Károly Váradi

Experimental and numerical evaluation of the mechanical properties of compacted wear debris layers formed between composite and steel surfaces in sliding contact

Abstract Microindentation experiments and non-linear FE contact analysis were used to study the hardness of a CF/PEEK (carbon fibre/polyetheretherketone) polymer composite material in the presence of a transfer film layer (TFL). The latter resulted from a sliding contact of the composite against a steel counterpart. The TFL plays an important role in the load transmission, and it therefore also affects the wear process. To study these mechanisms, requires reliable material data for the TFL. This study concentrates on TFL obtained at the beginning of a wear process on both components, i.e. the steel disk and the composite pin for two different fibre orientations relative to the sliding direction. Universal hardness values were evaluated from experimental load–depth profiles and their finite element analysis (FEA). The corresponding model contained contact elements that allowed prediction of the contact area, the contact pressure distribution, and the contact stresses and strains.

Evaluation of the real contact areas, pressure distributions and contact temperatures during sliding contact between real metal surfaces

Abstract A three-dimensional elastic contact algorithm has been developed to analyse the normal contact problems of bodies having rough surfaces. The algorithm can evaluate the real contact areas and contact pressure distributions using measured surface roughness data. Following an approximate elastic-plastic contact solution the analysis produces more realistic elastic and plastic contact areas; in addition results of contact pressure distributions can be predicted according to a given maximum plastic limit pressure. The technique can simulate (in an approximate way) the elastic-plastic sliding contact behaviour in the vicinity of asperities or concentrated contact areas by ignoring the effect of the tangential forces on the vertical displacement. Assuming a certain sliding speed and a particular coefficient of friction the local temperature distribution due to the heat generation over the real contact areas can also be calculated for ‘slow sliding’ problems. The results show the moving real contact areas and the contact temperature fields for an electric spark mechanical steel surface moving over a planed bronze surface. Changes of the rigid body displacement, as well as the average and maximum pressures are also presented during sliding. The micro-contact or asperity contact behaviour for bodies having large nominal contact area and the macro-contact behaviour for bodies being in ‘concentrated cotnact’ are also compared. In the latter case an ideal smooth steel ball was slid over the previously mentioned bronze surface.

Finite element analysis of a polymer composite subjected to a sliding steel asperity Part II: Parallel and anti-parallel fibre orientations

Finite element (FE) micro-models have been developed in order to determine contact, stress and strain conditions produced by a steel asperity sliding on the surface of a fibre-reinforced polymer composite. Two cases were studied, i.e. a parallel and an anti-parallel fibre orientation relative to the sliding direction. In order to get more realistic simulation results relating to the failure conditions in the composite structure, FE contact macro/micro-models were used, contrary to the so far widely applied anisotropic analytical or numerical macro-models. To model a “micro-environment” as part of a “macro-environment”, the displacement coupling technique was introduced. The contact analysis operates on both the macro- and the micro-level, applying node-to-node contact elements. The contact results, especially the contact pressure distribution, can characterize the real fibre/matrix micro-system. Displacement and strain results lead to explanations of fibre related phenomena, matrix shear effects, and fibre/matrix debonding events. On the basis of the stress results, conclusions were drawn on the possible wear mechanisms of the fibre-reinforced polymer composite. For parallel fibre orientation, fibre/matrix debonding as a result of shear stresses at the interface, matrix shear type failure and fibre thinning are the dominant sliding wear mechanisms. If an anti-parallel fibre orientation is considered, matrix shear, tension/compression type fibre/matrix debonding and fibre thinning, associated with fibre cracking events, are the most dominant wear mechanisms. To study the wear mechanisms experimentally, diamond tip scratch tests were carried out, showing that the predicted failure events occur also in reality.

Numerical and experimental contact analysis of a steel ball indented into a fibre reinforced polymer composite material

A three-dimensional anisotropic contact algorithm has been developed to analyse the contact behaviour of metal–composite bodies. The elements of the influence matrix are obtained by coupled finite-element anisotropic models. Linear elastic and approximate elastic–plastic techniques can evaluate the contact parameters (the normal approach, the size of the contact area and the contact pressure distribution) following different failure criteria. The contact technique is applied to the problem of a steel ball indented into a polymer composite material having either normal or parallel unidirectional fibre orientation. Finally, the contact results are verified by experimental evaluations. The latter were obtained by the use of a static testing machine, a laser profilometer, an optical microscope and a scanning electron microscope, and they illustrate the real response of the composite structure subjected to ball indentation. Good agreement between both methods could be demonstrated.

Finite Element Simulation of the Fiber–Matrix Debonding in Polymer Composites Produced by a Sliding Indentor: Part I – Normally Oriented Fibers:

To study the contact and debonding behaviors between a CF–PEEK fiber-reinforced polymer composite specimen and a sliding diamond indentor, finite element macro- and micro-models have been developed. Around each fiber, interface elements were introduced in order to detect the tension-type and also the shear-type debondings for different cases. If initial or final debonding has occurred, a “control algorithm” checked the limit strain conditions for the interface elements and changed the material properties according to the actual debonding condition. As a final result, it can be concluded, that the dominant debonding under compression is due to the shear loading conditions.

Comparison of stability in the operative treatment of pelvic injuries in a finite element model

Purpose of the studyThe comparison of the stability of four surgical methods for the treatment of vertically and rotationally unstable type C pelvic ring injuries.MethodsWe produced a type C pelvic ring injury (type Denis II fracture of the sacrum and symphysiolysis) on a finite element model, in the case of standing on both feet. We stabilized the symphysiolysis with a five-hole reconstruction plate; the sacrum fracture was fixed in the first experiment with two, two-hole reconstruction plates on the ventral surface, in the second one we applied dorsally the transsacral, narrow DC plate, in the third one with KFI-H plate, and in the last one with iliosacral screw. Finite element modeling was performed by the use of the ALGOR software. Not only bones and joints, but joints and mechanically important ligaments were modeled as well. We measured the shift between the two surfaces of the fracture gap, compared to the results of measurements accomplished on cadaver models.ResultsLarger shift could be elicited after transsacral plating than after direct plating. These results correspond to those of the parallel investigation of the bony ligamentous cadaver pelvis specimens. The shift values after KFI-H plating and iliosacral screw fixation are larger than after direct plating, but smaller than after transsacral plating. The tension created in the implants is less than the allowed values; therefore, the choice of operation should depend on the type of injury.ConclusionsThe finite element model may be utilized for the comparison of different methods of osteosynthesis for the treatment of injuries described above. Due to several difficulties in investigations performed on cadaver specimens, this model has undoubted utility.

Numerical and Finite Element Contact Temperature Analysis of Steel-Bronze Real Surfaces in Dry Sliding Contact

Numerical techniques have been developed and used to evaluate the contact temperature distribution between real surfaces in sliding contact. The contact temperature is evaluated for the entire range of Peclet numbers. In the case of “slow sliding” problems a stationary numerical technique was used, whereas for “intermediate and fast sliding” problems transient finite element (FE) solutions were preferred. At first, sliding contact of a single spherical steel asperity over a steel or bronze surface was modelled in order to study the contact temperature development on a microscopic level. Contact temperature results for real steel-bronze sliding surfaces are also presented in order to provide information about more realistic stress and wear conditions; the latter will be modelled in future works. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 26–28, 1998

A finite element analysis of bone plates available for prophylactic internal fixation of the radial osteocutaneous donor site using the sheep tibia model.

INTRODUCTION The strengthening effect of prophylactic internal fixation (PIF) with a bone plate at the radial osteocutaneous flap donor site has previously been demonstrated using the sheep tibia model of the human radius. This study investigated whether a finite element (FE) model could accurately represent this biomechanical model and whether stress or strain based failure criteria are most appropriate. METHODS An FE model of an osteotomised sheep tibia bone was strengthened using 4 types of plates with unilocking or bicortical screw fixation. Torsion and 4-point bending simulations were performed. The maximum von Mises stresses and strain failure criteria were studied. RESULTS The strengthening effects when applying stress failure criteria [factor 1.76-4.57 bending and 1.33-1.80 torsion] were comparable to the sheep biomechanical model [factor 1.73-2.43 bending and 1.54-2.63 torsion]. The strongest construct was the straight 3.5mm stainless steel unilocking plate. Applying strain criteria the strongest construct was the straight 3.5mm stainless DCP plate with bicortical screw fixation. CONCLUSIONS The FE model was validated by comparison with the sheep tibia model. The complex biomechanics at the bone-screw interface require further investigation. This FE modelling technique may be applied to a model of the human radius and other sites.

Refinements in osteotomy design to improve structural integrity: a finite element analysis study

Osteotomy cuts are typically made using a saw, and the meeting point acts as a focus for the concentration of stress and failure. We have studied the impact of different designs of osteotomy cut. Cadaver sheep tibias were scanned by computed tomography (CT) and transformed into a computer-aided design (CAD) model. A standard marginal resection defect was created and then modified, and a finite element analysis made. The relative stress concentrations at the intersection of osteotomy cuts were recorded using principal stresses S1, S3, and von Mises stress, von Mises under both 4-point bending and torsion testing. The osteotomy designs studied were: right-angled and bevelled osteotomy end cuts, overcutting, and a stop drill hole. Peak stress values for 4-point bending and torsion were 24-30% greater at the right-angled osteotomy than the bevelled end cut. Overcutting dramatically increased peak stress values caused by bending and torsion by 48% and 71%, respectively. Substantially lower concentrations of stress were noted with a stop hole using both a 90° (bending 38% and torsion 56%), and a tangential (bending 58% and torsion 60%) cut. A bevelled osteotomy has substantially lower concentrations of stress than a right-angled osteotomy. It is important to avoid creating an overcut as this causes an appreciable increase in the concentration of stress, while a stop drill hole substantially reduces the stress. The creation of a stop hole and the use of judicious bevelling techniques are modifications in the design of an osteotomy that are readily applicable to surgical practice.

Wear Simulation of a Reciprocating Seal

Modeling of the complex tribological behavior of the elastomer parts is required when designing sliding seal applications. Friction, wear, and Lubrication mechanisms of rubber-like materials differ from those in case of metals, ceramics, and rigid polymers; therefore, their modeling also requires other techniques. Tribological behavior of a sliding seal was investigated both experimentally and numerically. In the experimental setup, the counterpart of the seal was pressed and rubbed against the section of the seal in various lubrication conditions. The worn surface of the seal was inspected using white light profilometry. The test configuration was modeled by FEA. A wear algorithm (based on the linear wear theory) with an attached damage analysis was applied to the frictional contact simulation. The nonlinear and time dependent material behavior of the seal was also taken into account. The results of the tribological simulation (in which the internal friction and the effects of damage by rupture of the rubber material were considered) are in good agreement with the results of the surface inspections done on the worn seal specimens. The presented wear simulation technique of deactivating elements is suitable for modeling wear that is larger than the size of the elements in the FE mesh.