A quantitative criterion for predicting solid-state disordering during biaxial, high strain rate deformation

A criterion to predict the onset of disordering under biaxial loading based on a critical potential energy per atom was studied. In contrast to previous theories for disordering, this criterion incorporates the effects of strain rate and strain state. The strain state (or stress state) is defined by the combination of strain (or stress) magnitudes and directions that are applied to each sample during the simulation. Τhe validity of this criterion was studied using molecular dynamic (MD) simulations of Ag conducted over a wide range of biaxial strain rates, strain configurations, and crystal orientations with respect to the applied stress state. Biaxial strains were applied in two different planes, (112 ̅) and (001) in eight directions in each plane. Results showed that, when larger strain rates were applied, there was a transition from plastic deformation driven by the nucleation and propagation of dislocations to disordering and viscous flow. Although the critical strain rate to initiate disorder was found to vary in the range of ε ̇ = 1×1011 s-1 to ε ̇ = 4×1011 s-1, a consistent minimum PE/atom of -2.7 eV was observed over a broad range of strain states and for both crystallographic orientations that were studied. This indicates that the critical PE/atom is a material property that can be used to predict the onset of disordering under biaxial loading. Further, the results showed that this criterion can be applied successfully even when non-uninform strain states arise in the crystal.

Balanced ellipsoidal vortex equilibria in a background shear flow at finite Rossby number

We consider a uniform ellipsoid of potential vorticity (PV), where we exploit analytical solutions derived for a balanced model at the second order in the Rossby number, the next order to quasi-geostrophic (QG) theory, the so-called QG+1 model. We consider this vortex in the presence of an external background shear flow, acting as a proxy for the effect of external vortices. For the QG model the system depends on four parameters, the height-to-width aspect ratio of the vortex, $h/r$ , as well as three parameters characterising the background flow, the strain rate, $\gamma$ , the ratio of the background rotation rate to the strain, $\beta$ , and the angle from which the flow is applied, $\theta$ . However, the QG+1 model also depends on the PV, as well as the Prandtl ratio, $f/N$ ( $f$ and $N$ are the Coriolis and buoyancy frequencies, respectively). For QG and QG+1 we determine equilibria for different values of the background flow parameters for increasing values of the imposed strain rate up to the critical strain rate, $\gamma _c$ , beyond which equilibria do not exist. We also compute the linear stability of this vortex to second-order modes, determining the marginal strain $\gamma _m$ at which ellipsoidal instability erupts. The results show that for QG+1 the most resilient cyclonic ellipsoids are slightly prolate, while anticyclonic ellipsoids tend to be more oblate. The highest values of $\gamma _m$ occur as $\beta \to 1$ . For large values of $f/N$ , changes in the marginal strain rates occur, stabilising anticyclonic ellipsoids while destabilising cyclonic ellipsoids.

Excitation of airwaves by bubble bursting in suspensions : regime transitions and implications for basaltic volcanic eruptions

Basaltic magma becomes more viscous, solid-like (elastic), and non-Newtonian (shear-thinning, non-zero yield stress) as its crystal content increases. However, the rheological effects on bubble bursting and airwave excitation are poorly understood. Here we conduct laboratory experiments to investigate these effects by injecting a bubble of volume V into a refractive index-matched suspension consisting of non-Brownian particles (volumetric fraction $$\phi$$ ϕ ) and a Newtonian liquid. We show that depending on $$\phi$$ ϕ and V, airwaves with diverse waveforms are excited, covering a frequency band of $$f = {\mathcal {O}}(10-10^4)$$ f = O ( 10 - 10 4 ) Hz. In a suspension of $$\phi \le 0.3$$ ϕ ≤ 0.3 or in a suspension of $$\phi = 0.4$$ ϕ = 0.4 with a V smaller than critical, the bubble bursts after it forms a hemispherical cap at the surface and excites a high-frequency (HF) wave ($$f \sim 1-2 \times 10^4$$ f ∼ 1 - 2 × 10 4 Hz) with an irregular waveform, which likely originates from film vibration. However, in a suspension of $$\phi = 0.4$$ ϕ = 0.4 and with a V larger than critical, the bubble bursts as soon as it protrudes above the surface, and its aperture opens slowly, exciting Helmholtz resonance with $$f = {\mathcal {O}}(10^3)$$ f = O ( 10 3 ) Hz. Superimposed on the waveform are an HF wave component excited upon bursting and a low-frequency ($$f = {\mathcal {O}}(10)$$ f = O ( 10 ) Hz) air flow vented from the deflating bubble, which becomes dominant at a large V. We interpret this transition as a result of the bubble film of a solid-like $$\phi = 0.4$$ ϕ = 0.4 suspension, being stretched faster than the critical strain rate such that it bursts by brittle failure. When the Helmholtz resonance is excited by a bursting bubble in a suspension whose surface level is further below the conduit rim, an air column (length L) resonance is triggered. For L larger than critical, the air column resonance continues longer than the Helmholtz resonance because the decay rate of the former becomes less than that of the latter. The experiments suggest that a bubble bursting at basaltic volcanoes commonly excites HF wave by film vibration. The Helmholtz resonance is likely to be excited under a limited condition, but if detected, it may be used to track the change of magma rheology or bubble V, where the V can be estimated from its frequency and decay rate.

Steady and Oscillatory Shear Flow Behavior of Different Polysaccharides with Laponite®

The rheological behavior, in terms of steady and oscillatory shear flow, of Laponite® with different polysaccharides (alginate, chitosan, xanthan gum and levan) in salt-free solutions was studied. Results showed that a higher polymer concentration increased the zero-rate viscosity and decreased the critical strain rate (Cross model fit) as well as increasing the elastic and viscous moduli. Those properties (zero-rate viscosity and critical strain rate) can be a suitable indicator of the effect of the Laponite® on the shear flow behavior for the different solutions. Specifically, the effect of the Laponite® predominates for solutions with large critical strain rate and low zero-rate viscosity, modifying significantly the previous parameters and even the yield stress (if existing). On the other hand, larger higher polymeric concentration hinders the formation of the platelet structure, and polymer entanglement becomes predominant. Furthermore, the addition of high concentrations of Laponite® increases the elastic nature, but without modifying the typical mechanical spectra for polymeric solutions. Finally, Laponite® was added to (previously crosslinked) gels of alginate and chitosan, obtaining different results depending on the material. These results highlight the possibility of predicting qualitatively the impact of the Laponite® on different polymeric solutions depending on the solutions properties.

Technological Exercise of Cell Structure Forming

Single-layer and multi-layer cell structures are used for manufacturing of shells of liquid fuel tankers, as well as of "dry" shells of products, wings, fairings, etc. However, conventional methods of production by means of milling do not allow achieving the required specific strength. In this connection, diffusion bonding by means of gas pressure and gas forming at specified temperature and speed conditions are extremely important. Studies conducted by authors help model the processes and calculate the necessary processing parameters: pressure, critical strain rate, deformation rate (deformation time). This paper describes the manufacturing technology for these products, in which the solutions are based on theoretical and experimental studies, which provide: an increase in specific strength; reduction in weight of the product; reduction of labor intensity and increase in material utilization factor.

Investigation of the Portevin-Le Chatelier Effect in AlMgSi-Tailored Heat Treated Blanks

The dissolution of co-clusters in AlMgSi-alloys by a short term heat treatment can be used to locally adjust the mechanical properties of a blank for a following forming operation. This approach is known as Tailored Heat Treated Blanks (THTB) and allows to significant enhance the forming limits of AlMgSi-alloys. However, the dissolution of co-clusters leads to the observation of the Portevin-Le Chatelier (PLC) effect during deformation. The results are stretcher strain marks at the surface which are a limitation for potential applications of THTB. In contrast to AlMg-alloys, a critical strain rate above which no PLC effect occurs is not observed for the investigated alloys. This paper investigates various influence factors on the occurrence of the PLC effect for different AlMgSi-alloys and presents an approach under which conditions THTB can be used in applications with high demand on surface quality.

Effect of Pull-Out Rate on Interfacial Bonding Strength of Fiber

This paper investigates the effect of different pull-out rates on interface bonding strength. The experimental results show that rate effect is also another significant effect factor which contributes to optical interface bonding strength. In the uniaxial tensile process, the interface bonding strength is attributed to the interplay between the shear force and the friction force. The critical strain rate for leading role between shear force and friction force is 10-1.5, when less than that friction force dominates in the interface bonding mechanism, otherwise shear force dominates.

Correlation of Dynamic Response with Adiabatic Shearing Behavior in Ti–10V–2Fe–3Al Alloy

A Split Hopkinson Pressure Bar system was employed to investigate the compressive dynamic mechanical behaviors of Ti-10V-2Fe-3Al (Ti-1023) alloy with lamellar microstructure, over a broad strain rates ranging from 1500/s to 5100/s. The results reveal that the strain rate has a significant effect on the flow stress of Ti-1023 alloy, and there exists serious thermal softening as the strain rate exceeds 3200/s. The critical strain rate of fracture for this alloy is 2300/s. The microstructure examination indicated that adiabatic shear bands (ASBs) bifurcate more intensely with the increasing of strain rate. Micro-voids nucleate either in the ASB or interface between shear band and matrix bulk. Finally, fracture of this alloy proceeds through the nucleation, growth and coalescence of these voids and cracks along the ASBs.