Strain Rate Response and its effect in plane strain abrasion of metals by a wedge

Abstract

Understanding the wear process is important for preventing loss of material in industry as well as in everyday applications. Many models are available to calculate the wear rate. The simplest models are based on the interaction between a rigid asperity and a plastically compliant asperity. The interaction and deformation give rise to stress fields in the compliant material, and these fields form the basis for estimating the occurrence of material loss. However, the deformation and stress fields in such models are constructed assuming materials with relatively simple mechanical behaviour. Dynamic effects-such as the strainrate response of the material-do not feature in these models. Considering that asperity interactions can generate very high local strain rates even at nominally low sliding speeds, and engineering materials often exhibit significant strainrate sensitivity (e.g., adiabatic shear banding in certain strainrate and temperature regimes), the omission of such effects renders predictions based on these models inaccurate. The aim of the present work is to study the strainrate response of selected engineering materials, with a view to relating such responses qualitatively to the mode of deformation and the traction generated during sliding of wedges on these materials. The materials chosen for the study are titanium, copper, aluminium, and lead. The work has been carried out in three parts:

1. Estimation of average strain and strain rate during wedge sliding The upperbound method is used to estimate the average strain and strain rate generated during the sliding of a hard wedge on a flat surface.

2. Study of strainrate response in compression Since the stress field in deforming asperities is primarily compressive, compression tests were conducted at strain rates up to 100 s¹ (the estimated average strain rate during wedge sliding) and temperatures up to 400°C (representative of interface temperatures during sliding). Materials tested:

Hotrolled and annealed commercially pure titanium (1100 ppm oxygen) - rods and sheets (rolling and transverse directions) Ti6Al4V alloy (1100 ppm oxygen) - rods Coldrolled and annealed OFHC copper (10 ppm oxygen) - sheets Coldrolled and annealed superpurity aluminium (99.997%) - sheets Pure ascast lead (99.99%) - sheets

The applicability of certain constitutive models in predicting the strainrate response in this regime was also evaluated.

3. Wedgesliding experiments Wedge sliding experiments were conducted on the test materials using tungstencarbide wedges at attack angles up to 45°. The wedges were slid on smooth surfaces (CLA < 0.1µm), using constant normal loads and sliding speeds ranging from 1.66 × 10³ ms¹ to 1.66 × 10¹ ms¹, so that the strain rates in the plastically deforming zone did not exceed 100s¹. A custom singlepoint abrasion rig was designed and fabricated, with the capability to measure frictional force during experiments.

Key Findings From upperbound analysis

Average strain and strain rate increase with wedge attack angle and with increasing interfacial friction. The effect of increased interfacial friction is more pronounced at higher attack angles.

From compression tests

Titanium is prone to adiabatic shear banding at high strain rates. Criteria based solely on intrinsic material behaviour do not predict these instabilities; they are classified as geometric instabilities, caused primarily by the state of stress. For copper, aluminium, and lead, no geometric instabilities were observed; instabilities are intrinsic and are well predicted by existing models.

From wedgesliding experiments

The strainrate response of titanium correlates with:

bowwave morphology, and frictionforce recordings.

Titanium shows a highly distorted plastic deformation front, attributable to adiabatic shear band formation.

In other materials, strainrate effects are not strongly reflected in bowwave morphology because their instabilities are intrinsic, not geometric.

Wear Behaviour Implications

Materials prone to geometric instabilities (e.g., titanium at high strain rates) will exhibit wear phenomena localized at the surface or nearsurface layers.

Materials prone to intrinsic instabilities will experience plastic deformation that penetrates deeper, leading to subsurface damage.

Thus, the strainrate response of a material plays a crucial role in determining both the wear mechanism and the wear rate.