Influence Of Initial Porosity Research Articles

Study on pore structure evolution and water damage of asphalt mixture under cyclic loading

For a long time, the water damage issues of asphalt mixture pavements have been highly regarded by engineers. However, the existing research is difficult to accurately describe the fatigue damage process of asphalt mixture under hydrodynamic pressure environments. In order to reveal the influence of water damage on the asphalt mixture structure and its mechanism, this study combined low-field NMR technology to investigate the evolution law of asphalt mixture pore structure under cyclic loading with different parameters. The erosion effect of the coupled loading-dynamic water environment on asphalt mixtures and their failure mechanisms are analyzed. The results show that the initial damage dominated by harmful pores and major-harm pores provides space for water migration and storage, and improves the sensitivity of asphalt mixture to water damage. Under the action of repeated cycles of stress increase-decrease, the irreversible hydrodynamic extrusion-cracking and friction-stripping deformations in the asphalt matrix increase. The micro-cracks inside the materials continue to expand and the pores are interconnected. To comprehensively consider the influence of initial porosity, maximum cyclic stress and the number of cycles, the cumulative damage model of the material is optimized based on the mechanism of pore structure evolution, and obtained more accurate porosity prediction results. This study can provide scientific reference and guidance for solving the problem of asphalt pavement disease and durability design.

Influence of Initial Porosity on the Expansion Behavior of Electrodes in Lithium-Ion Batteries

When charging or discharging a lithium-ion-battery (LIB), lithiation or delithiation of the electrodes takes place. Especially in the case of anode active materials, lithiation often leads to a significant volume increase. The latter can cause a rearrangement of the particles. Although the volumetric changes of state-of-the-art cathode materials have been found to be smaller than for anodes, they remain relevant. The combined volumetric changes of anodes and cathodes are an important factor influencing the lifetime of LIBs. An electrochemical dilatometer was used to measure the thickness change of various electrode active materials under minimal constant pressure (≈ 16 kPa): graphite, silicon-graphite (SiG) composite electrodes, and high-voltage spinel lithium-nickel-manganese-oxide (HVS-LNMO). The influencing factors investigated included the initial porosity of the electrodes, the particle shape of graphite, and the silicon content in the case of the silicon-graphite composite electrodes. Regarding all investigated electrodes, the initial electrode porosity is shown to correlate negatively with the irreversible thickness change during the initial cycles. The thickness change in each electrode was constant over the post-formation cycles, regardless of the initial porosity. Spherical particles in graphite resulted in slightly higher thickness changes than flake-type particles. The thickness change of SiGs increased linearly with silicon content.

Influence of high initial porosity introduced by laser powder bed fusion on the fatigue strength of Inconel 718 after post-processing with hot isostatic pressing

Laser-based additive manufacturing (AM) of metallic materials can produce AM parts of high quality. The obtained density of metal parts manufactured by laser powder bed fusion (LPBF) is mostly greater than 99% and high mechanical strengths can be achieved; however, a long processing time is typically required to reach a high quality. Possibilities to shorten the LPBF processing time involve increasing either the scanning speed or the hatch distance, but both will result in greater porosity. In general, the porosity of AM parts can be reduced by subsequent post-densification of the materials by hot isostatic pressing (HIP). In this manner, not only can the LPBF process be accelerated but the fatigue strength of the parts can be increased by reliable reduction of porosity and the simultaneous homogenization of the microstructure. Nevertheless, argon porosity introduced by the LPBF process cannot be completely eliminated by HIP due to the insolubility of argon in metallic materials. In the present study, the influence of the initial porosity on the fatigue strength of IN718 specimens after HIP and different heat treatments were investigated with a view to understanding the impact of argon porosity. The fatigue strength under a rotating bending load was determined for four different test batches and one almost fully dense as-built state as a reference. For the test batches, two different initial porosities of 3.5% and 9.9% were introduced by increasing the hatch distance. These porous samples were subjected to the same HIP treatment but aged differently: half of the samples were annealed, quenched, and aged within the HIP vessel under pressure and the other half were annealed and quenched within the HIP vessel but aged in an ambient air atmosphere. HIP post-processing significantly improved the fatigue strength of IN718 samples produced by LPBF, even in the case of a high initial porosity and residual argon content. In the case of a 3.5% initial porosity, no difference was observed in fatigue behavior with respect to aging with or without pressure. In contrast, samples with an 9.9% initial porosity showed a large difference in fatigue strength according to different aging conditions.

Sintering of Cu–Sn–P–MoS2 Powder Samples at 780°C

The influence of initial porosity and MoS2 content of pressed Cu + 9Sn + 1.5P + (5, 10, 15%) MoS2 powder samples on their growth during sintering in hydrogen at 780°C is studied. The growth of samples with the same MoS2 content decreases with higher initial porosity and the growth of samples of the same size and same porosity after pressing increases with greater MoS2 content. Water vapors generated during the hydrogen reduction of oxides present on the starting tin powder and phosphorous-copper alloy powder cause swelling of the samples. Moreover, phase transformations in the material during sintering can increase the samples as new phases are formed (copper sulfide Cu2S) with a larger crystal lattice that that of the matrix and molybdenum disulfide.

Influence of Initial Porosity on Strength Properties of S235JR Steel at Low Stress Triaxiality

Abstract The paper discusses the influence of the initial parameters on the strength parameters of S235JR steel at low stress triaxiality. The analysis was performed using the Gurson-Tvergaard-Needleman (GTN) material model, which takes into consideration the material structure. The initial material porosity was defined as the void volume fraction f0. The fully dense material without pores was assumed and the typical and maximum values of porosity were considered for S235JR steel in order to analyse the porosity effect. The strength analysis of S235JR steel was performed basing on the force-elongation curves obtained experimentally and during numerical simulations. Taking into consideration the results obtained, the average values of the initial void volume fraction f0 = 0.001 for S235JR steel is recommended to use in a common engineering calculations for elements operating at low stress triaxiality. In order to obtain more conservative results, the maximum values of f0 = 0.0024 may be used.

Predictions for the Impact of a Granular Solid Having Spatially Non-uniform Bulk Porosity

Continuum equations are numerically integrated using a high-resolution method to characterize the thermomechanical response due to planar impact of initially stress-free granular solid having spatially varying bulk porosity. This response depends on strain history because the crush-up stress necessary for inelastic compaction increases with initial solid volume fraction (ϕ0). Emphasis is placed on characterizing the influence of initial porosity and piston speed on both the grain surface heat flux variation and peak temperature rise within compaction wave profiles for the granular explosive HMX (C4H8N8O8). Heat fluxes near 586 MW/m2, resulting in temperature rises in excess of 400 K, are predicted for weak compaction of granular HMX having an initially dense region adjacent to a highly porous region (ϕ0=0.99, 0.655). Phenomena associated with reflected and transmitted waves from material interfaces are highlighted. Predicted compaction zone end states and heat flux profiles are shown to agree well with those given by a steady-state theory.