Vikas Prakash

Frictional Response of Sliding Interfaces Subjected to Time Varying Normal Pressures

In the present investigation a plate-impact pressure shear loading device is employed to study frictional characteristics of sliding interfaces subjected to step changes in normal pressure. The present experimental configuration represents a significant improvement over the conventional tribology experiments by allowing the control of interfacial tractions through the use of pressure-shear loading waves instead of manipulating actuator motion. Moreover, the expermental configuration allows critical frictional parameters such as the applied normal pressure, the interfacial slip resistance, and the interfacial slip velocity to be interpreted by using the framework of one-dimensional plane wave analysis. The experimental results, deduced from the response to step changes imposed on the normal pressure at the frictional interface, reinforce the importance of including frictional memory in the development of the rate-dependent state variable friction models. The scope of the above experiments include technologically important combinations of workpiece materials such as 4340VAR structural steel and a commercially available titanium alloy (Ti-6Al-4V), and tool materials such as tungsten based tool cermets ( WC-Co alloys).

A pressure-shear plate impact experiment for investigating transient friction

A pressure-shear plate impact experiment is introduced to study time-resolved friction at interfaces subjected to high sliding speeds under relatively high normal pressures. The conditions of slip at the interface are varied by changing the surface roughness of the impacting plates and by varying the applied normal to shear stress ratio. The configuration offers the simplicity of allowing the interpretation of the experimental data by using the frame-work of one-dimensional plane wave analysis. The interfacial material pairs investigated in the present study are comprised of a wear-resistant grade of tungsten carbide and either an AISI 4340 steel or a Ti-6AI-4V alloy. The experimental results indicate that the coefficient of friction increases with the increase in surface roughness of the tungsten-carbide plates and with cumulative slip at the interface.

A modified torsional kolsky bar for investigating dynamic friction

This paper introduces an experiment to investigate dry sliding resistance of frictional interfaces at normal pressures up to 100 MPa, slip speeds up to 10 m/s and slip distances of approximately 10 mm. This new apparatus involves a novel modification of the conventional torsional Kolsky bar apparatus, employed extensively in the past for investigating high strain rate behavior of engineering materials. The new experimental configuration represents a significant improvement over conventional tribology experiments because it uses elastic torsional waves with a superimposed static compressive force to control the interfacial traction. Moreover, the apparatus allows critical frictional parameters such as the interfacial sliding resistance, slip speeds and slip without the use of transducers at the frictional interface. The usefulness of the device is demonstrated by presenting results of high-speed friction on 6061-T6 Al/1018 steel and Carpenter Hampden tool steel/7075-T6 Al tribo pairs.

Origin of pulverized rocks during earthquake fault rupture

The origin of pulverized rocks (PR) in surface outcrops adjacent to the fault cores of the San Andreas and other major faults in Southern California is not clear, but their structural context indicates that they are clearly associated with faulting. An understanding of their origin might allow inferences to be drawn about the nature of dynamic slip on faults, including rupture mechanisms and their speed during earthquakes. In the present study, we use split Hopkinson bar recovery experiments to investigate whether PR can be produced under dynamic stress wave loading conditions in the laboratory and whether PR is diagnostic of any particular process of formation. The results of the study indicate that in Westerly granite for transition from sparse fracture to pervasive pulverization requires high strain rates in excess of 250/s and that the formation of PR may be inhibited at the larger burial depths. The constraint imposed by field observations of the relatively low strains (1-3%) in PR recovered from the field and the laboratory derived threshold for the critical strain rate (similar to 250/s and higher) together indicate that a dynamic supershear-type rupture may be necessary for the origin of pulverized rocks at distances of tens of meters away from the fault plane as observed in the field for both large strike-slip-type and the relatively small dip-slip-type fault ruptures in nature.

Novel experimental techniques for investigating time resolved high speed friction

Abstract The paper presents some recent advances in experimental methods to investigate the phenomenon of high speed friction. In particular, two novel experimental methods are employed in the present study: (a) the plate impact pressure–shear friction experiment, and (b) the torsional Kolsky bar friction experiment. Using these experimental configurations, dynamic frictional characteristics of Carpentor Hampden tool-steel/Ti6Al4V and Carpenter Hampden tool-steel/7075-T6 Al are investigated under conditions of high interfacial normal pressures (100 MPa to 2 GPa), high slip speeds (1 to 60 m/s) and elevated temperatures. The results of these experiments provide some new insights into the role of the applied normal pressure, interfacial slip speeds, interfacial temperature, and the surface roughness in controlling the tribology of dry sliding interfaces.

Sensitivity of thermal conductivity of carbon nanotubes to defect concentrations and heat-treatment

In the present work, we use reverse non-equilibrium molecular dynamics with adaptive intermolecular reactive empirical bond order interatomic potential to investigate sensitivity of thermal conductivity in (6, 6) single-walled carbon nanotubes (SWCNTs) to side-wall defects and high temperature heat- treatment. Effects of two side-wall defect types and their concentrations are evaluated: chemisorbed hydrogen adatoms on the SWCNT side wall and point vacancy defects. The results of the simulations indicate that the degree of hydrogenation and vacancy concentrations have very similar detrimental effects on the thermal conductivity of (6, 6) SWCNTs. Vacancy repair is evident with heat treatment, and heat-treatment temperatures of 3000 °C for up to 22 ns are found to transform point vacancies into various non-hexagonal side-wall defects. The vacancy repair is accompanied by an approximately 10% increase in thermal conductivity. In addition, thermal conductivity measurements in both heat-treated and non-heat tre...

Time-Resolved Friction with Applications to High-Speed Machining: Experimental Observations

Pressure-shear plate impact experiments are conducted to study the kinetics of friction on a submicrosecond time scale. The configuration offers the simplicity of allowing the interpretation of the experimental results by using the framework of one-dimensional plane wave analysis. To address the problem of high-speed machining, one of the impacting plates is chosen to correspond to a wear-resistant grade of tungsten-carbide tool material and the other to a commercial-grade titanium alloy (Ti-6Al-4V). The interfacial conditions are varied from sticking to slipping by varying the angle of impact and/or the surface roughness. Also, within a single experiment, transitions from conditions of fully sticking to intermittent, to full sliding are introduced by subjecting the interface to step changes in normal pressure. The experimentally obtained time-resolved frictional response of the interface is used to examine the role of the interfacial normal pressure, slip velocity, slip distance, and surface roughness in...

Dynamic deformation and fracture behavior of novel damage tolerant discontinuously reinforced aluminum composites

Abstract Extrinsically toughened discontinuously reinforced aluminum composites are processed with the objective of enhancing damage tolerance of conventional particle reinforced aluminum composites. The approach consists of producing a composite microstructure in which discrete ductile phases have been incorporated into the particle reinforced metal matrix via traditional powder processing routes. The present study focuses on investigating the effects of volume fraction and flow strength of the ductile phase reinforcements in determining dynamic deformation and fracture characteristics of these extrinsically toughened composites. The dynamic compression behavior of the composites is examined by employing the split Hopkinson pressure bar. The measured dynamic stress–strain response of the composites is correlated with the macro- and micro-damage mechanisms inferred from post examination of the impacted specimens. The dynamic fracture characteristics of the composites are obtained by impact loading pre-cracked three point bend specimens in a modified Hopkinson bar apparatus. The measured load-point force versus load-point displacement curves are used to, (a) estimate the energy required for dynamic crack initiation, and (b) understand the interaction of the dynamically propagating crack tip with the ductile phase reinforcements. The results indicate that the extrinsically toughened DRA composites absorb significantly greater energy during the crack propagation as compared to the conventional DRA composites. Also, the level of extrinsic toughening introduced in the composites is affected by the location, volume fraction and mechanical properties of the ductile phase reinforcements. Amongst the relatively larger volume fraction ductile-phase reinforced composites, the ductile phase reinforcements comprising low flow strength commercial purity aluminum fail primarily in a ductile manner, whereas the ductile phase reinforcements comprising high strength Al alloy fail in a cleavage manner by inter-granular fracture.

Boxing and mixed martial arts: preliminary traumatic neuromechanical injury risk analyses from laboratory impact dosage data

OBJECT In spite of ample literature pointing to rotational and combined impact dosage being key contributors to head and neck injury, boxing and mixed martial arts (MMA) padding is still designed to primarily reduce cranium linear acceleration. The objects of this study were to quantify preliminary linear and rotational head impact dosage for selected boxing and MMA padding in response to hook punches; compute theoretical skull, brain, and neck injury risk metrics; and statistically compare the protective effect of various glove and head padding conditions. METHODS An instrumented Hybrid III 50th percentile anthropomorphic test device (ATD) was struck in 54 pendulum impacts replicating hook punches at low (27-29 J) and high (54-58 J) energy. Five padding combinations were examined: unpadded (control), MMA glove-unpadded head, boxing glove-unpadded head, unpadded pendulum-boxing headgear, and boxing glove-boxing headgear. A total of 17 injury risk parameters were measured or calculated. RESULTS All padding conditions reduced linear impact dosage. Other parameters significantly decreased, significantly increased, or were unaffected depending on padding condition. Of real-world conditions (MMA glove-bare head, boxing glove-bare head, and boxing glove-headgear), the boxing glove-headgear condition showed the most meaningful reduction in most of the parameters. In equivalent impacts, the MMA glove-bare head condition induced higher rotational dosage than the boxing glove-bare head condition. Finite element analysis indicated a risk of brain strain injury in spite of significant reduction of linear impact dosage. CONCLUSIONS In the replicated hook punch impacts, all padding conditions reduced linear but not rotational impact dosage. Head and neck dosage theoretically accumulates fastest in MMA and boxing bouts without use of protective headgear. The boxing glove-headgear condition provided the best overall reduction in impact dosage. More work is needed to develop improved protective padding to minimize linear and rotational impact dosage and develop next-generation standards for head and neck injury risk.

Effect of high strain rates on peak stress in a Zr-based bulk metallic glass

The mechanical behavior of Zr41.25Ti13.75Cu12.5Ni10Be22.5 (LM-1) has been extensively characterized under quasistatic loading conditions; however, its mechanical behavior under dynamic loading conditions is currently not well understood. A Split–Hopkinson pressure bar (SHPB) and a single-stage gas gun are employed to characterize the mechanical behavior of LM-1 in the strain-rate regime of 102–105/s. The SHPB experiments are conducted with a tapered insert design to mitigate the effects of stress concentrations and preferential failure at the specimen-insert interface. The higher strain-rate plate-impact compression-and-shear experiments are conducted by impacting a thick tungsten carbide (WC) flyer plate with a sandwich sample comprising a thin bulk metallic glass specimen between two thicker WC target plates. Specimens employed in the SHPB experiments failed in the gage-section at a peak stress of approximately 1.8 GPa. Specimens in the high strain-rate plate-impact experiments exhibited a flow stress i...