Dynamic Crushing Behavior of the High-Density Closed-Cell Foams

The face-centered cubic model is used to investigate the dynamic crushing behavior of high density closed-cell foams. The influences of the constant loading rate and the specimen aspect ratio on the crushing stress were discussed. It is demonstrated that the crushing stress is more sensitive to the constant loading rate than the specimen aspect ratio. To describe the dynamic crushing behavior of the foam theoretically, the idealized rigid-perfectly plastic-locking (RPPL) stress-strain model is extended to a more general case, in which both the density and the cross-section area are discontinuous. The good agreement between the finite element results and theoretical results confirms that the dynamic crushing behavior of foam can be described by the modified RPPL stress-strain model.

The strain-rate dependence of the nanoindentation stress of gold at 300 K: A deformation kinetics-based approach

Indentation tests involving a constant-loading rate stage followed by a constant-load stage were performed on annealed and 20% cold-worked Au to investigate the effect of indentation depth and initial dislocation density on the indentation deformation process. The indentation strain rate data were analyzed in terms of an obstacle-limited dislocation glide mechanism. The apparent activation energy was of the order of 0.16 μb3 and was neither a function of initial indentation depth nor cold work. The results of Haasen plot activation analysis and direct transmission electron microscopy (TEM) observations indicate that more mechanical work must be applied during the constant-loading rate stage due to the large amount of work hardening compared with the constant-load stage where considerably more dislocation recovery occurs.

Molecular Dynamics Simulation of Nanoindentation on Folded Chain Crystal of Polyethylene

Nanoindentation tests on a folded chain crystal of polyethylene are implemented with the molecular dynamics simulation. The orthorhombic crystal is made of the planar zig-zag chains and has the thickness of about 10nm. The ideal Berkovich indenter is plunged into upper surface of the crystal down to 2nm with the constant loading rate of 200m/s or 2000m/s. After the holding time of 1000fs at the maximum depth, the indenter is then pulled up with the same speed. The results are summarized as follows; a) The indentation of 2000m/s remains the residual depression while that of 200m/s recovers the hollow, b) No elastic component is found in the deformation under the both rate of 200m/s and 2000m/s, c) The crystal deforms statically under the indentation of 200m/s while that of 2000m/s shows delayed response.

Dynamic characterization of Portevin–Le Chatelier instabilities occurring in depth-sensing microhardness tests

Characteristic properties of plastic instabilities were studied using depth-sensing microhardness experiments on an Al–3.3 wt.% Mg alloy and computer simulations based on a macroscopic dynamic model of the experimental setup. A stepwise increase was observed in the indentation depth versus load (d-F) curves measured in constant loading rate mode, indicating hardness oscillations around a nearly constant value of the conventional dynamic microhardness. These oscillations were correlated with plastic instabilities starting from the contact surface between the sample and the indenter head. Taking into account the experimentally determined connection between the hardness oscillations and the indentation velocity, a dynamic model was proposed for the characterization of instability steps.

Hydrogen production from wastewater by acidogenic granular sludge

Sludge was granulated in a hydrogen-producing acidogenic reactor when operated at 26°C, pH 5.5 treating a sucrose-rich wastewater. The influence of hydraulic retention time (HRT) and sucrose concentration on hydrogen production by the acidogenic granular sludge was investigated at a constant loading rate of 25 g-sucrose/(láday). Results show that the gas composition was not greatly influenced by HRT or sucrose concentration. The hydrogen accounted for 57% to 68% of the biogas at HRT ranging 4.6-28.6 h and sucrose concentration ranging 4,800-29,800 mg/l. However, the hydrogen yield was more dependent on HRT and sucrose concentration. It ranged from 0.19 to 0.27 l/g-sucrose with the maximum yield occurring at HRT 13.7 h and sucrose concentration 14,300 mg/l in the wastewater. The acidified effluent was composed of volatile fatty acids and alcohols. The predominant products were butyrate (59-68%) and acetate (10-25%), plus smaller amounts of i-butyrate, valerate, i-valerate, caproate, methanol, ethanol, propanol, and butanol. The sludge yield averaged 0.2 g-VSS/g-sucrose. The carbon balance was 98-107% throughout the study.