Biaxial Stress Research Articles

The microstructural evolution of sputtered ZnO epitaxial films to stress-relaxed nanorods

Epitaxial ZnO thin films and nanorods were grown on c-sapphire substrate by reactive sputtering of Zn in Ar-O2 atmosphere at substrate temperatures ranging from near room temperature to 700 °C. Scanning electron microscopy showed that with increase in substrate temperature, the morphology transformed from columnar films to vertically aligned ZnO nanorods. High resolution x-ray diffraction revealed improvement in crystalline and epitaxial quality with increase in substrate temperature, along with the decrease of edge dislocation density from 5 × 1012 cm−2 to 8 × 1010 cm−2. The films grown near room temperature showed large hydrostatic strain (∼6 × 10−3) and compressive intrinsic biaxial stress (-0.8 GPa), which decreased substantially with increase in substrate temperature, due to the desorption of excess oxygen and reduction in edge dislocation density. Above the substrate temperature of 500 °C, the intrinsic biaxial stress reversed from compressive to mildly tensile in the case of nanorods, due to the intrinsic tensile stress, originating from crystallite coalescence. Raman measurements correlate well with the changes in biaxial stress and confirm the improvements in crystallinity and epitaxial quality of nanorods grown at higher temperatures. Room temperature photoluminescence of the ZnO films showed weak near-band-edge emission, which enhanced drastically in the case of nanorods due to their superior microstructure and epitaxial quality.

Comparison of internal stresses and air oxidation induced by quenching of partially Cr-coated and uncoated optimized ZIRLO™ part II: Stress assessment

The internal stress states of Optimized ZIRLO (OPZ) samples are evaluated post-quenching from increasing temperatures to determine the role of chromium coatings, oxidation, and microstructural reorienting on internal stress state. Before substantial oxide development (under 700 °C), uncoated tubular samples freely expand and do not accumulate significant global internal stress. As a stable, passivizing oxide develops (700–900 °C), a complex stress state develops with both the interior and exterior oxides in hoop compression. The internal tensile stress increases until the sample fully oxides, and biaxial stress is released through tortoiseshell-like cracking mechanisms. When a chromium coating is introduced, the initial stress states at low temperatures are higher than the uncoated samples due to Cr-coating application techniques and material differences between Cr and OPZ. At higher temperatures (700–1100 °C), the Cr-coated samples undergo a much simpler stress state than the uncoated samples due to the tensile hoop stress state of the Cr-coating caused by differing thermal expansion and the compressive state of the internal oxide caused by a Pilling-Bedworth ratio > 1. The subsequent film cracking mechanisms also differ from the uncoated samples. The smoother stress state experienced by the Cr-coated vs uncoated OPZ samples allows the Cr-coated samples to maintain the circular geometry that is vital to their protective application in nuclear reactors.

Influence of eccentric load introduction during launching with rockers using an elastomeric bearing

AbstractDuring launching, the superstructure acts as a continuous beam supported by different launching bearings on the piers. Just before reaching the next pier, a part of the superstructure temporarily behaves as a long cantilever. As a result, a large bending moment and reaction force at the last bearing occur. The interaction of the internal forces results in a biaxial stress state in the cross‐section. This load case is usually determining for the buckling check and the structural design of the webs. Additionally, the webs of the superstructure and the launching bearing often fail to share the same center lines. This eccentricity can be caused by misalignments of the launching bearings, the necessary air gap to the lateral guide as well as the different thicknesses of the panels along the superstructure. These eccentricities are not considered in the buckling check according to EN 1993‐1‐5 [1]. It is often discussed how these eccentricities affect the buckling behaviour and whether they must be considered in the buckling check. In the case of launching bearings like a rocker system which includes an elastomeric bearing between the superstructure and the launching beam, the reaction forces of the nonlinear material behaviour are additionally discussed. In [2] the effects on the buckling resistance in dependence of load eccentricity for some parameters for the different types of launching bearings are presented. [3] shows the comparison of the buckling resistance with investigations from [4] and [5] with the buckling check according to [1]. A comprehensive summary and the new proposal for considering an eccentric load application and clamping effects in the buckling zone can be read in [6]. This paper shows first investigations of the influence of the elastomeric bearing and the problem of the reaction of the nonlinear material behaviour and its definition in the numerical model. The results have shown that load eccentricities in combination with elastomeric bearings have no impact on the buckling behaviour.

Failure behavior and strength deterioration model of high-performance concrete under coupled elevated temperature, biaxial constraint and impact loading

This research investigates the effects of high temperatures, multiaxial stress constraints, and impact loads on the failure behavior and strength deterioration of high-performance concrete. Combined with scanning electron microscopy and ultrasonic testing, the microscopic deterioration characteristics of thermally damaged concrete were revealed. Dynamic compression tests were performed on concrete, after exposed to elevated temperatures (from 25 °C to 750 °C), using a true triaxial Hopkinson pressure bar test system. The study analyzes the effect of temperature and strain rate on dynamic failure behaviors of concrete under biaxial stress constraints. The study establishes models for predicting the evolution of dynamic strength deterioration and strain rate. Additionally, the research discusses the mechanisms of influence of stress constraint states on the dynamic increasing factor (DIF) and failure modes of concrete. Results indicate that concrete's strength under biaxial stress constraints increases with the load rate and is hardened or weakened by changes in temperatures (taking 300 °C as a demarcation point). The strain rate effect of concrete becomes more significant with increasing temperature. Stress constraint states significantly affect the dynamic strength of concrete, and the DIF of concrete grows quasi-linearly with the logarithm of strain rate at different temperatures. Under biaxial stress constraints, concrete is dominated by oblique shear failure, and damage is less severe with the increase in the intermediate principal stress σ2. Macro-fractures are unlikely to appear under true triaxial stress constraints.

Axial compressive behavior of GFRP tube-reinforced concrete-steel double skin tubular columns

Hybrid FRP-reinforced concrete-steel double-skin tubular columns (DSTCs) exhibit the benefits of high bearing capacity, sufficient ductility and excellent durability. However, there are fewer studies on reinforced DSTCs under axial compression compared to unreinforced DSTCs. Eleven circular cross-section columns were tested to study the compressive behavior of GFRP-reinforced concrete-DSTCs. The main parameters considered were the thicknesses of GFRP tubes, longitudinal reinforcement ratio, hollow ratio and concrete strength. The test results showed that reinforced DSTCs had higher bearing capacity and better ductility than traditional DSTCs, and their bearing capacity could be increased by 36%–48%. The bearing capacity and initial stiffness increased with the increase of longitudinal reinforcement ratio, thicknesses of GFRP tubes and concrete strength, while they decreased with the increase of hollow ratio. When the load was within the range of 55%Pu, the restriction of the GFRP tube to the concrete was not obvious. Furthermore, an analytical model was established to predict the full load-strain curves of reinforced DSTCs, which considered the restraining effect of stirrups and the bi-axial stress state of FRP tube. The established model was in good agreement with most of the test results.

Thermoelastic Stress Analysis in the Presence of Biaxial Stresses in Titanium: Effect of the Mean Stress on Errors in Stress Evaluation

BackgroundThermoelastic Stress Analysis (TSA) is a contactless technique capable of estimating superficial stresses on components subjected to dynamic loads. Surface stresses can be obtained by means of calibration methods that require the assessment of the thermoelastic constants. However, these methods can lead to errors in stress evaluation in those materials where the effect of the mean stress on the thermoelastic signal cannot be neglected (e.g., titanium).ObjectiveThe aim is the development of an analytic formulation of error made in first stress invariant amplitude evaluation for a biaxial stress state, when neglecting mean stress effect.MethodsBy considering the general theory of thermoelastic stress analysis accounting for the mean stress effect, the formulation of the thermoelastic effect in the presence of a biaxial stress state was obtained. The results were compared to those obtained by a numerical simulation and the proposed formulation has been validated for titanium by means of experimental tests.ResultsFirstly, the new formulation of thermoelastic temperature variations accounting for mean stress effect in presence of a biaxial stress state was provided. Secondly, an error analysis provided an analytical formulation for the error made in case mean stress effect is neglected for different case studies.ConclusionsThe error in stress evaluation can be considered as the error originating from the use of an incorrect calibration formula (traditional one)”. The new analytical formulation accounting for the general theory of thermoelastic stress analysis allows to account for the mean stress on titanium in the presence of a uniaxial and biaxial stress states and to evaluate the error made in neglecting such a second order effect when using TSA.

A compressive-shear biaxial stress field model for analyzing the anisotropic mechanical behavior of nanotwinned diamond

Synthetic nanotwinned diamond (Nt-D) with unprecedented hardness and toughness is an ideal material for manufacturing ultra-precision cutting tools. It is urgent to investigate the surface formation mechanism of Nt-D under complex stress fields. Here, we developed a compressive-shear biaxial stress field model to study the extreme anisotropic mechanical behavior of Nt-D. It was found that graphitization occurred in all shear directions of the Nt-D (0001) plane through the microstructure evolution of 2H-diamond, 4H-diamond, 6H-diamond, and 9R-diamond, in which the stress release mechanism was accompanied by partial slips in certain specific crystal directions such as 〈1010〉. In addition, the generalized stacking fault energy calculation indicates that the partial slip in 6H- and 9R-diamonds is mainly attributed to the low slip energy barrier between adjacent interlayer. These results expand our understanding of the surface formation mechanism of Nt-D and will contribute to the manufacturing of new-generation ultra-precision tools.

Experimental characterization of dynamic strength and failure behavior of saturated reef limestone concrete under biaxial stress constraint

On the basis of not destroying the island's ecological environment, concrete mixed with reef limestone aggregates in situ is beneficial to solve the problem of shortage of terrestrial rock resources in the island area. Considering the influence of water-saturated state, lateral stress constraint and strain rate, the dynamic mechanical properties and failure characteristics of reef limestone concrete under biaxial stress constraint are studied by using true triaxial Hopkinson bar test technology. The dynamic failure behavior of reef limestone concrete is analyzed from macro-meso level by means of SEM test method, and the stress intensity factor theory of crack tip is cited to reveal the enhancement effect of dynamic strength of water-saturated reef limestone concrete under the coupling effect of biaxial stress constraint and impact load. The results show that the dynamic compressive strength of reef limestone concrete increases with the strain rate in a power function, and the growth rate decreases gradually. Under the same impact load, the compressive strength of reef limestone concrete in water-saturated state increases more significantly than that in dry state. The lateral stress constraint has a significant enhancement effect on the dynamic strength of reef limestone concrete samples, and changes the Poisson's ratio of reef limestone concrete samples. In addition, based on the equivalent crack model theory, it is verified that under the combined action of biaxial stress constraint and high strain rate, the dynamic stress intensity factor of the crack tip of the reef limestone concrete sample in water-saturated state is larger than that in dry state.

Bulge test technique and digital image correlation for the determination of the biaxial behavior of polymeric films

Abstract Materials can display multiple mechanical behaviors when they are biaxially stressed. In this research work, films obtained through the blowing process were investigated in order to compare the mechanical properties and behavior of the film under uniaxial and biaxial stress conditions. Bulge test coupled with a continuous full-field strain measurement was undertaken in order to characterize polymeric film in a biaxial stress state. As far as this study is concerned, a laboratory-made pneumatic bulge test machine was used for the determination of the biaxial behavior of a 50 μm thickness PLSF (modified PLA) film. 3D-digital image correlation (3D-DIC) technique was used in order to compute the strain field continuously during the test. Tensile test was equally performed on the film along its machine direction (MD) and transverse direction (TD). Strain field distribution in the film sample was analyzed and compared in both tests. The tensile test results displayed an isotropic behavior of the film in both directions. However, a difference in the strain at break was noticed because of the thickness instability along the TD. The comparison between uniaxial and biaxial properties resulted in almost the same properties with respect to the Young Modulus as well as the tensile strength.

Applied in-plane strain effects on the polarization response of ferroelectric hafnium zirconium oxide thin films

The influence of biaxial stress on the maximum and remanent polarizations of 10 nm thick hafnium zirconium oxide thin films in metal–ferroelectric–metal capacitor structures has been quantified. In the as-prepared state with a nominal biaxial tensile strain of 0.20% and no applied extrinsic stress, remanent and maximum polarizations of 7.6 and 13.1 μC/cm2, respectively, were measured using a 2 MV/cm applied electric field. Reducing the intrinsic strain by 0.111% through the application of a compressive uniaxial stress results in a decrease in the remanent and maximum polarizations to 6.8 and 12.2 μC/cm2, respectively. The polarization dependence on strain is nearly linear between these values. The observed variation in polarization with strain is consistent with strain impacting ferroelastic switching whereby in-plane tension increases the fraction of the short polar axis orienting out-of-plane, hence increasing out-of-plane polarization. In contrast, reducing the in-plane strain through compression results in an increase in the fraction of the long non-polar axis orienting out-of-plane, thereby decreasing out-of-plane polarization.

A Visco-Hyperelastic Constitutive Model to Characterize the Stress-Softening Behavior of Ethylene Propylene Diene Monomer Rubber.

Uniaxial and biaxial cyclic tensile tests and stress relaxation tests were performed on the ethylene propylene diene monomer (EPDM) material to investigate its stress-softening effect. The experimental results reveal that the EPDM material presents a significant Mullins effect during the cyclic stretching processes. Furthermore, it is found that the deformation of the EPDM material does not return to zero simultaneously with the stress, due to the viscoelasticity of the EPDM material. Therefore, this study combines pseudo-elasticity theory and viscoelastic theory to propose a visco-hyperelastic constitutive model. The proposed model is used to fit and analyze the uniaxial and biaxial cyclic test results of EPDM and a comparison is conducted with the corresponding hyper-elastic constitutive model. The results show that the proposed model is in good agreement with the experimental data and superior to the hyper-elastic constitutive model, especially when it comes to the stress-softening unloading process. This work is conducive to accurately characterizing the stress-softening behavior of rubber-like materials at large deformation and can provide some theoretical guidance for their widespread application in industry.

Study on Dynamic Amplification Coefficient of U-Shaped Girder Based on Vehicle-Bridge Coupling Dynamics

In this paper, a vehicle-bridge coupling dynamic model is established considering the vertical wheel-rail tight contact and the lateral simplified wheel-rail creeping. Through the site test and the numerical calculation, the dynamic deflections and the stresses of the U-shaped girder in the Nanjing S6 Urban Rail Transit in China are obtained, and the effectiveness of the established numerical model is verified. By changing the vehicle types and speeds, the dynamic amplification coefficients (DAFs) of the vertical deflections and the biaxial stresses at the girder bottom including the key points at the plate and section intersections are calculated. The research shows that the distribution of the lateral stresses is more complex than that of the deflection and the longitudinal stresses. Based on the calculation results considering various vehicle types, it is suggested that the DAFs of the vertical deflections and the longitudinal stresses are taken as 1.30, and the DAF of the lateral stresses remains at 1.40 as stipulated by the code. The research of this paper is to provide a reference for the structural design and size optimization of the U-shaped girders for urban rail transit.

Chloride ion diffusion and distribution characteristics in recycled coarse aggregate concrete under uniaxial and biaxial pressure

Under uniaxial compression, recycled coarse aggregate leads to complex internal stresses (simultaneous uniaxial, biaxial, and multiaxial tensile and compressive stresses at the micro scale) and even micro cracks in concrete, which poses difficulties in accurately predicting the chloride ion diffusion of recycled concrete. By changing the compressive stress level and bi-directional stress ratio of concrete, a self-developed biaxial compression device was used to test the chloride ion diffusion coefficient of recycled concrete under different stress states. In order to analyze the influence of various factors on chloride ion diffusion, DIC digital image technology and microcrack statistics technology were used to analyze the influence of concrete micro deformation on its chloride ion diffusion coefficient under load, and X-ray fluorescence technology was used to accurately analyze the concentration distribution of chloride element in different positions of concrete. The pore structure inside recycled concrete was characterized through experiments, and a real three-dimensional multi-scale model of concrete and the statistical characteristics of porosity at different scales were obtained. Finally, a theoretical model was established to predict the chloride ion diffusion coefficient under biaxial compressive loads, achieving accurate prediction of chloride ion diffusion coefficient close to practical engineering.

Investigation on radial fracturing around borehole under combined static stress and blasting

Blast-induced radial fracturing is generally subjected to geo-stress. This study experimentally, theoretically and numerically investigates the radial fracturing behaviours around a rock borehole under combined static stress and blast loading by combining blast testing, elastic mechanics analysis and finite element modelling. The study begins with 11 blast tests on 100 mm cubic granite samples under various uniaxial and biaxial stress states based on a biaxial hydrostatic loading system. The fracture networks are subsequently image-processed in ImageJ where the morphology and fractal characteristics of radial fracturing in the rock surface are examined. Then, the variations in fracture patterns are analyzed using elastic mechanics, and the corresponding crack extension is numerically simulated in LS-DYNA. The mechanisms of the influence of static pressure on blast-induced radial cracking are revealed. The results demonstrate that the static stress greatly suppresses the blast-induced radial fracturing, resulting in fewer and shorter radial cracks with lower propagation velocity. This leads to a reduction in the fractal dimension of fractures and rock damage, which is attributed to the initial stress offsetting the blast-formed tangential tensile stress. Meanwhile, uniaxial pressure governs the propagation direction of radial fractures, as the cracks initiate at the borehole wall where the maximum hoop tensile stress presents and propagate along a path with the smallest circumferential compression. At last, the implications of the current results for practical blasting are discussed. It is advisable to leverage the directional cracking advantage offered by blasting under anisotropic in-situ stress to achieve a smoother excavation profile.

Influence of Biaxial Stress on Magnetic Behavior of Hot-Rolled Steels

On-line non-destructive magnetic monitoring (NDM) techniques are relevant to evaluate the quality of steels and their fluctuations during forming process. The on-line monitoring faces different issues. Due to magneto-mechanical coupling, stress that may be applied during NDM may have a strong effect on the magnetic behavior that may lead to erroneous estimations of the microstructure quality of the material. The multiaxial character of the stress state is another strong issue. In this communication, some new results showing the effect of a uniaxial and biaxial stress on the magnetic behavior of 3 grades of hot-rolled steels are presented (from low carbon steel to AHSS).

The effect of biaxial strain on the phonon-limited mobility in 4H-SiC MOSFETs

We investigate the effect of biaxial tensile stress at the interface of 4H-SiC metal-oxide-semiconductor field effect-transistors on the phonon-limited mobility using theoretical calculations. The band structure under strain is obtained by the first-principles calculations, and the electronic states and potential of the inversion layer at the interface are determined by the Schrödinger–Poisson method. The mobility is calculated using the Fermi's golden rules. We find that the biaxial strain reduces the energy difference between the first and second conduction band minima. This leads to an increase in polar-optical phonon scattering, which becomes the dominant factor in the mobility degradation under the tensile strain.

Study on Fatigue Cracking of Diaphragm's Arc Opening of OSD in Steel Bridges by Using Biaxial Stress Method.

Changes in loading position have a significant impact on the stress field of each vulnerable area of an orthotropic steel deck (OSD). The arc opening area of the diaphragm and the connecting area between the U-rib and the diaphragm under the moving load are prone to fatigue cracking. By comparing the stress responses under different methods, the hot spot stress (HSS) method is used as the main stress extraction method in fatigue performance evaluation. The control stress of fatigue cracking was analyzed by comparing the direction of the principal stress field with the crack direction in this experiment. According to the stress amplitude deviation under the biaxial stress state, a set of methods for evaluating the effects of in-plane biaxial fatigue was developed. An improved luffing fatigue assessment S-N curve was applied to analyze the fatigue life of the diaphragm's arc opening area. The results show that when the moving load is exactly above the connection of the deck and the web of the U-rib on one side, it is in the most unfavorable position in the transverse direction, and the diaphragm is mainly under the in-plane stress state. The longitudinal range of the stress influence line of the arc opening is approximately twice the diaphragm spacing. Two to three stress cycles are caused by one fatigue load. Fatigue crack control stress is the principal stress tangential to the arc opening's edge in this area. The normal direction of the principal stress in the model test is roughly consistent with the crack initiation direction. The variation in the stress amplitude deviation in this area is caused by changes in the action position of the moving load. When the moving load is at a certain distance from the involved diaphragm, it is reduced to zero, implying that the in-plane fatigue effect is the greatest in this area.

Electronic and optical properties of Be2C/graphene heterojunction from first-principles calculations

We study the electrical and optical properties of the van der Waals (vdW) heterojunction Be2C/graphene by first principles. The constructed heterojunction exhibits p-type Ohmic contact. It is found that the varied interlayer distance and biaxial stress can influence the electronic structure and charge transfer of heterojunction. While the small variety of the difference of electrostatic potential of the heterojunction induces the small change of charge transfer at the contact interface, so the p-type Ohmic contact can still be formed. Meanwhile, the decreased interlayer distance and tensile strain can enhance the optical absorption of heterojunction. Then we calculate the external quantum efficiency of the optoelectronic device based on Be2C/graphene that reaches to 27.8%. This work provides a way to design electronic and optoelectronic devices based on Be2C/graphene in the future.

Helium bubble size effects on the surface morphological response of plasma-facing tungsten

Abstract We report a simulation study on the effects of helium (He) bubble size on the morphological evolution and pattern formation on the surface of tungsten used as a plasma-facing component (PFC) in nuclear fusion devices. We have carried out a systematic investigation based on self-consistent dynamical simulations of surface morphological evolution according to an atomistically-informed, 3D continuum-scale model that captures well the relevant length and time scales of surface nanostructure formation in PFC tungsten. The model accounts for PFC surface diffusion, driven by the biaxial compressive stress originating from the over-pressurized He bubbles in the near-surface region of PFC tungsten as a result of He plasma exposure, combined with the formation of self-interstitial atoms in tungsten that diffuse toward the PFC surface and the flux of surface adatoms generated as a result of surface vacancy-adatom pair formation upon He implantation; this transport of surface adatoms contributes to the anisotropic growth of surface nanostructural features due to the different rates of adatom diffusion along and across step edges of islands on the tungsten surface. Our detailed analysis reveals that varying the average He bubble size plays an important role in the PFC surface growth kinetics as well as the resulting surface topography. Specifically, we find that an increase in the He bubble size leads to a deceleration in the growth rate of the tungsten nanotendrils that emanate from the PFC surface. We also find that the separation distance between the resulting surface features increases with increasing He bubble size, as well as over time. This coarsening effect is a thermally activated process resulting in an accurate description of the temperature dependence of the average surface feature separation by an Arrhenius relation.

A novel hydraulic bulge test in hot forming conditions

A novel hydraulic bulge test device was developed to evaluate high temperature biaxial stress–strain curves of quenchable boron steel sheets. The work mainly focuses on the resistance heating designed to assure a homogeneous temperature field in the dome area of the circular blank where the hydraulic pressure will be applied. The practical hot stamping conditions, including the heating and cooling steps, were reproduced to perform hydraulic bulge tests on the Usibor ®1500P steel for a temperature range between 700 to 900 °C, after an austenization step at 900 °C. Stress–strain curves were obtained from these expansion tests using the data extracted with a laser profilometer, due to the difficulties associated with the use of Digital Image Correlation at such high temperatures. Although the profilometer is a compromise solution, the comparisons between tensile and biaxial stress–strain results enable to verify the feasibility of the new device for the evaluation of the stress–strain curves at a high temperature in a biaxial state. The results point-out that, besides the difficulties with the acquisition of the strain fields and in the strain-rate control, there are also challenges in the interpretation of the metallurgical evolutions that can occur during the tests, which can affect the biaxial flow curves.