Interfacial Stress Concentrations Research Articles

Regulation interfacial energy by reducing the Young’s modulus of perovskite film in carbon-based HTM free MAPbI3 perovskite solar cell

In ambient air, a long-chain dodecafluorosuberic acid (DFSA) with lower Young’s modulus was introduced to improve the mechanical compatibility of perovskite/carbon interface. Firstly, the carbonyl groups CO and F– in DFSA can passivate the intrinsic defects of perovskite, the DFSA effectively reduces the Young’s modulus of the perovskite and mitigates residual stress generated during perovskite annealing procedure. Most importantly, DFSA significantly enhances the Young’s modulus compatible degree between DFSA-CH3NH3PbI3 (DFSA-MAPbI3, 35.8 GPa) and carbon (6.3 GPa), alleviating the interfacial stress concentration. Based on quantitatively analysis, interfical crack driving energy (Gd) is reduced from 0.63 to 0.43 J m−2, while the adhesive fracture energy (Ga) is increased from 0.47 to 0.54 J m−2, which reduces the probability of crack propagation and ductile delamination, and is conducive to the extraction and transmission of photogenerated carrior. This was also verified in finite element simulations. Finally, the photoelectric conversion efficiency (PCE) of the organic-inorganic hybrid perovskite (MAPbI3) PSC device is improved from 10.27 % to optimal 14.12 %. After service at room temperature with relative humidity (RH) of 50–55 % and high temperature of 85 °C for 720 h, the original efficiency is maintained at 72 % and 89 %, respectively.

Enhanced interfacial, mechanical, and anti-hygrothermal properties of carbon fiber/cyanate ester composites with the catalytic sizing agents of titanium epoxy

The mechanical properties of composites are closely related to the interfacial behavior, especially under the hygrothermal circumstance. A catalytic sizing agent of titanium epoxy is designed to enhance interfacial, mechanical, and anti-hygrothermal properties of high modulus carbon fiber (HMCF)/cyanate ester composites simultaneously. The mechanisms of interface enhancement and low hygroscopicity of composites are investigated. The titanium epoxy is synthesized and its catalytic effect on the curing of cyanate ester is proved. The interfacial properties of HMCF composites with catalytic sizing agents are improved to 95.5 MPa, which is attributed to the interphase with high crosslinking density and sufficient triazine rings and oxazolidinone structure due to preferential curing induced by interfacial catalysis, stimulating the smooth transition of interphase modulus. Further, the formed interphase exhibits few interface defects and low content of hydroxyl groups, which changes the moisture diffusion path and reduces saturated water absorption of composites to only 0.36 %, resulting in the release of interfacial wet stress concentration and high retention of mechanical properties in hygrothermal environment. The resultant composites with high stiffness, excellent temperature resistance, superior dimensional stability and low moisture absorption are expected to be applied to high-orbit space, aerospace, precision instruments.

Interfacial reaction induced mechanical behaviors in graphene reinforced Nb/Nb5Si3 composites

Fabricating high-toughness Nb/Nb5Si3 composites without sacrificing their strength is a challenge. Here, a novel and promising strategy is suggested for developing the graphene reinforced Nb/Nb5Si3 composites with high performance by adjusting the nano-sized Nb4C3 interfacial phase, and their mechanical behaviors including toughening and strengthening mechanisms were reported for the first time. The generated nano-Nb4C3 interfacial phase is tightly bonded with the graphene and matrix, which reduces the interfacial stress concentration and improves the interfacial bonding strength because of the improvement of interfacial compatibility. The high interfacial bonding strength caused by the nano-Nb4C3 can effectively promote crack deflection and bridging, and consume the more propagating energy of cracks, thus increasing the toughness. The primary strengthening mechanism in this composite is the load transfer, which is influenced by the nano-Nb4C3 interfacial phase. An appropriate amount of nano-Nb4C3 interfacial phase is crucial for achieving a good strength-toughness balance in this composite. The excellent combination of strength and toughness demonstrates that the strategy used in this work can make this composite stand out from the dilemma of strength toughness inversion. This work paves a new path for breaking the inverted relationship between the strength and toughness in graphene/metal matrix composites.

Hydrogen Bonds-Pinned Entanglement Blunting the Interfacial Crack of Hydrogel-Elastomer Hybrids.

Anchoring a layer of amorphous hydrogel on an antagonistic elastomer holds potential applications in surface aqueous lubrication. However, the interfacial crack propagation usually occurs under continuous loads for amorphous hydrogel, leading to the failure of hydrogel interface. This work presents a universal strategy to passivate the interfacial cracks by designing a hydrogen bonds-pinned entanglement (Hb-En) structure of amorphous hydrogel on engineering elastomers. The unique Hb-En structure is created by pinning well-tailored entanglements via covalent-like hydrogen bonds, which can amplify the delocalization of interfacial stress concentration and elevate the necessary fracture energy barrier within hydrogel interface. Therefore, the interfacial crack propagation can be suppressed under single and cyclic loads, resulting in a high interfacial toughness over 1650 J m-2 and an excellent interfacial fatigue threshold of 423 J m-2. Such a strategy universally works on blunting the interfacial crack between hydrogel coating and various elastomer materials with arbitrary shapes. The superb fatigue-crack insensitivity at the interface allows for durable aqueous lubrication of hydrogel coating with low friction.

Piezoelectric Interlayer Enabling a Rechargeable Quasisolid-State Sodium Battery at 0°C.

Solid-state sodium (Na) batteries (SSNBs) hold great promise but suffer from several major issues, such as high interfacial resistance at the solid electrolyte/electrode interface and Na metal dendrite growth. To address these issues, a piezoelectric interlayer design for an Na3Zr2Si2PO12 (NZSP) solid electrolyte is proposed herein. Two typical piezoelectric films, AlN and ZnO, coated onto NZSP function as interlayers designed to generate a local stress-induced field for alleviating interfacial charge aggregation coupling stress concentration and promoting uniform Na plating. The results reveal that the interlayer (ZnO) with matched modulus, high Na-adhesion, and sufficient piezoelectricity can provide a favorable interphase. Low interfacial resistances of 91 and 239 Ω cm2 are achieved for the ZnO layer at 30 and 0°C, respectively, which are notably lower than those for bare NZSP. Moreover, steady Na plating/stripping cycles are rendered over 850 and 4900h at 0 and 30°C, respectively. The superior anodic performance is further manifested in an Na2MnFe(CN)6-based full cell which delivers discharge capacities of 125mA h g-1 over 1600 cycles at 30°C and 90mA h g-1 over 500 cycles at 0°C. A new interlayer-design insight is clearly demonstrated for SSNBs breaking low-temperature limits.

Unraveling thermoelastic performance of the multi-directional functionally graded composite structure by data-driven deep-learning approach

Multi-directional functionally graded materials (MD-FGMs) are a brand-new family of sophisticated composites with continually changing structural and mechanical properties but no internal boundaries and interfacial stress concentrations. This study is the first to examine the effects of an instantaneous thermal shock on the thermo-elastodynamic response of a multi-directional functionally graded (MD-FG) doubly curved panel resting on an elastic substrate due to the remarkable properties of these advanced materials. The peripheral discontinuity at the boundaries between the loaded and unloaded surfaces as well as the shear coupling between the various Winkler-spring components are both taken into account in this proposed mode of the Kerr-type elastic substrate. Through the use of 3D-DQA, the system’s equations may be solved for both the simply supported and the completely clamped boundary conditions. For the first time, it is examined how several thermal shock models, such as cosinusoidal, sinusoidal, and Heaviside functions, impact how the system’s stress and deflection parameters fluctuate. The authors of this paper provide a unique strategy for enhancing the computing efficiency of the approach by constructing a deep-learning-based method on the data produced by 3D-DQT and Laplace transform at specified locations due to the dynamic nature of the issue. In order to decrease the mechanical excitation of the existing curved structure brought on by thermal shock, some advice for relevant industries is offered at the conclusion.

PVDF/6H-SiC composite fiber films with enhanced piezoelectric performance by interfacial engineering for diversified applications

Piezoelectric silicon carbide (SiC) has been quite attractive due to its superior chemical and physical properties as well as wide potential applications. However, the inherent brittleness and unsatisfactory piezoelectric response of piezoelectric semiconductors remain the major obstacles to their diversified applications. Here, flexible multifunctional PVDF/6H-SiC composite fiber films are fabricated and utilized to assemble both piezoelectric nanogenerators (PENGs) and stress/temperature/light sensors. The open circuit voltage (Voc) and the density of short circuit current (Isc) of the PENG based on the PVDF/5 wt% 6H-SiC composite fiber films reach 28.94 V and 0.24 µA cm–2, showing a significant improvement of 240% and 300% compared with that based on the pure PVDF films. The effect of 6H-SiC nanoparticles (NPs) on inducing interfacial polarization and stress concentration in composite fiber films is proved by first-principles calculation and finite element analysis. The stress/temperature/light sensors based on the composite fiber film also show high sensitivity to the corresponding stimuli. This study shows that the PVDF/6H-SiC composite fiber film is a promising candidate for assembling high-performance energy harvesters and diverse sensors.

Enhanced interfacial joining strength of laser wobble joined 6061-T6 Al alloy/CFRTP joint via interfacial bionic textures pre-construction

The joining of metals and carbon fiber reinforced thermoplastics composite (CFRTP) is recently considered to be an effective and promising way to meet the lightweight requirements of manufacturing in the aviation field. Herein, novel bionic textures (fishbone-like and nacre-like), constructed by laser pretreatment, are proposed to enhance the hybrid joints of 6061 Al alloy and CFRTP by the laser wobble joining method. The effects of the proposed bionic textures on the hybrid joints of 6061/CFRTP were uncovered through a systematic experimental method and finite element analysis (FEA). The results indicated that the strength and toughness of the hybrid joints with novel bionic textures were both distinctly improved compared with that of conventional textures (groove-like and circular-like). This strengthening effect can be essentially attributed to the buffering of interfacial stress concentration and the deflection behavior of cracks. The observed interfacial gaps and fractured resin appearance both distributed near the sides of the textures demonstrate that the cracks propagate inside the resin on one side of the texture and propagate along the interface on the other side. A new reinforcement strategy for hybrid metal/CFRTP joints has been proposed: regulating interfacial bionic textures to inhibit the initiation and propagation of interfacial cracks.

Influence of fiber hollowness on the local thermo-electro-elastic field in a thermoelectric composite

The fiber geometry is one of the important parameters in the effective conversion performance and local strength of thermoelectric composites. In this study, the plane problem of a hollow fiber embedded within a non-linear thermoelectric medium in the presence of a uniform remote in-plane electric current and a uniform remote energy flux is investigated based on the complex variable method. Closed-form expressions for all the potential functions characterizing the thermoelectric field and the associated thermal stress field in both the matrix and fiber are obtained by solving the corresponding boundary value problem. Numerical examples are presented to illustrate the effect of hollowness ratio of the fiber on the local energy conversion efficiency and interfacial thermal stress concentration. It is found that a higher conversion efficiency and a lower amount of thermal stress concentration around a hollow fiber than that around a solid fiber could be achieved simultaneously by appropriate selection of the hollowness ratio of the fiber. The results can be directly used for performance optimization and reliability evaluation in design of thermoelectric composites in engineering.

Stress analysis of adhesive-bonded tubular-coupler joints with optimum-stiffness-composite adherend under axial tension, pressure, and thermal loading

With descriptions of fiber orientation variation, variable-stiffness composites (VSCs) have opened many new opportunities for structural designs because they offer tailorable material properties to increase the performance of composite structures. Using the concept of VSCs, this study aimed to determine the optimum adhesive-bonded tubular coupler joints subjected to tension, pressure, thermal load, and/or combinations thereof. The joint consists of three axisymmetric cylindrical adherends. The two inner adherends are made of a linear elastic isotropic material, whereas the outer adherend is composed of a symmetric balanced laminate. As the most complicated cases among others, owing to various stress components induced in the adherends and adhesive, the joints under these dominant in-plane normal force loadings are modeled to efficiently minimize stress concentrations in the thin adhesive layer. The unified theory of adhesive-bonded tubular joints was modified one step further to compute the optimum fiber angle variation using the constraint of linear distribution of the axial resultant forces in the adherends. From the methodological viewpoint, the results unveiled that the interfacial shear stress concentration in the adhesive layer was considerably diminished or even vanishes under all loading scenarios. The possibility of multiple optimum composite joints can be achieved, which otherwise are not straightforward to determine when using other analytical tubular joint models and optimization methods. Parametric studies of geometrical variables were also performed to provide a design guideline for optimum coupler joints with efficient mitigation of peak adhesive interfacial stresses.

The Mechanics of Bioinspired Stiff-to-Compliant Multi-Material 3D-Printed Interfaces.

Complex interfaces that involve a combination of stiff and compliant materials are widely prevalent in nature. This combination creates a superior assemblage with strength and toughness. When combining two different materials with large stiffness variations, an interfacial stress concentration is created, decreasing the structural integrity and making the structure more prone to failure. However, nature frequently combines two dissimilar materials with different properties. Additive manufacturing (AM) and 3D printing have revolutionized our engineering capabilities concerning the combination of stiff and compliant materials. The emergence of multi-material 3D-printing technologies has allowed the design of complex interfaces with combined strength and toughness, which is often challenging to achieve in homogeneous materials. Herein, we combined commercial 3D-printed stiff (PETG) and compliant (TPU) polymers using simple and bioinspired interfaces using a fused deposition modeling (FDM) printer and characterized the mechanical behaviors of the interfaces. Furthermore, we examined how the different structural parameters, such as the printing resolution (RES) and horizontal overlap distance (HOD), affect the mechanical properties. We found that the bioinspired interfaces significantly increased the strain, toughness, and tensile modulus compared with the simple interface. Furthermore, the more refined printing resolution elevated the yield stress, while the increased overlap distance mostly elevated the strain and toughness. Additionally, 3D printing allows the fabrication of other complex designs in the stiff and compliant material interface, allowing various tailor-designed and bioinspired interfaces. The importance of these bioinspired interfaces can be manifested in the biomedical and robotic fields and through interface combinations.

Effects of MnS inclusions on mechanical behavior and damage mechanism of free-cutting steel: A molecular dynamics study

In order to research the effect pattern of MnS inclusions on free-cutting steel, we study the microstructure evolution, the damage mechanism and the mechanical properties in free-cutting steel in the presence of MnS inclusions. Spindle shaped MnS is added as inclusions within the free-cutting steel. The mechanical properties were found to change when inclusions were present. The gained results show that the formation of voids causing fracture starts from the interface inside the matrix close to the MnS. From the point of view of nanocomposite strength, the main effect of MnS inclusions is related to stress concentration, leading to the effect of increased stresses near the interface between the interior of the matrix and the inclusions. The inclusions have lower Young's modulus and lower dislocation activity, resulting in smaller deformation of the alloy system, larger interfacial stress concentrations and earlier hole formation. The maximum strain and stress regions of the alloy also appear near the MnS inclusions, which leads to the formation of defects near the MnS inclusions and then fracture of the alloy. MnS inclusions adversely affect the tensile properties of the alloy, such as Young's modulus, yield stress and yield strain. By comparing the stress-strain curves of single crystal iron and alloy containing MnS inclusions, it is indicated that the yield strength of the latter decreases. Slip bands and dislocation lines are first generated around the MnS inclusion, and the phase transition is induced from the original single BCC structure to FCC, HCP and amorphous structures, and the atoms of FCC, HCP and amorphous structures increase with increasing strain, while those of BCC structure decrease, especially after yield strain. This study is significant for understanding the effect of inclusions on the mechanical laws and fracture mechanisms of the alloy.

Octopus-like carbon nanomaterial for double high stretchable conductor

One dimensional conductive nanowire is an ideal component for efficient percolation network, which is used to construct highly conductive stretchable elastomer in wearable electronics. The percolation network frequently meets with the trade-off between high conductivity and high stretchability (“double high”) due to the interfacial stress concentration between conductive nanowire and elastomer matrix. Inspired by the octopus' structure, a hierarchical carbon nanostructure of carbon nanotubes riveted on carbon sphere (CNTs-CS) is proposed to synthetically couple high conductivity with high stretchability. As the ''octopus body'', carbon sphere anchor into elastomer matrix to allow the uniform distribution of stretching stress. As the ''octopus feet'', highly conductive carbon nanotubes remain robust linkage state under stretching strain, which ensures the generation of highly efficient percolation network. The stretchable conductor (CNTs-CS in Dragonskin) exhibits good conductivity (1.7 × 10 4 S m −1 ), stretchability (>550%) and mechanical stability (>5000 cycles). The stretchable conductor is successfully assembled in many stretchable electronic devices, including a LED-based illuminating system without obvious brightness changes under various deformations, and strain sensor exhibiting excellent sensing performance in the detection of human physical signs. This unique design strategy reveals a great application prospect in the wearable electronics.

The Effect of Carbon Content on the Microstructure and Mechanical Properties of Cemented Carbides with a CoNiFeCr High Entropy Alloy Binder.

CoNiFeCr high entropy alloy (HEA) was used as a binder in cemented carbides for developing a new high-performance binder. The microstructure of WC-HEA cemented carbides with different binder components and carbon contents was subsequently studied. It was observed that the (Cr,W)C phase precipitated at the WC/HEA interface, and a coherent interface with a low degree of misfit was formed between WC and (Cr,W)C, thereby resulting in a significant reduction in the interfacial energy and stress concentration. The (Cr,W)C phase exerted a pinning force (Zener-drag) on the moving grain boundaries, which effectively inhibited the growth of WC grains. As a result, compared with WC-Co, WC-CoNiFeCr had smaller WC grain size, smoother grain shape and larger mean free path (MFP) of the binder, which resulted in slightly lower hardness and higher transverse rupture strength (TRS) and fracture toughness. The lower limit of carbon content in WC-CoNiFeCr was higher than that of WC-Co. With the addition of Ni, the width of the two-phase region became wider, whereas the width of the two-phase region became narrower with the addition of Fe and Cr.

A novel micromechanical model to study the influence of cure process on the in-plane tensile properties of z-pinned laminates

This paper proposes a novel micromechanical model to study the curing effects on the in-plane tensile properties and failure mechanisms of carbon/epoxy laminates reinforced with z-pins. The microstructures including fiber distortion and resin-rich pockets caused by z-pins are characterized. The results indicate that cure-induced residual stresses greatly exist with increasing z-pin density. Thermal expansion and volume shrinkage mismatch between z-pins and resin-rich regions attribute to the high stress concentration. The existence of interfacial stress concentration and resin-rich channels produce a loss of tensile strength and lead to an initial pathway for crack growth under tension. The failure mode changing from the interfacial cracking to softening and damages of both interface and resin-rich regions is also revealed with increasing z-pin density.

Achieving high strength-ductility synergy through high-density coherent precipitation in twin-roll cast Al–Zn–Mg–Cu strips

Achieving dense coherent precipitates in 7xxx aluminum alloys during artificial aging to attain high strength-ductility synergy remains a challenging task as they are unstable and tend to dissolve or transform to the semi-coherent phase. Here, we successfully obtained high-density (1.32 × 1024 m−3) and fine-sized (∼4.1 nm) coherent precipitates in the twin-roll cast Al-6.3Zn-3.3Mg-1.8Cu (wt.%) alloys through pre-strain and subsequent artificial aging, i.e. strain aging. Therefore, the peak-aged sample exhibited a satisfactory tensile strength of ∼600 MPa and extraordinary fracture elongation of ∼15%. The inconspicuous interface between coherent precipitates and the Al matrix reduces the tendency of interfacial stress concentration, which ensures high room temperature ductility. The strengthening mechanism was calculated by the dislocation shearing model, indicating that coherent precipitates contribute ∼80% of the total yield strength. This work could provide a short-process and low-budget method for preparing high-performance 7xxx series alloy strips.

Numerical simulation of the interfacial stress between epoxy resin and metal conductor of power equipment during epoxy curing

There are many metal and epoxy resin interfacial structures in power equipments, which can generate interfacial stress concentrations and may induce accidents due to epoxy curing. In this paper, through simplifying interface structure into a coaxial cylindrical model, the formation mechanism of interfacial stress concentration of this interface structure was studied during epoxy resin curing process. The strain and temperature at the interface were measured by the strain and temperature measurement system. Then a model for calculating the interface stress distribution was established, including the heat conduction module, curing dynamics module and curing deformation module. The validity of the model was verified by comparing the calculated results with the measured results. In addition, the air gap defects at the interface, appearing after the curing process, were simulated. The results show that, after the curing process, the first principal stress at the centre of the interface gradually increases from the centre to the edge, reaching 41.06 and 40.20 MPa at the upper and lower edges respectively. The existence of air gap leads to partial discharge at low voltage. The calculation model can be extended to the stress prediction between the metal and epoxy resin in other power equipments.

The effectiveness of the bonding layer to attain reliable thermoelectric structures

The reliable interface plays a significant role in improving the performance of thermoelectric devices. Due to the interface stability, thermoelectric interfacial layers are prone to multilayer and elastic thick layers. This paper develops a new theoretical model to investigate how an elastic bonding layer affects the behaviors of thermoelectric structures. The governing integral and integro-differential equation involving thermoelectric effects are obtained for the cases without and with the bonding layer and solved by collocation methods numerically. The results demonstrate that the geometric and material parameters, the total electric current, and the total energy flux have significant effects on the interfacial shear stress and normal stress. The interfacial stress concentration degradation can appear because the interface is introduced by a larger thick bonding layer. The proposed model can explain the experimental properties qualitatively, and provide theoretical guidance for designing thermoelectric structures.

Divisions in a Fibrillar Adhesive Increase the Adhesive Strength.

To realize the potential of bioinspired fibrillar adhesive applications ranging from biomedical devices to robotic grippers, there has been a significant effort to improve their adhesive strength and understanding of the underlying adhesion and detachment mechanisms. These efforts include changes to the backing layer, which connects the roots of all of the pillars in the fibrillar adhesive. However, previous approaches such as thickness or elastic modulus changes are selectively advantageous to the adhesive strength depending on the substrate condition because of the trade-off between conformity to misaligned/rough surfaces and increased interfacial stress concentrations. In this work, we explore mechanical divisions (cuts) in the backing layer as a new approach to improve the adhesive strength without this trade-off. We combine experiments and finite element analysis (FEA) to study the effect of the divisions, which decouples the mechanical interaction between the pillars on the divided layers, and show that the adhesive strength can be improved regardless of the substrate condition. Tensile adhesion experiments show increased adhesive strength with cuts to a micropost array (150 μm diameter posts) by approximately 25% for 4 divisions. In situ imaging of pillar detachment shows a transition of the detachment process from a peel-like detachment to a random detachment sequence. FEA simulations of the detachment process suggest that the increased strength may originate from a simultaneous enhancement of the load distribution between the pillars and the compliance of the backing layer.

Phase morphology, mechanical, and thermal properties of fiber-reinforced thermoplastic elastomer: Effects of blend composition and compatibilization.

In this work, recycled high density polyethylene (rHDPE) was compounded with regenerated tire rubber (RR) (35–80 wt.%) and reinforced with recycled tire textile fiber (RTF) (20 wt.%) as a first step. The materials were compounded by melt extrusion, injection molded, and characterized in terms of morphological, mechanical, physical, and thermal properties. Although, replacement of the rubber phase with RTF compensated for tensile/flexural moduli losses of rHDPE/RR/RTF blends because of the more rigid nature of fibers increasing the composites stiffness, the impact strength substantially decreased. So, a new approach is proposed for impact modification by adding a blend of maleic anhydride grafted polyethylene (MAPE)/RR (70/30) into a fiber-reinforced rubberized composite. As in this case, a more homogeneous distribution of the fillers was observed due to better compatibility between MAPE, rHDPE, and RR. The tensile properties were improved as the elongation at break increased up to 173% because of better interfacial adhesion. Impact modification of the resulting thermoplastic elastomer (TPE) composites based on rHDPE/(RR/MAPE)/RTF was successfully performed (improved toughness by 60%) via encapsulation of the rubber phase by MAPE forming a thick/soft interphase decreasing interfacial stress concentration slowing down fracture. Finally, the thermal stability of rubberized fiber-reinforced TPE also revealed the positive effect of MAPE addition on molecular entanglements and strong bonding yielding lower weight loss, while the microstructure and crystallinity degree did not significantly change up to 60 wt.% RR/MAPE (70/30).