Search Results (35)

Plastic forming in magnesium alloy sheet products is becoming a hot topic because of its potential in light-weight structural designs. Due to the special anisotropic and tension–compression asymmetrical properties of magnesium alloys, traditional modeling methods based on the von Mises yield criterion and using only uniaxial tensile properties for bending-dominated process simulations are not able to produce accurate predictions. In this study, two kinds of tensile tests (uniaxial and biaxial) and some compressive tests were performed along three material directions to obtain anisotropic and asymmetric properties, based on which the parameters of the Hill48 and Verma yield criteria were obtained. Then, the user subroutine VUMAT was developed, and the roll-forming process for magnesium alloys was simulated with the established anisotropic and asymmetric yield criteria. Finally, a roll-forming experiment on AZ31 magnesium alloy was performed. Compared with the experiments, it was found that roll-forming and springback predictions based on the Verma yield criterion had higher accuracy than those based on the von Mises and Hill48 yield criteria FEM models, which ignore anisotropy and asymmetry. This study provides an important FEM modeling idea that considers not only anisotropy but also asymmetry in the bending-dominated forming processes of magnesium alloys in which tension and compression exist simultaneously. Full article

Proton-conducting solid oxide fuel cells (P-SOFCs) are widely studied for their lower working temperatures than oxygen-ion-conducting SOFCs (O-SOFCs). Due to the elevated preparation and operation temperatures varying from 500 °C to 1500 °C, high mechanical stresses can be developed in the electrolytes of SOFCs. The stresses will in turn impact the electrical conductivities, which is often omitted in current studies. In this work, the mechanical–conductive behaviors of Y-doped BaZrO₃ (BZY) electrolytes for P-SOFCs under high temperatures are studied through molecular dynamics modeling. The Young’s moduli of BZY in fully hydrated and non-hydrated states are calculated with different Y-doping concentrations and at different temperatures. It is shown that Y doping, oxygen vacancies, and protonic point defects all lead to a decrease in the Young’s moduli of BZY at 773 K. The variations in the conductivities of BZY are then investigated by calculating the diffusion rates of protons in BZY at different triaxial, biaxial, and uniaxial strains from 673 K to 873 K. In all cases, the diffusion rate present a trend of first increasing and then decreasing from compression state to tension state. The variations in elementary affecting factors of proton diffusion, including hydroxide rotation, proton transfer, proton trapping, and proton distribution, are then analyzed in detail under different strains. It is concluded that the influences of strains on these factors collectively determine the changes in proton conductivity. Full article

Different hyperelastic material models (Mooney-Rivlin, Yeoh, Gent, Arruda-Boyce and Ogden) are able to estimate Treloar’s test data series containing uniaxial and biaxial tension and pure shear stress-strain characteristics of rubber. If the rubber behaviour is only determined for the specific load of the product, which, in the case of rubber bumpers, is the compression, the time needed for the laboratory test can be significantly decreased. The stress-strain characteristics of the uniaxial compression test of rubber samples were used to fit hyperelastic material models. Laboratory and numerical tests of a rubber bumper with a given compound and complex geometry were used to determine the accuracy of the material models. Designing rubber products requires special consideration of the numerical discretization process due to the nonlinear behaviours (material nonlinearity, large deformation, connections, etc.). Modelling considerations were presented for the finite element analysis of the rubber bumper. The results showed that if only uniaxial compression test data are available for the curve fitting of the material model, the Yeoh model performs the best in predicting the rubber product material response under compressive load and complex strain state. Full article

In the present study, the mechanical response and deformation behavior of a Mg AZ31 plate with different types of pre-twins was systematically investigated under biaxial tension along the normal direction (ND) and transverse direction (TD) with different stress ratios. The results show that significant hardening was observed under biaxial tension. The yield values in the direction of larger stress values were higher than those under uniaxial loading conditions, and the solute atom segregation at twin boundaries generates more obvious strengthening effect. Noting that, for TRH (with cross compression along the rolling direction (RD) and TD and annealing at 180 °C for about 0.5 h) sample, the strength effect of the RD yield stress ${\mathsf{\sigma }}_{\mathrm{RD}}:{\mathsf{\sigma }}_{\mathrm{ND}}$ = 2:1 was higher than that of the ND yield stress under stress ratio ${\mathsf{\sigma }}_{\mathrm{RD}}:{\mathsf{\sigma }}_{\mathrm{ND}}$ = 1:2. There is a complex competition between twinning and detwinning under biaxal tension along the ND and TD of the pre-twinned samples with the variation in the stress ratio along the TD and RD. The variation in the twin volume fractions for all samples under biaxial firstly decreases and then increases with a higher stress ratio along the ND. As for the TDH sample (precompression along the TD and annealing), the changes of the twin volume fraction were lower than that of the TR sample (cross compression along the TD and RD). However, the amplitude of variation in twin volume fraction of the TRH sample is higher than that of the TR sample. This is because the relative activity of detwinning decreases and that of twinning increases, as the ND stress mainly leads to the growth of pre-twins and the TD stress often promotes detwinning of primary twins. With a higher stress ratio along the ND, the activity of twinning deformation increases and that of detwinning decreases. Full article

Post-filling coarse aggregate concrete (PFCC) is a new type of concrete that achieves energy-saving and emission-reduction goals through optimizing material proportions. The post-filled coarse aggregates can save the amount of cement material used, improve the strength and elastic modulus, prolong the service life of the material, and reduce expenses. We conducted a biaxial tension–compression test on PFCC cubic specimens, analyzed the strength and stress–strain curve regularity under different post-filling ratios (PFRs) and stress ratios, and proposed a new failure criterion suitable for PFCC. The results demonstrated that the tensile strength and compressive strength of each post-filling ratio concrete specimen under biaxial tension–compression action are lower than its uniaxial tensile and uniaxial compressive strength under the same post-filling ratio. Under the same stress ratio, the variation pattern of the post-filling ratio was the same as that under uniaxial stress, with the maximum value occurring at a PFR of 20%. The strength change rule was affected by both the stress ratio and the post-filling ratio. From the stress–strain curve, it can be seen that the presence of tensile stress significantly reduces the stiffness and ductility of PFCC under biaxial tensile and compressive loading. The strain corresponding to the peak strength of the σ₃/f_c-ε₃ curve was much smaller than the peak strain under uniaxial compression. For example, at a stress ratio of (0.05:1), the strain ε₃ in the compression direction was on average about 50% to 60% of the uniaxial compression strain under the same PFR. The stress–strain curve of PFCC under biaxial tensile and compressive loading was approximately linear throughout the loading process. A failure criterion for PFCC under biaxial tension–compression loading was established, and the calculated values agreed well with the test values. This paper provides references and research data for the study of PFCC under complex stress conditions. Full article

Conventional scar treatment options of single pressure garment therapy (PGT) or silicone gel sheeting (SGS, Cica-Care^®, Smith and Nephew, London, UK) alone lack mechanical property tunability. This article discusses a scar healing composite (PGF-Biopor^®AB, Dreve Otoplastik GmbH, Unna, Germany) and how its mechanical properties can be tuned for improved mechanotherapy. A balance between compression and tension was achieved by tuning the tensile and shear properties, facilitating tension shielding and pressure redistribution for scar therapeutics. Biopor^®AB-wrapping on biaxial-tensioned pressure garment fabric (PGF) allowed compression therapy and internal pressure redistribution. The Biopor^®AB surface, with a coefficient of friction close to 1, strategically localizes stress for effective tension shielding. A substantial five-fold reduction in silicone tension, amounting to 1.060 N, achieves tension shielding and pressure redistribution. Simultaneously, a dynamic internal pressure-sharing mechanism distributes 0.222 kPa from each SPK-filament bundle, effectively managing internal pressure. Alongside the principle compression-silicone dual therapy, this composite design with dynamic internal pressure sharing and mechanical property tunability provides an additional pressure-relieving strategy for multiple scar therapeutics. Full article

When subjected to impact loading, aircraft composite structures are usually in a specific preloading condition (such as tension and compression). In this study, ballistic tests were conducted using a high-speed gas gun system to investigate the effect of biaxial in-plane tensile preload on the delamination of CFRP laminates during high-speed impact. These tests covered central and near-edge locations for both unloaded and preloaded targets, with the test speeds including 50 m/s, 70 m/s, and 90 m/s. The delamination areas, when impacting the center location under 1000 με, show a 14.2~36.7% decrease. However, the cases when impacting the near-edge location show no more than a 19.3% decrease, and even more delamination areas were observed. In addition, in order to enhance the understanding of experimental phenomena, numerical simulations were conducted using the ABAQUS/Explicit solver, combined with the user subroutine VUMAT with modified Hou criteria. The experimental and simulation results were in good agreement, and the maximum error was approximately 12.9%. The results showed that not only the preloading value but also the impact velocity have significant influences on the delamination behavior of preloaded CFRP laminated plates. Combining detailed discussions, the biaxial tensile preload enhanced the resistance to out-of-plane displacement and caused laminate interface stiffness degradation. By analyzing the influence of the preloading value and impact velocity on competing mechanisms between the stress-stiffening effect and interface stiffness degradation effect, the complex delamination behaviors of laminates under various preloading degrees and impact velocities at different impact locations were reasonably explained. Full article

Sheet metals usually experience various loading paths such as uniaxial tension, uniaxial compression, biaxial tension, and simple shear during the forming process. However, the existing constitutive models cannot always accurately describe blanks’ anisotropic yield and plastic flow behavior of blanks under all typical stress states. Given this, this paper improves the Eyld2000-2d yield criterion by introducing hydrostatic pressure to the A-Eyld2000-2d yield criterion that can describe the strength differential effect of materials. Meanwhile, to control the curvature of the yield surface more effectively, the near-plane strain yield stresses were added in the parameter identification process to calibrate the exponent m, so that the exponent is no longer considered as a constant value. Taking the widely used AA6016-T4, AA5754-O, DP980, and QP980 blanks in the automotive stamping industry as an example, the effectiveness of the new model and different parameter identification methods was verified by predicting experimental data under various simple and complex loading paths. Subsequently, the new model employing the optimal parameter identification strategy was compared with four widely used asymmetric yield criteria under associated and non-associated flow rules, including CPB06, LHY2013, S-Y2004, and Hu & Yoon2021, to further verify the accuracy of the proposed constitutive model. The results indicate that parameter identification strategy with variable exponent can significantly improve the flexibility of the yield criterion in describing the plastic anisotropy of blanks. Compared to the other yield criteria examined in this work, the new model provides the best prediction accuracy for the yield stresses and plastic flows of all blanks, especially in the near-plane strain and simple shear stress states. Modeling under the concept of anisotropic hardening can more accurately capture the evolving plastic behavior of blanks than isotropic hardening. Full article

A statistical damage model (SDM) of fully-graded concrete was created using statistical damage theory, based on the mechanical properties of axial tension and axial compression of the material. The SDM considers two damage modes, fracture and yield, and explains the intrinsic connection between the mesoscopic damage evolution mechanism and the macroscopic nonlinear mechanical behavior of fully-graded concrete. The artificial bee colony (ABC) algorithm was used to obtain the optimal parameter combination through an intelligent search of parameters ${\epsilon }_a$, ${\epsilon }_h$, ${\epsilon }_b$ and $H$ in the constitutive model by taking the test data as the target value, and the sum of the squares of the differences between the target value and the predicted value as the objective function. The SDM numerical simulation model of fully-graded concrete is proposed by compiling subroutines in FORTRAN by constructing two modules of data model and damage analysis. The numerical results under uniaxial and biaxial forces are in agreement with the experimental results, which verifies the accuracy of the program. The model also analyzes the characteristics of mesoscopic damage evolution and predicts the mechanical properties under triaxial forces. The results show that the proposed numerical simulation model can reflect the salient features for fully-graded concrete under uniaxial, biaxial and triaxial loading conditions, and the evolution law of mesoscopic parameters. Therefore, the proposed model serves as a basis for the refined finite element analysis of hydraulic fully-graded concrete structures and reveals the mesoscopic damage mechanism of concrete under different load environments. Full article

Aging infrastructure is increasingly costing taxpayers due to increased repair and replacement costs. Ultra-high performance concrete (UHPC) has recently been recognized as a viable material for both the repair of concrete and steel infrastructure as well as a replacement material for new structures due to its enhanced mechanical and durability properties. Such uses require a much better understanding of the multiaxial tensile properties of UHPC to utilize the material more efficiently. This study focused on developing a novel apparatus capable of subjecting specimens to tensile forces in each of the three principal directions simultaneously. Such an apparatus could collect data for a portion of the failure surface that currently only has a small dataset to establish trends. The “Looney Bin” was designed to test 50-mm cube specimens in triaxial tension, biaxial tension, tension-compression, and tension-tension-compression stress states. Once the apparatus and fixtures were designed and fabricated, trial tests were conducted on a non-proprietary UHPC without steel fibers to establish a test method for each of the stress states evaluated. Data were then collected for different stress states using the established procedures and plotted against previously published failure models for UHPC to verify that the collected data were reasonable. Full article

Using experimental methods to study the influence of hole and cracks on the mechanical properties and fracture characteristics of rock-like mortar materials under biaxial compression conditions. The double crack specimens with hole depths from 0–100 mm are prefabricated to study the strength and deformation characteristics of the specimens under different lateral loads σ₂ = 0–6 MPa. The evolution process of secondary crack initiation, development, and connection of the hole-crack specimens are recorded. The results show that: (1) One type of rock mortar test material is prepared, and its main physical and mechanical parameters are all within the range of sandstone, which can effectively simulate the stress deformation characteristics of sandstone. (2) When the depth of the holes in cracked samples exceeds 50% of the length, the strength and deformation of the samples undergo a sudden change. When the depth of the hole in the crack specimen increases from 40 mm to 60 mm, the peak stress decreases most significantly. Moreover, the maximum values of the strain value at peak strength and lateral strain both occur at a hole depth of 60 mm. (3) When the cracked specimen contains through-holes, the failure mode is composite fracture and shear composite fracture. When the depth of the hole is different, the fracture forms include tension composite fracture, shear composite fracture, and composite fracture. Full article

To meet the measurement needs of multidimensional high-g acceleration in fields such as weapon penetration, aerospace, and explosive shock, a biaxial piezoresistive accelerometer incorporating tension–compression is meticulously designed. This study begins by thoroughly examining the tension–compression measurement mechanism and designing the sensor’s sensitive structure. A signal test circuit is developed to effectively mitigate cross-interference, taking into account the stress variation characteristics of the cantilever beam. Subsequently, the signal test circuit of anti-cross-interference is designed according to the stress variation characteristics of the cantilever beam. Next, the finite element method is applied to analyze the structure and obtain the performance indices of the range, vibration modes, and sensitivity of the sensor. Finally, the process flow and packaging scheme of the chip are analyzed. The results show that the sensor has a full range of 200,000 g, a sensitivity of 1.39 µV/g in the X direction and 1.42 µV/g in the Y direction, and natural frequencies of 509.8 kHz and 510.2 kHz in the X and Y directions, respectively. Full article

The buckling enhancement of the negative Poisson’s ratio (NPR) effect on a laminated plate under uniaxial compression with an in-plane translational restraint is investigated in this paper. According to the buckling equation of an orthotropic plate under biaxial compression, the critical buckling load of an NPR-laminated composite under uniaxial compression can be increased due to the induced tension force on the unloaded direction. Instead of layer angles and stacking sequence, the NPR envelope and buckling load enhancement are studied using lamination parameters in this paper. The Poisson’s ratio contours are given in the feasible region of membrane lamination parameters. The results show that the negative Poisson’s ratios are more sensitive to $V_3^A$, which represents the unbalance degree of the laminate. Furthermore, the buckling loads for various Poisson’s ratio layups are investigated, and it is concluded that the buckling load increases with a decrease in Poisson’s ratio for the laminated rectangular plate considering in-plane translational restraint. Finally, the inverse problem of deciding the laminate configuration to target the lamination parameters is solved using the particle swarm optimization (PSO) algorithm. Full article

Molecular dynamics simulation is used to study and compare the mechanical properties obtained from compression and tension numerical tests of multilayered graphene with an increased interlayer distance. The multilayer graphene with an interlayer distance two-times larger than in graphite is studied first under biaxial compression and then under uniaxial tension along three different axes. The mechanical properties, e.g., the tensile strength and ductility as well as the deformation characteristics due to graphene layer stacking, are studied. The results show that the mechanical properties along different directions are significantly distinguished. Two competitive mechanisms are found both for the compression and tension of multilayer graphene—the crumpling of graphene layers increases the stresses, while the sliding of graphene layers through the surface-to-surface connection lowers it. Multilayer graphene after biaxial compression can sustain high tensile stresses combined with high plasticity. The main outcome of the study of such complex architecture is an important step towards the design of advanced carbon nanomaterials with improved mechanical properties. Full article

Rubber-like materials exhibit stress softening when subject to loading–unloading cycles, i.e., the Mullins effect. However, this phenomenon can be recovered after annealing the previously stretched sample under a stress-free state. The aim of this paper is to establish a constitutive model with thermodynamic consistency to account for the stress softening and thermal recovery. Towards this goal, (i) an explicit form of Helmholtz free energy can be found such that the restrictions from thermodynamic law can be satisfied; (ii) a compressible, multi-axial strain-energy function considering energy dissipation is proposed by introducing specific invariants; (iii) a unified shape function based on the symmetry property of the test data in a one-dimensional case with stress softening and thermal recovery is provided by introducing a weight variant; (iv) it is proven that the new potential can automatically reduce to the one-dimensional case, i.e., uniaxial tension, equal biaxial, or plane strain; (v) numerical results for model validation are exactly matched with classical experimental data. Full article