Timothy C. Ovaert

Indentation of an Orthotropic Half-Space by a Rigid Ellipsoidal Indenter

Anisotropic materials represent a unique class of materials, including crystals, wood, thin-films, and composites. Existing work in the literature has provided the engineering community with an in-depth understanding of isotropic contact mechanics, and the methodologies for solving isotropic contact problems have been fully developed. Anisotropic material systems, however, are more complex and their analysis is less fully-developed in the literature. Presented here is the analysis of indentation by a rigid ellipsoidal indenter against an orthotropic half-space, with the surface of the half-space parallel to two of the axes of material symmetry, derived, from stress equilibrium. A numerical method has been used to solve for the contact parameters (contact dimensions and normal approach) for both orthotropic and transversely isotropic material systems.

On the Wear Behavior of Longitudinally (Parallel) Oriented Unidirectional Fiber-Reinforced Polymer Composites

A simple model for failure and wear of continuous unidirectional fiber-reinforced polymer composites with fiber orientation longitudinal (parallel) to the contact plane and sliding direction is presented. Failure or weakening of the immediate surface region, caused by fiber cracking, with subsequent fiber-matrix separation and wear, is the basis for the proposed model. The composite is modeled as an anisotropic (transversely isotropic) half-space whose effective elastic properties are estimated from composite micro-mechanical considerations. The counterface is modeled as an ideal rigid hemispherical indenter sliding against the composite. In this contact configuration, indentation by a hemispherical asperity produces a generally elliptical contact region whose geometry depends on the elastic constants of the composite. The individual fiber is modeled as an infinite beam on an elastic foundation, with the foundation stiffness approximated from the results of the anisotropic contact simulation. From these r...

A Rough Surface Contact Model for General Anisotropic Materials

A method for solving the two-dimensional (2-D) isothermal rough surface contact problem of general anisotropic materials with friction is presented. By using Strohs formalism, the surface displacements of an elastic half-space due to uniform distributions of traction over a strip are derived from the surface Greens function. The surface displacement and subsurface stresses of the anisotropic half-space due to the distributed contact pressure may then be calculated by superposition. The real contact area and the contact pressure are determined via an iteration scheme using the conjugate gradient method.

Wear of Unidirectional Polymer Matrix Composites with Fiber Orientation in the Plane of Contact

In a previous paper, a simple model for failure and wear of unidirectional fiber-reinforced polymer composites was proposed. The model relates fiber stresses and sliding contact conditions to bulk wear rates for fiber orientation parallel to the sliding direction. In this work, the model has been extended to examine the case where the fibers are oriented transverse to the sliding direction, while still parallel to the plane of contact. When simulating the counterface contact with an ideal hemispherical asperity, the model overpredicts wear rates for the transverse fiber orientation. However, when the counter-face contact is simulated according to observed composite material transfer and corresponding surface topographical changes, indicating deviation from an ideal hemispherical asperity to a more general elongated or ellipsoidal form, the model correlation compares more favorably with existing friction and wear test data in the literature. Presented at the 50th Annual Meeting in Chicago, Illinois May 14–...

Wear Particle Formation of Polymers Against Smooth Hardened Steel Counterfaces

Previous work in the literature has demonstrated the application of a non-Hertzian contact modeling and stress calculation approach for polyetheretherketone (PEEK) sliding on smooth soft steel counter-faces in the three-pin-on-disk configuration. In this investigation, the non-Hertzian model was applied to PEEK, acrylobutadiene-styrene (ABS), and polyamide-imide (PAI) pins after sliding against smooth hardened steel counterfaces. The experimentally determined wear particle thicknesses for PEEK and ABS display a correlation with the location of a relative maximum subsurface von Mises stress. For PAI, wear particles of measurable size could not be collected due to the very low wear rate and due to the formation and adherence of a thin transfer film on the counterface during sliding. Fatigue/delamination was believed to be the dominant wear mechanism due to the observed wear particle morphology. In this investigation, the Paris law was used in conjunction with wear test data and the observed fatigue/delamina...

Effect of asperity-scale tensile stresses on the wear behavior of normally oriented fiber-reinforced polymer composites

Wear of unidirectional continuous fiber-reinforced polymer composites with fiber orientation normal to the contact plane, sliding over scribed stainless steel disks having a controlled surface topography, is investigated in relation to the tensile stresses developed near the contact region. The composite is modeled as a transversely isotropic half-space whose effective elastic moduli are estimated from composite micro-mechanical considerations. The scribed disk is treated as a rough surface whose controlled topographical features serve as model hemispherical indenters against the composite. With friction coefficients obtained from, the wear experiments, the tensile stress field at and below the composite surface is estimated. From this, an estimate of the theoretical depth of fiber-matrix separation (fiber debonding) is calculated based on the composite transverse tensile strength. A correlation between the wear rate and theoretical depth of debonding was shown for several composites. Presented at the 48t...

Theoretical Estimates of Asperity-Scale Stresses in Normally-Oriented Continuous Fiber-Reinforced Composites

Fiber-reinforced polymer matrix composites represent a class of materials with enhanced mechanical and tribological capabilities over the non-reinforced matrices. Past studies have shown that fiber orientation in relation to the plane of contact has a significant effect on tribological performance. In general, when the fibers are oriented normal to the sliding interface, wear is minimized. In this study, stresses in the near-surface region to a depth on the scale of the asperity dimensions are estimated for normally-oriented continuous fiber-reinforced composites. The composite is treated as a transversely isotropic half-space whose properties are estimated from the micro-mechanical considerations of structural composites. Asperity contacts are represented by a hemispherical indentor on the transversely isotropic half space with a known sliding friction coefficient. The surface tensile stresses and their effect on wear are discussed in light of the results. Presented at the 47th Annual Meeting In Philadel...

Three-Dimensional Rough Surface Contact Model for Anisotropic Materials

By applying the line integral of Barnett-Lothe tensors on oblique planes, the three-dimensional rough surface contact problem for a semi-infinite anisotropic elastic half-plane in contact with a rough rigid sphere is formulated. The conjugate gradient technique of analytical continuation was employed to determine the contact parameters. The general solutions due to varying degrees of anisotropy and mechanical boundary conditions are obtained, and examples with fiber-reinforced composites are presented.

Optimal Design of Layered Structures Under Normal (Frictionless) Contact Loading

A methodology is presented for the optimization of a tribological contact configuration, namely, a multi-layered elastic structure under normal (frictionless) point contact loading. This work is aimed at developing an algorithm by which the finite element (FE) mesh and the corresponding structure may be generated automatically for each variation in the vector of design variables during optimization iterations. The FE model for contact analysis may be developed in a given commercial solver such as ABAQUS or ANSYS. To do this, a flexible mesh generator, which interfaces with the FE model and the optimizer, was developed. The optimization scheme is implemented using a simulated annealing (SA) algorithm as the optimizer with an axisymmetric (point contact) FE indentation model in the commercial finite element solver ABAQUS. The results suggest that conventional optimization methods may be employed to examine the design of tribological contact configurations such as multi-layered structures, working seamlessly within the operating system shell (e.g., Unix), and the finite element solver.

Effect of Coating Geometry on Contact Stresses in Two-Dimensional Discontinuous Coatings

Recent experimental investigations have shown that discontinuous coatings, characterized by island-like coating deposits on dissimilar substrates, can exhibit improved tribological performance over equivalent continuously-coated substrates. In this analysis, the effrect of coating geometry on the normal contact pressure profile was examined for several two-dimensional discontinuous coatings using a numerical elastic stress model. Normal pressure singularities were found for discontinuous regions were examined, the normal pressure singularities were reduced or eliminated. Interfacial tensile stress, due to an imposed tangential friction force, was also investigated. The magnitude of this tensile stress (and stress singularities due to edge configuation) was most affected by the friction coefficient and by the discontinuous coating geometry in the middle of the contact region, where the normal contact pressure was the highest. The discontinuous coating has the potential to provide reservoirs of sacrificial solid lubricants, which wear away with the coating itself, providing a friction-reduction mechanism over the life of the coating reducing the interfacial tensile stress which can lead to premature coating failure.