Central Contact Research Articles

Image based weighted center of proximity versus directly measured knee contact location during simulated gait

To understand the mechanical consequences of knee injury requires a detailed analysis of the effect of that injury on joint contact mechanics during activities of daily living. Three-dimensional (3D) knee joint geometric models have been combined with knee joint kinematics to dynamically estimate the location of joint contact during physiological activities—using a weighted center of proximity (WCoP) method. However, the relationship between the estimated WCoP and the actual location of contact has not been defined. The objective of this study was to assess the relationship between knee joint contact location as estimated using the image-based WCoP method, and a directly measured weighted center of contact (WCoC) method during simulated walking. To achieve this goal, we created knee specific models of six human cadaveric knees from magnetic resonance imaging. All knees were then subjected to physiological loads on a knee simulator intended to mimic gait. Knee joint motion was captured using a motion capture system. Knee joint contact stresses were synchronously recorded using a thin electronic sensor throughout gait, and used to compute WCoC for the medial and lateral plateaus of each knee. WCoP was calculated by combining knee kinematics with the MRI-based knee specific model. Both metrics were compared throughout gait using linear regression. The anteroposterior (AP) location of WCoP was significantly correlated with that of WCoC on both tibial plateaus in all specimens (p<0.01, 95% confidence interval of Pearson׳s coefficient r>0), but the correlation was not significant in the mediolateral (ML) direction for 4/6 knees (p>0.05). Our study demonstrates that while the location of joint contact obtained from 3D knee joint contact model, using the WCoP method, is significantly correlated with the location of actual contact stresses in the AP direction, that relationship is less certain in the ML direction.

Investigation on the Stress Behaviour of Coating-Substrate Interface under Contact Loading

This study is intended to predict the stress behavior of thick hard coating at the interface with the changes of coating stiffness and thickness to the substrate of Ti-6Al-4V and SCMV. The elastic mismatch between the coating and the substrate is presented in the value of Dundurs parameter α. The prediction is done using simple geometry of a cylinder-on-flat model in 2D analysis subjected to normal and tangential loading. Tangential stress distribution along the coating-substrate interface is then obtained from the FE modelling after a finite sliding of the cylinder. It is predicted that the maximum tangential stress value predicted at the interface which relates to coating fracture failure is increasing as stiffer coating is used on compliant substrate (i.e. increasing α values). The location of the maximum tangential stress predicted also changes from the trailing edge to the center of contact with increasing α values. Effect of changes of coating thickness on the predicted maximum tangential stress value is more significant for high positive α values. Risk of coating fracture at the interface is therefore predicted to increase with the increase of coating thickness and stiffness.

In Vivo Gliding and Contact Characteristics of the Sigmoid Notch and the Ulna in Forearm Rotation

To investigate shifting of the contact center over the surfaces of 2 opposing bones of the distal radioulnar joint during forearm rotation. We recruited 8 volunteers and used their right wrists. Serial computed tomography scans were obtained with the forearm at neutral position and 6 other positions of forearm rotation. We reconstructed 3-dimensional images and mapped contact regions of both the sigmoid notch and ulnar head by calculating the shortest distance between the 2 opposing bones. The center of contact was also defined and plotted against the distal radioulnar joint rotation to determine the sliding distance over the surfaces of the 2 bones. During forearm rotation, the maximal sliding of the sigmoid notch over the ulnar head was 7.4 mm in forearm pronation and 9.2 mm in forearm supination, which occurred in volar-dorsal direction primarily. Sliding of the ulnar head over the sigmoid notch was more limited, measuring 4.7 mm during pronation and 2.3 mm during supination. Most of the motion occurred between 30° pronation and 60° supination. In the proximal-distal direction, the contact site of the sigmoid notch with the ulnar head translated distally 1.6 mm during pronation and proximally 0.7 mm during supination. During forearm rotation, the sigmoid notch slides substantially against the ulnar head at each part of the forearm rotation arc. The sliding of the ulnar head over the sigmoid notch is smaller, most of which is at the range from moderate forearm pronation to slight supination. The contact site of the sigmoid notch with the ulnar head moves slightly distally during forearm pronation and proximally during supination. The in vivo findings provide more detailed information and insight into distal radioulnar joint motion kinematics.

Analytical and experimental investigation on friction of non-conformal point contacts under starved lubrication

This paper presents an analytical and experimental study on the friction of non-conformal point contacts (ball-on-disk) under starved lubrication. Theoretical models were developed to simulate the behavior of friction and separation versus the degree of starvation while experimental measurements were conducted by using a Tribometer equipped with a torque sensor and a digital camera, which provided the possibility to use the optical interferometry technique simultaneously with measuring the friction. The effect of air-oil meniscus distance from the center of Hertzian contact on the friction between non-conformal surfaces was observed under starved conditions in sliding motion by capturing interferometric images. In addition, the reduction of friction by artificially-produced shallow micro-dents was investigated under severely starved and fully flooded conditions. Results show that the coefficient of friction increases dramatically when the air-oil meniscus starts to touch the circle of Hertzian contact and there is not such a significant difference in the friction between starved and fully flooded contacts since the air-oil meniscus is far away from the borders of Hertzian contact. In other words, the starvation of lubrication causes a high level of fatigue and wear of machine components when there is interference between the air-oil meniscus and the Hertzian contact. However, it was found that shallow micro-dents are helpful in reducing the friction under severe starvation conditions while the benefits of micro-dents are negligible for fully flooded conditions.

Force and Contact Location Shading Methods for Use Within Two- and Three-Dimensional Polygonal Environments

Current state-of-the-art haptic interfaces only provide kinesthetic (force) feedback, yet studies have shown that providing tactile feedback in concert with kinesthetic information can dramatically improve a person's ability to dexterously interact with and explore virtual environments. In this research, tactile feedback was provided by a device, called a contact location display (CLD), which is capable of rendering the center of contact to a user. The chief goal of the present work was to develop algorithms that allow the CLD to be used with polygonal geometric models, and to do this without the resulting contact location feedback being overwhelmed by the perception of polygonal edges and vertices. Two haptic shading algorithms were developed to address this issue and successfully extend the use of the CLD to 2D and 3D polygonal environments. Two experiments were run to evaluate these haptic shading algorithms. The first measured perception thresholds for rendering faceted objects as smooth objects. It was found that the addition of contact location feedback significantly increased user sensitivity to edges and that the use of shading algorithms was able to significantly reduce the number of polygons needed for objects to feel smooth. The second experiment explored the CLD device's ability to facilitate exploration and shape recognition within a 3D environment. While this study provided a validation of our 3D algorithm, as people were able to identify the rendered objects with reasonable accuracy, this study underscored the need for improvements in the CLD device design in order to be effectively used in general 3D environments.

A numerical study of impact force caused by liquid droplet impingement onto a rigid wall

Liquid droplet impingement (LDI) erosion could be regarded to be one of the major causes of unexpected troubles occasionally occurred in the inner bent pipe surface. Evaluating the LDI erosion is an important topic of the thermal hydraulics and structural integrity in aging and life extension for nuclear power plants. One of the causes of LDI erosion is the impact pressure by the impingement of droplets in the involved steam. We investigated a simple droplet impingement to a rigid wall using volume of fluid (VOF) model, which is a two-phase Eulerian–Eulerian approach. The impact of a single water droplet with a high velocity towards a solid surface is examined numerically. The high Reynolds number value implies inertia dominated the phenomena and supports an inviscid approach to the problem. The high Weber number is justifying that an assumption to neglect the surface tension effect is adopted. We show that the compressibility of the liquid medium plays a dominant role in the evolution of the phenomenon. Both generation and propagation of shock waves are directly computed by solving the fluid dynamics continuity and momentum equations. In the simulation we employed a front tracking solution procedure, which is particularly suitable for two-phase free surface computation. The numerical results show that critical maximum pressure is not highest at the center of droplet contact on the surface at the first instantaneous moment but highest behind the contact angle later before jet eruption. It agrees generally well (within 20%) with the mathematical analysis. Finally, a droplet impact angle function is proposed for the global LDI erosion prediction.

Adhesive Elastic–Plastic Microcontact Analysis of Truncated Cylinder-on-Flat with Asperities

Adhesion interaction during micro-contacts of deformable rough bodies is characterized in this study using finite element based methodology. The interacting bodies are a truncated cylinder and a flat. Surfaces roughness is simulated by multiple asperities on a cylindrical segment. Lennard–Jones potential methodology is used to calculate adhesion forces by volume integration over the contacting bodies as well as over all asperities, both undeformed and deformed. Cylindrical segment of the asperity is deformed basically in two cylindrical segments. The flat body is deformed the least at the center and maximum at the outer portion of contact width, causing concave bending of the upper body with multi-asperity interfaces. Radius of asperities decreases from the center to outer periphery along the contact width. Adhesion force at each asperity is minimum at the center and maximum at the outer portion of the contact width in the deformed state after complete unloading. The total adhesion force between all asperities and lower body depends on number of asperities, and its maximum value decreases after deformation and occurs at separation distance greater than that for undeformed contact. Its maximum value increases with increasing number of asperities for the undeformed state and decreases with increasing number of asperities for the deformed state. On the other hand, total adhesion force between asperities, truncated cylindrical body and lower body increases with increasing number of asperities for both undeformed and deformed states due to the contribution of truncated segment/lower body interaction, and its location shifts towards the center of contact with increasing number of asperities.

The Effects of Interfacial Particles on the Contact of an Elastic Sphere With a Rigid Flat Surface

In chemical mechanical polishing (CMP), a rigid wafer is forced on a rough elastomeric polishing pad, while a slurry containing abrasive particles flows through the interface. One of the important factors that influence the material removal rate in CMP is the magnitude of contact force transmitted to the abrasive particles trapped at the contact interface. The total push-down force is distributed to the direct contact between the wafer and the pad, and to the three-body contact between the wafer, the pad, and the abrasive particles. The presence of the abrasive particles alters the asperity contact, which otherwise can be described by Hertz contact relationships. In this study, the effect of the interfacial particles on the single asperity contact is investigated. An approach used by Greenwood and Tripp (1967, “The Elastic Contact of Rough Spheres,” ASME J. Appl. Mech., 34, pp. 153–160) to study the contact of rough spheres is utilized since the presence of the particles provides a rough character to the contact. The results show that the contact behavior becomes non-Hertzian with decreasing contact force and increasing elastic modulus, particle size, and particle concentration. The role of the interfacial particles is to spread the contact over a larger area while lowering the maximum contact pressure at the center of contact predicted by Hertz contact. The conditions required to transfer the contact force on the particles effectively are also described.

A Numerical Algorithm for Determining the Contact Stress of Circular Crowned Roller Compressed between Two Flat Plates

The main purpose of this paper is to explore a numerical algorithm for determining the contact stress when a circular crowned roller is compressed between two plates. To start with, the deformation curve on a plate surface will be derived by using the contact mechanical model. Then, the contact stress distribution along the roller which occurs on the plate surface is divided into three parts: from the center of contact to the edge, the edge and apart from the contact line. The first part is calculated by the elastic contact theorem for the contact subjected to nominal stress between non-crowned parts of roller and plates, the second part is obtained by the classical Hertzian contact solution for the contact between crowned parts of roller and plates, and the third part is simulated as exponential decay. In order to overcome the defect of the half space theorem, in which a plate with infinite thickness is assumed initially, a weighting method is introduced to find the contact stress of the plate with finite thickness. Comparisons with various finite element results indicate that the algorithm for estimating the contact stress of a circular crowned roller compressed between two plates derived in this paper can be a reasonably accurate when a heavy displacement load is applied. This is because the contact area is large under a heavy load, and the effect of stress concentration is smaller in comparison with the case under a light load.

Close Packing of Helices 3 and 12 of 16 S rRNA Is Required for the Normal Ribosome Function

The along-groove packing motif is a quasi-reciprocal arrangement of two RNA double helices in which a backbone of each helix is closely packed within the minor groove of the other helix. At the center of the inter-helix contact, a GU base pair in one helix packs against a Watson-Crick base pair in the other helix. Here, based on in vivo selection from a combinatorial gene library of 16 S rRNA and on functional characterization of the selected clones, we demonstrate that the normal ribosome performance requires that helices 3 and 12 be closely packed. In some clones the Watson-Crick and GU base pairs exchange in their positions between the two helices, which affects neither the quality of the helix packing, nor the ribosome function. On the other hand, perturbations in the close packing usually lead to a substantial drop in the ribosome activity. The functionality of the clones containing such perturbations may depend on the presence of particular elements in the vicinity of the area of contact between helices 3 and 12. Such cases do not exist in natural 16 S rRNA, and their selection enriches our knowledge of the constraints imposed on the structure of ribosomal RNA in functional ribosomes.

Study of quantum point contact via low temperature scanning gate microscopy

Two types of quantum point contacts have been studied by low temperature scanning gate microscopy. In addition to the usual bright spot, which corresponds to a large conductance change at the constriction, ring structures are observed near the center of the quantum point contact. The ring diameter shrinks with increasing base conductance when the side gate voltage is changed. The rings are thought to relate to the observation of impurity potentials in the constriction region.

Ab initiostudy of the electric transport in gold nanocontacts containing single impurities

By employing a real-space formulation of the Kubo-Greenwood equation based on a Green's function embedding technique combined with the fully relativistic spin-polarized Korringa-Kohn-Rostoker method a detailed investigation of the electrical transport through atomic-scaled contacts between two Au(001) semi-infinite systems is presented. Following a careful numerical test of the method the conductance of Au nanocontacts with different geometries is calculated and, in turn, correlated with the thickness of the constriction. Particular emphasis is paid to the influence of transition metal impurities (Pd, Fe, and Co) placed on various positions near the center of a particular contact. We found that the conductance is very sensitive to the position of the magnetic impurities and that the mechanism for the occurring relative changes can mainly be attributed to the impurities' minority $d$-band inducing resonant line shapes in the $s$-like density of states at the center of the contact.

The Effects of a Stationary Surface Pocket on EHL Line Contact Start-Up

In this paper, the start-up process of line contact elastohydrodynamic lubrication with a stationary central surface pocket is studied. The solution of the pressure profile and film thickness distribution in the contact area is achieved by using a mixed lubricated-solid contact model. The mass balance of pocket lubricant flow is enforced by using a simple algorithm which adjusts the pocket cavitation boundary. With the surface pocket in the center of contact, the Hertzian contact area is divided into two micro-EHL contacts: the inlet micro-EHL contact and the outlet micro-EHL contact. The amount of lubricant trapped inside the pocket determines the overall start-up behavior. If the lubricant trapped is less than a critical amount, the two micro-EHL contact would be in start-up condition in serial: the lubricant film builds up in the inlet micro-EHL contact area first and then the lubricant film builds up in the outlet micro-EHL contact after the inlet lubricant flow fills the pocket. The start-up time in this case is longer than the start-up time for smooth surface start-up. If the lubricant trapped is more than the critical amount, the two micro-EHL contact would be in start-up condition in parallel: a lubricant film builds up in the inlet micro-EHL contact and at the same time a lubricant film builds up in the outlet micro-EHL contact. The start-up time in this case is much smaller than that of the smooth surface start-up. Compared with the smooth surface start-up condition, because the solid contact vanishes faster, there would be less frictional heat generated in the contact area and thus the surface temperature rise during the start-up process would be much smaller. These effects could prove beneficial in applications with frequent start and stop or oscillating running conditions, in which direct solid to solid contact occurs at the start (restart) of motion.

Location of the active contact within the subthalamic nucleus (STN) in the treatment of idiopathic Parkinson's disease

Background Chronic electrical stimulation of the subthalamic nucleus (STN) has been shown to be safe and effective in the treatment of medically refractory idiopathic Parkinson's disease. The clinically most relevant location of stimulation within the physiologically defined STN has not been confirmed. We reviewed the locations of active electrical contacts in 33 patients who underwent simultaneous bilateral STN deep brain stimulator (DBS) implantation. Methods The location of the microelectrode-defined dorsal STN border was compared to the location of the center of the active contact(s) employed in achieving optimal clinical results 6 to 18 months postoperatively. Furthermore, the location of this optimal contact was determined with respect to each individual patient's midcommissural point. Bilateral monopolar stimulation was employed in 30 patients using quadripolar DBS electrodes. Results After a minimum follow-up period of 6 months, the motor subscores (UPDRS Part III) in the postoperative on-stimulation/off-medication state were 64 ± 18% (mean ± SD) improved as compared to the preoperative off-medication state ( p < 0.01). Additionally, an improvement of 53 ± 38% was noted when comparing the postoperative on-stimulation/on-medication state to the preoperative on-medication state ( p < 0.01). On average, the center of the optimal contact was 13.3 mm lateral, 0.5 mm posterior, 0.1 mm inferior to the mid-commissural point and was 0.1 ± 2.1 mm dorsal to the physiologically defined dorsal STN border. Conclusions While the achieved clinical results are comparable to those published in the literature, it appears that monopolar electrical stimulation at the anterior dorsolateral border of the STN yields optimal clinical results. Further studies are crucial in determining the precise mechanism of various modes of DBS in an effort to maximize clinical outcome.

Measurement and Prediction of Tape Cupping Under Mechanical and Hygrothermal Loads and Its Influence on Debris Generation in Linear Tape Drives

Magnetic tapes, which may be modeled as three-ply laminates, exhibit transverse curvature, or cupping, as manufactured and when mechanical and hygrothermal loads are applied. Among other things, this cupping affects debris generation since it influences the contact between the flawed tape edge and head, the point where much of the debris generation occurs. This influence on debris generation is demonstrated experimentally in this study. Much more debris accumulates near the tape edge-head contact than at other contact locations. No difference in debris generation was found for two tapes with slightly different residual cupping (which is controlled during manufacturing). The target residual cupping is usually negative, which means that the tape bows out towards the tape so that the edges are farther away from the head than the center of contact is, so as to reduce contact pressure with the tape edges. However, cupping generally changes upon application of a tension and generally reduces the importance of residual cupping, which accounts for the failure to find a difference in debris generation for tapes with slightly different residual cupping. A finite element method model that uses laminate shell elements and accounts for in-plane stress stiffening, thus making it suitable for thin laminate modeling, was created. This modeling demonstrates that application of tensile and normal (used to simulate head contact) loads leads to cupping movement in the positive direction, which indicates a more severe edge contact, for an increase in front coat Young’s modulus and/or an increase in front coat thickness. The same trends hold for an increase in back coat Young’s modulus and/or an increase in back coat thickness. Modeling also demonstrates that cupping moves in the positive direction for an increase in the substrate’s Young’s modulus in the transverse direction for MP and ME tapes. An analytical model demonstrates that increases in temperature and front coat thermal expansion coefficient leads to cupping movement in the negative direction. The same trends hold for changes in relative humidity.

Analysis of Relative Motion between Femoral Head and Acetabular Cup and Advances in Computation of the Wear Factor for the Prosthetic Hip Joint

The amount and type of wear produced in the prosthetic hip joint depends on the type of relative motion between the femoral head and the acetabular cup. Wear particles removed from the bearing surfaces of the joint can cause adverse tissue reactions resulting in osteolysis and ultimately in loosening of the fixation of the implant. When designing a simulator for evaluation of prospective materials for artificial hip joints it is important to verify that the type of relative motion at the articulation is similar to that produced in walking, involving continually changing direction of sliding. This paper is an overview of recent research done at Helsinki University of Technology on the analysis of the relationship between relative motion and wear in the prosthetic hip joint.To analyze the relative motion, software for computing tracks, referred to as slide tracks, drawn on the counterface by marker points on the bearing surface was developed and experimentally verified. The overall relative motion of the joint was illustrated by a slide track pattern, produced by many points. The patterns resulting from walking motion and from motion produced in ten contemporary hip simulator types were compared. The slide track computations were not limited to illustrational purposes but offered a basis for computing variations of sliding distances, sliding speeds and direction of sliding during a cycle. This was done for the slide track termed the force track, drawn by the resultant contact force. In addition, the product of the instantaneous load and increment of sliding distance was numerically integrated over a cycle. This track integral of load had so far not been determined for the majority of contemporary hip simulators. The track integral can be used in determining the wear factor, making it possible to compare clinical wear rates with those produced by hip simulators. The computation of the wear factor was subsequently improved by replacing the track integral of the resultant contact force with a surface integral computed as the sum of track integrals of a large number of smaller normal forces obtained by discretizing the contact pressure distribution. The slide track software could also be utilized in the conceptual design of new simulators because it was possible to rapidly investigate the effect of changes to the motion waveform amplitudes or phases, or of omitting certain waveforms to simplify the design of a simulator.The slide track analysis showed that walking motion produced mainly open tracks on the center of contact, implying continually changing direction of sliding. This phenomenon, which is crucial for obtaining the correct wear mechanisms for acetabular cups made of polyethylene, was reproduced by simulators having abduction-adduction motion in addition to flexion-extension motion. In the force track computations involving contemporary simulators with the common femoral head size of 28 mm, the sliding distance per cycle and the force track integral per cycle ranged from 19.7 to 34.3 mm and from 17.4 to 43.5 N m, respectively. The average sliding speed ranged from 19.7 to 49.0 mm/s. The sum of track integrals computed with forces obtained by discretizing the contact pressure distribution reached a substantially higher value than the track integral obtained with the resultant contact force only. This suggests that the wear factor is actually overestimated when computed in the conventional way by dividing the wear rate with the force track integral.

Flat medial–lateral conformity in total knee replacements does not minimize contact stresses

The potential for wear in UHMWPE components for total knee replacements can be reduced by decreasing the stresses and strains arising from tibial–femoral contact. The conformity of the articular surfaces has a large effect on the resultant stresses, and components that achieve flat medial–lateral contact have been assumed to produce the lowest stresses due to their perfect conformity. We computed the stresses arising from curved and flat contact on a half-space using two-dimensional, plane strain elasticity solutions and finite element analyses to compare the performance of curved and flat indenters. These indenters were represented by a polynomial so the profiles could be continuously varied from curved to flat. Curved contact resulted in maximum stresses at the center of contact, while flat contact produced maximum stresses at the edge of contact. In addition, three contemporary tibial configurations (flat-on-flat, curved-on-flat, and curved-on-curved geometries) were analyzed using the finite element method with nonlinear material properties. The maximum contact stress, von Mises stress, and von Mises strain were lowest for the curved-on-curved model. The other configurations resulted in higher contact stresses, von Mises stresses, and von Mises strains. The perfect conformity arising from flat contact did not reduce the contact stresses in the UHMWPE component. The tensile stresses, however, were lowest for the flat-on-flat geometry compared with the other two configurations. Relating these distinct differences could prove useful in interpretation of data from simulator and retrieval studies.

New tactile sensor chip with silicone rubber cover

We report on a new tactile sensor chip developed for measuring distribution of forces on its surface. The chip has eight force-sensitive areas, called “taxel” with a pitch of 240 μm. Surface micromachining techniques are used to produce small cavities that work as pressure-sensitive capacitors. The process is CMOS compatible, therefore, on-chip switched capacitor circuits can be used for signal amplification. To enable transduction of normal forces to the sensitive areas, we cover the sensor chip surface with silicone rubber. First, measurements show that the sensor's output can be explained by results from contact mechanics. We demonstrate this by the simple case of a hard sphere pressed in the silicone rubber cover. The center of contact can be measured within 2 μm precision. The radius of the sphere and the load working on it can be estimated with high precision from the tactile sensor output data.

Observation of stable cylindrical modes in electrically pumped circular grating-coupled surface-emitting lasers

We present the results of a comprehensive experimental study of the modal behavior of electrically pumped circular grating-coupled surface-emitting lasers with resonant second-order gratings. Our results show stable cylindrical mode operation with azimuthal orders m = 1 and m = 52. We have found that, when a passive, unpumped 0.7-microm-deep indentation is included at the center of the circular resonator contact, the modal behavior becomes more stable with respect to increasing current. This result manifests itself in the lasing of two orthogonal m = 1 modes along with one higher-order mode from threshold to 2.2 times threshold or more. The spatial coherence of the cylindrical modes over the area of the resonator leads to better beam quality. Lasers without passive central indentations exhibit mode hopping between quasi-linear modes and higher-order cylindrical modes as the current is increased.

Elastohydrodynamic Lubrication of Air-Lubricated Rollers

Abstract The lubricating air film between two rotating rollers in close contact was studied numerically. The numerical model used in this study accounts for the effects of air compressibility, material deformation, and the slip flow which occurs when the air film thickness is not much larger than the mean-free-path of the air molecules. The air film profiles and the pressure profiles for the nip region between the rollers were calculated. It was found that the calculated air film thicknesses are lower than predicted by the liquid elastohydrodynamic calculation. From this study, equations for the minimum air film thickness, the air film thickness at the center of contact, and the amount of air that passes through the nip were obtained. This study has application to the prediction of the amount of air entrained in a winding roll.