Effect Of Contact Stress Research Articles

Effect of contact stresses on the phase composition, strength, and tribological properties of nanocrystalline structures formed in steels and alloys under sliding friction

Compressive stresses (pressures) arising in the zone of sliding friction in iron alloys are evaluated. The effects of compressive and tensile contact stresses on the structure, hardness, and tribological characteristics of iron-manganese alloys, austenitic steels, and high-carbon steel U13 with pearlitic structure are determined.

THE IMPORTANCE OF CONTACT STRESS IN KNEE HEMIARTHROPLASTY DESIGN: A TRIBOLOGICAL SIMULATION

An alternative type of joint replacement for the young, active patient is hemiarthroplasty, which preserves bone stock for future revision surgery. However, this treatment has had varied success to date [Amstutz, 1994; Sisto, 2005]. The tribological regime in both healthy and those replaced with a hemiarthroplasty is very complex. The mechanisms of wear and degradation of AC are not completely understood. The aim of this study was to investigate the effects of contact stress on friction and wear of articular cartilage, using a tribological simulation of the medial compartment of a bovine knee. Materials and Methods A pendulum friction simulator was used to apply physiological loads and motion, in this tribological simulation of the medial compartment of the natural knee. Bovine medial femoral condyles (18 month old and with approximate radius of 38-42 mm) were articulated against tibial surfaces or stainless steel plates of different conformity to create different stress conditions. Several simulations were carried out as shown in table 1. Simulation Tibial Counterface Natural Tibia +meniscus intact (AC-AC +meniscus) Meniscectomy Tibia (meniscus removed) (AC-AC no meniscus) Hemi (flat) Flat SS plate – AC-SSFP Hemi (50mm radius) 50mm conforming plate AC-SSCP50 Hemi (100mm radius) 100mm conforming plate AC-SSCP100 Table 1:Five different tibial counterfaces were used simulating the natural, meniscectomised and hemiarthroplasty-replaced joint. Each simulation was tested at high (1000N) and low (250N) load. The lubricant used was 25% bovine serum in saline. The loading and motion profiles were derived from BS 14243-3:2004. A dynamic axial load was applied (peak loads are referred to in this paper). A motion amplitude between -10o 13o was used, with a frequency of 1Hz. The coefficient of friction ( ) was measured through a piezoelectric transducer. Contact stresses between bearing surfaces were measured using pressure sensitive Fuji Film and a spectrodensitometer. Results

Evaluation of the service life of 15kp steel under conditions of fretting with variable level of contact stresses and the size of the counterbody

We perform the experimental investigation of the fretting fatigue of a 15kp steel-45 steel couple in the case where both the size of the counterbody and the level of contact stresses are variable. The proposed second-order polynomial model of regression analysis fairly exactly approximates the experimental data. On the basis of this model, we predict the effect of contact stresses and scale factor on the service life of the couple under conditions of fretting fatigue. It is shown that the service life decreases as the contact pressure increases up to a certain fixed level and that this effect becomes more pronounced for high amplitudes of stresses. The scale factor effect can be described as the decrease in the service life of a fretting couple with smaller sizes of the counterbody for high amplitudes of stresses. In order to explain the indicated prediction, we study the corresponding mechanism of interaction between the specimen, counterbody, and fretting products.

Quantification of the Joint Effect of Wheel Load and Tire Inflation Pressure on Pavement Response

Most pavement design and analysis procedures predict performance on the basis of expected pavement damage under traffic loads expected during design life. Some failure criteria are primarily dependent on wheel loads and almost independent of contact stresses. Others are primarily dependent on normal and shear stresses, not on load magnitude. Wheel load is used as a proxy for tire pressure to account for the effect of contact stresses indirectly. In most pavement design methods, tire-pavement contact stress is assumed to be equal to tire inflation pressure and to be uniformly distributed over a circular area. A methodology that explicitly accounts for the effect of tire inflation pressures and the corresponding contact stresses on pavement response is not available. In this research, pavement responses of typical pavement structures under the combined actions of variable wheel loads and tire pressures were evaluated. A multilayer, linear-elastic computer program was used to estimate three critical pavement responses: longitudinal and transverse tensile strains in asphalt and compressive strains in the subgrade. The differences of the strains estimated by the two models were statistically analyzed to quantify the effect of the assumption of uniform stress over a circular shape. The traditional model proved to be reliable to estimate compressive strains in the subgrade layer. The tensile strains in the asphalt layer under actual contact stress, however, were quite different from those under uniform constant stress. Contrary to initial expectation, for the general case, the assumption of uniform stresses is a conservative approach.

Effect of contact stress in bones of the distal interphalangeal joint on microscopic changes in articular cartilage and ligaments.

To examine articular cartilage of the distal interphalangeal (DIP) joint and distal sesamoidean impar ligament (DSIL) as well as the deep digital flexor tendon (DDFT) for adaptive responses to contact stress. Specimens from 21 horses. Pressure-sensitive film was inserted between articular surfaces of the DIP joint. The digit was subjected to a load. Finite element models (FEM) were developed from the data. The navicular bone, distal phalanx, and distal attachments of the DSIL and DDFT were examined histologically. Analysis of pressure-sensitive film revealed significant increases in contact area and contact load at dorsiflexion in the joints between the distal phalanx and navicular bone and between the middle phalanx and navicular bone. The FEM results revealed compressive and shear stresses. Histologic evaluation revealed loss of proteoglycans in articular cartilage from older horses (7 to 27 years old). Tidemark advancement (up to 14 tidemarks) was observed in articular cartilage between the distal phalanx and navicular bone in older clinically normal horses. In 2 horses with navicular syndrome, more tidemarks were evident. Clinically normal horses had a progressive increase in proteoglycans in the DSIL and DDFT. Load on the navicular bone and associated joints was highest during dorsiflexion. This increased load may be responsible for microscopic changes of tidemark advancement and proteoglycan depletion in the articular cartilage and of proteoglycan production in the DSIL and DDFT Such microscopic changes may represent adaptive responses to stresses that may progress and contribute to lameness.

Guidelines for performing storage modulus measurements using the TA Instruments DMA 2980 three-point bend mode II. Contact stresses and machine compliance

The effects of contact stresses and instrument compliance on the storage modulus, E′, measurements for a rectangular sample with span-to-thickness ratio of about 17 is presented. Using finite element analysis, it has been found that contact stress effects are negligible even when the applied force is near the maximum limit. Machine compliance, however, appears to affect the stiffness, and hence E′ values, particularly at static loads less than 4 N. An analytical model describing machine compliance effects has been developed and can be used for correcting the stiffness measurements of the geometry studied. The model has been verified for three different rigid thermoplastics.

The influence of stress conditions on the wear of UHMWPE for total joint replacements.

In vitro studies of the effect of contact stress on the wear of ultra high molecular weight polyethylene (UHMWPE) in orthopaedic applications have produced contradictory results which predict both increased and decreased wear with increasing contact stress. In vivo studies of functioning hip prostheses have reported that 22 mm femoral heads generate lower linear and volumetric wear rates than 32 mm femoral heads. The effect of decreasing the head size will increase the contact stress but decrease the sliding distance per motion cycle. The present study consists of wear experiments under a range of contact stress magnitudes and application conditions in order to simulate the wear processes occurring in vivo. The results from these tests indicated that the wear factor actually decreases with increasing contact stress if the stress was not varied with time. If a time dependent or spatially varying stress was applied, the wear factor can increase greatly when compared to similar magnitude constant contact stress. This effect may be due to the complex relationship between the rate of wear particle generation and the rate at which the particles are released from the interface. The results of these wear experiments are discussed in terms of the influence of the stress conditions upon potential wear processes in total hip and knee prostheses.

The influence of contact stress on the wear of a carburized steel case with a high content of retained austenite

The effect of contact stress on the action of retained austenite in the wear of a carburized steel case under conditions of rolling-sliding contact was investigated. There is a limiting contact stress for the negative and positive effects of retained austenite. Retained austenite has an inhibiting effect on the nucleation and propagation of fatigue cracks during frictional contact.