Effect Of Mechanical Constraint Research Articles

A continuum large-deformation theory for the coupled modeling of polymer–solvent system with application to PV recycling

Nowadays recycling of photovoltaics (PV) using the solvent method is becoming a very hot topic as massive products deployed in the last century have approached the end of their service lifetime. The key problem in the recycling of end-of-life PV modules is the nondestructive recovery of precious silicon wafers for the manufacturing of new products. However, the attempt to comprehensively understand the polymer–solvent system in the PV recycling process is completely lacking. In this work, a thermodynamically consistent large-deformation theory is proposed to model the coupled behavior of this system. The development of continuum theory accounts for the solvent permeation, swelling and elastic deformation, as well as shrinking effects due to the initial crosslinking of ethylene-co-vinyl acetate (EVA). The crosslinking of EVA influences the stiffness of the polymer network, and interacts with the diffusive kinetics of solvents. Also, given the effects of mechanical constraint, the two-way coupling between the EVA deformation and solvent diffusion is established on the basis of thermodynamic arguments. The proposed modeling method is firstly applied to simulate the swelling experiments of cylindrical EVA samples in solvents Toluene, Tetrahydrofuran, and Octane, and good agreement has been achieved between the numerical prediction and available testing data. Then the second example demonstrates the capability of this modeling framework to describe the influences of initial crosslinking and mechanical constraints on the time history evolution of swelling and elastic deformation. Finally, the complete PV laminate in the 3D setting is modeled for the investigation of solvent penetration induced deformation in the silicon cell layer during the PV recycling process, and comparison has been made to showcase the spatial distribution of maximum principal stress of the silicon cell layers in solvents with different solubility parameters and molar volumes. With this computational tool at hand, it is possible to provide guidance to the design of suitable experimental procedures for the structure-intact recovery of silicon wafers in PV recycling with the solvent method.

Mechanical Constraint Effect on DNA Persistence Length.

Persistence length is a significant criterion to characterize the semi-flexibility of DNA molecules. The mechanical constraints applied on DNA chains in new single-molecule experiments play a complex role in measuring DNA persistence length; however, there is a difficulty in quantitatively characterizing the mechanical constraint effects due to their complex interactions with electrostatic repulsions and thermal fluctuations. In this work, the classical buckling theory of Euler beam and Manning's statistical theories of electrostatic force and thermal fluctuation force are combined for an isolated DNA fragment to formulate a quantitative model, which interprets the relationship between DNA persistence length and critical buckling length. Moreover, this relationship is further applied to identify the mechanical constraints in different DNA experiments by fitting the effective length factors of buckled fragments. Then, the mechanical constraint effects on DNA persistence lengths are explored. A good agreement among the results by theoretical models, previous experiments, and present molecular dynamics simulations demonstrates that the new superposition relationship including three constraint-dependent terms can effectively characterize changes in DNA persistence lengths with environmental conditions, and the strong constraint-environment coupling term dominates the significant changes of persistence lengths; via fitting effective length factors, the weakest mechanical constraints on DNAs in bulk experiments and stronger constraints on DNAs in single-molecule experiments are identified, respectively. Moreover, the consideration of DNA buckling provides a new perspective to examine the bendability of short-length DNA.

Real-time tracking of root hair nucleus morphodynamics using a microfluidic approach.

Root hairs (RHs) are tubular extensions of root epidermal cells that favour nutrient uptake and microbe interactions. RHs show a fast apical growth, constituting a unique single cell model system for analysing cellular morphodynamics. In this context, live cell imaging using microfluidics recently developed to analyze root development is appealing, although high-resolution imaging is still lacking to enable an investigation of the accurate spatiotemporal morphodynamics of organelles. Here, we provide a powerful coverslip based microfluidic device (CMD) that enables us to capture high resolution confocal imaging of Arabidopsis RH development with real-time monitoring of nuclear movement and shape changes. To validate the setup, we confirmed the typical RH growth rates and the mean nuclear positioning previously reported with classical methods. Moreover, to illustrate the possibilities offered by the CMD, we have compared the real-time variations in the circularity, area and aspect ratio of nuclei moving in growing and mature RHs. Interestingly, we observed higher aspect ratios in the nuclei of mature RHs, correlating with higher speeds of nuclear migration. This observation opens the way for further investigations of the effect of mechanical constraints on nuclear shape changes during RH growth and nuclear migration and its role in RH and plant development.

Effects of mechanical constraint on thermally induced reverse martensitic transformation in granular shape memory ceramic packings

Zirconia-based ceramics exhibit shape memory and superelastic effects based on the reversible martensitic transformation between tetragonal and monoclinic crystal structures. In the form of granular packings, these shape memory ceramics can be scaled up for bulk applications despite their intrinsic brittleness, while displaying drastically different transformation characteristics than the monolithic counterparts. Here, we present a comparative study to understand the thermally induced reverse martensitic transformation in granular packings and the influence of mechanical constraints. This study employs ZrO2–CeO2 shape memory ceramics of the same composition but with different degrees of mechanical constraints. The explored material forms include loose and jammed granular packings, themselves consisting of polycrystalline or single crystal particles, as well as sintered bulk polycrystals. Except for the latter, no endothermic peak is observed in the heat flow measurement of the reverse transformation process. This unusual behavior is shown to stem from the weak inter-particle mechanical constraint and the transformation heterogeneity among individual particles, rather than stress relaxation or particle rearrangement. To compare, conspicuous endothermic peaks only appear in bursting-type transformations under a strong mechanical constraint. For granular packings, the intra-particle mechanical constraint does not affect the presence of any endothermic peaks in thermal reversion but can influence the austenite start temperature.

Simultaneous effect of particle size and location on stress development in the electrodes of lithium‐ion batteries

In this study, stress generation at the electrode in Li-ion batteries was studied using a two-dimensional cell-scale model that includes multiple active particles during galvanostatic discharge. Numerical simulations were performed using an electrochemical–mechanical coupled model to elucidate the simultaneous effects of particle size and location, lithium intercalation kinetics and binder constraints on the stress. The simulation results showed that when different sizes of particle are considered in the electrode, the small particles were discharged more than the large particles, resulting in higher level of stress in the smaller particles. In addition, the closer the particles were located to the separator, the larger the stresses that were developed in those particles. Therefore, a layered structure, where the particle size gradually increases as the distance from the particles to the separator decreases, can alleviate stress on the electrode. When binder constraints were considered for the electrode particles, the stress was increased at the anode and alleviated at the cathode upon discharge. This indicates that the effect of mechanical constraints on stress generation in the particles differs in the lithiation and delithiation process.

Microstructural considerations of enhanced tensile strength and mechanical constraint in a copper/stainless steel brazed joint

The microscopic and macroscopic material strengthening mechanisms encountered in brazed joints were experimentally investigated and quantified in this work. Microstructural contribution towards the overall strength of a stainless steel brazed joint was evaluated by conducting multi-scale microstructural characterisations. Theoretical evaluation of the collected microstructural data suggests a Cu–Mn solid-solution strengthening enhancement of ~200 MPa towards the overall joint strength. The mechanical constraint effect was considered as the macroscopic strengthening mechanism, as revealed by comparing two identical joints but with their interfaces orientated at 90° and 45°with respect to the applied load. Bridgman necking criteria was applied to derive the longitudinal flow stress for the 90° joint configuration. The discrepancy (~330 MPa) between the calculated and experimentally determined strengths can thus be concluded as the contribution of mechanical constraint. In addition, the pile-up of geometrically necessary dislocations (GNDs) was observed at the base-filler metal interface for the 90° joint, but a homogeneous GND distribution was revealed for the 45° one. This observation indicates that GNDs were introduced to accommodate deformation incompatibility imposed by the mechanical constraint. This finding is thus considered as an experimental (microscopic) evidence for strain inhomogeneity due to the presence of mechanical constraint.

Tearing and reliability of photovoltaic module backsheets

AbstractThe backsheet in photovoltaic modules belongs to an important class of layered materials where the tearing behavior of the individual layers does not necessarily represent the tearing behavior of the entire backsheet. Such characteristic arises from the interaction between the individual layers during the tearing process, where one layer of the backsheet is mechanically constrained by its neighboring layers and the layers may debond from each other. The mechanical constraint and debonding change the amount of energy dissipated during tearing and affect the overall tearing energy. In this work, we exposed a wide selection of backsheets with polymers including ethylene‐vinyl acetate (EVA), polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), and ethylene tetrafluoroethylene (ETFE) to damp heat (85°C/85%RH) for up to 2000 hours. We report on the effect of damp heat on the tearing energy as a function of damp heat exposure. We developed a model that describes the tearing energy of a layered structure by accounting for the tearing of the individual layers in the backsheet, the effect of mechanical constraint, and the adhesive debonding between the layers. Additionally, we explore the relationship between the microstructural change in the polymers which resulted from the damp heat exposure and the mechanical properties using modulated differential scanning calorimetry (MDSC), small and wide angle X‐ray scattering (SAXS and WAXS), and tensile testing.

A Simultaneous Multiscale and Multiphysics Model and Numerical Implementation of a Core-Shell Model for Lithium-Ion Full-Cell Batteries

The increasing significance on the development of high-performance lithium-ion (Li-ion) batteries is calling for new battery materials, theoretical models, and simulation tools. Lithiation-induced deformation in electrodes calls attention to study the multiphysics coupling between mechanics and electrochemistry. In this paper, a simultaneous multiscale and multiphysics model to study the coupled electrochemistry and mechanics in the continuum battery cell level and the microscale particle level was developed and implemented in comsolmultiphysics. In the continuum scale, the porous electrode theory and the classical mechanics model were applied. In the microscale, the specific particle structure has been incorporated into the model. This model was demonstrated to study the effects of mechanical constraints, charging rate, and silicon/C ratio, on the electrochemical performance. This model provides a powerful tool to perform simultaneous multiscale and multiphysics design on Li-ion batteries, from the particle level to full-cell level.

Increased female resistance to mating promotes the effect of mechanical constraints on latency to pair.

Size‐assortative mating, defined as a positive linear association of body size between members of mating pairs, can arise from mechanical constraints on pairing efficiency, particularly when mating success is affected by males’ mate‐grasping force. In this context, female resistance is predicted to have an important role in changing the threshold force necessary for males to hold females, thereby contributing to the effect of mechanical constraints. Thus, increased female resistance is expected to increase the paring success of an optimally sized male relative to the female body size (sexual size ratio = male body size/female body size = 0.86), which leads to positive size‐assortative mating. However, very little is known about the extent to which female resistance affects mechanical constraints on mate grasping. Here, using the water strider Gerris gracilicornis (Hemiptera: Gerridae), we tested whether the level of female resistance affected the relationship between the sexual size ratio and latency to pair. We found that optimally sized males mated sooner than other males when females resisted a male's mating attempts. When females did not resist, an effect of sexual size ratio on latency to pair was not found. Our results thus imply that increased female resistance to male mating attempts may strengthen the pattern of size‐assortative mating. We provide clear empirical evidence that female resistance to mating influences the effect of mechanical constraints on size‐assortative mating under sexual conflict. This result further suggests that patterns of size‐assortative mating can be altered by a variety of ecological circumstances that change female resistance to mating in many other animal species under sexual conflict.

Computational Study on the Effects of Mechanical Constraint on the Performance of Si Nanosheets as Anode Materials for Lithium-Ion Batteries

Lithiation process of Si has been widely studied, while most of the previous works neglected the effects of mechanical constraint from the accessory materials such as binder and conductive additive on Si. In this paper, we carry out molecular dynamics simulations to investigate the effects of mechanical constraint on the performances, such as capacity and lithiation rate, of Si nanosheets as the anode material for lithium-ion batteries. Our results show that strong mechanical constraint would largely affect the capacity and lithiation rate of Si nanosheets. Remedial strategies for improving the capacity and lithiation rate of the constrained Si nanosheets are proposed, giving rise to guidelines for the design of Si-based anode material for high-performance lithium-ion batteries.

On the response function and range dependence of manual estimation.

Manual estimates without vision of the hand are thought to constitute a form of cross-modal matching between stimulus size and finger opening. However, few investigations have systematically looked at how manual estimates relate a perceived size to the response across different ranges of stimuli. In two experiments (N = 18 and N = 14), we sought to map out the response properties for (1) manual estimates of visually presented stimuli as well as (2) visual estimates of proprioceptive stimuli, and to test whether these properties depend on the range of stimuli. We also looked at whether scalar variability is present in manual estimates, as predicted by Weber's Law for perceptual tasks. We found that manual estimates scale linearly and with a slope of close to 1 with object sizes up to 90mm, before participants' hand size limited their responses. In contrast, we found a shallower response slope of about 0.7 when participants performed the inverse task, adjusting the size of a visual object to match a not actively chosen, induced finger opening. Our results were mixed with regards to scalar variability in large objects. We saw some indication of a plateau, but no evidence for an effect of mechanical constraints in the range studied (up to 90mm). Participants also showed a clear tendency to overestimate small differences when a set of objects differed little in size, but not when stimulus differences were more pronounced.

Effect of mechanical constraint on the hydration properties of Na-montmorillonite: study under extreme relative humidity conditions

The evaluation of the performance of a geological barrier, consisting essentially of a clay matrix, in the context of industrial and household waste confinement must go with the study of its hydration behavior respectively under extreme atmospheric conditions and variable mechanical soil constraints. Na-montmorillonite (Swy-2) is used, as starting materials, in order to establish the link between applied externals strain (variable relative humidity rate %RH and axial mechanical constraint) and the hydration material response. All constraints are realized at the laboratory scale. This work is achieved using oedometric testing and quantitative X-ray diffraction (XRD) analysis, based on the modeling approach, which consists in the comparison of experimental 00l reflections with the calculated ones deduced from structural models. This approach allows us to quantify the interlamellar space configuration and all structural changes along the c* axis. Obtained results show a decrease for the void ratio e value along the compaction/reswelling process. The “insitu” XRD analysis realized at 5%RH demonstrates hydration shift, from dehydrated water layer (i.e. 0W) to monohydrated water layer (i.e. 1W), attributed to the applied mechanical constraint. At 90%RH, the sample hydration state remains at tri-hydrated water layer (3W) with a clear interstratified trends.

Effect of Mechanical Constraint on Tearing Energy of Polymer Membranes

The role of mechanical constraint on the tearing behavior of perfluorosulfonic acid and polyethylene membranes is examined. A constrained tearing test is employed to measure the tearing energy when the formation of a plastic zone ahead of the tear tip is constrained. It is shown that the tearing energy decreases with increasing constraint associated with a smaller plastic zone and is much lower than the tearing energy with no constraint. Furthermore, it is demonstrated that the tearing rate has a strong effect on the tearing energy when the membrane is unconstrained, but a much smaller effect when the membrane is constrained as the viscoelastic relaxation processes related to the plastic zone are suppressed. The results highlight the significant role of mechanical constraint on the tearing behavior of membranes during both testing and in application. image

Effect of Mechanical Constraint on Domain Reorientation in Predominantly {111}‐Textured Lead Zirconate Titanate Films

Ferroelectric/ferroelastic domain reorientation was measured in 2.0 μm thick tetragonal {111}‐textured PbZr0.30Ti0.70O3 thin films using synchrotron X‐ray diffraction (XRD). Lattice strain from the peak shift in the 111 Bragg reflection and domain reorientation were quantified as a function of applied electric field amplitude. Domain reorientation was quantified through the intensity exchange between the 112 and 211 Bragg reflections. Results from three different film types are reported: dense films that are clamped to the substrate (as‐processed), dense films that are partially released from the substrate, and films with 3% volume porosity. The highest amount of domain reorientation is observed in grains that are misoriented with respect to the {111} preferred (domain engineered) orientation. Relative to the clamped films, films that were released from the substrate or had porosity exhibited neither significant enhancement in domain reorientation nor in 111 lattice strain. In contrast, similar experiments on {100}‐textured and randomly oriented films showed significant enhancement in domain reorientation in released and porous films. Therefore, {111}‐textured films are less susceptible to changes in properties due to mechanical constraints because there is overall less domain reorientation in {111} films than in {100} films.

Effects of Mechanical Constraint on Director Configuration of Monodomain Nematic Elastomers under Temperature Variation

Effects of Mechanical Constraint on Director Configuration of Monodomain Nematic Elastomers under Temperature Variation

Geometry effect on mechanical performance and fracture behavior of micro-scale ball grid array structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints

Solder joint integrity has long been recognized as a key issue affecting the reliability of integrated circuit packages. In this study, both experimental and finite element simulation methods were used to characterize the mechanical performance and fracture behavior of micro-scale ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints with different standoff heights (h, varying from 500 to 100μm) and constant pad diameter (d, d=480μm) and contact angle under shear loading. With decreasing h (or the ratio of h/d), results show that the stiffness of BGA solder joints clearly increases with decreasing coefficient of stress state and torque. The stress triaxiality reflects the mechanical constraint effect on the mechanical strength of the solder joints and it is dependent on the loading mode and increases dramatically with decreasing h under tensile loading, while the change of h has very limited influence on the stress triaxiality under shear loading. Moreover, when h is decreased, the concentration of stress and plastic strain energy along the interface of solder and pad decreases, and the fracture location of BGA solder joints changes from near the interface to the middle of the solder. Both geometry and microstructure greatly affect the shear behavior of joints, the average shear strength shows a parabolic trend with decreasing standoff height. Furthermore, the brittle fracture of BGA solder joints after long-time isothermal aging was investigated. Results obtained show that, under the same shear force, the stress intensity factors, KI and KII, and the strain energy release rate, GI, at the Sn–3.0Ag–0.5Cu/Cu6Sn5 interface and in the Cu6Sn5 layer obviously decrease with decreasing h, hence brittle fracture is more prone to occur in the joint with a large standoff height.

Damage evolution and distribution of interpenetrating phase composites under dynamic loading

The objective of this paper is to investigate the damage evolution of ceramic-metal interpenetrating phase composite (IPC) under dynamic loading. Uniaxial dynamic compression loading was performed to characterize the failure of SiC3D/Al composite with 20% porosity using a modified Split Hopkinson Pressure Bar (SHPB). A micro CT technology is utilized to acquire SiC skeleton scanning images. 3D FE (finite element) model of SiC3D/Al was generated and was applied based on the experimental data. SEM and computer simulation results were employed to study the damage evolution in IPC. The experimental and simulation results demonstrated that there were double cones formed within the IPC cylinder without lateral confinement under uniaxial dynamic compression. The damage initiation and damage extension direction were determined by the shear stress. The synergistic mechanical constraint effect provided by the interpenetrating metal and ceramic phase conduced the fracture cone feature, which was significantly different from the splitting feature of ceramic materials.

Effect of Mechanical Constraints in Thin Ceramic LTCC/PZT Multilayers on the Polarization Behavior of the Embedded PZT

The effect of direct integration of PZT into LTCC by sintering on its polarization behavior is studied. Due to the typically small thermal expansion of PZT, compressive stresses are expected after cooling, which should promote poling. However, measurements show a strongly reduced remanent polarization compared to free PZT. Analysis of thermal expansion reveals that thermal shrinkage of the PZT is larger than that of LTCC. The impact of this finding on the poling behavior is simulated for the LTCC/PZT module, confirming the experimental observations and allowing for conclusions concerning the suitability of the PZT for integration into LTCC structures.

Research on Low Cycle Fatigue Behavior of BGA Structure Lead-free Solder Joints

A plastic strain energy density methodology is proposed to evaluate the initiation and propagation of fatigue crack in lead-free solder joints. The relationships among the plastic strain, plastic strain energy, continuum damage mechanics(CDM) and fatigue life are clarified. Crack growth correlation constants for micro-scale ball grid array(BGA) structure solder joints(with standoff height h in the range 100 to 500 μm and a pad diameter 480 μm) are determined by a combination of experimental estimation and numerical calculation. The results show that the cycle numbers of crack initiation and propagation have power function relationship with the plastic strain energy density generated in each fatigue cycle. Crack propagation rate is affected by stress triaxiality, which is dependent on loading modes, i.e., stress triaxiality increases dramatically with decreasing h under tensile load because of the mechanical constraint effect arising from interfaces and package structure, while under shear load the standoff height has very limited effect on stress triaxiality. Furthermore, crack growth correlation constants identified in solder joints with h=100 μm can be well used to predict fatigue life of solder joints with different geometries, indicating that the fatigue life prediction method proposed in this study can effectively prevent the influence of plastic strain energy concentration caused by structural and volume factors on the prediction of fatigue life of BGA structure solder joints.

Examination of zwitterionic polymers and gels subjected to mechanical constraints

The effect of mechanical constraints on the anti-polyelectrolyte behavior of zwitterionic polymers was studied systematically in this work via viscosity examination of uncross-linked polymer solutions, swelling of cross-linked polymer gels and swelling of elastomers compounded with these gels. Zwitterionic polymers, poly(sulfobetaine methacrylate) (polySBMA), poly(2-methacryloyloxyethyl phosphorylcholine) (polyMPC) and copolymers of polySBMA with a polyelectrolyte, poly(sodium methacrylate) (polyNaMA) were used in this study. Anti-polyelectrolyte effects were observed both in solution and as gels. However, when the zwitterionic gels were compounded with hydrogenated acrylonitrile butadiene rubber (HNBR), the anti-polyelectrolyte behavior of the native polymer was lost. This is attributed to mechanical constraints imposed by the elastomeric HNBR matrix that dominate Coulombic interactions. Without the manifestation of chain relaxation and extension afforded by the charge screening, the zwitterionic polymers serve as hydrophilic agents that facilitate water diffusion into rubber.