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Purpose Oil-free heat pumps that use the system refrigerant gases as lubricants are preferred for thermal management in future space applications. This study aims to numerically and experimentally investigate the static performance of externally pressurized thrust bearings lubricated with refrigerant gases. Design/methodology/approach The refrigerant gases R22, R410A and CO2 were chosen as the research objects, while N2 was used for comparison. Computational fluid dynamics was used to solve the full 3 D Navier–Stokes equations to determine the load capacity, static stiffness and static pressure distribution in the bearing film. The numerical results were experimentally verified. Findings The results showed that the refrigerant-gas-lubricated thrust bearings had a lower load capacity than the N2-lubricated bearings, but they presented a higher static stiffness when the bearing clearance was less than 9 µm. Compared with the N2-lubricated bearings, the optimal static stiffness of the R22- and CO2-lubricated bearings increased by more than 46% and more than 21%, respectively. The numerical and experimental results indicate that a small bearing clearance would be preferable when designing externally pressurized gas thrust bearings lubricated with the working medium of heat pump systems for space applications. Originality/value The findings of this study can serve as a basis for the further investigation of refrigerant gases as lubricants in heat pump systems, as well as for the future design of such gas bearings in heat pump systems for space applications.

Graphical abstract, Introduction As an important advanced functional material, the memory effect of nitinol shape memory alloy (SMA) is the focus of research. According to the current research, the memory function of the alloy decreases after welding, and there is no sufficient explanation for the phenomenon. Objectives For the problem, this research is to explore the underlying causes of the decrease of shape memory function after welding by analyzing the microstructure and micro defects. Methods The vacuum electron beam welding tests of 1 mm thick Ni50Ti50 alloy plate was carried out to determine the appropriate welding process parameter. And the shape memory function of the welded joint was compared with that of the base metal to analyze the change of memory function. Results It was found that the shape memory function of the welded joint decreased significantly under different strain variables. And the phase transition temperature also changed. Conclusions This was due to the micro stress field produced by non-equilibrium solidification in molten pool promoted the formation and propagation of dislocations, increasing the dislocation density in the martensite. Dislocations entangled with each other in the martensite, showing a grid-like distribution, which destroyed the integrity of martensite substructure. At the same time, the twin substructure of martensite was often accompanied by vacancies, dislocations, stacking faults, and a consequently large stress field formed between twin planes due to lattice distortion. Secondary twin was identified inside martensite under micro shear stress, where the martensite showed the bending state. The habitual relationship between martensite phase and parent phase was destroyed, resulting in the decrease of shape memory function.

Developing a practical strategy to fabricate an anti-abrasion and durable superhydrophobic wood surface with ultraviolet (UV) resistance has great practical significance for expanding the application of natural wood. In this study, a robust superhydrophobic layer with a hierarchical micro/nano-roughness structure was modified on the wood surface through in-situ mineralization and polymerization using a simple sol–gel method along with efficient electron beam (EB) curing technology. Hydrophobic agent (polydimethylsiloxane, PDMS), and crosslinking monomer (γ-methacryloxypropyl trimethoxysilane, MAPS) form new covalent bonds between TiO2 particle layer and wood substrate after EB radiation, which endows robust superhydrophobicity and remarkable UV resistance on the wood surface. The as-prepared wood exhibited a water contact angle of approximately 165.7° and obvious repellency to many aqua-phase liquids (cola, strongly acidic, alkaline droplets etc.). Furthermore, the hierarchical micro/nano-protrusion structures remained unchanged and micro/nano particles aggregated tightly on the as-prepared wood surface under harsh external environments (sandpaper abrasion and, ultrasonic treatment), confirming the desirable anti-abrasion and mechanically durable performance of the superhydrophobic surface. After the 18-day UV accelerated weathering test, the TiO2 particle layer conspicuously retained the discoloration and maintained its exceptional repellency toward water. The biomimetic superhydrophobic wood with excellent mechanical durability and UV resistance reveals its potential application in the furniture and architecture fields.

Alloys with combined outstanding strength and excellent ductility are highly desirable for many structural applications. However, alloys subjected to deformation at very high strain rates and/or cryogenic temperatures usually suffer from very limited ductility. Here, we demonstrate that a bulk CrCoNi medium-entropy alloy presents exceptional combination of high strength and excellent ductility during deformation at high strain rates over a wide temperature range. Full tensile stress-strain curves at a high strain rate of 2000 s−1 and temperatures down to 77 K were successfully obtained using an electromagnetic Hopkinson tension bar system attached with a cooling device, revealing high true ultimate tensile strength (σUTS,T) of 1.8 GPa and true strain of ∼54% at σUTS,T. These outstanding mechanical properties were mainly attributed to profuse deformation twinning. Both high strain rate and cryogenic temperature promoted deformation twinning. Grain refinement caused by deformation twinning, dislocation slip and dynamic recrystallisation added to work hardening and the excellent tensile strain.

Cu foams with 75% and 98% porosity were employed as reinforcing structures to enhance the performance of the Sn3.0Ag0.5Cu(SAC305)/Cu solder joints. The effects of Cu foam porosity and ultrasonic power on the microstructure and shear properties of solder joints were investigated. The results suggested that the Cu substrate produced excellent metallurgical reactions with both composite solder foils, but more Cu6Sn5 was generated in 75% porosity Cu foam/SAC305 solder joints. The increase of ultrasonic power would accelerate the dissolution of Cu skeleton and effectively refine and homogenize the intermetallic compound grains. Higher power ultrasonic vibration optimized the microstructure of the solder joint, so the shear strength of the joint was increased, but excessive ultrasonic power would cause microcracks at the interface of the solder joint in turn reducing the shear strength.

Glutathione and 2-aminopyridine are used as carbon sources to prepare carbon dots (CDs) by a one-step hydrothermal reaction. The results show that the average particle diameter of CDs is 8.64 nm with uniform size distribution and the fluorescence quantum yield is 13.62%. We further demonstrate that novel CDs possess highly selective sensing of Fe3+ from 0.2 to 200 μM with a low detection limit (0.194 μM). Meanwhile, the fluorescence of CDs can be repeated many times by the addition of S2−. Moreover, the CDs are used for biological imaging of living cells with well cell penetrability and low toxicity. Furthermore, it is successfully applied for anti-counterfeiting and information encryption. More interestingly, it can be doped with hydrogel and filter paper to prepare solid-phase sensors exhibiting high sensitivity and fast response, demonstrating their tremendous potential for the simple, rapid, and low-cost monitoring of Fe3+ and S2−.

Microwave absorption materials (MAMs), which can simultaneously show high reflection loss (RL) and broad effective absorbance band (EAB) at a thinner matching thickness, were meaningful to the settlement of current electromagnetic wave pollution. Herein, all-polymer derived carbon foam with unique molecular-level interpenetrating carbon network (ICN) heterojunction interface was constructed. The heterostructure was fabricated through carbonizing the molecular-level miscible interpenetrating polymer networks (IPN) of polyimide foam (PIF) and bismaleimide (BMI), while target heterojunction was proved through several characterizations. Benefitting from stronger polarization loss gene derived from countlessly molecular-level heterojunction interface of ICN, higher RL value could be realized at a thinner matching thickness. As a result, when only orthogonal polymerizing 5 wt% BMI with PIF, maximum RL value of corresponding carbon foam (CPIF-CBMI-5%) could achieve −72.8dB at the matching thickness of only 2.05 mm. In addition, the matching thicknesses of CPIF-CBMI-5% that covered the whole X band and Ku band were significantly decreased by 25% (3.75 mm to 2.8 mm) and 21% (2.6 mm to 2.05 mm) respectively, which was also superior to most foam materials that decorated by traditional loading or doping. Moreover, maximum EAB of CPIF-CBMI-5% could also achieved 6.2 GHz at 2.05 mm and its density was as low as 0.026 g/cm3.

This paper aims to investigate the influence of composite flux on the interfacial reaction, wettability, and shear strength evolution of Sn-3.0Ag-0.5Cu (SAC305) solder joint; the ultrasonic vibration (USV) was performed to the solder joint during reflowing. Cu-modified graphene nanosheets (Cu-GNSs)-doped flux were prepared by chemical modification. It was found that the wettability of the solder was enhanced after the addition of Cu–GNSs, and the wetting angle was reduced up to 14.43%. While, the presence of Cu–GNSs could reduce the thickness of the intermetallic compound (IMC) layer and the size of IMC grain within the solder joint by 4.15% and 3.19%, respectively. Interestingly, the solder joint was prepared with 0.1 wt% Cu–GNSs-doped flux which had the optimal effect on enhancing the wettability of the solder and inhibiting the growth of the IMC layer. The application of USV during reflow could enhance the wettability of the solder and decrease the thickness and grain size of IMC within the solder joint. Shear test results stated that the employment of Cu–GNSs and USV made the shear strength of Cu/SAC305/Cu solder joint enhanced by up to 9.34% and 9.76%, respectively. Furthermore, as the content of Cu–GNSs doped in the flux increased, the fracture type of solder joints without USV treatment gradually changed from the ductile–brittle mixed type to the ductile type, whereas, the fracture type of all the solder joints-relayed USV treatment was the ductile type.

In this paper, a micromechanics-based constitutive model is proposed for linear viscoelastic particle-reinforced composites with interphase based on the theoretical homogenization framework in the time domain that we recently proposed for viscoelastic composites. All the composite's phases (i.e., the matrix, particle and interphase) are considered as linear viscoelastic materials, whose constitutive responses are governed by the general Maxwell model. The interphase with varying viscoelastic property is approximated by the multiple homogenized layers. The constitutive model of the composites is multiplicatively decomposed as two parts: (1) the effective relaxation function characterizes the time-dependent behaviors, and (2) the referred elastic part refers to the long-term responses. The classical composite-sphere model and three-phase model are extended to derive the variables involved in the constitutive model for the bulk and shear behaviors, respectively. The comprehensive numerical simulations, including the effects of the number of interphase layer, interphase thickness, particle content and loading condition, are carried out to validate the proposed constitutive model. The results reveal that the constitutive model can predict well the viscoelastic behaviors of the particle-reinforced composites with interphase. The validated constitutive model is then applied to identify the viscoelastic properties of the interphase by fitting the predictive results and the experimental data of the “overall” composites. The results show the good consistency for the interphase properties between the theoretical identification and the experimental observation. The discussion of the interphase percolation is then carried out, and the findings demonstrate again that the constitutive model can also give good predictions under the percolation situation.

A generalized analytical model is developed to predict progressive failure behavior of several types of textile composites, including plain weave composites, twill weave composites, two-dimensional tri-axially braided composites and warp-reinforced 2.5-dimensional braided composites. In this model, the unit cell (UC) of composite is firstly identified and reconstructed into a refined lamina structure with multiple equivalent lamina elements (ELEs) based on apt geometrical approximation and assumptions. Secondly, two-way coupled stress-strain responses within the UC (macro-scale) and ELE (meso-scale) are established through a universal series-parallel model (SPM). Finally, a progressive damage model, which consists of damage initiation criteria and a stiffness evolution strategy, is employed to predict damage behavior of the ELE. The analytical results including mechanical properties and progressive failure process are validated against the existing numerical and experimental ones in literature. The validated analytical model is then used to study the effects of global fiber volume fraction, braided angle, shear failure coefficient and selected failure criteria on stiffness, strength and failure process. The present results demonstrate the efficiency and generic capability of the present analytical model for predicting the mechanical responses of a range of textile composites.

This work presents mechanical properties of the NiTi polycrystalline superelastic shape memory alloys (SMA) of 5 different grain sizes under high-speed impacts. The amorphous, nanocrystalline (40, 80, 120 nm) and coarse grain (20 μm) sheets are manufactured with cold rolling and suitable heat treatments. A Hopkinson tensile bar is used to perform tests up to 45 m/s. High-speed camera system and digital image correlation method are used to get the strain field and particle velocity field at a sampling frequency of 2×106 frames/s with a resolution of 924×768 pixels. Nominal stress-strain curves are obtained for all the sheets with a strain rate of about 1000 s−1 and they have a similar evolution to the quasi-static case but with much higher stress levels. The rate sensitivity is increased with the grain size and the stress level can reach up to a 70% growth for a coarse grain sheet but be totally insensitive for the amorphous sheet in the strain rate from 10−4 to 103 s−1. A single transformation front can be found under high-speed impact (45 m/s) at the early loading stage. The speed of the transformation front is calculated from strain time histories and the highest front speed of 811 m/s is observed which is never observed before. It also reveals that the front speed depends also on the grain size. With the same loading speed, the bigger the grain size is, the slower the transformation front speed is.

Metal foams are typically subjected to quasi-static or dynamic shear-compression combined loading in applications such as energy absorbers and structure protectors. The yield behavior of a metal foam under dynamic and quasi-static shear-compression combined loadings is investigated in this study. First, quasi-static and dynamic compression-shear combined tests at different loading angles are conducted using a universal testing machine and a rotatable Hopkinson bar system, respectively. Shear deformation reduces the plateau stress as the loading angle increases. Subsequently, the yield modes of the metal foam under combined loadings are investigated. Only one yield band occurs under a combined loading with large loading angles (mode I), whereas several yield bands occur under a combined loading with small loading angles (mode II). Finally, the yield surface plot of metal foam indicates significant enhancement in terms of normal stress and shear stress under dynamic loading. Quasi-static and dynamic phenomenological yield criteria for a shear-normal stress space are established to provide a brief and precise prediction of the behavior of metal foam under quasi-static and dynamic combined loadings.

In this study, we developed a microfluidic paper analysis device (μPAD) for distance-based detection of Ag+ in water. The μPAD was manufactured by wax printing method on filter paper. Then, a layer of gold nanoparticles (AuNPs) was deposited and ascorbic acid was printed on the channel. In the detection, Ag+ was reduced by ascorbic acid and coated on the surface of the AuNPs on the channel, forming Au@Ag core/shell nanoparticles. Based on the capillary flow principle, different concentrations of Ag+ formed different distances of color ribbons. Thus, quantitative detection of Ag+ can be achieved by measuring the distance of the color ribbon. The detection limit of this method was as low as 1 mg·L−1 within 15 min and the interference of common metal ions in water can be eliminated. In conclusion, this method had successfully realized the leap from colorimetry to direct reading, realizing fast read and easy manipulation with low-cost.

Conventional aromatic compounds tend to exhibit the formation of sandwich-shaped excimers and exciplexes between their excited and ground states at high concentrations or in their aggregated states, causing their fluorescence to weaken or disappear due to the aggregation-caused quenching (ACQ) effect. This limits their applications in concentrated solutions or solid materials. Herein, for the first time, ACQ-based pyrene (Py) units are covalently connected to the surface of polyethylene/polypropylene nonwoven fabric (PE/PP NWF) via electron beam preradiation-induced graft polymerization followed by chemical modification. The matrix can be considered a solid solvent and Py units as a solid solute, such that the amount of Py units can be controlled by varying the reaction time. The obtained fluorescent fabric not only exhibits remarkable fluorescence properties with high fluorescence intensity, high quantum yield (>90%), and excellent fluorescence stability after laundering or in harsh chemical environments, but the fluorescence color and intensity, quantum yield, and lifetime can also be regulated by employing the ACQ effect. Additionally, the as-prepared fluorescent fabric can effectively distinguish common monocyclic aromatic hydrocarbons via a simple fluorescence response test.

Cu modified carbon nanotubes (Cu-CNTs) doped flux were prepared in this paper, the influence of the composite flux on the evolution of microstructure and shear strength of Sn-3.0Ag-0.5Cu (SAC305) solder joints were investigated, the ultrasonic vibration (USV) was applied for the solder joint during reflowing. Experiments were designed to modify the CNTs with Cu nanoparticles through electroless modification process. Transmission electron microscopy (TEM) was utilized to obtain the graph of modified CNTs. Using X-ray photoelectron spectroscopy (XPS) spectra to prove that the CNTs were successfully modified with Cu nanoparticles. Compared with the solder joint without USV, the application of USV on the solder joint would alter the morphologies of interfacial IMC layer and grains, reduce the IMC thickness and grain size by 16.6 % and 65.6 %, respectively. Reflow process was implemented to evaluate the microstructure evolution and interfacial reaction of SAC305 solder joint with different content of Cu-CNTs doped flux with USV. It was found that the presence of Cu-CNTs could reduce the thickness and grain size of interfacial intermetallic compound (IMC) within solder joint by 3.1 % and 5.25 %, respectively. After addition of the Cu-CNTs in the flux, the thickening of IMC layer and the coarsen of interfacial IMC grain were suppressed, the optimal inhibition effect was gained when the content of Cu-CNTs was 0.1 wt.%. The shear test was conducted for the Cu/SAC305/Cu solder joint with flux containing Cu-CNTs under different soldering condition, the results indicated that as the content of the Cu-CNTs in the flux increased, the shear strength was significantly enhanced by up to 16.8 %. Furthermore, the application of USV could enhance the shear strength by up to 14.2 % as well, The fracture type of solder joints without USV treatment also changed from ductile-brittle mixed type to ductile type as the content of Cu-CNTs in the flux increased. While, the fracture type of all the solder joints with USV treatment was ductile type.

Micromechanics-based constitutive models offer superior ability to estimate the effective mechanical properties for the composites, which greatly promote the computational efficiency in the multiscale analysis for composite structures. In this work, a thermo-viscoelastic model for particle-reinforced composites is proposed to estimate their thermal–mechanical coupling behaviors in terms of a micromechanics-based homogenization method in the time domain. The matrix and particles of the composites are modeled as “thermo-rheologically complex” viscoelastic materials. The temperature-dependent effective elastic strain energy ratios of particle to composite are proposed to evaluate the contributions of the matrix and particles. The thermo-viscoelastic model for the composites is then formulated by superposing the matrix and particle’s contributions. Finite element simulations based on the representative volume element models are employed to validate the constitutive model under various thermal–mechanical coupling loads. The effects of the loading rate, viscous parameter and particle content on the effective thermal–mechanical responses of the composites are also comprehensively discussed. The experimental data from literature are also employed to verify the constitutive model. The findings show that the proposed thermo-viscoelastic model can accurately predict the thermal–mechanical coupling behaviors for the particle-reinforced composites.

The dry friction structure is a commonly used vibration-damping method for railway vehicles. Insufficient vibration damping performance will cause excessive vibration of the vehicle, which is not conducive to the safety of the vehicle. However, the mechanism of vibration damping and the cause of clamping stagnation have not been well resolved. This paper uses the analytical method, numerical method, and finite element method to analyze the vertical dynamic characteristics of the simple suspension system with dry friction and demonstrates that the numerical method is an effective method to study the dry model. The conditions for the system to produce sticking events were analyzed by the numerical method. The analysis shows that the system's excitation is too small, which causes clamping stagnation to the system. The reduction of the wedge angle and the friction coefficient are conducive to eliminating sticking. A negative side frame angle is conducive to reducing the high-frequency energy of the excitation. Decreasing spring stiffness or increasing system mass to reduce system frequency can reduce sticking events. The mutual verification of different methods confirms the correctness of the analysis method and analyzes the cause of sticking or clamping stagnation from the mechanism, which provides a new idea for the design and improvement of the dry friction damping system of railway vehicle bogies.

Chemical regulation of fluorinated graphene (FG) is of great importance for promoting its practical application but is still challenging due to the complexity of its chemical structure. Here, we demonstrate that the introduction of nitrogen (N) into graphene sheets is a feasible method to regulate the bonding nature and location of C–F bonds in the corresponding FG product. The fluorination reactivity of graphene materials is improved by doping N atoms into graphene sheets, which can even be increased by 2.7 times. Under the same F/C ratio, more covalent C–F bonds existed in fluorinated N-doped graphene than in fluorinated reduced graphene without doped N atoms. Results of DFT calculations, polarized ATR-FTIR, and WAXD confirmed that doped N atoms could significantly enhance the fluorination reactivity of their surrounding C atoms and thus localized C–F bonds around them. The local enrichment of C–F bonds certainly improved the covalent bonding nature; even the F/C ratio is not high enough. Meanwhile, this work provides a method for preparing N, F bifunctional graphene in large quantities, and an ultrahigh-bifunctional graphene with N/C ratio of 0.24 and F/C ratio of 0.56 has been obtained here, which has potential contributions to the industrialization of the preparation of ultrahigh-functional graphene.

The thermal stability of fluorinated graphene (FG) plays an important role in its application and research, and thus it is necessary to conduct in-depth research on the thermal stability of the C–F bond in FG. Herein, FG with different types and distributions of C–F/C(–F)2 bonds were synthesized, and the correlation between the C–F/C(–F)2 bonds and thermal stability of these FG samples was monitored via in situ heating infrared spectroscopy (in situ FTIR). The stability of the different types and distributions of C–F/C(–F)2 bonds in FG and the temperatures at which these C–F/C(–F)2 bonds were eliminated were determined. In terms of C–F bonds in FG, the most stable type is that in C(–F)2 of perfluorinated FG, followed by the C–F bonds in perfluorinated FG. The thermal stability of isolated C–F bonds and C(–F)2 bonds adjacent to the conjugated structure was the worst, which would be detached from FG at low temperature (≤82 °C). Furthermore, the evolution of the conjugated structures in FG during thermal annealing was also affected by the type and distribution of the C–F bonds.

Vapor compression heat pump technology has great potential in future space thermal management application. Considering microgravity in space, this paper proposes using aerostatic bearings for the lubrication of the heat pump compressor. An experimental apparatus was built to study the performance of a rotor supported by aerostatic bearings subjected to microgravity. The gravity-independent experiment was conducted by changing the angle between the rotor axis and the direction of gravity on the ground. The gas consumption and rotor stability at different rotor orientations were investigated and compared. Results showed that the largest difference in gas consumption due to the change in rotor orientation was 3% for the journal bearing and 5.56% for thrust bearings. The difference in rotation speed at the second peak amplitude under different rotor orientations was smaller than 4.2%. The experimental results imply that the aerostatic bearing-rotor system is independent of gravity. Aerostatic gas lubrication technology is thus a viable option for the vapor compression heat pump used in a space thermal management system.

In the work, the process of unsteady contact interaction of rigid stamp and elastic half-space having a recessed cavity of arbitrary geometry and location with a smooth boundary was investigated. Three variants of contact conditions are considered: free slip, rigid coupling, and bonded contact. The method for solving the problem is constructed using boundary integral equations. To obtain boundary integral equations, the dynamic reciprocal work theorem is used. The kernels of integral operators are bulk Green functions for the elastic plane. Because of straight-line approximations of the domain boundaries with respect to the spatial variable and straight-line approximations of the boundary values of the desired functions with respect to time, the problem is reduced to solving a system of algebraic equations with respect to the pivotal values of the desired displacements and stresses at each time interval. One of the axes is directed along the regular boundary of half-space, the second - deep into half-space.

High-performance photodetectors based on an interface modified hybrid perovskite film are reported. Large crystalline grains are formed in perovskite layer with less PbI2. The on/off switching ratio higher than 105 is achieved. Owing to the enhanced efficiency in carrier transport and extraction, the photodetector with a modified interface shows a transient response time as fast as 110 ns. In addition, such devices show an X-ray sensitivity up to $67\mu \text{C}$ Gy−1 cm−2. Our work indicates that the use of hybrid perovskite film with interface engineering is a promising method for efficient photon detection in visible light and X rays mainly because of their fast carrier extraction, low cost and easy preparation.

Nano-Cu-based materials have many applications in the microelectronics and aerospace industries, which not only maintain the advantages of copper materials, but also make up for the shortage of cop...

Optical glass materials have a lot of superior properties such as outstanding optical transparency, light weight, low manufacturing cost and high impact resistance. Thus, they are widely utilized in the transparent armors, windshields of high-speed train and airplane as well as mobile phone touch screens. Glass structures are often subjected to loading, including quasi-static and dynamic loading during their service. It is well known that the mechanical properties of engineering materials under impact loading are usually much different from those under static loading. To guide the optimal design of glass structures, it is necessary to conduct a thorough and in-depth study of the mechanical properties of glass materials. However, as a kind of typical brittle material, the failure strain of inorganic glass is extremely small. Therefore, measuring the dynamic mechanical properties of glass material has always been a quite difficult problem in the fields of experimental mechanics and impact dynamics. This paper would propose a new experimental technique-electromagnetic split Hopkinson pressure bar (ESHPB) to solve the afore-mentioned problem. In this paper, quasi-static compressive tests were first conducted using the electronic universal testing machine at a strain rate 10–3 s–1, followed by dynamic unidirectional and bidirectional compressive tests by the ESHPB apparatus under an average strain rate of 300 s−1 for soda-lime silicate glass, respectively. During dynamic compressive tests, the ultra-high-speed camera and flashlights were utilized to observe the damage/crack initiation, propagation and fracture process inside the glass samples. The feedback signal of the ultra-high-speed camera is input into the digital acquisition system, aiming to associate accurately the stress history of the sample with the high-speed image and to study the dynamic failure process in depth. The following conclusions are made based on the test results: (1) When using the ESHPB set-up to measure the dynamic mechanical properties of glass materials, stress equilibrium and approximate constant strain rate loading condition can be achieved without using the pulse shaping technique. It provides a simpler and easier test method for studying the dynamic mechanical properties of brittle materials. (2) Compared with the unidirectional loading tests, bidirectional symmetric loading experimental technique can significantly shorten the time duration for achieving the condition of stress equilibrium and constant strain rate deformation. Therefore, more effective information can be obtained from the test results and the accuracy and reliability of the test results can be greatly improved. (3) Compared with the quasi-static test results, it is found that the compressive strength of the tested glass material increases significantly under dynamic impact loading, showing a strong strain rate effect. The dependence of compressive strength on strain rate was quantitatively characterized by the strain rate sensitivity exponent. (4) The deformation images of samples captured by ultra-high-speed camera indicated that when the cracks first appeared, the specimens did not completely lose the load bearing capacity. The cracks continue to nucleate, propagate and converge. When the cracks fill almost the entire sample, the material loses the load bearing capacity completely and breaks into powders. By comparison, the samples are split into a few columns under quasi-static loading.

Micro-cracks generated by hard body scratch are a major cause of strength decrease for silicate glass. The influence of normal scratch load on the cracking patterns and flexural strength of annealed glass (AG) and chemically strengthened glass (CSG) were studied. With the increase of the normal load, the load capacity of scratched AG specimens decreased to about 40 MPa at 20gf immediately. However, the residual strength of CSG decreased to a steady value of 145 MPa as the scratch load increased to 500gf. Then the effect of hydrofluoric acid (HF) etching on the surface morphology and mechanical properties of the 500gf scratched glass were investigated. After 8min (for CSG) and 16 min (for AG) acid treatment, the flexural strength of CSG and AG increased to a considerable value of 900 MPa, which is 3.6 and 5.5 times higher than the flexural strength of undamaged specimens. Microscopic observations show that the blunting and eliminating of median cracks as well as the formation of new surfaces are the main causes of strength enhancement.

The effects of peak temperature (483 K and 533 K) and substrate surface roughness on wetting characteristics and interfacial structure between Sn–35Bi–1Ag solder and Cu substrates were studied. Results revealed that the wettability of Sn–35Bi–1Ag solder was enhanced with increasing surface roughness of Cu substrates and temperature. However, the surface topography characteristics (i.e., anisotropic or isotropic) had little effect on wettability of Sn–35Bi–1Ag solder. The liquid solder had a preferential spreading direction, which caused an elliptical triple-line pattern when Sn–35Bi–1Ag solder spread over an anisotropic surface, while the solder spread as almost a perfect circle when the surface of Cu substrate was isotropic. Besides, the surface roughness had little effect on the interfacial structure, while the interfacial IMCs increased with increasing the temperature. Moreover, the spreading process of Sn–35Bi–1Ag solder spreading on Cu substrates with different surface roughness was similar at 483 K and 553 K. The whole spreading process was featured with four distinguishable stages: (i) initial stage, (ii) rapid spreading stage, (iii) limited spreading stage and (iv) steady stage. Wetting kinetics of the rapid spreading stage and limited spreading stage were described by the Rn ~ t model and the values of n of the rapid spreading stage and limited spreading stage decreased with increasing surface roughness of Cu substrates, which indicated increasing surface roughness of Cu substrates increased slightly the spreading rate. However, the temperature and surface topography characteristics had little effect on the spreading rate. Meanwhile, the dynamic contact angle and time t could be characterized by an equation, i.e., cosθF – cosθ = (cosθF – cosθ0) e−kt. The value of k decreased with increasing surface roughness of Cu substrates, while the surface topography characteristics and the temperature had little effect on the value of k.

Typography-like templates for polydimethylsiloxane (PDMS) microfluidic chips using a fused deposition modeling (FDM) three-dimensional (3D) printer are presented. This rapid and fast proposed scheme did not require complicated photolithographic fabrication facilities and could deliver resolutions of ~100 μm. Polylactic acid (PLA) was adopted as the material to generate the 3D-printed units, which were then carefully assembled on a glass substrate using a heat-melt-curd strategy. This craft of bonding offers a cost-effective way to design and modify the templates of microfluidic channels, thus reducing the processing time of microfluidic chips. Finally, a flexible microfluidic chip to be employed for cell-based drug screening was developed based on the modularized 3D-printed templates. The lithography-free, typography-like, 3D-printed templates create a modularized fabrication process and promote the prevalence of integrated microfluidic systems with minimal requirements and improved efficiency.

The relevance of the work is due to the need for experimental studies to determine the mechanical characteristics of epoxy resin samples, which can be used to check the correctness of the choice of parameters and criteria for the onset of crack growth within the framework of elastic fracture mechanics, cohesive models, models such as virtual crack closure technique, extended finite element method, etc. Thus, the article is aimed at determining the parameters of fracture toughness of samples of brittle epoxy resin with applied eccentric cracks. The leading method for the study of this problem is the experimental method, which makes it possible to determine the critical stress intensity factor for three-point bending of samples with an edge crack, as well as to study samples with an eccentric (relative to the center of the sample) location of cracks. The paper presents the results of experimental studies to determine the critical stress intensity factors for samples of brittle epoxy resin L285 with hardener H 285 (Hexion), obtained without the addition of a plasticizer. The results of testing samples with asymmetric cracks are compared with the results of numerical modeling within the framework of elastic fracture mechanics with the energy fracture criterion. The materials of the article are of practical value, first of all, for the calibration of fracture mechanics models.

This article is relevant, as changes during the external loading may affect the stress state of the materials. The aim of this paper is to consider the numerical modeling of heating for circular cylinders in the frame of the theory of elastic materials with voids. A numerical solution is build using COMSOL Multiphysics software, where the implementation of the considered theory is realized based on the direct equation-definition approach. Constitutive relations were written in General form partial differential equation module. A matrix form of the equations for the two-dimensional case was used. Scale effects arising in considered problems are discussed. The classical solution is the particular case of the considered theory, when the coupling number tends to asero, i.e. when the micro-dilatation effects are small and do not affect the material's stress state. The limiting case in the case of the small value of the coupling number is the classical thermoelasticity solution.

The solar power is one of the most promising renewable energy resources, but the high cost and complicated preparation technology of solar cells become the bottleneck of the wide application in many fields. The most important parameter for solar cells is the conversion efficiency, while at the same time more efficient preparation technologies and flexible structures should also be taken under significant consideration [1]. Especially with the rapid development of wearable devices, people are looking forward to the applications of solar cell technology in various areas of life. In this article the flexible solar cells, which have gained increasing attention in the field of flexibility in recent years, are introduced. The latest progress in flexible solar cells materials and manufacturing technologies is overviewed. The advantages and disadvantages of different manufacturing processes are systematically discussed.

In this work, three-dimensional thermal simulations of single 18650 lithium-ion battery cell and 75 V lithium-ion battery pack composed of 21 18650 battery cells are performed based on a multi-scale multi-domain (MSMD) battery modeling approach. Different cooling approaches’ effects on 18650 lithium-ion battery and battery pack thermal management under fast discharging and external shorting conditions are investigated and compared. It is found that for the natural convection, forced air cooling, and/or mini-channel liquid cooling approaches, the temperature of battery cell easily exceeds 40 °C under 3C rate discharging condition. While under external shorting condition, the temperature of cell rises sharply and reaches the 80 °C in a short period of time, which can trigger thermal runaway and may even lead to catastrophic battery fire. On the other hand, when the cooling method is single-phase direct cooling with FC-72 as coolant or two-phase immersed cooling by HFE-7000, the cell temperature is effectively limited to a tolerable level under both high C rate discharging and external shorting conditions. In addition, two-phase immersed cooling scheme is found to lead to better temperature uniformity according to the 75 V battery pack simulations.

In recent years, plastic packaging has become the main form of electronic packaging technology due to its low cost, miniaturization, and simple producing process. Epoxy molding compound (EMC) are widely used in electronic packaging as encapsulants of electronic devices. However, in the actual service environment, due to the material properties of epoxy molding compounds, there are many factors cause the package structure failure. In this paper, we design the orthogonal experiment to systematically investigate the impact of thermal stress on the reliability of a molded underfill (MUF) FCBGA (flip-chip ball grid array) structure by using the finite element analysis. In the orthogonal experiment, the F test is used to estimate whether the experiment is reliable, and the T test is used to select the most important factors from the samples. The statistical analysis indicates that the coefficient of thermal expansion (CTE) and modulus of EMC have the significant influence on the thermal stress of the EMC package, and static max thermal stress is at the interface between the chip and EMC. In addition, we find that the node of the static max stress and the one of the max displacement are different.

Selective laser melting (SLM) can be used to manufacture complex parts, however, it is difficult to make large parts due to the size limitation of the SLM equipment. In application, smaller selective laser-melted (SLMed) Ti-6Al-4V (TC4) parts can be brazed or welded to form larger components. In the brazing, AgCuTi is often used to braze TC4. However, the wettability of AgCuTi on the SLMed TC4 should be evaluated before joining the SLMed TC4 parts. As a result, wetting and spreading tests and brazing experiments should be undertaken to successfully join the SLMed TC4 parts. In this study, a LINKAM TS 1500 high-temperature hot stage was used to test the brazability of the AgCuTi on the surface of SLMed TC4. Different temperatures and dwell times were used: (i) 850 °C 900 °C and 950 °C, holding for 120 s, were used to study the temperature effects, (ii) 20 s, 120 s and 200 s were used at 850 °C to study the dwell time effects. The R~t model was used to describe the wetting and spreading process. The results of this study can provide basic data for the joining of SLMed TC4 in industry.

The viscous heat of gas bearings and heat generated by the motor cause a large temperature difference between the bearing surface and lubricating gas influencing the bearing performance in gas-bearing applications. This study investigated the effects of the bearing surface temperature and supply gas temperature on the static characteristics of a six-orifice externally pressurized gas thrust bearing under air, CO2, and R22 lubrication, numerically and experimentally. The results show that the load increases as the bearing surface temperature increases, while the static stiffness increases at first but decreases later. In contrast, the load and static stiffness decrease with increasing supply gas temperature. The findings can provide insights on designing gas bearings using refrigerant gases.

The adiabatic shear instability of ductile materials has attracted more and more attentions of researchers and groups, who have been sparing no effort in further understanding of the underlying mechanism since the first experimental depiction of adiabatic shear instability by Zener and Hollomon. As for the adiabatic shear instability, many factors account for its occurrence, including heat conduction, inertia effect, microstructure effect and so on. However, lots of experimental evidence has shown that metal materials display a strong size effect when the characteristic length scale is in the order of microns. The size effect has also been observed in the analysis of shear band in the ductile materials because the order of the bandwidth stays within the microscale range. However, a comprehensive understanding of the whole process of adiabatic shear banding (ASB), including the early onset and the subsequent evolution, is still lacking. In this work, a gradient plasticity model based on the Taylor-based nonlocal theory feasible for the linear perturbation analysis and convenient for numerical calculation is proposed to investigate the strain gradient on the onset of ASB and the coupling effect of heat conduction, inertia effect and strain gradient at the early stage, as well as on the subsequent evolution process at later stages. As for the onset of ASB, the linear perturbation method is used to consider the effect on the initial formation of ASB. After the investigation of the onset of ASB, the characteristic line method is applied to describe the subsequent nonlinear evolution process of ASB. Three stages of ASB evolution are clearly depicted during the evolution process, and the significance of size effect on the ASB nonlinear evolution process of ASB at different stages is analyzed. With the help of linear perturbation analysis and characteristic line method, a comprehensive description of the role of strain gradient in the ASB from the early onset to the end of the evolution is provided.

The damage propagation and fracture process of monolithic annealed and chemically strengthened aluminosilicate (ALS) glass plates are characterized using Edge-on impact (EOI) testing technique. The EOI experiments on two types of ALS glass plates at a medium impact velocity of ∼180 m/s with spherical and cylinders steel projectiles were completed using pneumatic gas-gun. The high-speed images in shadowgraph mode during the impact process were recorded at the framing rate of 1,000,000 frames per second (fps) to visualize the fracture and damage front in annealed and strengthened glass plates. The real-time captured shadowgraphs in case of annealed glass (AG) showed that the damage front initially starts to develop from the impact end and then after traveling some distance arrested in few microseconds and measured average front velocity against cylindrical and spherical projectile was 2658.91 m/s and 1710.0 m/s, respectively. The tensile wave reflection in AG impact case against both types of projectiles was observed. In a chemically strengthened glass (CSG), the damage front shape and propagation against both types of projectiles were quite different than the AG, front traveled in a self-sustained mode due to the release of elastic stored energy with the velocity of 1746.92 m/s and 1690.12 m/s, respectively. The damage evolution curves plotted from captured high-speed images show that damage traveled slower in case of the spherical projectile which suggests that the shape of the projectile has a great influence on the fracture process of annealed and strengthened glass. The damage front shape in CSG against cylindrical projectile impact was more dominant along the glass boundaries and the spallation effect was not observed in CSG against spherical projectile impact. Additionally, the damage-free zones and fracture surfaces were studied using a scan electron microscope (SEM) from collected post-test debris to discuss the fracture process.

Cold metal transfer welding/brazing of AA5754 aluminum alloy and Q235 low carbon steel was performed in a lap joint configuration using an AlSi12 filler wire. The effect of heat input on intermetallic compounds (IMC) formations and mechanical properties has been investigated. At a low heat input of 157 J/mm, a layer of IMC, with a composition of Al7.2Fe1.8Si was generated at the fusion zone/steel interface. Low interfacial energy and good interfacial bonding between fusion zone and Al7.2Fe1.8Si led increased joint strength. An additional Fe(Al,Si)3 IMC formed at the interface in conjunction to Al7.2Fe1.8Si at a high heat input of 201 J/mm. The joint strength was significantly decreased and attributed to a high interfacial energy and poor interfacial bonding between Al7.2Fe1.8Si and Fe(Al,Si)3. The results showed that the interfacial energy at interphase boundaries had remarkable effect on the joint strength.

The effects of aging time and temperature on the formation and growth behavior of interfacial intermetallic compound (IMC) of Sn-16Sb/Cu(wt%) solder joints prepared by using dip soldering were investigated. The results show that the major IMCs between Sn-16Sb solder and Cu substrate after thermal aging are Cu3Sn and Cu6Sn5. The thickness of the interfacial IMC in Sn-16Sb/Cu is linearly against the square root of aging time, which indicates that the growth of IMC is mainly controlled by diffusion between Cu and Sn atoms. By using linear regression method, the growth rate constants of interfacial IMC layers are 1.254×10-18, 8.821×10-18 and 1.22×10-17 m2s-1 for Sn-16Sb/Cu joints aged at 120, 150 and 170 °C, respectively. Besides, the activation energy of the interfacial IMC growth was also calculated to be 68.27 kJ/mol. The IMC grain diameters after aging treatment increase with the increasing aging time, with i e, d = 0.492t0.106, d = 0.543t0.143 and d = 0.290t0.263 for aging temperatures of 120, 150 and 170 °C, respectively. Besides, by using nanoindentation, the softening of Sn-16Sb solder was found during aging treatment. Moreover, the U-shape evolution of the values in hardness and Young's moduli was found in this work.

The step-edge type high temperature superconductor (HTS) radio frequency (rf) superconducting quantum interference devices (SQUIDs) were fabricated from YBa2Cu3O7−δ thin films with a field sensitivity of 66 fT/Hz1/2. Even though the sensitivity of the rf SQUID sensor used in the experiments was degraded to 77 fT/Hz1/2 in the magnetic shielded room (MSR), the performance of the rf SQUID sensor was good enough for the ultra-low field (ULF) nuclear magnetic resonance (NMR) experiments at a measurement field of 61.5 μT. With the help of a strictly controlled fast switching circuit and a coupled LC circuit which acted as flux transformer, the signal-to-noise ratio (SNR) of the rf SQUID-based ULF NMR system with different operating points was measured. The optimum SNR was 40 dB which increased significantly by coupling an input coil closely beneath the rf SQUID sensor. The extracted 1/T1 value for pure water sample at 23 ∘C showed the rf SQUID based ULF NMR was an effective system for ULF NMR researches. All above, the direct coupled method and the control circuit show a great potential in the future rf SQUID-based ULF NMR &MRI system.

A Ni layer was first electroplated on the surface of Ti-6Al-4V (TC4) to enhance surface hardness and reduce the negative impact of a powder addition, and then, the preplated TC4 was surface remelted by a high energy electron beam. The associated Ti-Ni composite coating, heat affected zone and substrate were characterized by XRD, SEM, TEM and nanoindentation. The results showed that the surface integrity of the coating increased, the surface hardness improved, and new intermetallic phases of Ti2Ni and TiNi were distributed in the remelted surface area. The coating grains were refined to a nanocrystalline structure with a distorted lattice, which was rich in dislocations or stacking faults. The maximum hardness of the modified coating was 7.03 GPa. Through first-principles calculations, the related properties of the Ti-Ni binary compounds of Ti2Ni, TiNi and TiNi3 were obtained, including crystal structure, mechanical properties and electronic properties.

The reactive wetting of AgCuTi filler metal over a TC4 substrate was conducted using real-time in situ hot stage microscopy. In addition to the commonly observed bulk liquid, a rapidly spreading precursor film was observed. Both the precursor film and bulk liquid were studied and characterized. The wetting kinetics of the bulk liquid within the main spreading stage was predominantly controlled by chemical reactions, in which Ag-based solid solutions (Ag(s,s)), Ti-based solid solutions (Ti(s,s)) and intermetallic compounds (Ti2Cu) were formed within the central domain. The high temperature phase transition of α-Ti to β-Ti induced by the diffusion of Ag into the TC4 substrate was also observed at the edge domain. In the mid-to late-stage spreading process, a thin precursor film, with an extremely rapid spreading speed, appeared at the front of the triple line. According to compositional analysis, the precursor film was predominantly composed of Ag and exhibited similar spreading characteristics to pure Ag on TC4. The formation mechanism of the precursor film can be explained by the mechanism of rapid adsorption followed by the film overflow; based on this mechanism, a physical model was proposed.

With the development of weak magnetic detection methods, the sensors, like superconducting quantum interference device (SQUID), become more and more sensitive. The magnetic shielding takes a more important part in weak signal detection systems than before. Traditional magnetic shielding methods are expensive, not flexible, and hard to prevent the low-frequency magnetic field clearly. For the superconducting magnetic shield, the operating temperature of a low critical temperature superconducting magnetic shield has high demand on refrigerating system. The high critical temperature superconducting magnetic shielding would introduce thermal magnetic noise. Due to the excellent superconductive property, the medium-temperature superconductor MgB2 has a huge potential to build the magnetic shielding system. In this article, we fabricate an MgB2 superconducting magnetic shielding cup with Mg Liquid Infiltration method. The highest TC of the sample is 39.4 K. The highest JC is 10 6 A/cm 2 at 4.2 K. The shielding effect of the cup has been tested at axial dc weak magnetic field and the shielding factor exceeds 2000.

Miscible thermosetting/thermoplastic aromatic polyimide foam is prepared by in situ simultaneous orthogonal polymerizations, where the linear polyimide (PI) is formed by condensation polymerization from polyester ammonium salt (PEAS) and the cross-linked bismaleimide (BMI) is synthesized through addition polymerization from 4,4′-bismaleimide diphenylmethane (BDM). A unique pleated cellular structure is formed after the polyblend foam is cured at high temperature; here, 2D FT-IR correlation analysis is employed to detect the detailed chemical reactions during the thermal foaming progress. The simultaneous orthogonal polymerization is confirmed, and the cross-linking reaction of BDM is found to be divided into two stages. The pleated structure formed in the second stage significantly improves the thermal insulation property of the polyblend foam—the effective thermal conductivity decreases from 427.5 to 77.5 mW·m–1·K–1 at 300 °C with 15 wt % BMI. Meanwhile, the polyblend is miscible and foams only have one ...

The CuO-Al2O3 composite powders were synthesized by chemical coprecipitation method, then, after sintering in tube furnace of 400 °C for 1 h at atmosphere of hydrogen and nitrogen, the comp...

The effects of atmosphere conditions on microstructural and mechanical properties of stainless steel 17-4PH components fabricated by laser direct manufacturing (LDM) were investigated through measurements on phase constitution, porosity, tensile strength, fracture morphology, hardness and evolution of substrate temperature. Results showed that the samples produced in air atmosphere condition possessed higher tensile strength and hardness for both as-deposited and heat-treated states than that in Ar chamber condition, due to dispersion strengthening effect of amorphous oxide particles and nitrogen solution strengthening as a result of higher content of oxygen and nitrogen. The temperature of substrate heat accumulation was higher in Ar chamber condition, leading to dramatically lower porosity and more reverse austenite, which also contributed to the lower strength and hardness.

This paper presents a feasible and universal approach to fabricate regionally controllable nitrogen-heteroatom-doped multiwalled carbon nanotubes with a skin-core heterostructure (N-o-FMWCNT) with the assistance of defluorination in outer-layered fluorinated MWCNTs (o-FMWNCT). First, o-FMWCNT with outer fluorinated tubes encapsulating intact inner tubes was prepared through well-designed direct heating fluorination with temperatures from 25 to 180 °C. Subsequently, nitrogen-doping was implemented at a mild temperature under NH3 atmosphere (400 °C). According to the results of X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and elemental analysis, about 4.97 atom % nitrogen-heteroatom with primary pyridine-nitrogen configuration was successfully doped into the carbon skeleton at the outer tube of o-FMWCNT with the recombination of C–N bonds. Because of the integrity of its inner tubes, the N-o-FMWCNT was endowed with a relatively high-level conductivity (5.46 × 10–2 S·m–...

The effects of different Bi contents on the properties of Sn solders were studied. The interfacial reaction and growth behavior of intermetallic compounds (IMCs) layer (η-Cu6Sn5 + e-Cu3Sn) for various soldering time and the influence of Bi addition on the thermal behavior of Sn-xBi solder alloys were investigated. The Cu6Sn5 IMC could be observed as long as the molten solder contacted with the Cu substrate. However, with the longer welding time such as 60 and 300 s, the Cu3Sn IMC was formed at the interface between Cu6Sn5 and Cu substrate. With the increase of soldering time, the thickness of total IMCs increased, meanwhile, the grain size of Cu6Sn5 also increased. An appropriate amount of Bi element was beneficial for the growth of total IMCs, but excessive Bi (⩾ 5 wt%) inhibited the growth of Cu6Sn5 and Cu3Sn IMC in Sn-xBi/Cu microelectronic interconnects. Furthermore, with the Bi contents increasing (Sn-10Bi solder in this present investigation), some Bi particles accumulated at the interface between Cu6Sn5 layer and the solder.

This work presents a comprehensive study of microstructure evolution, crack development and hardness performance of Fe-based bulk metallic glass parts processed by laser 3D printing. The combination of a low scan speed and high energy density generates a low temperature gradient, leading to supercooling and the formation of coarse dendrite and cellular crystal microstructure. Columnar dendrites grow in a single direction with large stress, and cracks are easily formed between the dendrites. The stress intensity factor (SIF) caused by the crack surface tension is small, the crack healing effect is weak, and it is difficult to prevent crack propagation. Cellular grain growth is uniform and more easily accommodates the strain, preventing crack initiation and propagation. In contrast, use of a high scan speed with low energy input produces a fine and uniform dendrite or even nanocrystalline, microstructure with microcracks. The SIF caused by the crack surface tension is large, and the crack healing effect is strong and prevents crack initiation and propagation. For low energy density, the hardness of the sample is similar to the hardness of the cast amorphous. This result indicates that the printing sample retained some amorphous characteristics.

This paper investigates experimentally the dynamic compressive behavior of short fiber reinforced polyetheretherketone (PEEK) composites, with a specific focus on the effect of strain rate, fiber reinforcement and fabrication process. Hopkinson compressive bar system is employed to test the specimens with strain rate varies from 800–2500/s. Ultra-high speed camera system is used to record the deformation and failure process. Scanning electron microscopy (SEM) is introduced to examine the fracture surface, and to analyze the failure mechanism and its rate dependency. Three different types of PEEK composites are studied and compared to investigate the sensitivity of failure mechanism to fiber reinforcement and fabrication method (injection and extrusion). The results indicate that the ultimate stresses of PEEK and PEEK composites present different levels of sensitivity against strain rates, while the failure strain of PEEK composites decrease with the increase of strain rate. The strain rate dependency is associated with the increase of interface toughness under higher strain rate. The experimental results also identify the presence of phase transformation for matrix material during the high-rate compressive failure. The sensitivity of failure behavior against fabrication process is found to associate with the difference of fiber distributions.

Laser welding/brazing of AA5182 Al to ZEK100 Mg in a lap joint configuration was performed by using a Ni interlayer. The stick-out width of Ni interlayer (WNi) had significant effects on the weld a...