Effect Of Void Distribution Research Articles

Synergistic effects of aggregate-void distribution on low-temperature fracture performance and crack propagation of asphalt concrete under mode I and mode II fracturing

Mesoscopic structures of asphalt concrete play an important role in low-temperature cracking behavior. Existing studies mainly focus on a single feature of the mesoscopic structure, neglecting the spatial distribution and synergistic effects of aggregates and voids, especially in Mode II fracturing. The results show that: in the case of −5 °C and Mode I fracturing, the fracture trend is less affected by the spatial distribution of mesoscopic structure, and tending to spread through aggregate and asphalt mastic along the shortest path. In the case of −5 °C and Mode II fracturing, the crack is easily affected by the distribution of large particle size aggregates in the initial zone, but not by the uneven distribution of voids. In the case of 10 °C and Mode I fracturing, the dominant large aggregate particle size influence on the path is weakened, and the effect of void distribution on the cracking path is perceptibly increased. In the case of 10 °C and Mode II fracturing, the location of large-particle aggregate determines the main streak of fractures, and the uneven distribution of void space determines the occurrence of fracture bending.

Multiscale modelling of composite laminates with voids through the direct FE2 method

A concurrent multiscale method, namely the direct FE2, is developed to analyse the mechanical response of fibre-reinforced composites with several void fractions in a nested manner, one at the macro and another one at the microscale. The FE2 is developed through the finite element (FE) mesh at the macro scale, wherein there is a representative volume element (RVE) at every Gauss point where the strains and stresses are evaluated. The RVEs, located at macroscopic FE mesh, with linear boundary conditions applied to the boundary nodes, are linked to macro-scale nodes via Multi-Point Constraints (MPCs). This procedure enables solving a single equilibrium problem, with boundary conditions and loadings at the macro scale and provides data for all defined RVEs. The methodology is applied to a composite laminate under transverse tension. Initially, the method is evaluated to compare the results from the FE2 with those from analyses where the entire domain is discretised (conventional FE full-scale modelling). The stresses obtained from the FE2 method match traditional FE analyses but utilise a significantly lower number of finite elements, resulting in substantial computational cost savings. It follows a parametric study of void distribution effects within the RVE. Afterwards, the stress distribution within the RVE is assessed and then failure of the composites is predicted at the macro scale through the stresses and strains concurrently calculated at the micro-scale.

Flow Boiling Pressure Drop Characteristics in Rectangular Channels under Uniform and Non-Uniform Heating

Pressure drop characteristics in flow boiling in a vertical rectangular channel (2.0 mm × 60 mm × 1000 mm) are reported under uniform and non-uniform transverse wall heating. The test section has symmetrical wall power, and the effective heating area is 56 mm × 700 mm. A wide range of operating conditions is obtained by varying the inlet pressure and mass flow. Results show that the variation of the pressure drop gradients with thermal parameters is similar under uniform and non-uniform wall heating. The pressure drop gradient decreases as the saturation pressure increases and increases significantly as the mass flow rate increases. It remains nearly constant or slightly decreases with increasing vapor quality at low mass flux, while it decreases slightly first and then rises rapidly at high mass flux. The homogeneous flow model using the equivalent viscosity assumption significantly underestimates experimental values, and the data dispersion is relatively large. The Beattie-Whalley correlation has a relatively good prediction, with a weighted mean absolute percent error (WMAPE) of 24.6%. The Muller-Steinhagen-Heck and Li-Wu correlations are found to be the best with WMAPE of 11.1% and 12.2% respectively, and most of the predicted values are within a ±20% error band. A new correlation based on the forms of Muller-Steinhagen-Heck correlation is proposed, the Bond number is introduced, and measurements are within ±8% of predictions. The lateral power distribution has an important influence on the flow boiling pressure drop, which can be attributed to the effects of void distribution and transverse flow. The non-uniform wall power distributions increase the pressure drop gradient compared to that of the uniform distribution.

Research on establishing numerical model of geo material based on CT image analysis

Geotechnical engineering material is a kind of heterogeneous composite material. The stone, mineral, gravel, hole, and crack contained in it have different physical and mechanical properties, and their responses to external forces are very different. For example, stress distribution, crack propagation, and failure mode are closely related to material heterogeneity and microstructure. This paper presents a finite element model method for processing CT images of geotechnical materials by using digital image technology. This paper presents a finite element model method for processing CT images of geotechnical materials by using digital image technology. The theory of digital image processing is applied to geotechnical CT image processing to realize pseudo-color enhancement of CT image, and the histograms of different geotechnical CT numbers are obtained. Canny operator is used to detect the edge of geotechnical CT image, and the binary image of geotechnical microstructure is obtained. According to the color change of pseudo-color enhanced image and CT histogram, the meso-structure and variation of rock and soil can be quantitatively analyzed. The finite element model established by this method can fully consider the heterogeneity of geotechnical materials, especially the effect of void distribution on the mechanical properties of geotechnical materials.

Biomorphic C/SiC 복합재료의 기계적 물성 연구

본 논문에서는 Biomorphic C/SiC 복합재료에 대하여 단위구조해석을 수행하였다. 소나무와 뉴송을 탄화하고 실리콘을 함침해 제조한 복합재료의 미세조직을 사각배열과 육각배열로 가정해 단위구조를 정의하고 등가물성치를 계산하였다. 단위구조의 크기가 동일하지 않은 경우도 고려하였고, 또한 공극의 배열에 따른 물성치의 변화를 몬테카를로 시뮬레이션을 통해 조사하였다. In this paper, mechanical property of biomorphic C/SiC composite was calculated by unit cell analysis. The microstructural arrangements of carbonized pine and radiata pine which were impregnated with silicon, were idealized as square and hexagonal arrays. Unit cell was then defined and equivalent elastic constants were calculated. A single and double unit cell structures were considered. The effect of void distribution was also studied by monte carlo simulation.

Void growth and interaction in crystalline materials

Abstract An inelastic rate-dependent crystalline constitutive formulation and specialized computational schemes have been developed and used to obtain a detailed understanding of the interrelated physical mechanisms that can result in ductile material failure in rate-dependent porous crystalline materials subjected to finite inelastic deformations. The effects of void growth and interaction and specimen necking on material failure have been investigated for a single material cell, with a discrete cluster of four voids, where geometrical parameters have been varied to result in seven unique periodic and random void arrangements. The interrelated effects of void distribution and geometry, strain hardening, geometrical softening, localized plastic strains and slip-rates, and hydrostatic stresses on failure paths and ligament damage in face centered cubic (f.c.c.) crystalline materials have been studied. Results from this study are consistent with experimental observations that ductile failure can occur either due to void growth parallel to the stress axis, which results in void coalescence normal to the stress axis, or void interaction along bands, which are characterized by intense shear-strain localization and that intersect the free surface at regions of extensive specimen necking.

Cavity distribution effects on superplastic ductility of a eutectic Pb-Sn alloy

The study illustrates the importance of the distribution and alignment of holes as a factor in determining tensile elongations in Pb-Sn alloys. It is possible for a sample with more holes to give a higher elongation than ones with fewer holes. This seemingly counter-intuitive result can occur due to void distribution effects, with strain localization occurring at more cross-sections. A modification of the earlier model to incorporate simultaneous deformation of non-cavitated and cavitated sections resulted in predictions of failure elongations which are in fair agreement with experimental trends and results. This can also be used to explain the higher elongations obtained in some Al-Mg-Mn alloys with elongated cracked stringers than those without, under otherwise similar conditions.

An analysis of void distribution effects on the dynamic growth and coalescence of voids in ductile metals

T he effects of void distribution on dynamic failure are studied by modeling a discrete set of randomly distributed cylindrical voids and subjecting them to a tension wave. The model problems are solved numerically with an Eulerian finite element program to allow for the generation of new free surfaces during coalescence. Unlike previous work, the growth and coalescence of small clusters of voids are simulated, i.e. the analysis is continued until the ligaments between the voids are broken and a fracture surface is formed. The peak transmitted stress is found to be insensitive to the magnitude of the tension wave and sensitive to the void distribution, but the time for the voids to coalesce into a fracture surface and the path of the fracture surface are found to be functions of the magnitude of the tension wave. For the small number of voids considered, the effects of the void distribution on the failure stresses were equivalent to the effects of doubling the initial void fraction.

Sensitivity analysis of an impedance void meter to the void distribution in annular flow: a theoretical study

Impedance void meters are frequently used to measure the area-averaged void fraction in pipes. This is primarily for two reasons: firstly, this method is non-intrusive since the measurement can be made by electrodes flush mounted in the walls, and secondly, the signal processing equipment is simple. Impedance probes may be calibrated by using a pressure drop measurement or a quick closing valve system. In general, little attention is paid to void distribution effects. It can be proved that in annular flow, the departure from radial symmetry has a strong influence on the measured mean film thickness. This can be easily demonstrated by solving the Laplace equation for the electrical potential by simple analytical methods. When some spatial symmetry conditions are encountered, it is possible to calculate directly the conductance of the two-phase medium without a complete calculation of the potential. A solution of this problem by using the separation of variables technique is also presented. The main difficulty with this technique is the mixed nature of the boundary conditions: the boundary condition is both of Neumann and of Dirichlet type on the same coordinate curve. This formulation leads to a non-separable problem, which is solved by truncating an infinite algebraic set of linear equations. The results, although strictly valid in annular flow, may give the correct trends when applied to bubbly flow. Finally, the theory provides an error estimate and a design criterion to improve the probe reliability.

A numerical study of void distribution effects on dynamic, ductile crack growth

Rapid crack propagation through a material containing two populations of second phase particles is analysed numerically. An elastic-viscoplastic model of a ductile porous material is used to represent small second phase particles that nucleate cavities at large strains as well as larger inclusions with low strength, which result in large voids near the crack tip at an early stage. Various random distributions of the larger inclusions in front of the crack tip are considered, while the small second phase particles are taken to be uniformly distributed. Analyses are carried out for a plane strain double edge cracked specimen under symmetric impulsive loading at the ends and adiabatic heating and the resulting thermal softening are accounted for. The predicted crack growth velocities are entirely based on the ductile failure criterion incorporated in the material model, and thus the present results give a direct estimate of the effect of various random inclusion distributions, compared to results for a uniform distribution. The resistance to crack growth increases and the average crack speed decays with increasing deviation from a uniform inclusion spacing. The dependence of the average crack speed on the root mean square particle spacing is found to be approximately linear over the range considered.

The effect of a non uniform void distribution on grain boundary void growth during creep

The effect of the distribution of voids over a grain boundary facet, on the void growth process, has been modelled. Assuming the grains to behave as rigid slabs which move apart during cavitation at a uniform rate, any non-uniformity of void spacing results in stress redistribution over the facet. The net result is a decrease in the overall damage rate compared to that expected for a uniform array at the same damage level, and a large variation in the growth rate of individual voids. The effect increases as the distribution of voids becomes clustered. The model shows that the effects of void distribution are significant and that they must be included in any attempt to predict times to failure.

Transient response of a packed bed for thermal energy storage

An analysis, with experimental results, for the transient response of a packed bed thermal storage unit is presented. The analysis is in two spatial dimensions and considers the influence of both axial and radial thermal dispersion for arbitrary time and radial variations in the inlet fluid temperature. Both charging and recovery modes are included. Spatial variations in void fraction are found to have significant influence on the dynamic response of both fluid and solid temperatures. An unconditionally stable numerical model is developed that predicts the two-dimensional transient response of both solid and fluid phases. The influence of radial velocity variations, wall heat capacity and wall thermal losses are considered. The analysis is valid for fluids of various Prandtl numbers. Experimental measurements of temperature distributions in a randomly packed bed of uniform spheres, with air as the working fluid, compare favorably with the analytical results over a broad range of Reynolds numbers. Results are presented for full-size rock beds which indicate pronounced effects of void distribution in such systems.