Quantitative In Situ Analysis of Deformation in Sliding Metals: Effect of Initial Strain State

Abstract

Using in situ, high-speed imaging of a hard wedge sliding against pure aluminum, and image analysis by particle image velocimetry, the deformation field in sliding is mapped at high resolution. This model system is representative of asperity contacts on engineered surfaces and die–workpiece contacts in deformation and machining processes. It is shown that large, uniform plastic strains of 1–5 can be imposed at the Al surface, up to depths of 500 μm, under suitable sliding conditions. The spatial strain and strain rate distributions are significantly influenced by the initial deformation state of the Al, e.g., extent of work hardening, and sliding incidence angle. Uniform straining occurs only under conditions of steady laminar flow in the metal. Large pre-strains and higher sliding angles promote breakdown in laminar flow due to surface fold formation or flow localization in the form of shear bands, thus imposing limits on uniform straining by sliding. Avoidance of unsteady sliding conditions, and selection of parameters like sliding angle, thus provides a way to control the deformation field. Key characteristics of the sliding deformation such as strain and strain rate, laminar flow, folding and prow formation are well predicted by finite element simulation. The deformation field provides a quantitative basis for interpreting wear particle formation. Implications for engineering functionally graded surfaces, sliding wear and ductile failure in metals are discussed.

Appendix: Finite Element Analysis

As seen in the experiments, the initial WP deformation state has a major influence on the subsequent sliding-induced deformation. This aspect had been neglected in prior analyses of sliding deformation using slipline field (SLF) [10, 11, 21–23, 30, 31] and FEA [23]. To address this deficiency, a specially devised 2-pass sliding process with WPs of ɛ _o  = 0, 0.4 was analyzed. The simulation of the first pass provided the strain, strain rate and other flow details for ɛ _o  = 0, 0.4. Since the surface strain imposed in the first sliding pass in these two WPs was ~1, the simulation of the second pass provided deformation data for ɛ _o in the 1–2 range. In the analysis, the cumulative surface strain at the end of the first sliding pass was used as the ɛ _o for the second pass. Thus the entire range of ɛ _o used in the experiments (0 to 2) was covered in the simulation.

ABAQUS/EXPLICIT solver was used to simulate the sliding-induced deformation in the aluminum 1100-0. The wedges were modeled as stiff elastic bodies with θ = 20°, 25°. In the interest of computational efficiency, the 2-pass sliding was simulated in a single FEA pass by having two wedges, separated by a horizontal distance of 4.25 mm, traverse the WP surface in sequence. About 120,000 four-nodded, plane strain elements with reduced integration (CPE4R) and aspect ratio of 0.7 were used to mesh the specimens. The element size is smallest in a fine-meshed region close to the WP surface. This region consists of 2250 elements along the length of the WP, and 48 elements into the depth, with an element size of 10.16 µm. The wedges were held fixed in the horizontal direction, while the WP was pushed against the wedges with a velocity of 5 mm/s. An initial depth h _o  = 100 µm was used for the leading wedge. To better replicate experiment conditions, where the compliance of the wedge-tool holder gradually relaxes the initial h _o into a lower value, the leading wedge was provided with a programmed upward motion. This resulted in gradual retraction of the leading wedge after the formation of a leading prow of approximately the height h _p observed in the experiments. The trailing wedge was held fixed, providing an effective h _o as selected in the second sliding pass of the experiments.

The annealed Al (ɛ _o  = 0) was modeled as a rate-dependent plastic solid with yield stress data from [39] and rate-dependent yield stress ratios from [40]. The ɛ _o  = 0.4 WP was simulated by shifting the flow–stress curve of the ɛ _o  = 0 material leftward by a plastic strain of 0.4. This ensured a first yield stress σ _Y of about 130 MPa at a strain rate of 1/s.

Contact constraints at the wedge–WP interface were enforced kinematically. A capped Coulomb friction model was used for all contacts, with a friction coefficient µ = 0.1 and maximum interfacial shear stress τ _max of 127 MPa. Since sliding occurs under essentially quasi-static conditions, adequate mass scaling was used to ensure a reasonable minimum steady time increment and thus overall simulation time.