface layer are selected to resist mechanical loading without an attempt to affect concrete pavement shrinkage, ride quality, or noise attenuation adversely. An alternative approach is to design sublayers within the concrete pavement surface that have specific functions and thus to achieve higher performance at a lower cost. The objective of this research was to address the structural benefits of functionally graded concrete materials (FGCMs) for rigid pavements by testing and modeling the fracture behavior of different combinations of layered plain concrete materials and concrete materials reinforced with synthetic fibers. The three-point bending-beam test was used to obtain the softening behavior and fracture parameters of each FGCM. The peak loads and initial fracture energy between the plain, fiber-reinforced, and FGCMs were similar; this signified similar crack initiation. The total fracture energy clearly indicated the improvements in fracture behavior of FGCM relative to full-depth plain concrete. The fracture behavior of FGCM depended on the position of the fiber-reinforced layer relative to the starter notch. The fracture parameters of both the fiber-reinforced and plain concrete were embedded into a finite element–based cohesive zone model. The model successfully captured the experimental behavior of the FGCMs and now can be implemented to predict the fracture behavior of proposed FGCM configurations and structures such as rigid pavements. This integrated approach (testing and modeling) is promising and demonstrates the viability of FGCM for designing layered concrete pavement systems.

Comparing the cyclic behavior of concrete cylinders confined by shape memory alloy wire or steel jackets

Shape memory alloy (SMA) wire jackets for concrete are distinct from conventional jackets of steel or fiber reinforced polymer (FRP) since they provide active confinement which can be easily achieved due to the shape memory effect of SMAs. This study uses NiTiNb SMA wires of 1.0 mm diameter to confine concrete cylinders with the dimensions of 300 mm × 150 mm (L × D). The NiTiNb SMAs have a relatively wider temperature hysteresis than NiTi SMAs; thus, they are more suitable for the severe temperature-variation environments to which civil structures are exposed. Steel jackets of passive confinement are also prepared in order to compare the cyclic behavior of actively and passively confined concrete cylinders. For this purpose, monotonic and cyclic compressive loading tests are conducted to obtain axial and circumferential strain. Both strains are used to estimate the volumetric strains of concrete cylinders. Plastic strains from cyclic behavior are also estimated. For the cylinders jacketed by NiTiNb SMA wires, the monotonic axial behavior differs from the envelope of cyclic behavior. The plastic strains of the actively confined concrete show a similar trend to those of passive confinement. This study proposed plastic strain models for concrete confined by SMA wire or steel jackets. For the volumetric strain, the active jackets of NiTiNb SMA wires provide more energy dissipation than the passive jacket of steel. (Some figures in this article are in colour only in the electronic version)

Inadvertent scleral perforation after strabismus surgery: incidence and association with refractive error

BACKGROUND Inadvertent scleral perforation is a rare but serious complication of extraocular muscle surgery for strabismus. We evaluated the incidence of unwanted scleral penetration after strabismus surgery and its association with refractive error. METHODS This cross-sectional noncomparative case series comprised 453 eyes from 236 patients who underwent strabismus surgery. After dilation, each patient had fundus examination using indirect binocular ophthalmoscope. Chorioretinal scars around the suture area, which presumed the previous scleral perforation, were detected. We assessed whether the presence of these scars was related to the degree of preoperative refractive error. RESULTS Mean postoperative follow-up was 52.44 (SD 30.07) months. Overall incidence of inadvertent penetration of the sclera was 1.77%. A total of 8 eyes (6 patients) showed chorioretinal scars, and all were myopic. Of these, 3 eyes were classified as mild myopia, 3 as moderate myopia, and the other 2 as severe myopia. The relationship between inadvertent scleral penetration and degrees of refractive error was weak but statistically significant (p = 0.001). INTERPRETATION Inadvertent scleral perforation after strabismus surgery seems to be associated with myopic refractive error.

Thermal Cracking Prediction Model and Software for Asphalt Pavements

Thermally induced cracking in asphalt pavements remains to be one of the prominent distress mechanisms in regions with cooler climates. At present, the AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) is the most widely deployed pavement analysis and design procedure. For thermal cracking predictions, MEPDG utilizes a simplified one-dimensional stress evaluation model with a simple Paris-law (i.e. linear elastic fracture mechanics) based crack propagation procedure. The user-friendly graphical interface for MEPDG makes it an attractive design procedure of choice, however, the over simplicity of the model and lack of a physicsbased representation to accurately capture the nonlinear fracture behavior of ratedependent asphalt concrete reduce(s) the reliability of predictions. This study presents an interactive thermal cracking prediction model that utilizes a nonlinear finite element based thermal cracking analysis engine which can be easily employed using a user-friendly graphical interface. The analysis engine is comprised of (1) the cohesive zone fracture model for accurate simulation of crack initiation and propagation due to thermal loading and (2) the viscoelastic material model for time and temperature dependent bulk material behavior. The graphical user interface (GUI) is designed to be highly interactive and user-friendly in nature, and features screen layouts similar to those used in the AASHTO MEPDG, thus minimizing transition time for the user. This paper describes the individual components of the low temperature cracking prediction software (called LTC Model) including details on the graphical user interface, viscoelastic finite element analysis, cohesive zone fracture model, and integration of various software components for thermal cracking predictions.

Long-Term Intraocular Pressure Control of Trabeculectomy and Triple Procedure in Primary Open Angle Glaucoma and Chronic Primary Angle Closure Glaucoma

To evaluate the long-term intraocular pressure (IOP) control of trabeculectomy and triple procedure (cataract extraction by phacoemulsification, intraocular lens implantation and trabeculectomy), 1,542 eyes of 900 patients with primary open angle glaucoma (POAG) or chronic primary angle closure glaucoma (CPACG) were included. When success was defined as an IOP reduction of at least 30% from baseline, with or without antiglaucoma medications, the overall probability of success of trabeculectomy and triple procedure was 0.613 and 0.733 at 15 years, respectively. The success probability of trabeculectomy and triple procedure in reducing IOP below 18 mm Hg was 0.748 and 0.825 at 15 years, respectively. In POAG and CPACG patients, trabeculectomy and triple procedure were effective in reducing IOP for up to 15 years after surgery.

Role of Heat Shock Protein 47 in Transdifferentiation of Human Tenon's Fibroblasts to Myofibroblasts

BackgroundHeat shock protein 47 (Hsp47) is a well-known molecular chaperone in collagen synthesis and maturation. The aim of this study is to investigate its putative role in the transdifferentiation of Tenon’s fibroblasts to myofibroblasts.MethodsPrimary cultured human Tenon’s fibroblasts were exposed to transforming growth factor-β1 (TGF-β1) for up to 48 hours. The mRNA levels of Hsp47 and α smooth muscle actin (αSMA) were determined by quantitative real time RT-PCR. After delivery of small interfering RNA (siRNA) molecules targeting Hsp47 into the cells, the expression of Hsp47 and αSMA proteins was determined by western immunoblotting.ResultsTGF-β1 increased the mRNA expressions of both Hsp47 and αSMA in human Tenon’s fibroblasts, as determined by quantitative real time RT-PCR. However, it induced the protein expression of only αSMA but not Hsp47, as determined by western immunoblots. When siRNAs specific for Hsp47 were introduced into those cells, the TGF-β1-induced expression of αSMA was significantly attenuated on western immunoblots; after 48 hours of exposure to TGF-β1, the relative densities of immunobands were 11.58 for the TGF-β1 only group and 2.75 for the siRNA treatment group, compared with the no treatment control group (p < 0.001).ConclusionsOur data suggest that Hsp47 may be related to the TGF-β1-induced transdifferentiation of human Tenon’s fibroblasts to myofibroblasts.

Relationship between the Retinal Thickness Analyzer and the GDx VCC Scanning Laser Polarimeter, Stratus OCT Optical Coherence Tomograph, and Heidelberg Retina Tomograph II Confocal Scanning Laser Ophthalmoscopy

Purpose To assess the relationship between the retinal thickness analyzer (RTA) parameters, and those of the GDx VCC scanning laser polarimeter (GDx VCC), Stratus OCT optical coherence tomography (Stratus OCT), and Heidelberg retinal tomograph II confocal scanning laser ophthalmoscopy (HRT II). Methods Twenty-nine primary open-angle glaucoma patients were retrospectively included in this study. Measurements were obtained using the RTA, GDx VCC, Stratus OCT, and HRT II. We calculated the correlation coefficients between the parameters of RTA and those of the other studies. Results Among the optic disc parameters of RTA, the cup volume was best correlated with Stratus OCT (R=0.780, p<0.001) and HRT II (R=0.896, p<0.001). Among the posterior pole retinal thickness parameters, the posterior pole abnormally thin area (PPAT) of the RTA and the inferior average of the GDx VCC were best correlated (R=-0.596, p=0.001). The PPAT of the RTA and the inferior maximum of the Stratus OCT were best correlated (R=-0.489, p=0.006). The perifoveal minimum thickness (PFMT) of the RTA and the cup shape measurement of the HRT II were best correlated (R=-0.565, p=0.004). Conclusions Many RTA optic disc parameters were significantly correlated with those of the Stratus OCT and HRT II. The RTA posterior pole retinal thickness parameters were significantly correlated with those of the GDx VCC, Stratus OCT and HRT II. The RTA optic disc and posterior pole retinal thickness parameters may be valuable in the diagnosis of glaucoma.

Reproducibility of Peripapillary Retinal Nerve Fiber Layer Thickness Measured by Spectral Domain Optical Coherence Tomography in Pseudophakic Eyes

Purpose To assess the reproducibility of circumpapillary retinal nerve fiber layer (cpRNFL) thickness measurement (measurement agreement) and its color-coded classification (classification agreement) by Cirrus spectral domain optical coherence tomography (OCT) in pseudophakic eyes. Methods Two-hundred five participants having glaucoma or glaucoma suspected eyes underwent two repeated Cirrus OCT scans to measure cpRNFL thickness (optic disc cube 200 × 200). After classifying participants into three different groups according to their lens status (clear media, cataract, and pseudophakic), values of intra-class coefficient (ICC), coefficient of variance, and test-retest variability were compared between groups for average retinal nerve fiber layer (RNFL) thicknesses and that corresponding to four quadrant maps. Linear weighted kappa coefficients were calculated as indicators of agreement of color code classification in each group. Results ICC values were all excellent (generally defined as 0.75 to 1.00) for the average and quadrant RNFL thicknesses in all three groups. ICC values of the clear media group tended to be higher than those in the cataract and pseudophakic groups for all quadrants and average thickness. Especially in the superior and nasal quadrants, the ICC value of the cataract group was significantly lower than that of the clear media and pseudophakic groups. For average RNFL thickness, classification agreement (kappa) in three groups did not show a statistically significant difference. For quadrant maps, classification agreement (kappa) in the clear media group was higher than those in the other two groups. Conclusions Agreement of cpRNFL measurement and its color code classification between two repeated Cirrus OCT scans in pseudophakic eyes was as good as that in eyes with clear crystalline lens. More studies are required to ascertain the effect of lens status on the reproducibility of Cirrus OCT according to different stages of glaucoma patients.