Strong Shear Component Research Articles

Analysis of the Strain Distribution and Texture Measurements in Asymmetric Rolling (AR) and Asymmetric Accumulative Roll Bonding (AARB)

Severe plastic deformation (SPD) with strong shear component is required to promote both grain refinement and texture randomization. When Asymmetric rolling (AR) is applied as asymmetric accumulative roll bonding (AARB), it enables the production of architectured microstructures and metallic composites. Finite element (FE) simulations of AR and AARB were employed to understand the influence of pass thickness reduction (PTR) on the through thickness variation of the velocity gradient. The influence of the PTR up to a total thickness reduction of 50% and the effect of a single 50% reduction step in a bi-layer bonding condition was analyzed. The influence of these process parameters on the strain and rigid body rotation components was compared with the experimental data obtained on an AA1050 aluminum. A better shear to compression ratio across the sheet thickness is achieved by PTRs lower than 30%; at a PTR of 50% the texture is dominated by the frictional shear generated at the roll-sheet interface and the process has a stronger compressive character. This indicates that simple ARB followed by AR with smaller PTRs should generate a better shear distribution than AARB alone.

Brittle‐Ductile Deformation and Tensile Rupture of Dome Lava During Inflation at Santiaguito, Guatemala

AbstractGas‐and‐ash explosions at the Santiaguito dome complex, Guatemala, commonly occur through arcuate fractures, following a 5‐ to 6‐min period of inflation observed in long‐period seismic signals. Observation of active faults across the dome suggests a strong shear component, but as fault propagation generally proceeds through the coalescence of tensile fractures, we surmise that explosive eruptions require tensile rupture. Here, we assess the effects of temperature and strain rate on fracture propagation and the tensile strength of Santiaguito dome lavas. Indirect tensile tests were conducted on samples with a porosity range of 3–30% and over diametral displacement rates of 0.04, 0.004, and 0.0004 mm/s. At room temperature, the tensile strength of dome rock is rate independent (within the range tested) and inversely proportional to the porosity of the material. At eruptive temperatures we observe an increasingly ductile response at either higher temperature or lower displacement rate, where ductile deformation is manifest by a reduction in loading rate during constant deformation rate tests, resulting in slow tearing, viscous flow, and pervasive damage. We propose a method to conduct indirect tensile tests under volcanic conditions using a modification of the Brazilian disc testing protocol and use brittleness indices to classify deformation modes across the brittle‐ductile transition. We show that a degree of ductile damage is inevitable in the lava core during explosions at the Santiaguito dome complex and discuss how strain leading to rupture controls fracture geometry, which would impact gas pressure release or buildup and regulate explosive activity.

A brittle failure model for long‐period seismic events recorded at Turrialba Volcano, Costa Rica

AbstractA temporary seismic network, consisting of 23 broadband and six short‐period stations, was installed in a dense network at Turrialba Volcano, Costa Rica, between 8 March and 4 May 2011. During this time 513 long‐period (LP) events were observed. Due to their pulse‐like waveforms, the hypothesis that the events are generated by a slow‐failure mechanism, based on a recent new model by Bean et al. (2014), is tested. A significant number (107) of the LPs are jointly inverted for their source locations and mechanisms, using full‐waveform moment tensor inversion. The locations are mostly shallow, with depths < 800 m below the active Southwest Crater. The results of the decompositions of the obtained moment tensor solutions show complex source mechanisms, composed of high proportions of isotropic and low, but seemingly significant, proportions of compensated linear vector dipole and double‐couple components. It is demonstrated that this can be explained as mode I tensile fracturing with a strong shear component. The source mechanism is further investigated by exploring scaling laws within the data. The LPs recorded follow relationships very similar to those of conventional earthquakes, exhibiting frequency‐magnitude and corner frequency versus magnitude relationships that can be explained by brittle failure. All of these observations indicate that a slow‐failure source model can successfully describe the generation of short‐duration LP events at Turrialba Volcano.

Statistical properties of passive scalar in a random flow with a strong shear component

We study mixing of passive scalar by a chaotic velocity field with a relatively strong regular shear component. We show that the tail of partition distribution function (PDF) of coarse-grained passive scalar field differs qualitatively from the corresponding asymptotics in the case of isotropic flow statistics.

Polymer dynamics in chaotic flows with a strong shear component

We consider the dynamics of a polymer molecule injected into a chaotic flow with a strong mean shear component. The polymer experiences aperiodic tumbling in such flows. We consider a simplified model of the chaotic velocity field given by the superposition of a steady shear flow and a large-scale isotropic short-correlated random component. In the framework of this model, we present a detailed study of the statistical properties of single-polymer dynamics. We obtain the stationary probability distribution function of the polymer orientation, find the distribution of time periods between consequent events of tumbling, and find the tails of the polymer size distribution.

Microstructural development in asymmetric processing of tantalum plate

Sputtered tantalum (Ta) and TaN are employed as barrier layers in modern integrated circuits to enable reliable use of Cu as an interconnect material. The directional properties of sputtering Ta can result in nonuniform film thicknesses (from heavily textured plate) and unpredictable sputtering rates (from plates with through thickness texture gradients). This results in film thicknesses larger than necessary because of the sputtering being unpredictable. This presentation reports on an effort to increase textural and grain size uniformity in Ta by deforming the plate under conditions that simulate asymmetric rolling. This is accomplished by using a channel die configuration with uneven friction on the top and bottom plates so that a strong shear component is added to the plane strain condition enforced by the channel dies. Results indicate a trend towards less severe texture banding and more uniform structure in the plate processed by asymmetric friction conditions.

Theory and experiment of martensitic nucleation in ZrO2 containing ceramics and ferrous alloys

An in situ TEM experiment on martensitic nucleation was performed using sub-micron ZrO2 particles which were initially defect free. Extrinsic, spontaneous and heterogeneous nucleation was induced by introducing dislocation loops with a strong shear component and Hertzian contact stresses into selected particles. The critical characteristics of these defects which were required to trigger barrierless nucleation, in terms of the loop radius, Burgers vector, contact stress, and contact area were in good agreement with an analytic elasticity model. Along with dislocation walls, these defects which have a short range stress field can also account for intrinsic nucleation in ZrO2 containing ceramics and ferrous alloys. The essential features of nucleating defects, which may be classified into intrinsic or extrinsic ones depending on their origins, and short range or long range ones depending on their stress fields, are analyzed and contrasted in view of their experimental evidence.