Anirban Mahato

Surface folding in metals: a mechanism for delamination wear in sliding

Using high-resolution, in situ imaging of a hard, wedge-shaped model asperity sliding against a metal surface, we demonstrate a new mechanism for particle formation and delamination wear. Damage to the residual surface is caused by the occurrence of folds on the free surface of the prow-shaped region ahead of the wedge. This damage manifests itself as shallow crack-like features and surface tears, which are inclined at very acute angles to the surface. The transformation of folds into cracks, tears and particles is directly captured. Notably, a single sliding pass is sufficient to damage the surface, and subsequent passes result in the generation of platelet-like wear particles. Tracking the folding process at every stage from surface bumps to folds to cracks/tears/particles ensures that there is no ambiguity in capturing the mechanism of wear. Because fold formation and consequent delamination are quite general, our findings have broad applicability beyond wear itself, including implications for design of surface generation and conditioning processes.

Geometric flow control of shear bands by suppression of viscous sliding.

Shear banding is a plastic flow instability with highly undesirable consequences for metals processing. While band characteristics have been well studied, general methods to control shear bands are presently lacking. Here, we use high-speed imaging and micro-marker analysis of flow in cutting to reveal the common fundamental mechanism underlying shear banding in metals. The flow unfolds in two distinct phases: an initiation phase followed by a viscous sliding phase in which most of the straining occurs. We show that the second sliding phase is well described by a simple model of two identical fluids being sheared across their interface. The equivalent shear band viscosity computed by fitting the model to experimental displacement profiles is very close in value to typical liquid metal viscosities. The observation of similar displacement profiles across different metals shows that specific microstructure details do not affect the second phase. This also suggests that the principal role of the initiation phase is to generate a weak interface that is susceptible to localized deformation. Importantly, by constraining the sliding phase, we demonstrate a material-agnostic method—passive geometric flow control—that effects complete band suppression in systems which otherwise fail via shear banding.

Nucleation and propagation of solitary Schallamach waves.

We isolate single Schallamach waves--detachment fronts that mediate inhomogeneous sliding between an elastomer and a hard surface--to study their creation and dynamics. Based on measurements of surface displacement using high-speed in situ imaging, we establish a Burgers vector for the waves. The crystal dislocation analogs of nucleation stress, defect pinning, and configurational force are demonstrated. It is shown that many experimentally observed features can be quantitatively described using a conventional model of a dislocation line in an elastic medium. We also highlight the evolution of nucleation features, such as surface wrinkles, with consequences for interface delamination.

Kinematic flow patterns in slow deformation of a dense granular material

The kinematic flow pattern in slow deformation of a model dense granular medium is studied at high resolution using in situ imaging, coupled with particle tracking. The deformation configuration is indentation by a flat punch under macroscopic plane-strain conditions. Using a general analysis method, velocity gradients and deformation fields are obtained from the disordered grain arrangement, enabling flow characteristics to be quantified. The key observations are the formation of a stagnation zone, as in dilute granular flow past obstacles; occurrence of vortices in the flow immediately underneath the punch; and formation of distinct shear bands adjoining the stagnation zone. The transient and steady state stagnation zone geometry, as well as the strength of the vortices and strain rates in the shear bands, are obtained from the experimental data. All of these results are well-reproduced in exact-scale non-smooth contact dynamics simulations. Full 3D numerical particle positions from the simulations allow extraction of flow features that are extremely difficult to obtain from experiments. Three examples of these, namely material free surface evolution, deformation of a grain column below the punch and resolution of velocities inside the primary shear band, are highlighted. The variety of flow features observed in this model problem also illustrates the difficulty involved in formulating a complete micromechanical analytical description of the deformation.

Surface flow in severe plastic deformation of metals by sliding

An in situ study of flow in severe plastic deformation (SPD) of surfaces by sliding is described. The model system – a hard wedge sliding against a metal surface – is representative of surface conditioning processes typical of manufacturing, and sliding wear. By combining high speed imaging and image analysis, important characteristics of unconstrained plastic flow inherent to this system are highlighted. These characteristics include development of large plastic strains on the surface and in the subsurface by laminar type flow, unusual fluid-like flow with vortex formation and surface folding, and defect and particle generation. Preferred conditions, as well as undesirable regimes, for surface SPD are demarcated. Implications for surface conditioning in manufacturing, modeling of surface deformation and wear are discussed.

Vehicle Lightweighting: Challenges and Opportunities with Aluminum

Rising energy costs, consumer preferences and regulations drive requirements for fuel economy, performance, comfort, safety and cost of future automobiles. These conflicting situations offer challenges for vehicle lightweighting, for which aluminum applications are key. This paper describes product design needs and materials and process development opportunities driven by theoretical, experimental and modeling tools in the area of sheet and castings. Computational tools and novel experimental techniques used in their development are described. The paper concludes with challenges that lie ahead for pervasive use of aluminum and the necessary fundamental R&D that is still needed.

In Situ Study of Plastic Flow at Sliding Metal Surfaces

An in situ study of deformation and flow at sliding metal interfaces is described. The model system used—a hard wedge sliding against a metal surface—is representative of wear, and surface conditioning processes typical of manufacturing. By combining high speed imaging with image analysis, important characteristics of unconstrained plastic flow intrinsic to these processes are highlighted. These characteristics include unusual fluid-like flow, surface folding, wear particle formation by folding, and large plastic strains on the surfaces. Implications for sliding wear and quality of surfaces generated by manufacturing processes are discussed.

Understanding deformation on machined surfaces

The deformation history and state of machined surface in cutting of metals are characterized using high-speed image analysis, complemented by hardness, microstructure and texture measurements. Large surface strains are observed, due to the severe plastic deformation (SPD) intrinsic to chip formation. The deformation history and microstructure/texture of the chip and surface are found to be equivalent. The surface strain distribution is shown to scale with undeformed chip thickness and influenced by the rake angle. Based on the observations, and related prior work on process-microstructure-property correlations, a framework is suggested for controlling deformation levels, microstructure and texture on machined surfaces. The results offer scope also for validation of machining simulations and multi-scale modeling of machining.Copyright

Unconstrained plastic flow at surfaces in sliding and cutting

An in situ study of deformation and material removal at high–resolution in a model system of a hard steel wedge sliding against metals is described. The mesoscale system is representative of wear, and material removal processes typical of manufacturing. By combining high speed imaging with image correlation analysis, important characteristics of unconstrained plastic flow intrinsic to these processes are highlighted. These characteristics include unusual fluid–like flow, surface folding, a new mechanism of wear particle formation by folding, tears and crack–like features triggered by non–laminar surface flow, and large plastic strains on the surfaces and chip/wear particles. Implications for sliding wear and quality of surfaces generated by manufacturing processes are discussed. The results demonstrate also the power of the direct observational approach for characterising flow phenomena in these engineering systems.