Greater Contact Pressure Research Articles

Importance of microstructure modeling for additively manufactured metal post-process simulations

This work investigates the significance of microstructure-level modeling by simulating the material response of an inhomogeneous selective laser melted (SLM) Inconel 625 specimen subjected to two different post-process operations, namely micro-milling and laser shock peening (LSP). A physics-based thermal finite element simulation is executed to obtain the SLM thermal history from which a 3-dimensional inhomogeneous microstructure representative volume element (RVE) is generated via the Dynamic Kinetic Monte Carlo predictive model. A Johnson–Cook plasticity definition coupled with Hall–Petch strengthening is used to define unique yield surfaces for individual grains based on their major diameters. Micro-milling and LSP simulations are subsequently executed with and without considering an inhomogeneous microstructure RVE in attempt to elucidate differences in the plastic strain, temperature, induced stress magnitude and distribution, as well as differences that arise during material removal for the micro-milling only. The micro-milling simulations reveal a greater volumetric distribution of plastic strain and temperature for the inhomogeneous case, although the homogeneous case with isotropic assumption reveals greater heat dissipation at the tool-workpiece interface with 27% greater contact pressure and 39% greater frictional shear stress. Examining the ductile and shear damage progression at a specific time increment reveals that the inhomogeneous model has a slightly lower damage propensity in comparison the homogeneous case, despite having identical damage models and boundary conditions. Variation in the SLM process-dependent yield surfaces, for grains at different locations, results in spatial variations of the computed stress triaxiality, which influences the material removal, as well as the stress concentrations developed near the tool-workpiece interface. Thus, a process-structure-property relationship is captured with the microstructure modeling. This work is the first to illuminate the importance of capturing SLM-induced anisotropy, considering the additively manufactured grain structure subject to micro-milling and LSP post-processes.

Biomechanics and Clinical Outcomes of Partial Meniscectomy.

Partial meniscectomy for meniscus tears is one of the most common procedures performed by orthopaedic surgeons. Much research has been done to evaluate the biomechanical consequences and clinical outcomes from meniscus débridement. Biomechanically, as the portion of the meniscus that is removed increases, greater contact pressures are experienced by the cartilage, which leads to altered knee mechanics. The use of partial meniscectomy to manage degenerative meniscus tears in knees with mild preexisting arthritis and mechanical symptoms may be beneficial; however, its routine use in the degenerative knee over physical therapy alone is not supported. In younger populations, partial meniscectomy may provide equal long-term symptom relief, earlier return to play, and lower revision surgery rate compared with meniscal repair. Partial meniscectomy may result in earlier development of osteoarthritis. Treatment should be patient specific in a shared-decision making process with the patient after discussion about known outcomes.

Indentation and flattening of rough surfaces spherical asperities

The paper indicates that the application of roughness models and the theories of contacting rough surfaces developed by Greenwood-Williamson and N.B. Demkin for solving the problems of hermetology leads to significant errors. This is explained by much greater contact pressures than for the tribology problems, by describing only the initial part of the reference surface curve, the lack of allowance for the plastic extrusion of the material. A brief review of methods for describing the introduction of a sphere into an elastoplastic reinforced half-space is given. The properties of the elastoplastic reinforced material are described by the power law of Hollomon. To describe the indentation and flattening of single spherical asperity, the results of finite element modeling are used. The cases of contacting a rigid rough surface with an elastoplastic half-space and a rigid smooth surface with a rough surface are considered. To determine the relative contact area, the discrete roughness model is used in the form of a set of spherical segments distributed along the height in accordance with the curve of the reference surface.

Nanotribological behavior of ZnO films prepared by atomic layer deposition

We used atomic layer deposition to form ZnO thin-film coatings on Si substrates and then evaluate the effect of pile-up using the nanoscratch technique under a ramped mode. The wear volume decreased with increasing annealing temperature from room temperature to 400°C for a given load. Elastic-to-plastic deformation occurred during sliding scratch processing between the groove and film for loading penetration of 30nm. The onset of non-elastic behavior and greater contact pressure were evident for loading penetration of 150nm; thus, full plastic deformation occurred as a result of a substrate effect. We suspect that elastic–plastic failure events were related to edge bulging between the groove and film, with elastic–plastic deformation attributable to adhesion discontinuities and/or cohesion failure of the ZnO films.

Finite Element Analysis and Simulation of the Sealing Performance of Y-Ring Rubber Seal

Application of super elasticity theory and nonlinear theory, based on finite element method and the Abaqus software, the performance of Y ring rubber seal, such as the Von Mises stress, the sealing ring deformation and the contact pressure were simulated and analyzed under the influence of changeable hydraulic pressure. According to seal failure criterion, the effect of hydraulic pressure changes on the sealing performance were studied. Results show that, the maximum stress appears in the intersection of lips, the largest deformation area near the inner lip. The root has greater contact pressure, which would result in bitten phenomenon or aggravate wear.

Finite Element Analysis and Simulation of the Sealing Performance of Y-Ring Rubber Seal

Application of super elasticity theory and nonlinear theory, based on finite element method and the Abaqus software, the performance of Y ring rubber seal, such as the Von Mises stress, the sealing ring deformation and the contact pressure were simulated and analyzed under the influence of changeable hydraulic pressure. According to seal failure criterion, the effect of hydraulic pressure changes on the sealing performance were studied. Results show that, the maximum stress appears in the intersection of lips, the largest deformation area near the inner lip. The root has greater contact pressure, which would result in bitten phenomenon or aggravate wear.

The Effect of the Slip Ratio in Sagittal Plane on the Wear of UHMWPE Bearings in TKR

Reducing the sliding distance by decreasing slip ratio which occur during knee articulation helps to reduce the wear rate of the ultra-high molecular polyethylene (UHMWPE) bearings in total knee replacements (TKR). However, in the limited space in knee, a smaller sagittal radius of femoral articular surface which leads to greater contact pressure that may aggravate wear is needed to make a smaller slip ratio possible. This paper described a model for evaluating the effect of slip ratio in sagittal plane on the wear depth of UHMWPE bearings. The results showed that decreasing the sagittal radius of femoral articular surface properly was an effective approach to reduce the wear of UHMWPE bearings.

The effect of implant size and device keel on vertebral compression properties in lumbar total disc replacement

Background Context Vertebral end plate support is necessary for successful lumbar total disc replacement (TDR) surgery. Failure to achieve anterior column support as a result of lumbar TDR device undersizing could lead to implant subsidence and fracture. Purpose The purpose of the study was to examine the compressive biomechanical behavior of the vertebral end plate with varying sizes of disc replacement implants. Study Design The study design comprises a biomechanical investigation using a human cadaveric lumbar spine model. Methods Fifty-six vertebrae with intact posterior elements were prepared from 13 fresh frozen lumbar spines. Peripheral quantitative computed tomography was performed to assess regional bone density. Vertebrae were potted and subjected to nondestructive compression testing with a small, medium, and large custom-made implants with the footplate geometry of the ProDisc-L TDR (Synthes Spine, West Chester, PA, USA) system and having no keel. Failure testing was performed using the ProDisc-L implant with an intact keel. Pressure sensor film was used to assess contact pressure and distribution. Results There was a linear correlation between percent coverage of the end plate and implant-end plate stiffness (p=.0001) and an inverse correlation with displacement (p=.01). The difference in implant-end plate stiffness between small-medium, medium-large, and small-large implants was 10.5% (p=.03), 10.2% (p=.02), and 19.6% (p<.0001), respectively. Failure analysis revealed similar trends for implant sizing, but only bone density was found to significantly correlate with failure properties (r=0.76, p<.0001). There was a significant reduction in implant-end plate stiffness of 18% when the keel was intact compared to without the keel (range 6–27%, p=.0008). Pressure film analysis revealed that the implant was loaded peripherally and did not have central contact during nondestructive loading. There was a trend toward greater contact pressure with the small implant when compared with the medium implant (p=.06) and the large implant (p=.06). Conclusions Although larger implants reduce end plate displacement, increase apparent implant-end plate stiffness, increase the implant-end plate contact area, and decrease the peak contact pressures, low bone density reduces failure properties. The keel introduces a reduction in stiffness to the implant-end plate interface, the clinical significance of which is currently unknown.

Double-Row Rotator Cuff Repairs

Advances in surgical technique and implants for arthroscopic rotator cuff repair have generated an interest in the indications and application of double- and single-row repairs. Recently, a variation of the double-row technique, known as the transosseous equivalent repair, has also been described. Previous reports of unacceptably high failure rates after arthroscopic repair of large and massive rotator cuff tears have led to the scrutiny of both this technique and the implants used. Biomechanically, double-row techniques are superior to single-row with respect to restoring the anatomic footprint, minimizing gap formation, and providing a greater load to failure at time equals zero. Furthermore, the recently described transosseous equivalent fixation technique provides improved footprint restoration, greater contact pressure across the bone-tendon interface, and an increased load to failure when compared to double-row repair. Although the biomechanical data are encouraging, clinical studies are still in progress to assess the efficacy of double-row versus single-row techniques, particularly with regard to tendon healing. The potential advantages may be offset by the added surgical time, required technical expertise, and potential increase in cost. The purpose of this study is to review the biomechanical, histological, and in vivo results of double-row and transosseous equivalent arthroscopic rotator cuff repairs while also discussing our indications and technique for double-row repairs.

Lesiones producidas por la ablación con radiofrecuencia del istmo cavotricuspídeo en un modelo experimental

Experimental studies have shown that deeper and wider lesions (up to 10 mm long or deep) can be safely created using an 8 mm or irrigated tip catheter for ablation to treat atrial flutter. However, potential damage to the tricuspid valve or inferior cava vein has not been systematically evaluated. The cavotricuspid isthmus was ablated in 26 pigs (body weight 26-52 kg), with a total of 187 radiofrequency pulses. Standard 4 mm, 8 mm and irrigated tip catheters were used at random. For each ablation, energy, impedance and temperature were recorded continuously. The lesions were larger with irrigated tip and 8-mm catheters than with standard ones. In 7 animals (1 with an irrigated tip, 4 with an 8-mm, and 2 with a standard tip) the tricuspid valve was damaged. The tricuspid valve was severely damaged in 3 pigs and lesions were moderate in 4. In animals with tricuspid valve lesions, maximal energy was higher (59 +/- 27 vs. 51 +/- 24 W; p=0,03) and higher temperatures were reached (63 +/- 4 vs. 55 +/- 11 degrees C; p<0.001). Low energy pulses measured before ablation were also more intense in animals in which damage was produced (0.55 +/- 0.24 vs. 0.35 +/- 0.29; p=0.001), indicating greater contact pressure. The tricuspid valve may be severely damaged during the ablation of the cavotricuspid isthmus for atrial flutter: damage was seen most often with high energy pulses and with 8-mm catheters, but can also occur with usual energy levels and standard catheters. To minimize damage this technique should not be used from the inside of the right ventricle just above the tricuspid valve.

Kinetic analysis of the autohesion of raw SBR samples

AbstractA kinetic analysis of the autohesion of raw, cold, emulsion SBR rubbers having number‐average molecular weights ranging from 74,000 to 169,000 was undertaken by using the modified Wallace tackmeter. The autohesion was observed to be a reversible process following first‐order kinetics with respect to the formation of elementary area bonds. The application of greater contact pressures during autohesion increase in the kinetic work, bond strength, and time required for bond rupture. A practical aspect of the study shows that the contact time necessary to form a complete area bond is reduced by: (1) the annealing of samples to release internal stresses; (2) production of clean and very smooth surfaces; (3) equilibrating samples under pressure prior to joining them; (4) lowering the sample temperature. The raw SBR offering the best potential autohesive properties has a Mn of 133,000 and ML‐4 (212°F.) of 66.