Large Deformation Region Research Articles

Poisson’s Ratio of Selected Metallic Materials in the Elastic–Plastic Region

Poisson’s ratio is one of the fundamental characteristics in the material models that are used. In engineering practice, its values are assumed to be constant in the elastic and in the plastic region. In this paper, the conventionally used values of this number for steel materials and aluminum alloys are confronted with experimental results. By using non-contact strain measurements with the DIC (digital image correlation) method, the evolution of the Poisson ratio value in the regions of transition from the elastic to the plastic region as well as in the regions of large plastic deformations was documented. The obtained experimental results are graphically compared using the proposed strain scaling. The gradient of the Poisson ratio changes in the vicinity of the yield stress is significant, indicating the need for a refinement of the material models in this region. Deviations from the conventionally used value of this number were found in the large plastic deformation region. In conclusion, a possible approach for improving the accuracy of simulations in FEM softwares was formulated.

Porous Fabrication of White Metal Using Ultrasonically Generated Microbubbles

Conventional methods for producing porous metals involve the use of chemicals such as thickeners and foaming agents under high temperatures and pressures. However, these methods are costly and pose a risk of dust explosion. Thus, the objective of this research is to achieve the cost-effective and safe production of porous metals by introducing microbubbles generated by ultrasonic oscillation into the molten metal. One end of an ultrasonic horn was inserted into three different molten metals—white metal, Pb-free solder, and zinc—and microbubbles were generated at the horn end by the strong ultrasonic oscillation in the molten metals. The microbubbles that contained molten metal changed phase to porous metal through solidification, and the diameter, porosity, and stress–strain curve of the generated porous metals were measured. The results indicate that the porosity of white metal, Pb-free solder, and zinc foams reached 54%, 76%, and 48%, respectively, and these porous metals had many micropores less than 1 mm in diameter. It was also observed that the higher the melting point, the larger the pore diameter and the lower the porosity. Furthermore, in the case of white metal, a plateau region of large deformation at constant stress was observed in the stress–strain curve.

Seismic Performance of Ordinary and Damage-tolerant Concrete Beam-column Joints under Reversed Cyclic Loading

The adoption of the capacity design principle in modern codes of practice allows for multi-storey frame structures to exhibit ductile behaviour in a major earthquake without collapsing. Whilst the risk of shear failure in regions of large inelastic deformation can be largely reduced, the detailing requirements in capacity design often leads to reinforcement congestion that adds difficulty during the construction process, due to the high amount of transverse reinforcement required to provide shear resistance and confinement. The work presented in this paper forms a part of a wider research programme that aims to simplify the detailing requirements of transverse reinforcement in the joint and plastic hinge regions of moment-resisting frame structures. To this end, damage-tolerant concrete known as engineered cementitious composite (ECC) was used to substitute concrete in these critical regions. In this article, the results of reversed cyclic tests on two half-scale exterior beam-column joints, constructed with ordinary concrete and ECC, are presented. Key features of the response such as the reversed cyclic hysteresis loops, stiffness degradation and cumulative energy dissipation are discussed, along with crack development under reversed cyclic loading in the form of digital strain maps.

An Enhanced Offset Tracking Method: Providing Auxiliary Information for DInSAR Phase Filtering in Urban Areas.

In the application of synthetic aperture radar differential interferometry in urban environments, it is easy to regard the phase change in the deformation band of buildings under construction as noise that requires filtering. This introduces an error into the surrounding area while over-filtering, resulting in an error in the magnitude of the deformation measurement results for the entire region and the loss of deformation details in the surrounding area. Based on the traditional DInSAR workflow, this study added a deformation magnitude identification step, determined the deformation magnitude by using enhanced offset tracking technology, supplemented the filtering quality map and removed the construction areas that affect the interferometry in the filtering stage. The enhanced offset tracking technique adjusted the ratio of contrast saliency and coherence via the contrast consistency peak in the radar intensity image, which was used as the basis for adjusting the adaptive window size. The method proposed in this paper was evaluated in an experiment on a stable region using simulated data and in an experiment on a large deformation region using Sentinel-1 data. The experimental results show that the enhanced method has a better anti-noise ability than the traditional method, and the accuracy rate is improved by about 12%. The supplemented quality map can effectively remove the large deformation area to prevent over-filtering while ensuring the filtering quality, and it can achieve better filtering results.

Damage Evolution of Antibacterial Stainless Steel at Different Loading Rates Based on Digital Image Correlation Method

Using Shimadzu AGS‐100KN tensile machine, LEICA‐LM/DM optical microscope, and electron backscatter diffractometer (EBSD), combined with digital image correlation (DIC) analysis method, the damage evolution behavior of copper‐containing antibacterial stainless steel in large deformation area and small deformation area under different loading rates is investigated. The results show that with the increase of the loading rate, the temperature rise induces local adiabatic shear bands in the microstructure, increasing work hardening, and a decrease in ultimate stress and full elongation. The damage evolves in the large deformation area, the sample is mainly deformed rapidly, the grain deformation degree is large, and the deformation basically stops in the small deformation area. The damage factor of the specimens in the large deformation zone and the small deformation zone increases with the increase of strain, and the damage deformation rate and the critical damage factor decrease with the increase of the loading rate. Based on the DIC method, the damage evolution equations of copper‐containing antibacterial stainless steel samples in large and small deformation regions under different loading rates are established, which effectively reflect the damage evolution laws at different loading rates.

Focus on the crucial deformation region to adjust the comprehensive performance of poly (L-lactic acid) stent

The fully biodegradable polymer stent is considered as the fourth-generation vascular implant with good biocompatibility and long-term therapeutic potential. It has attracted much attention because it overcomes the disadvantage of the permanently implanted metal stent. However, compared with the metal stent, its mechanical properties are slightly inferior, which is an urgent problem. Based on previous studies, fully biodegradable polymer stents are prone to experience cracks and damage in large deformation region during the crimping and expansion process. The large deformation region is mainly located at the ring bend of the stent. We supposed that these damages are the leading causes of weakening the mechanical performance of polymer stents and are mainly affected by the crucial deformation region. For this purpose, this work studies the relationship between different crucial deformation regions and the mechanical performance of the polymer stent. Firstly, the volume of the crucial deformation region is improved by increasing the ring width. Although the radial strength of the stent is enhanced with the increase in ring width, the radial stiffness also increases, and correspondingly, the flexibility of the stent decreases. To obtain acceptable comprehensive mechanical performance, two types of slotting design in critical deformation region were proposed. The proposed slotted stent with a bulge has sufficient radial strength and low radial stiffness, having a good radial support capacity and flexibility. In other words, the proposed stent has improved the radial support without sacrificing flexibility. Overall, different crucial deformation regions cause different degrees of damage to the stent during crimping and expansion, which affects the mechanical properties of the stent. Reasonable structural design of the crucial deformation region is the key to adjust the comprehensive performance of the stent.

Study on Improvement of Pipe-Soil Spring Model in Mountain Slope Area

For the interaction between the pipeline and the soil under the condition of large surface displacement, the displacement load is often transmitted to the pipeline through soil springs. The selection of soil spring parameters is based on laboratory research results, which may not be effective for pipe-soil interface simulation. In addition, the load and soil spring coefficient of the pipeline in the slope should vary with space and time, and these factors are ignored by the resistance formula of the current general spring model. The adaptability and temporal load distribution of the pipe-soil spring model in regions of large surface deformation are largely unknown. In this paper, we deduced the soil spring resistance formula when the surface is inclined, established an improved soil spring model for the slope, and obtained the variation law of soil spring resistance in different directions at different slope angles. At the same time, a comparison of the improved soil spring model with the standard soil spring model recommended by international associations and codes was carried out. The rationality of the improved soil spring model proposed in this paper in the calculation of the pipe-soil interaction in the slope section is further verified by small-scale physical model tests. It has a certain guiding significance for the calculation of pipe-soil interaction in slope sections.

Study on microstructure and tensile damage evolution of 2205 duplex steel at different solution temperatures

2205 dual-phase steel was subjected to solution heat treatment experiments at 1000℃～1160℃. Electron backscatter diffraction (EBSD) was used to analyze the evolution of its microstructure; the evolution of its surface damage was analyzed by a tensile testing machine combined with digital image correlation (DIC). The results show that the FCC phase in the microstructure of 2205 dual-phase steel grows into strips after solid solution treatment. With the increase of the solution temperature, local phase boundary dissolution and fracture of the FCC phase occur, the yield strength and tensile strength of the stainless steel plate after heat treatment are the same, showing a downward trend. 2205 dual-phase steel presents the morphological characteristics of ductile fracture. The average strain factor of the critical large deformation region and the small deformation region of the original specimen are both maximum. The average strain factor at 1080℃ specimen is the smallest. With the increase in annealing temperature, the speed of damage deformation becomes slower. At different solution temperatures, in 2205 dual-phase steel, the austenite texture type changes to a certain extent, but eventually, a typical brass texture is formed; that is, the brass texture is the main texture, accompanied by a certain strength, the S texture. The grain orientations in the BCC phase are concentrated between {100}<011> and {112<110>, forming a significant a-fiber texture (<110>//RD).

Dynamic response of wind turbine blade surface material under water droplet high velocity impact

When a wind turbine blade exposure to some degree of rainfall, water droplets in the atmosphere impact the blade at high velocity will causes erosion damages of the leading edge. The damages have significant influence to the aerodynamic performance of the blade, and also the cost of power generation. In this paper, a high velocity impact dynamics model of water droplet impact on wind turbine blade is develops based on SPH-FEM coupling method, where SPH model is used to model the large deformation region of the fluid domain, and FEM method is used to model the small deformation region of the structure domain. Together with properties of gelcoat material on the blade surface, effects of varying impact velocities and angles on the impact response of the gelcoat material are analysed through numerical simulation. The results show that during a high velocity impact event of water droplet, the contact force history appears two peaks. Also contact forces and strains on the blade material decreases with decreasing droplet impact velocity and impact angle, the total contact duration of the droplet during the impact decreases with increasing droplet impact velocity and angle. The present study is expected to provide theoretical guidance for enhancing the erosive capacity of wind turbine blade gelcoat material.

Energy dissipation in phase change salogels under shear stress

Phase change salogels are physical networks designed to shape stabilize salt hydrate phase change materials (PCMs) and inhibit the leakage of molten PCM. In contrast to chemically crosslinked gels, salogels can restructure by breaking of prior and reforming of new bonds. Therefore, salogels are susceptible to dissipate energy during deformations due to the nature of physical bonds. In the present work, different concentrations of diethylenetriamine, as a physical crosslinker, were added to a constant concentration of polyvinyl alcohol (PVA) in a high-salinity environment of a fluid inorganic PCM (lithium nitrate trihydrate), and network viscoelasticity was studied using oscillatory rheology. The energy dissipated was calculated by computing the area inside Lissajous stress-strain curves performed at various strain amplitudes and a constant frequency of 1 rad/s. The results of energy dissipation as a function of salogels' crosslinker concentration has a minimum when crosslinker concentration is ≅ 5–7 wt%. tes with the maximum point in the salogels' shear modulus graph. Lissajous plots' intracycle moduli analysis were used to quantify the observed non-linear behavior within a cycle of oscillation and proposed internal structure changes during deformation. Three regions based on the variation of dissipation energy with strain amplitudes (γ) for each salogel network were identified: region I (γ < ∼10%): linear viscoelastic region, region II (∼10% < γ < ∼100%): intermediate strain region with shear thickening behavior and, region III (γ > ∼100%): large deformation region with shear thinning behavior. Our results show that while most bonds are breaking at high strain amplitudes, there is a possibility of interchain reformation at intermediate shear strains due to the polymer chain alignment, which can slow down energy dissipation in this region. This study enhances the fundamental understanding of salogels’ microstructural response to non-linear deformation, which would benefit energy storage applications subjected to large deformations such as conformable packs and pillows.

Lightweight auxetic metamaterials: Design and characteristic study

In the past decades, auxetic metamaterials have attracted extensive attention due to their desirable properties. In order to further improve the energy absorption properties of auxetic metamaterials, this work investigates the design of lightweight auxetic metamaterials based on the elliptic perforated plate. The material in the large deformation region is retained and the material in the small deformation region is removed as much as possible. The experimental and numerical results indicate that the mechanical properties of the novel metamaterials are almost unchanged, but the specific energy absorption is substantially enhanced owing to the reduced material overall and the distribution of the optimized materials to the most needed area. In this paper, the influence of the amount and mode of the material removal on the mechanical properties of the auxetic metamaterial is explored through the numerical method. The proposed auxetic metamaterials with lightweight characteristics have potential applications in many fields, e.g., civil engineering, aerospace and vehicle engineering.

A hyperelastic-damage model based on the strain invariants

We develop a unified framework to describe the hyperelastic and damage behaviors of soft materials. The free energy density consists of both the crosslinked part of Langevin chains and the entanglement part described by a tube model. The micro–macro transition is obtained by using the averaging value over a microsphere. Consequently, the free energy density only depends on the strain invariants. The damage mechanism is then incorporated via the network alteration theory for the crosslinked part, while the deformation can also induce dilation of the tube and subsequent a decrease in the modulus for the entangled part. The hyperelastic model captures the Mooney Rivlin plot with a first decrease in the small deformation regime and then an increase in the large deformation region. The hyperelastic model can also accurately reproduce the stress–strain relationships in the uniaxial and biaxial loading conditions for various elastomer systems. The damage model can describe the stress response of filled rubbers in uniaxial and biaxial loading conditions, which shows a considerable improvement compared with the damage model without the entanglement effect. Since the model only depends on the strain invariants, the model can be easily implemented into the commercial finite element software Abaqus through a UHYPER. The finite element model can accurately reproduce the experimentally measured strain distribution of filled rubbers with complex geometry in the loading process. These results clearly show that the hyperelastic-damage model developed in this work has strong potentials to predict the mechanical responses of soft materials for practical applications.

Optimizing the local microstructure and mechanical properties of variable section particulate reinforced titanium matrix composites component based on numerical simulation and isothermal forming

To solve the precision forming problem of titanium matrix composites (TMCs) precision components with variable cross-sections, isothermal compression model of (TiB + La2O3)/IMI834 composite was established to investigate the deformation behaviors, determine forming strategy, predict and optimize the local microstructure. The subsequent isothermal precision forming experiment proved the precision forgings can be successfully fulfilled with good surface quality in one step using a cylindrical billet following the simulation strategy. Referring to simulation results, the local microstructure and performance were successfully optimized by controlling the deformation degree in different section regions to meet the service requirements. Fine-grained regions and reinforcement-rich regions were formed in the small cross-section (large deformation) regions due to the rotation and directional arrangement of the reinforcements and the dynamic recrystallization stimulated by the reinforcements, where the engineering strength and elongation were both improved from 1137 MPa and 2.55% to 1176.5 MPa and 4.0%, respectively, compared with the large cross-section (small deformation) regions. This research provided an important guidance for improving the fabrication efficiency and optimizing the local microstructure and performance of TMCs precision components by isothermal forming.

Electron Backscatter Diffraction Investigation of Heat Deformation Behavior of 2205 Duplex Stainless Steel

Herein, the thermal compression of 2205 duplex stainless steel is simulated using a thermal simulation testing machine at 850–1100 °/0.01 s−1 and 850–1100 °/10 s−1, and the large deformation zone (LDZ) and free deformation zone (FDZ) of the structure are characterized by electron backscatter diffraction (EBSD). The study results show that the stress–strain curve with a strain rate of 10 s−1 hardens continuously. In addition, ferrite undergoes deformation mostly when the strain is less than 0.33, and strain hardening occurs in austenite after the strain reaches 0.33. As a result, the curve ascends and steepens, and secondary hardening occurs. Continuous dynamic recrystallization (CDRX) is distinctly observed in original ferrite grains, whereas the austenite grains are recrystallized discontinuously due to the bulging high‐angle grain boundaries (HAGBs). Meanwhile, the recrystallization fraction in the large deformation region of both ferrite and austenite is significantly higher than that in the free deformation zone. The orientation distribution function (ODF) diagram analysis reveals that ferrite is mainly composed of the rotation cube {001}&lt;111&gt; texture, and austenite mainly consists of the rolled α‐fiber texture, including brass {011}&lt;211&gt;, Gauss {011}&lt;100&gt;, and {011}&lt;111&gt;.

Design of a Kirigami Structure with a Large Uniform Deformation Region.

We designed a kirigami structure with a particular shape at both ends to provide a large uniform deformation region when stretched. When a kirigami structure is stretched, non-deformation regions, where the regions’ cuts do not open, and non-uniform deformation regions, where the regions’ cuts are not uniformly deformed, are produced. The extent of the non-deformation and non-uniform deformation regions increases in proportion to the number of cut cycles in the width direction nw this reduces the percentage of the uniform deformation region. We propose a method that increases the uniform deformation region in a kirigami structure by deforming the shape of the ends from a rectangle to a trapezoid when stretched. The proposed kirigami structure has separation lines at both ends that separate cuts in the width direction, and the position of contacts at both ends are moved to the center. The proposed kirigami structure has a large uniform deformation region, even when nw is large, as evidenced by calculating the area of open cuts under stretching. The product of our study realizes a stretchable electro device with a large area, which maintains the position of evenly mounted functional elements when stretched.

Monotonic and hysteretic model for H-shaped beams incorporating deterioration behavior owing to the local buckling

To investigate the collapse behavior of steel frames under the random earthquake, it is essential to grasp the hysteretic behavior of beam members. However, in the large-deformation region, the behavior of beams is difficult to be captured precisely because of the local buckling of the flange plate and the web plate. This paper presents a method to obtain the monotonic curve and the hysteretic curve efficiently for H-shaped beams subjected to bending moment. 96 models for H-shaped beams were designed and analyzed under the monotonic and cyclic loading using finite element analysis. Parameter study was implemented to grasp the influence of width-to-thickness ratio, shear span, and the beam depth. Depending on the numerical analysis, a monotonic model was proposed which can obtain the monotonic curve precisely and efficiently. Consequently, through combining with the modified Ibarra-Medina-Krawinkler hysteretic model, the accuracy for predicting the hysteretic behavior of beams was verified through comparing with the numerical results.

Microstructure formation and elevated temperature mechanical properties of directionally solidified Ti44Al6Nb1Cr alloy

To improve room and elevated temperature mechanical properties of high-Nb TiAl alloys, Ti44Al6Nb1Cr alloy has been prepared by cold crucible directional solidification (CCDS) technique under loaded power of 40 kW and pulling rate of 8.3 μm/s. Macro/micro-structure and phase composition have been characterized. Tensile properties have been tested at room and elevated temperature and the related mechanisms have been revealed. Results show that equiaxed grains change to columnar grains growing up along the axial direction of ingot after CCDS. Phase component is still composed of α2, γ and B2 phases. Microstructure consists of lamellar clusters and network structure. The lamellar clusters in a grain possess the same orientation. The content and size of B2 phases decrease and some casting defects with small size decrease as well due to sufficient diffusion of Nb and Cr atoms under action of electromagnetic force during CCDS. Tensile results indicate that comprehensive mechanical properties of Ti44Al6Nb1Cr alloy are improved significantly after CCDS. At room temperature, ultimate tensile strength and total strain are 523 MPa and 3.03%, respectively, which are improved by 18.1% and 64.7% compared with that of as-cast Ti44Al6Nb1Cr alloy (443 MPa and 1.84%). Moreover, fracture morphology also indicates that there are many deep ravines between cleavage steps and some similar dimples microstructure on cleavage steps. They are beneficial to improving comprehensive mechanical properties. At 973 K, ultimate tensile strength of Ti44Al6Nb1Cr alloy does not decrease at all, still reaching 530 MPa after CCDS. When service temperature rises to 1173 K, the strain is less than 4.28% showing excellent mechanical stability under tensile stress less than 436 MPa. The excellent stability for mechanical properties is due to thin lamellar phases, directional microstructure and large plastic deformation region.

Measurement of High Strain Rate Indentation-Induced Deformations in Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;/SiC&lt;sub&gt;p&lt;/sub&gt; Composite Ceramics

Al2O3/SiCp composite ceramics with 2 vol%, 5 vol% and 10 vol% SiC nanoparticles additions were hot pressed at 1650 °C. The polished ceramic discs were prepared and indented at high strain rate using a compressed gas gun with tiny tungsten carbide bullets. The microstructure and mechanical properties were obtained to explain the dynamic deformation behavior of Al2O3/SiCp ceramics. Cr3+ fluorescence mapping was used to examine the residual stress and plastic deformation induced on the surface of each target. The residual compressive stress area around the crater was evenly distributed, while the greatest plastic deformation was found at the hitting point of the bullet tip. It can be calculated that the high temperature of 1400K may be produced at the instant of the bullet impact and result in large plastic deformation region and low residual stress of the Al2O3/SiCp ceramic.

Microstructure Characteristic and Fatigue Damage Behaviors of 2060 Al-Li Alloy Thin Plate

The microstructure characteristic, fatigue damage behaviours and evolution of microstructure during the fatigue testing of 2060 Al-Li alloy thin plate were studied by fatigue tester equipped with in-situ observation device, OM, EBSD, SEM and TEM. The results show that recrystallized grains with irregularity shape and large size are observed, the Goss and Cube texture are the mainly types, abundant of T1 with size of 50~60nm are found in grains. During axial loaded fatigue testing, a crack initiates from the edge of sample and extend into grain along the direction with maximum Schmid factor. When the crack reaches and crosses the first grain boundary, the extending direction deflects from 45° degree to perpendicular with loading direction, and propagates along this direction till the sample quickly fracture. A large size deformation region contains numerous slip strips around the crack tip during the initiation and extending process is observed. The dislocations cut across the T1 precipitates located in grains during deformation, and release the stress concentration on the crack tip. The fatigue crack extends across the grains with serrated shape can result from the big orientation difference of grains and easy screwed T1 precipitates.

Field-Dependent Stiffness of a Soft Structure Fabricated from Magnetic-Responsive Materials: Magnetorheological Elastomer and Fluid

A very flexible structure with a tunable stiffness controlled by an external magnetic stimulus is presented. The proposed structure is fabricated using two magnetic-responsive materials, namely a magnetorheological elastomer (MRE) as a skin layer and a magnetorheological fluid (MRF) as a core to fill the void channels of the skin layer. After briefly describing the field-dependent material characteristics of the MRE and MRF, the fabrication procedures of the structure are provided in detail. The MRE skin layer is produced using a precise mold with rectangular void channels to hold the MRF. Two samples are produced, namely with and without MRF, to evaluate the stiffness change attributed to the MRF. A magnetic field is generated using two permanent magnets attached to a specialized jig in a universal tensile machine. The force-displacement relationship of the two samples are measured as a function of magnetic flux density. Stiffness change is analyzed at two different regions, namely a small and large deformation region. The sample with MRF exhibits much higher stiffness increases in the small deformation region than the sample without MRF. Furthermore, the stiffness of the sample with MRF also increases in the large deformation region, while the stiffness of the sample without MRF remains constant. The inherent and advantageous characteristics of the proposed structure are demonstrated through two conceptual applications, namely a haptic rollable keyboard and a smart braille watch.