Mean Contact Pressure Research Articles

A method to take account of the geometrical imperfections of quasi-spherical indenters

Perfect indenter geometry is quite difficult to manufacture, especially in the nano and macro scales. Indentation curves obtained with imperfect indenter geometry can show strong differences with those obtained with assumed perfect indenter geometry, thereby leading to erroneous data exploitation results.This numerical study brings out the effect of imperfect spherical indenter geometry on indentation load–penetration depth curves, and on the mechanical properties identified by a reverse analysis model based on ideal spherical geometry. It is shown that a method to take account of geometrical imperfections is essential. Two correction methods based on geometrical and physical considerations are assessed, as well as the relevance of the use of the penetration data or the contact data. A method based on the equality of mean contact pressures and indenter volumes under the contact surface is found to be most relevant, as confirmed by the quality results obtained after application of a reverse analysis model.The proposed method is of particular interest in the case of the use of an imperfect indenter whose profile is accurately known.

Elastic–plastic spherical indentation: Deformation regimes, evolution of plasticity, and hardening effect

Deformation of an elastic–plastic half-space by a rigid spherical indenter was examined with the finite element method. Simulation results show that the post-yield deformation behavior comprises four deformation regimes: linear elastic–plastic, nonlinear elastic–plastic, transient fully plastic, and steady-state fully plastic, representing different stages of the plastic zone evolution. Equations for the boundaries between neighboring deformation regimes are given in terms of the dimensionless mean contact pressure and indentation depth. The development of the plastic zone in the nonlinear elastic–plastic deformation regime is interpreted in terms of two characteristic modes of evolution that depend on the effective elastic modulus-to-yield strength ratio. General constitutive relations of the dimensionless mean contact pressure and contact area are derived for each deformation regime from finite element simulation results. The effect of strain hardening on the indentation behavior is shown to be significant for indentation depths larger than those corresponding to linear elastic–plastic deformation. An equation of the effective strain is introduced, and generalized constitutive relations are obtained using the effective yield strength, which accounts for the effect of strain hardening on the yield strength. The constitutive relations of the deformation behavior at the asperity scale derived in this study can be incorporated in contact analyses of elastic–plastic rough surfaces.

Pre-sliding Behaviour of Single Asperity Contact

The pre-sliding and static friction force behaviour at asperity level between a smooth ball and a smooth flat surface at different normal loads, as well as friction behaviour during full slip has been studied. The normal load dependence of the friction force and the preliminary displacement is discussed when the mean contact pressure is kept under 100 MPa. The theoretical model to calculate the shear stress and the preliminary displacement in the contact is discussed and the experimental data were used to verify the model. The results show that for low applied normal loads the adhesion force has an influence on the friction force measurements. Furthermore, the results for the friction force and preliminary displacement show good agreement with the theoretical trends. The experiments along with the model can be used to analyse the tangential traction in the contact and the behaviour of the stick–slip area. The measurement results along with the model were used to calculate the maximum shear stress at the point of sliding for different applied normal loads. It is also shown that at low applied normal loads the shear stress is not constant as compared to relatively high applied normal loads due to the presence of adhesion force.

Pilot validation study on a quasi-static weight-bearing knee rig

This article presents a pilot study on a quasi-static knee rig designed to investigate the influence of pathologies and surgical interventions on the patellofemoral kinetics of cadaveric knees. The knee rig allows cadaveric knees to flex and extend under a simulated body weight by transmitting a force to the quadriceps tendon. During the squat simulation, the ground reaction force stays within physiological values. Before using this device to answer clinical questions, two knee specimens were tested to assess the repeatability of the rig. Four repeated flexion-extension cycles were performed under a simulated body weight of 700 N, with an isolated force on the quadriceps tendon up to 2700 N and with a ground reaction force close to 350 N. The resulting patellofemoral contact area shifted from distal to proximal during knee flexion. From 20 degrees to 60 degrees of knee flexion, the mean contact area and pressure increased from 80.2 +/- 3.3 to 349.5 +/- 10.1 mm2 and from 0.9 +/- 0.2 to 5.9 +/- 0.7 MPa, respectively. The transmitted force on the quadriceps tendon, the ground reaction force and the patellofemoral contact area and pressure were continuously measured and showed a relative variability of 1.6%, 2.4%, 2.8% and 3.2%, respectively. The presented knee rig shows a good repeatability that allows us to use this knee rig to quantify the influence of anatomical changes on the patellofemoral contact area and pressures during a squat simulation.

Finite Element based Parametric Study of Elastic-plastic Contact of Fractal Surfaces

The present investigation utilizes the commercial finite element software ANSYS to study the effect of material properties such as yield strength (Y), modulus of elasticity (E) on the contact parameters like contact area, normal displacement and mean contact pressure in both perfect slip and full stick contact of fractal surfaces. The results indicate the nonlinear relationship among the contact parameters that are affected by the yield strength and the contact conditions; however the same E/Y ratio yields identical contact parameters. Load carrying capacity in full stick contact condition is slightly higher than that in perfect slip contact condition. The results are validated with experimental ones and also compared with single asperity finite element results.

A deterministic model-based contact analysis of Gaussian and non-Gaussian rough surfaces using finite element method

Rough surfaces having skewness values of –0.3, 0 and +0.3 and kurtosis ranging from two to four, are developed using FFT and analysed using FEM based ANSYS package. The continuity of elastic, elasto-plastic and plastic deformations of near realistic feature-based asperities and effects of asperity interactions are inherently included in the present deterministic contact approach. The results showed that the rough surface having positive skewness leads to minimum area of contact but attains the maximum value of mean contact pressure (Pmean) earlier and consequently can lead to severe wear when compared with Gaussian and negatively skewed surfaces. The area of contact and contact load decrease whereas the Pmean increases with an increase in kurtosis value. In the platykurtic surfaces, the contact load is shared by most of the contacting asperities and the neighbouring asperity interactions restrain the asperities to remain in elastic state mostly even at high loads.ASME Journal of Tribology, Wear, Materials Letters and Tribology International.

Finite element analysis of different contact bearing couples for human hip prosthesis

The main concern limiting the long-run success of the total hip replacement is the limited service life. The success of the implant is mainly depends upon the proper material selection and design of the prosthesis. The present work is to develop the two dimensional finite element model of acetabular component to investigate the maximum von Mises stress, contact pressure, contact radius and mean contact pressure to yield strength ratio developed for the different material combinations. The different bearing couple considered for the analysis are metal in contact with plastic, metal on metal, metal on ceramic, ceramic on plastic, ceramic on metal and ceramic on ceramic combinations. The result reveals that the combination with the metal in contact with plastic i.e. cobalt chromium femoral head paired with UHMWPE cup reduces von Mises stress by 90%, contact pressure by 91% and additionally provides thrice the contact area in comparison with other combinations. The finite element results are validated with the experimental data’s and found to be in close correlation between them.

Material selection of acetabular component in human hip prosthesis using finite element concepts

Hip joint replacement is an important issue in orthopaedic surgery. The success of the artificial hip replacement depends upon proper material selection, structure and shape of the prosthesis. Many orthopaedic analyses have been tried with different materials, but ended with partial success on the application side. The main objective of this paper is to extend the acetabular component finite element model of human hip prosthesis to investigate the maximum von Mises stress, contact pressure, contact radius and mean contact pressure to yield strength ratio for different combinations of bio-compatible materials. Eight different geometrical parameters are considered in the current study. The materials used are Co-Cr alloy, Co-Cr-Mo alloy, ultra high molecular weight polyethylene (UHMWPE) and alumina. To guarantee good clinical results in total hip replacement (THR), it is predicted from this extensive analysis that cobalt chromium paired with UHMWPE provides optimum performance in all the conditions. The analysis demonstrates that the implant with larger head radius, cup radius, and UHMWPE cup thickness with decrease in radial clearance value yields better mechanical performance. The finite element results are also compared with the experimental data of Jin et al. (1999) and found to be in good agreement with each other.

Finite-Element-Based Multiple Normal Loading-Unloading of an Elastic-Plastic Spherical Stick Contact

The repeated normal elastic plastic contact problem of a deformable sphere against a rigid flat under full stick contact condition is investigated with a commercial finite element software ANSYS. Emphasis is placed on the effect of strain hardening and hardening model with the maximum interference of load ranging from elastic to fully plastic, which has not yet been reported. Different values of tangent modulus coupled with isotropic and kinematic hardening models are considered to study their influence on contact parameters. Up to ten normal loading-unloading cycles are applied with a maximum interference of 200 times the interference required to initiate yielding. Results for the variation of mean contact pressure, contact load, residual interference, and contact area with the increasing number of loading unloading cycles at high hardening parameter as well as for low tangent modulus with two different hardening models are presented. Results are compared with available finite element simulations and in situ results reported in the literature. It is found that small variation of tangent modulus results in same shakedown behavior and similar interfacial parameters in repeated loading-unloading with both the hardening rules. However at high tangent modulus, the strain hardening and hardening rules have strong influence on contact parameters.

Ex vivoContact Mechanics and Three‐Dimensional Alignment of Normal Dog Elbows after Proximal Ulnar Rotational Osteotomy

To evaluate the effect of proximal ulnar rotational osteotomy (PURO) on the contact mechanics and 3-dimensional (3D) alignment of cadaveric normal dog elbows. Ex vivo biomechanical study. Unpaired thoracic limbs from 12 dogs (mean ± SD weight, 26 ± 4 kg). PURO consisted of a transverse osteotomy with 30° external rotation of the proximal segment. With the limb under 200 N axial load, contact area (CA), mean contact pressure (CP) and peak contact pressure (PCP) were measured using digital pressure sensors in the medial and lateral compartments and 3D static elbow poses were obtained, before and after PURO. Each specimen was tested at 115°, 135°, and 155° elbow flexion, with the antebrachium in neutral rotation, in 28° supination, and in 16° pronation. Repeated measures ANOVAs with post-hoc Bonferroni (P ≤ .05) were performed. PURO caused significant changes, mostly at 135° elbow flexion and neutral antebrachial rotation, characterized by decreased CP (10%) and PCP (10%) in the medial compartment, and increased CP (27%) and PCP (23%) in the lateral compartment. The apex of the medial coronoid process translated 3.4 mm caudally, 1.7 mm abaxially, and rotated 1.8° externally relative to the radial head. Humeroradial varus angulation reduced by 6.7°. In limited poses, PURO displaces the medial coronoid process caudally and abaxially and shifts contact pressures towards the lateral elbow compartment by decreasing varus angulation.

The Effect of Medial Opening Wedge Proximal Tibial Osteotomy on Patellofemoral Contact

Background: It has been suggested that patellofemoral contact pressures and contact forces may be altered secondary to an opening wedge high tibial osteotomy, yet few data are available that quantify the effect of varying degrees of medial opening wedge osteotomy on the patellofemoral joint contact characteristics. Hypothesis: Opening wedge medial proximal tibial osteotomy will increase patellofemoral contact force and pressure. Study Design: Controlled laboratory study. Methods: Nine human cadaver knees were used. Pressure-sensitive film was placed in the suprapatellar pouch, leaving the patellar tendon and medial and lateral retinacula intact. The quadriceps tendon was attached to a materials testing machine along the axis of the femur, whereby a pulley mechanism generated 950 N of force. Patellofemoral contact characteristics were measured with pressure-sensitive film at 30°, 60°, 90°, and 120° of flexion for the native knee and after subsequent 10-mm and 15-mm medial opening wedge proximal tibial osteotomies. The film was analyzed with imaging software. Results: There was a statistically significant increase (P < .05) in mean contact pressure at 30° and 120° between the 10-mm osteotomy and native knee and across all flexion angles between the 15-mm osteotomy and native knee. Furthermore, a significant difference was seen in peak pressures when native knees were compared with 10-mm and 15-mm opening wedge osteotomies at all flexion angles. Conclusion: There was a significant increase in patellofemoral pressures at varying degrees of knee flexion after medial opening wedge proximal tibial osteotomies of only 10 mm; a larger osteotomy resulted in a greater increase. Clinical Relevance: When performing a medial opening wedge proximal tibial osteotomy, the surgeon should consider the negative effects of increased patellofemoral peak pressure.

Tibiofemoral contact mechanics following posterior root of medial meniscus tear, repair, meniscectomy, and allograft transplantation

The purposes of this study were to evaluate the effect on tibiofemoral contact mechanics of repair of the posterior root of the medial meniscus and the effect of meniscal allograft transplantation (MAT) with medial collateral ligament (MCL) release at different flexion angles. Ten fresh-frozen human cadaveric knees (five pairs) were used. A digital pressure sensor was inserted by capsulotomy, and experiments were performed serially under the following six conditions, that is, with an intact medial meniscus (normal controls), with a root tear, after root repair, after total meniscectomy, after MAT, and after MAT plus MCL release. During each experiment, knees were positioned at 0°, 30°, 60°, and 90° of flexion, and peak pressure (kPa) and contact area (cm2) were measured. At 0° of flexion, contact pressure did not differ among the six experimental settings. However, at 30° and 60° of flexion, contact pressure differed significantly between root tear and root repair specimens (p = 0.04 and 0.03, respectively), and between total meniscectomy and MAT specimens (p = 0.02 and 0.03, respectively). On the other hand, mean contact pressures were different between normal (476.7 ± 473.1 and 573.3 ± 479.1 kPa) and root repair (575.7 ± 357.8 and 598.6 ± 415.8), and between normal and MAT (635.7 ± 437.4 and 674.3 ± 533.2). At 0°, 30°, 60°, and 90° of flexion, contact areas differed significantly between normal and total meniscectomy specimens (p = 0.02, 0.01, 0.02, and 0.02, respectively), and between MAT and total meniscectomy specimens (p = 0.03, 0.02, 0.02, and 0.03, respectively). Contact areas differed significantly between root tear and root repair specimens at 60° of flexion (p = 0.04), and between normal control and root repair specimens at 60° and 90° of flexion (p = 0.03 and 0.04, respectively). The effects of MAT plus MCL release on contact mechanics were not different from the effects of MAT alone (n.s.). Both meniscal root repair and transplantation of meniscus improved contact mechanics, but it did not appear that repair of the meniscal root or transplantation of meniscus restores the biomechanical function back to normal level. The MAT plus MCL release was similar to those after MAT alone. Therefore, it is better to preserve meniscus and MCL release could be done during the MAT.

Contact Mechanics and Three‐Dimensional Alignment of Normal Dog Elbows

To evaluate the effects of antebrachial rotation at 3 elbow flexion angles on contact mechanics and 3-dimensional (3D) alignment of normal dog elbows. Ex vivo biomechanical study. Unpaired thoracic limbs from 18 dogs (mean ± SD weight, 27 ± 4 kg). With the limb under 200 N axial load, digital pressure sensors measured contact area (CA), mean contact pressure (MCP), peak contact pressure (PCP), and PCP location in the medial and lateral elbow compartments, and 3D static poses of the elbow were obtained. Each specimen was tested at 115°, 135°, and 155° elbow flexion, with the antebrachium in a neutral position, in 28° supination, and in 16° pronation. Repeated measure ANOVAs with post-hoc Bonferroni (P ≤ .0167) were performed. Both pronation and supination decreased CA by 16% and 8% and increased PCP by 5% and 10% in the medial and lateral compartments, respectively. PCP location moved 2.3 mm (1.8-3.2 mm) closer to the apex of the medial coronoid process in pronation and 2.0 mm (1.8-2.2 mm) farther away in supination. The radial head and medial coronoid process rotated 5.4° and 1.9° internally during pronation and 7.2° and 1.2° externally during supination. Contact mechanics and 3D alignment of normal dog elbows varied significantly at different elbow poses.

Comparison of nano-f`retting and nano-scratch tests on biomedical materials

Nano-scratch and nano-fretting tests were performed on highly polished biomedical grade Ti6Al4V, 316L stainless steel and CoCr alloy samples using a 3.7μm sphero-conical diamond indenter in a commercial nanomechanical test system (NanoTest). Over a wide range of experimental conditions the CoCr alloy showed significantly better wear resistance. The scratch recovery and mean contact pressure for plastic deformation in a low pass repetitive scratch test both correlated with mechanical properties determined from nanoindentation testing. Decreases in friction during the initial wear cycles of this low-cycle test were consistent a reduction in ploughing. In the extended nano-fretting tests Ti6Al4V and 316L stainless steel showed delamination wear.

Plastic flattening of a sinusoidal metal surface: A discrete dislocation plasticity study

The plastic flattening of a sinusoidal metal surface is studied by performing plane strain dislocation dynamics simulations. Plasticity arises from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. By analyzing surfaces with constant amplitude we found that the mean contact pressure is inversely proportional to the wavelength. For small wavelengths, due to interaction between plastic zones of neighboring contacts, the mean contact pressure can reach values that are about 1/10 of the theoretical strength of the material, thus significantly higher than what is predicted by simulations that do not account for size dependent plasticity. Surfaces with the same amplitude to period ratio have a size dependent response, such that if we interpret each period of the sinusoidal wave as the asperity of a rough surface, smaller asperities are harder to be flattened than large ones. The difference between the limiting situations of sticking and frictionless contacts is found to be negligible.

Effect of External Rotational Humeral Osteotomy on the Contact Mechanics of the Canine Elbow Joint

To evaluate the effect of 15° external rotational humeral osteotomy (ERHO) on canine elbow joint contact mechanics. Ex vivo biomechanical study. Unpaired cadaveric canine thoracic limbs (n = 8) Digital pressure sensors placed in a subchondral osteotomy distal to the elbow joint were used to measure contact area, peak and mean contact pressure, peak pressure location, center of pressure, and total force in the subchondral bone of the radius and the ulna. Measurements were taken in the following sequence: (i) neutral and (ii) after 15° external rotation. The distal aspect of the humerus was externally rotated by a middiaphyseal osteotomy and stabilized with an internal fixator. Data were normalized and analyzed using paired t-tests; P ≤ .05 was considered significant. Peak pressure location and center of pressure shifted 37.5 ± 15.9% and 21.5 ± 6.8% laterally after ERHO (P < .001 for both). Both were situated over the subchondral bone of the ulna in neutral position and over the radius after rotation in all 8 specimens (P < .001). Pressure measured in the ulnar part of the osteotomy was reduced from 58.7 ± 9.1% to 27.1% after ERHO (P < .001). Contact area, peak and mean contact pressure, and total force did not vary significantly between conditions. ERHO shifts the peak pressure location and the center of pressure laterally, toward the radial head and reduced the pressure acting on the ulna. The lateral shift of peak pressure may be beneficial in dogs with medial compartment disease.

Effects of Serial Sectioning and Repair of Radial Tears in the Lateral Meniscus

Background: Radial transection of the peripheral fibers of the meniscus could render it nonfunctional; however, the biomechanical consequences of a complete lateral meniscal radial tear and repair in human specimens have not been elucidated. Hypothesis: A complete radial tear will exhibit knee contact mechanics approaching those of total meniscectomy. Repair of complete radial tears will re-create normal load transmission across the joint. Study Design: Controlled laboratory study. Methods: Five matched pairs of fresh-frozen human cadaveric knees were tested in axial compression (800 N) at 2 knee flexion angles (0° and 60°). Six meniscal conditions were sequentially tested: (1) intact lateral meniscus; radial width tears of (2) 50%, (3) 75%, and (4) 100%; (5) meniscal repair; and (6) total meniscectomy. Repairs were pair matched and used either an inside-out or all-inside technique. Tekscan sensors measured tibiofemoral contact pressure, peak contact force, and contact area in the lateral meniscus and medial meniscus. Results: Complete radial tears of the lateral meniscus produced significant increases in mean contact pressure (P = .0001) and decreased contact area (P < .0001) compared with the intact state. This effect was significantly less than that of total meniscectomy (P < .0023). Lesser degrees of radial tears were not significantly different from the intact state (P > .3619). Mean contact pressure after either repair technique was not significantly different from the intact state (P = .2595) or from each other (P = .4000). Meniscal repair produced an increase in contact area compared with a complete tear but was still significantly less than that of the intact meniscus (P < .0001). The medial compartment showed no significant difference between all testing conditions for 0° and 60° of flexion (P ≥ .0650). Conclusion: A complete radial meniscal tear of the lateral meniscus has a detrimental effect on load transmission. Repair improved contact area and pressure. Contact pressures for repaired menisci were not significantly different from the intact state, but contact area was significantly different. Biomechanical performance of repair constructs was equivalent. Clinical Relevance: Repair of complete radial tears improves joint mechanics, potentially decreasing the likelihood of cartilage degeneration.

The friction coefficient on polycarbonate as a function of the contact pressure and nanoscale roughness

AbstractDuring dry friction the rheological behavior at the interface between a polymer surface and a rigid indenter is linked to the local pressure and the roughness of the surface of the indenter. This study on polycarbonate reports experimental results obtained at ambient temperature using rigid, spherical glass indenters of various radii. In a first step, a plot of the experimental friction coefficient μ versus the mean contact pressure pmean showed that below the yield stress (pmean &lt; 100 MPa) the intrinsic friction coefficient follows a master curve for smooth indenters. In a second step, roughened indenters were prepared by chemical etching, which allowed the monitoring of nanoscale roughness parameters. From 5.5 to 140 nm Rrms, the friction coefficient μ progressively fell to a plastic‐like constant value, indicating that the nanoroughness mediates the friction. These results form the basis for a study of the rheology of confined polymer layers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

The biomechanical effects of a deepened articular cavity during dynamic motion of the wrist joint

BackgroundA deepened articular cavity of the distal radius due to a metaphyseal comminution zone is associated with early osteoarthritis and reduced joint motion. As this deformity has not been investigated biomechanically, the purpose of this study was to evaluate the effects of a deepened articular cavity on contact biomechanics and motion range in a dynamic biomechanical setting. MethodsSix fresh frozen cadaver forearms were tested in a force controlled test bench during dynamic flexion and extension and intact mean contact pressure and contact area as well as range of motion were evaluated. Malunion was then simulated and intraarticular as well as motion data were obtained. Intact and malunion data were compared for the scaphoid and lunate facet and the total radial joint surface. FindingsDue to malunion simulation, cavity depth increased significantly. Motion decreased significantly to 54–69% when compared to the intact state. Malunion simulation led to a significant decrease of contact area in maximum extension for all locations (by ~50%). In maximum flexion and neutral position, contact area decrease was significant for the scaphoid fossa (by 51–54%) and the total radial joint surface (by 47–50%). Contact pressure showed a significant increase in maximum extension in the scaphoid fossa (by 129%). InterpretationAlready a small cavity increase led to significant alterations in contact biomechanics of the radiocarpal joint and to a significant range of motion decrease. This could be the biomechanical cause for degenerative changes after the investigated type of malunion. We think that restoration of the normal distal radius shape can minimize osteoarthritis risk post trauma and improve radiocarpal motion.

Hydration lubrication: exploring a new paradigm

Lubrication by hydration shells that surround, and are firmly attached to, charges in water, and yet are highly fluid, provide a new mode for the extreme reduction of friction in aqueous media. We report new measurements, using a mica surface-force balance, on several different systems which exhibit hydration lubrication, extending earlier studies significantly to shed new light on the nature and limits of this mechanism. These include lubrication by hydrated ions trapped between charged surfaces, and boundary lubrication by surfactants, by polyzwitterionic brushes and by close-packed layers of phosphatidylcholine vesicles. Sliding friction coefficients as low as 10(-4) or even lower, and mean contact pressures of up to 17 MPa or higher are indicated. This suggests that the hydration lubrication mechanism may underlie low-friction sliding in biological systems, in which such pressures are rarely exceeded.