Impact Of Voids Research Articles

Impact of Void in CFRP on Dark-field Images Derived via X-ray Talbot-Lau Interferometer

Impact of Void in CFRP on Dark-field Images Derived via X-ray Talbot-Lau Interferometer

Molecular-Dynamics Analysis of the Mechanical Behavior of Plasma-Facing Tungsten.

We report a systematic computational analysis of the mechanical behavior of plasma-facing component (PFC) tungsten focusing on the impact of void and helium (He) bubble defects on the mechanical response beyond the elastic regime. Specifically, we explore the effects of porosity and He atomic fraction on the mechanical properties and structural response of PFC tungsten, at varying temperature and bubble size. We find that the Young modulus of defective tungsten undergoes substantial softening that follows an exponential scaling relation as a function of matrix porosity and He atomic content. Beyond the elastic regime, our high strain rate simulations reveal that the presence of nanoscale spherical defects (empty voids and He bubbles) reduces the yield strength of tungsten in a monotonically decreasing fashion, obeying an exponential scaling relation as a function of tungsten matrix porosity and He concentration. Our detailed analysis of the structural response of PFC tungsten near the yield point reveals that yielding is initiated by emission of dislocation loops from bubble/matrix interfaces, mainly 1/2⟨111⟩ shear loops, followed by gliding and growth of these loops and reactions to form ⟨100⟩ dislocations. Furthermore, dislocation gliding on the ⟨111⟩{211} twin systems nucleates 1/6⟨111⟩ twin regions in the tungsten matrix. These dynamical processes reduce the stress in the matrix substantially. Subsequent dislocation interactions and depletion of the twin phases via nucleation and propagation of detwinning partials lead the tungsten matrix to a next deformation stage characterized by stress increase during applied straining. Our structural analysis reveals that the depletion of twin boundaries (areal defects) is strongly impacted by the density of He bubbles at higher porosities. After the initial stress relief upon yielding, increase in the dislocation density in conjunction with decrease in the areal defect density facilitates the initiation of dislocation-driven deformation mechanisms in the PFC crystal.

Impact of voids and backfill on seismic wave velocity-preliminary results

In this study, laboratory experiments were conducted on discrete physical models that mimic mining effects to better understand the impact of continuous changes in mining environments on seismic wave velocities. The discrete physical models are represented by concrete and granite cubic samples of different sizes with holes of different diameters filled and unfilled with cemented sand backfill of different cement-sand content ratios. The hole diameters range from 0 to 150 mm in block sizes ranging from 150 mm to 450 mm in increments of 75 mm. The increasing hole size mimics increasing extraction in the mine with time. Cemented sand fills at cement contents ranging from 0 to 20% are used to fill the voids after testing them empty and retesting the same at different backfill cured ages. The SAEU3H AE eight-channel system is used in the study. Preliminarily results show that the impact of continuous changes in mining environments significantly affects the seismic wave velocities. The impact of voids and their contents on the seismic wave velocity depends on the sensor location relative to source and void, and it backfills cement content with time.

Three dimensional waves propagation in thermo-viscoelastic medium with two temperature and void

A dynamic model comprising anisotropic viscoelastic solid with voids along with two temperature and three phase lag is used to study the propagation of harmonic waves. The system of differential equation is made dimensionless by incorporating suitable quantities. Harmonic type plane wave solution is assumed to determine the Christoffel equation. From this equation five quasi-waves are noticed which vibrate with distinct speed in infinite medium and are under the impact of void, phase lag and viscosity. The velocity and attenuating factors of waves are computed numerically in three dimensional space. The influence of viscosity and void have been seen on the characteristic of waves. Different unique cases are also determined.

Effect of Voids Behind Lining on the Failure Behavior of Symmetrical Double-Arch Tunnels

The presence of voids behind lining seriously affects the safety of the symmetrical double-arch tunnels during service life. It is essential to find out the impact of voids on tunnel safety due to the increasing demand for the construction and maintenance of symmetrical double-arch tunnels. Model tests and numerical analyses were conducted in this research. The results attained were explored including earth pressure, internal force, and lining failure. Results reveal that the presence of voids has a large influence on the internal force in the lining of symmetrical double-arch tunnels, generally in the form of asymmetrical failure patterns of the lining. The failure patterns of the lining are greatly influenced by the size and location of voids with respect to the symmetrical double-arch tunnel circumference. Significant changes in the lining internal forces were found at the areas in the close vicinity of the void whereas a few changes were found at the bottom of the sidewall, the invert, and the central wall far away from the void. The propagation laws of lining cracks of asymmetrical double-arch tunnels are more complicated than symmetrical tunnels with a void behind the central wall. The location of the initial cracking of symmetrical and asymmetrical double-arch tunnels is the same, while the lining failure of the large-section tunnel is the most complicated.

Robustness of BGAs: Parametric study of voids' distribution in SAC solder joints

The impact of voids on the mechanical behaviour of a BGA solder joint can be evaluated by simulation. The Finite Element (FE) method is adapted to achieve such a task where an infinite number of different void distributions is possible. A tool is proposed in this paper with a modelling process involving a global calculation of a thermal load and a refined sub model. The global model represents a quarter of an electronic package soldered to a piece of PCB while the submodel is limited to a single ball with copper pads and defects under investigation i.e. intermetallic compounds (IMC) and voids. Creep properties are taken into account to model mechanical response of the lead-free solder SAC 305. The simulation of a thermal cycle is performed on the global model with only elastic properties. The nodal displacements are extracted from the results to be applied to the submodel's mesh. True viscoplastic mechanical behaviour is simulated in this refined model to get the solder material response with respect to different defects' considerations. The void distribution is handled with 4 parameters driving the height and thickness of the layer of distribution, the size and the proximity of voids. Simulating the effects of these parameters can afford to sort them for future reliability assessments via Finite Element methods.

Voids and compositional inhomogeneities in Cu(In,Ga)Se2 thin films: evolution during growth and impact on solar cell performance

ABSTRACTStructural defects such as voids and compositional inhomogeneities may affect the performance of Cu(In,Ga)Se2 (CIGS) solar cells. We analyzed the morphology and elemental distributions in co-evaporated CIGS thin films at the different stages of the CIGS growth by energy-dispersive x-ray spectroscopy in a transmission electron microscope. Accumulation of Cu-Se phases was found at crevices and at grain boundaries after the Cu-rich intermediate stage of the CIGS deposition sequence. It was found, that voids are caused by Cu out-diffusion from crevices and GBs during the final deposition stage. The Cu inhomogeneities lead to non-uniform diffusivities of In and Ga, resulting in lateral inhomogeneities of the In and Ga distribution. Two and three-dimensional simulations were used to investigate the impact of the inhomogeneities and voids on the solar cell performance. A significant impact of voids was found, indicating that the unpassivated voids reduce the open-circuit voltage and fill factor due to the introduction of free surfaces with high recombination velocities close to the CIGS/CdS junction. We thus suggest that voids, and possibly inhomogeneities, limit the efficiency of solar cells based on three-stage co-evaporated CIGS thin films. Passivation of the voids’ internal surface may reduce their detrimental effects.

Void Formation and Their Effect on Reliability of Lead-Free Solder Joints on MID and PCB Substrates

Since the introduction of lead-free solder alloys, the impact of voids in solder joints has rapidly gained in importance. Particularly in power electronics, voids in solder joints can reduce their reliability due to less heat dissipation and a significant reduction of the cross-sectional area. While void formation in the printed circuit board (PCB) technology has been the focus of many research projects, the increasing application of molded interconnect devices (MIDs) demands a vivid understanding of void formation and its impact on reliability. This report will show that those processes specific to MID technology do not significantly influence the void formation in SAC305 solder deposits. The processed different roughness levels on Vectra, Vestamid, and FR4 substrate materials show no significant impact on void formation. The surface finish metallization was detected as the most important factor. Especially, the chemical-Sn (iSn) surface finish is inclined to having more voids compared to a nickel–phosphor–gold (NiP-Au) finish. Furthermore, three solder processes were investigated: convection, condensation, and vacuum-condensation soldering. The vacuum-condensation processes significantly reduced the voids in the Sn–3.0Ag–0.5Cu (simplified as SAC305) solder joints of CR0805, MLF20, and switches on all the investigated substrates. An accelerated thermal cycling test of these components according to JESD22-A104D demonstrates no significant effect of the void content of <19% on the solder joints’ reliability. Finally, the result of accelerated thermal cycling shows the possibility to achieve an equivalent lifetime of the solder joints on Vectra in comparison to the FR4 substrate.

Effects of Die-Attach Voids on the Thermal Impedance of Power Electronic Packages

Thermal runaway is among the major failure mechanisms of power semiconductor packages. Thermal dissipation of power electronics depends on the quality of the solder die-attach, any defects such as voids, and delamination-impede heat dissipation that results in hot spots that can cause thermal runaway and/or a premature device failure. Therefore, investigation of thermal impact due to die-attach defects is indispensable for improved device performance and reliability. In this paper, transient dual interface measurement is used to characterize the thermal path of a power electronic package consisting of an insulated gate bipolar transistor (IGBT) and copack diode. The die-attach defects were identified by X-ray imaging. The impact of voids on thermal dissipation was quantified by junction-to-case thermal resistance and structure function analysis. The junction-to-case thermal resistance (IGBT) values were found to be increasing with the increase in void%. The largest void% instead of the total void% has a greater impact on the thermal dissipation of the devices. However, die-attach voids under the IGBT die have no impact on junction-to-case thermal resistance (diode), suggesting that the copack diode heat path is independent of the IGBT heat path.

Impact of Voids on Mechanical Stability of Well Cement

Techniques of well construction currently used in oil industry and for underground CO2 storage typically involve cementing a casing or liner in a borehole. Cement integrity affects the overall integrity of the well. Developing new sealing materials and predicting long-term integrity of CO2 injection wells requires a good understanding of defects in well cement, and how these defects may affect the well integrity. In this paper, voids and channels are observed in the cement near different cement-steel interfaces using a scanning electron microscope (SEM). The effect of voids on the stress state under mechanical or thermal loading of the near-well area is assessed numerically using a finite-element model of the casing-cement-rock system.

Status of steam explosion understanding and modelling

The main results of the major international activities related to fuel–coolant interactions (FCI) of the last 4-year period are presented and a summary of the knowledge gained regarding understanding and the improvements of modelling is provided. At first, the major outcomes of the OECD SERENA-2 program are presented and discussed. Important clarifications were obtained on the so-called material effect and on FCI code capabilities. We then summarise complementary analytical analyses and experimental programs performed in the frame of the SARNET community. The focus was put on the role of melt fragmentation and solidification, the impact of void on the intensity of an explosion and the triggering mechanisms. As a conclusion, tables summarising the improvements are proposed as well as research priorities.

Numerical simulation of elastic linear micropolar media based on the pore space length scale assumption

The 3D micropolar theory numerical simulations have been performed on the brittle isotropic materials (amorphous glass, brittle rock and two different lightweight concretes) with different pore sizes using the cylindrical models under uniaxial compressive loading. To pursue this goal, it is assumed that first, second and third microrotation constants (α, β, and γ), which appear in the couple stress equilibrium equation, are proportional to the square of average pore diameter or so called characteristic length. Unexpectedly such an assumption leads to a constant polar ratio and consequently, the polar ratio cannot be accounted for as a material constant. The present phenomenon substantiates the existence of a redundant material constant for the 3D micropolar media. Accordingly, the micropolar shear constant κ is a material constant. Different coupling numbers N, with relevant domain are numerically investigated to explore the characteristic features of the micropolar shear constant κ. According to the results obtained, the present methodology shows a very good convergence and is consistent with the physically accepted results for the heterogeneous and homogeneous materials including nano-and microscale pores, whereas several unconverted or discontinuous stress fields are found out when using mesoscale pores. The latter disadvantage is believed to be caused by the impact of voids’ ratio variation under quasistatic loading.