Void Generation Research Articles

Degassing and layers variation effect on composite processing by vacuum assisted resin transfer moulding

Vacuum assisted resin transfer molding (VARTM) is a fiber reinforced composite (FRC) making process in which resin is impregnated to fabric by application of vacuum. This process is also known as vacuum infusion process. The critical issue in VARTM process is void generation. Voids form due to variety of reasons, most of which can be avoided. Vacuum degassing is one of the solutions which will reduce air entrapped inside resin during impregnation. In this work six laminates from jute and polyester resin were prepared, three with degassing and three without degassing with variation in number of jute layers 5, 10 and 15 respectively. Microscopic examination and mechanical properties have been observed before and after degassing. It was observed that degassing improves mechanical properties of composite laminates and reduce void content. It was observed that the thickness variation in laminate increased as number of layer increased.

Evolution of Interfacial Microstructure During Resistance Spot Welding of Cu and Al With Ni-P Coating

Abstract Dissimilar materials of copper (Cu) to aluminum (Al) with nickel-phosphorus (Ni-P) coatings were joined using resistance spot welding. The Ni-P coatings were electroless plated on the Al surfaces to eliminate the formation of brittle Cu-Al intermetallic compounds (IMCs) at the faying interface of Cu to Al. Three welding schedules with various heat input were used to produce different interfacial microstructure. The evolution of interfaces in terms of phase constitution, elemental distribution, and defects (gaps and voids) was characterized and the formation mechanisms were elucidated. During the welding, the bonding between Cu and Ni-P form through solid-state diffusion, while the faster diffusion rate of Cu relative to Ni and P atoms promotes the generation of sub-micro voids. As the heat input increases, gaps at the Cu/Ni-P interface diminish accompanied by increase of sub-micro voids. A moderate schedule helps to remove the gaps and inhibit the voids formation. An Al3Ni layer and nanovoids were found around the interface of Ni-P/Al. The increased heat input decreases the grain size of Al3Ni at the interface by eutectic remelting and increases the nanovoids by enhanced nanoscale Kirkendall effect.

On the mechanism of onset of significant void in subcooled flow boiling

Theoretical investigation was carried out for a possible mechanism of the onset of significant void (OSV) or net vapor generation (NVG) in subcooled flow boiling. Based on available knowledge on the two-phase flow regime transition from bubbly to slug flows, it was postulated that bubble coalescence is intensified substantially when the local void fraction near the heated wall exceeds a critical value. It was discussed that this can be a main cause of a rapid increase in the cross-sectional area-averaged void faction since the condensation rate decreases suddenly due to a sudden decrease in the interfacial area of vapor bubbles. It was demonstrated that if the vaporization rate on the heat transfer surface is evaluated using a reliable wall heat transfer correlation, the OSV conditions calculated by the present model agree with those by a widely-used empirical correlation well.

Soil biostabilisation and interaction with compaction processes for earthen engineering structures production

Interaction between microbially induced calcium carbonate precipitation (MICP) and compaction procedures to stabilise raw soil materials has been studied with the aim of producing earthen engineering structures. Initial tests to optimise MICP in aqueous medium and in selected soils were performed. MICP and compaction were finally applied to assess medium-size elements. The main result was that sandy soils should be compacted before irrigation treatment to close the existing voids and prevent bacterial sweeping, whereas clayey soils should be compacted after irrigation treatment to avoid the plugging effect. MICP improved small sand soil compressive strength by up to 32% over the value reached by compaction alone. However, MICP had no positive effect on coarse soils and soils with an optimum particle size distribution: MICP treatment was not able to fill large connected voids in the first case and it caused little void generation due to bacteria sporulation in the second.

Numerical investigation of microstructure and failure of lithiated silicon under biaxial tension

To develop high-capacity Li-ion batteries, volume expansion and material degradation of Si anodes have been attracting increasing attention. However, the effects of phase structures and geometrical constraints on the failure mechanism of lithiated Si have not been well understood. Considering phase boundaries formed during electrochemical cycles, we conducted molecular dynamics simulations to investigate the failure behaviour and microstructural evolution of single-phase and dual-phase (core–shell) lithiated Si structures when subjected to biaxial tensile stress. During lithiation, lithiated Si structures show a brittle-to-ductile transition. The brittle fracture is associated with nanoscale cavitation and void propagation. The ductility of lithiated Si structures increases with Li content due to the rearrangement of Li atoms. Compared with single-phase lithiated Si, dual-phase structures tend to fracture owing to inhomogeneous deformation and nanoscale cavitation. On the other hand, the ductility of lithiated Si decreases during delithiation, especially in dual-phase structures. This work provides a better understanding of the structure–property relationship of lithiated Si in charging-discharging cycles and strategies to improve the electrochemical performance of Si-based electrodes.

Numerical study on thermal-hydraulics of external reactor vessel cooling in high-power reactor using MARS-KS1.5 code: CFD-aided estimation of natural circulation flow rate

This paper presents a numerical investigation of two-phase natural circulation flows established when external reactor vessel cooling is applied to a severe accident of the APR1400 reactor for the in-vessel retention of the core melt. The coolability limit due to external reactor vessel cooling is associated with the natural circulation flow rate around the lower head of the reactor vessel. For an elaborate prediction of the natural circulation flow rate using a thermal-hydraulic system code, MARS-KS1.5, a three-dimensional computational fluid dynamics (CFD) simulation is conducted to estimate the flow rate and pressure distribution of a liquid-state coolant at the brink of significant void generation. The CFD calculation results are used to determine the loss coefficient at major flow junctions, where substantial pressure losses are expected, in the nodalization scheme of the MARS-KS code such that the single-phase flow rate is the same as that predicted via CFD simulations. Subsequently, the MARS-KS analysis is performed for the two-phase natural circulation regime, and the transient behavior of the main thermal-hydraulic variables is investigated.

Out-of-plane tensile failure behavior of fiber reinforced composites due to lay-up temperature induced intra-ply and inter-ply voids

The focus of this paper is to understand the effect of voids on the damage mechanism of the composite under out-of-plane tensile load with different lay-up temperature conditions. A micro-scale model for ply-by-ply structure is established containing intralaminar and interlaminar domain in which voids are generated by a random irregular voids generation algorithm to explore the role of voids (intra-ply voids and inter-ply voids) on the transverse damage behavior of the composite. Then, a macro-scale model containing elements with and without void is established to capture crack propagation behavior in the specimen. Analysis of voids in variation layup temperatures are conducted experimentally and numerically. The results show that with the increasing lay-up temperature, the porosity increases from 1% to 4.5%, and the void distribution changes from the intra-ply voids dominant to the inter-ply voids dominant, while the out-of-plane tensile strength decreases by about 30%.

Characterization of Three-Dimensional Internal Structure Evolution in Asphalt Mixtures during Freeze–Thaw Cycles

This paper aims to characterize the three-dimensional (3D) internal structure evolution of asphalt mixtures under freeze–thaw cycles. Asphalt mixtures with three levels of design void content (3%, 5%, and 7%) were prepared in the laboratory. Subsequently, X-ray computed tomography (CT) tests were conducted to capture two-dimensional (2D) images of the internal structure of samples before and after freeze–thaw testing. A set of image processing techniques for reconstructing 3D images of the internal structure were utilized to extract the internal structure properties, which were then used to analyze the changes in the air void distributions and to evaluate the internal structure evolution under freeze–thaw cycles. 3D images reconstructed from X-ray CT images illustrated a dramatic degradation in the internal structure after cyclic freeze–thaw exposure. The change in internal structure occurs mainly in three ways: (1) expansion of existing individual voids, (2) combination of two separated air voids, and (3) generation of new voids. In addition, the parametric analysis of the three-dimensional reconstructed voids revealed that the asphalt mixture void ratio increased with the number of freeze–thaw cycles, while the larger the initial void content, the more pronounced the increase in the specimens. Therefore, asphalt mixture freeze–thaw resistance should be optimized in relation to the design void content.

Micro-mesoscopic prediction of void defect in 3D braided composites

Void defects will inevitably occur in 3D braided composites during Resin Transfer Molding (RTM) process, which may reduce mechanical properties of 3D braided composites. Therefore, this paper focuses on void generation in 3D braided composites and optimizes its manufacturing technology. First, the basic governing equations considering flow, temperature changing and resin curing process are derived to describe the conservation of momentum, energy and mass of resin during RTM process. Second, the whole RTM process for 3D braided preform cell is simulated by means of finite element method (FEM), which indicates the distribution of resin flow speed, pressure and void content. Third, influences of resin injection flow volume, outlet pressure, volatile content in resin and curing temperature on void distributions and contents in 3D braided composite cell are investigated. The results will provide theoretical basis for analyzing void generation and evaluating the quality of 3D braided composites made by RTM technology.

Elaboration of porosity for the alumina particle surfaces/ bimodal PP composite cast films under continuous stretching

AbstractEfforts have been focused on developing an interrelationship between machine direction orientation (MDO)/material variables and the porosity of Al2O3 particulate‐filled orientable PP for various applications. Composite films were prepared by cast‐extrusion followed by stretching up to 100% and 200% by the MDO machine. Cavitation could occur during film stretching when the adhesion between dispersed alumina and continuous polypropylene phase failed. Composite films with modified micro‐particles and also nano ones could not generate any voids. On the other hand, composite specimens of unmodified micro‐alumina particles generated micro‐pores in the joint between dispersed particles and polymer during the hot stretching. Beta nucleation in these films, plays an important role in the generated ductility of the specimens. Cavitation effectiveness (no. of voids) or film permeability depended on the size and amount of alumina particles, in addition to the ratio and temperature of stretching. Cavitation particles with a size range of 0.7–3 μm, create cavities of about 3–7 μm. Generation of voids, by drawing environmentally friendly and antimicrobial active alumina particles, can be utilized in the fabrication of sustainable hygiene polypropylene films with desirable separation abilities for gases or small molecules. Furthermore, the polypropylene/alumina composite films have the potential for microwaveable packaging.

Molecular dynamics study on the fracture mechanism in bimodal nanotwinned Cu with a composite structure

Utilizing molecular dynamics simulations, the authors investigate the notion of bimodal nanotwinned (NT) microstructure in strength-ductility trade-off. Using a coarse circular grain with twins embedded in a sea of nano-grains (without twins) as a surrogate, the paper explores three scenarios pertaining to the strength and failure strain mapping: (i) the effect of twin boundary (TB) inclination; (ii) the effect of coarse-grain size; (ii) the effect of twin boundaries spacings (TBSs). The central result is that there exists an optimum grain size at which the strength and failure strain are simultaneously improved. Meanwhile, the fracture mechanisms of bimodal NT microstructure are demonstrated by analyzing dislocation-TB interactions, which involve TB migration, deformation twinning, void generation, etc.

Study of wavelet entropy for airport pavement inspection using a multistatic ground-penetrating radar system

The multilayer nature of airport pavement structures is susceptible to the generation of voids at the bonding parts of the structure, which is also called interlayer debonding. Observations have shown that the thickness of the resulting voids is usually at the scale of millimeters, which makes it difficult to inspect. The efficient and accurate characteristics of ground penetrating radar (GPR) make it suitable for large area inspections of airport pavement. In this study, a multistatic GPR system was used to inspect the interlayer debonding of a large area of an airport pavement. A special antenna arrangement can obtain common-midpoint (CMP) gathers during a common-offset survey. The presence of interlayer debonding affects the phase of the reflection signals, and the phase disturbance can be quantified by the wavelet transform. Therefore, we have developed an advanced approach that uses the average entropy of the wavelet transform parameters in CMP gathers to detect the interlayer debonding of airport pavement. The results demonstrate that the regions with high entropy correspond to regions where tiny voids exist. The new approach introduced in this study was then evaluated by a field-base experiment at an airport taxiway model. The results show that our approach can detect interlayer debonding of the pavement model accurately and efficiently. On-site coring results confirm the performance of our approach.

Coarse-grained molecular dynamics simulations of void generation and growth processes in the fracture of the lamellar structure of polyethylene.

We investigate the void generation and growth process in semicrystalline polymers by large-scale coarse-grained molecular dynamics simulations. Voids are generated in the amorphous layers and grow spherically and then cylindrically, consistent with the results of previous experiments. Interestingly, the fusion of voids is indicated to enlarge the voids in the direction perpendicular to the stretching direction, but not beyond the crystalline layers. The orientational order along the stretching direction increased sharply before void generation, but the increase leveled off afterward. The simulations also clearly reveal that the crystalline layers break but do not bend in the fragmentation process. The dependence of the void growth process on stretching velocity and the stability levels of voids at constant strain are also discussed.

A hierarchical scheme from nano to macro scale for the strength and ductility of graphene/metal nanocomposites

Recent experiments have shown that grain size of metals in graphene/metal nanocomposites has a significant effect on their strength and ductility. As the grain size decreases, the strength tends to increase while the ductility tends to decrease. But quantitative assessment of these observations remains a challenge. In this paper, we establish a hierarchical scheme from nano to macro scale to evaluate the dependance of these properties on the grain size and graphene volume concentration. In the nano scale, the elastic properties of graphene nanofiller and metal matrix are evaluated via the density functional theory (DFT). In the micro scale, the plasticity of the ductile metal is described by a dislocation density-based constitutive equation, and its degradation process is accounted for by the generation of micro voids. In the macro scale we consider the system of randomly distributed graphene nanosheets in the degraded metal matrix. The grain-size dependent stress-strain relation of the overall graphene/metal nanocomposites are calculated by a two-scale homogenization method with the assistance of the field-fluctuation approach. In this process, the grain-size dependent thermodynamic driving force for the progressive generation of micro voids and the influence of an ultrathin imperfect interphase surrounding the graphene nanofiller are also considered. It is demonstrated that the predicted results from the developed hierarchical scheme agree well with the experimental data of graphene/aluminum nanocomposites. The developed multiscale theory can provide a useful guideline for the design and applications of graphene/metal nanocomposites through the grain-size controlled process.

Development of the automatic void generation module in GEOUNED conversion tool

The radiation transport simulation of complex nuclear facilities, such as ITER, involves the process to adapt the highly complex engineering CAD models to radiation transport computational codes. An essential feature of this process is the generation of void space. In this article, the automatic void generation module of the new GEOUNED conversion tool from CAD to MCNP geometry is presented. The module combines common approaches, such as the division of void space into void cells, with two new features: fusion of void cells and independent void regions definition with individual fill material assignation. In order to evaluate the performance of the module and the capabilities of the new features, the CAD model of the IFMIF-DONES Accelerator Systems (AS) is converted with different void mode configurations. The best configurations are determined comparing the computational performance and the void quality of the converted models.

Deformation and fracture behavior of a laser powder bed fusion processed stainless steel: In situ synchrotron x-ray computed microtomography study

Tensile plastic deformation and fracture behavior of a laser powder bed fusion processed 316L stainless steel alloy was investigated in situ using high-resolution synchrotron x-ray computed microtomography (sXCT). Three different cases were studied: (1) specimen A1, near full-density with initial porosity of 0.04%, (2) specimen B1, initial lack-of-fusion (LOF) porosity of about 3% with the major axis of defects aligned nearly perpendicular to the tensile loading direction (LD), and (3) specimen B, similar to B1 but with the major axis of LOF defects aligned nearly parallel to LD. The effects of type, density, and orientation of defects in as-printed samples on plastic deformation and failure modes were studied by analyzing sXCT image sequences and quantitative defect properties. Specifically, the evolutions of porosity, size, aspect ratio, and angle of gas pores and LOF defects were characterized as a function of applied strain. Moreover, interactions between as-printed defects and deformation-induced crack propagation were investigated. The tensile behavior of specimen A1 was comparable to that of a commercial wrought plate. The quantitative in-situ sXCT results showed a void-growth phenomenon in A1 but without noticeable coalescence, resulting in a ductile cup-cone failure. Specimen B1 showed damage-driven brittle failure with limited plasticity due to early and rapid propagation of open voids. On the other hand, specimen B, with a similar amount of LOF defects as B1, showed deceivingly strong and ductile tensile behavior similar to A1 without any major crack propagations due to the favorable orientation relationship between LOF defects and the LD.

Crustal Fault Zones (CFZ) as Geothermal Power Systems: A Preliminary 3D THM Model Constrained by a Multidisciplinary Approach

The Pontgibaud crustal fault zone (CFZ) in the French Massif Central provides an opportunity to evaluate the high-temperature geothermal potential of these naturally permeable zones. Previous 2D modeling of heat and mass transfer in a fault zone highlighted that a subvertical CFZ concentrates the highest temperature anomalies at shallow depths. By comparing the results of these large-scale 2D numerical models with field data, the depth of the 150°C isotherm was estimated to be at a depth of 2.5 km. However, these results did not consider 3D effects and interactions between fluids, deformation, and temperature. Here, field measurements are used to control the 3D geometry of the geological structures. New 2D (thin-section) and 3D (X-ray microtomography) observations point to a well-defined spatial propagation of fractures and voids, exhibiting the same fracture architecture at different scales (2.5 μm to 2 mm). Moreover, new measurements on porosity and permeability confirm that the highly fractured and altered samples are characterized by large permeability values, one of them reaching 10-12 m2. Based on a thermoporoelastic hypothesis, a preliminary 3D THM numerical model is presented. A first parametric study highlights the role of permeability, stress direction, and intensity on fluid flow. In particular, three different convective patterns have been identified (finger-like, blob-like, and double-like convective patterns). The results suggest that vertical deformation zones oriented at 30 and 70° with respect to the maximum horizontal stress direction would correspond to the potential target for high-temperature anomalies. Finally, a large-scale 3D numerical model of the Pontgibaud CFZ, based on THM coupling and the comparison with field data (temperature, heat flux, and electrical resistivity), allows us to explore the spatial geometry of the 150°C isotherm. Although simplified hypotheses have been used, 3D field data have been reproduced.

Reliability analysis for TO-247 multilayered power module packaging under mechanical oscillation based on finite element method

This paper presents the simulation and hardware test results for determining the fatigue life of the solder joints of a printed circuit board (PCB) including a DC to AC inverter circuit with six power metal oxide semiconductor field effect transistors (MOSFETs) by using the finite element method (FEM) under different vibration effects. This board is exposed under different angles by a vibration machine. The selected angles were performed on the power module to find the maximum stress points of the power module for different vibration frequencies. The results indicate that the maximum stress is observed at the corners of the solder layer. The stress of the solder joints significantly increases when the input frequency increases, and the failure, voids generation and crack formation and propagation are more observable under this condition. Furthermore, the loading direction changes show that the peeling stress of the solder layer is directly affected by lower angles. The shear stress occurs in lower angles of loading directions, and this situation results in the peeling stress. The simulation investigations are proved by the experimental results in the study. According to the observations, the fatigue effects result in the coalescence and the void growths in lower angles of vibration loadings. The paper also investigates the normal loading condition of the power inverter circuit, and the highest reliability is obtained compared to all other experimental tests.

Molecular Insights into the Mechanical Properties of Polymer–Fullerene Bulk Heterojunctions for Organic Photovoltaic Applications

We investigate the mechanical properties of $\pi$-conjugated polymeric materials composed of regioregular poly(3-hexylthiophene) (P3HT) and fullerene C$_{60}$ using coarse-grained molecular dynamics simulations. Specifically, we perform tensile simulations of P3HT:C$_{60}$ composites with varied degrees of polymerization and C$_{60}$ mass fractions to obtain their stress-strain responses. Decomposition of stress tensor into kinetic energy and virial contributions indicates that the tensile moduli of the pure P3HT samples are greatly dependent on non-bonded interactions and on bonded interactions associated with bond-stretching, while the addition of C$_{60}$ leads to an increase in the tensile modulus originating from enhanced non-bonded interactions associated with C$_{60}$. Additionally, the tensile strength of the P3HT:C$_{60}$ samples correlates well with molecular chain entanglements, which are characterized by the average number of kinks per chain obtained from primitive path analysis. We also find that the upper and lower yield points characterizing strain softening become more pronounced with increasing C$_{60}$ mass fraction. Persistent homology analysis indicates that the emergence of the yield points correlates well with the coalescence of microvoids in the course of tensile deformation, resulting in the generation of larger voids. These results provide a fundamental understanding of the molecular determinants of the mechanical properties of $\pi$-conjugated polymer-fullerene composites, which can also help to interpret and predict the mechanical properties of other polymer composites containing fullerene.

Molecular dynamics simulation on (001) interfacial fracture of Fe/Ni and Fe/Pd/Ni and deformation mode analysis by eigenvector of atomic elastic stiffness matrix

Tensile fracture simulations are performed on the (001) interfaces of Fe/Ni and Fe/Pd/Ni by molecular dynamics method, in order to discuss the interfacial strength by means of the eigenvalue and eigenvector of atomic elastic stiffness, Baij=Δσai/Δεj. Three models are considered, Fe/Ni stacked layers (Bimetal), 2 atomic interlayers of Pd between Fe and Ni phases (Pd 2layer) and same thickness Fe/Pd/Ni/Pd blocks (Trimetal). Before tensile simulation, distribution of ηa(1) < 0 atoms (ηa(1) is the 1st eigenvalue of the solution of BaijΔεj=ηaΔεi) and residual atomic stress in each atomic layers are discussed. Then tensile simulations are implemented; the peak strain and stress of Pd2layer are significantly reduced from that of Bimetal. Trimetal shows slightly lower maximum stress at slightly higher strain than Bimetal. Bimetal shows fracture in Ni phase near the interface, while Pd2layer and Trimetal do in Pd phase near the Ni/Pd and Fe/Pd interface, respectively. Void generation process or the initial stage of the fracture is then discussed with the distribution of ηa(1) < 0 atoms and corresponding eigenvector, {Δε1,Δε2,・・・,Δε6} ={Δεxx,Δεyy,・・・,Δγxy}. In all models the void is formed at the intersection between the interface and slip deformation, so that we can’t find clear mode for atomic plane opening but complicated modes for plastic deformation and inhomogeneous heterometal interface. However, large deformation modes or large negative ηa(1) definitely emerge at the opening void edges.