Local Deformation Research Articles

Energy absorption characteristics of stepped multi-cell columns subjected to transverse loading

Automotive bumper beam is a vital component that shields passenger and vehicle from harm and damage generated by catastrophic collapse. Previous investigations on its bending behavior have largely concentrated on multi-cell tubes with the same length, whereas stepped multi-cell structures have received less attention. In this paper, a novel stepped multi-cell configuration is proposed to improve the energy absorption characteristics of thin-walled structures under transverse loading. The finite element method is employed to analyze the crushing behaviors of the stepped multi-cell tubes. The numerical results reveal that the stepped multi-cell structures (SM2 to SM5) can reduce the initial peak force by 23.44–45.91% while increasing the energy absorption capacity, crush load efficiency, and specific energy absorption by 5.87–29.51, 38.29–139.45, and 5.87–29.51%, respectively, when compared to a conventional square tube (M1). In addition, the effects of wall thickness, section width, load angle, punch radius, and punch shape on the bending behaviors and energy absorption characteristics are examined. The results indicate that these factors have a considerable influence on the deformation features of M1 and SM2, which leads to a significant reduction in their bending energy absorption characteristics. These variables have no influence on the deformation modes of SM3, SM4, and SM5, and they present local indentation deformation with a high energy absorption efficiency. Increasing the number of layers improves the comprehensive performance of stepped multi-cell tubes, with SM5 exhibiting the best energy absorption characteristics under transverse loading.

Corrugation by Local Plastic Deformation

Corrugation by Local Plastic Deformation

Additively manufactured copper alloy with heterogeneous nanoprecipitates-dislocation architecture for superior strength-ductility-conductivity synergy

Employing a single strategy that overcomes the strength-ductility-conductivity trade-off in copper alloys has proven to be challenging. In this study, we introduced a novel heterogeneous nanoprecipitate–dislocation (HND) architecture in CuCrNb alloy consisting of a multi-modal core–shell grain structure and an interconnected dislocation network pinned by abundant nanoprecipitates. Our CuCrNb-HND alloy exhibited superior strength–ductility synergy at both room and elevated temperatures. In particular, aging treatment-induced high-density coherent Cr secondary nanoprecipitates into the HND skeleton endowed the CuCrNb-HND450 alloy with a high tensile strength of over 1 GPa and a conductivity of ∼50%, surpassing those of most of the reported additively manufactured copper alloys. An in situ transmission electron microscopy heating experiment revealed the superior thermal stability of the HND architecture. Hierarchical strengthening contributed to the enhancement of mechanical properties. At the micrometer scale, the harmonic grain structure with a strong fine-grained shell and a ductile coarse-grained core effectively improved mechanical properties by suppressing localized plastic deformation. At the nanometer scale, the synergistic effect of the nanoprecipitate–dislocation network further improved the alloy strength by slowing down dislocation movement. Overall, our proposed HND architecture provides an efficient pathway for developing high-strength and high-conductivity copper alloys.

Hydrogen enhanced localised plasticity of single grain α titanium verified by in-situ hydrogen microcantilever bending

The effect of hydrogen on the micromechanical characteristics of pure grade 2 titanium was investigated by comparing prenotched microcantilever bending in air versus in-situ electrochemical hydrogen charging. The nanoscale interaction between hydrogen and the metal's defects was analysed in three different grain orientations, i.e. the top surface parallel to {101‾2}, {0001}, and {202‾1}. The subsequent high resolution post-mortem characterisation enabled to characterise the interplay between hydrogen in solid solution and the dislocations in the local deformation process. The results showed that hydrogen facilitated a localised plasticity mechanism, causing hydrogen assisted degradation at the applied low hydrogen concentration below the hydride formation threshold. Meanwhile, hydrogen provoked a reduction in defect formation energy, following the defactant concept, which was responsible for a softening effect on the yield stress and flow stress. Furthermore, due to the anisotropic characteristics of the α titanium hexagonal close packed crystal lattice, the susceptibility to hydrogen effects also depended on the grain orientation. This single grain in-situ micromechanical testing is complementing the existing macroscaled identification of hydrogen effects in α titanium.

A Thermodynamically Based Modified Cam‐Clay Model for Post‐Bifurcation Behavior of Deformation Bands

AbstractCompaction bands are a type of localized deformation that can occur as diffuse or discrete bands in porous rocks. While modeling of shear bands can replicate discrete and diffusive bands, numerical models of compaction have so far only been able to describe the formation of discrete compaction bands. In this study, we present a new thermodynamic approach to model compaction bands that is able to capture both discrete and diffuse compaction band growth. The approach is based on a reaction‐diffusion formalism that includes an additional entropy flux. This entropic velocity regularizes the solution, by introducing a characteristic diffusion length scale and controlling the mode change from discrete to diffusive post‐localisation growth. The approach is used to model compaction band growth in highly porous carbonates. The model can replicate the areas of material damage exhibiting reduced porosity which are often observed as nuclei for the growth of compaction bands in experiments. The model also has the versatility to predict the formation of diffuse compaction bands, which is a significant advance in the field of compaction band modeling. The method can potentially be used for investigating the effect of material heterogeneities on compaction band growth and is heuristic for developing new methodologies for forecasting compaction band formation.

Achieving synergistic improvement of wear and mechanical properties in Ti–6Al–4V alloy by multiple cryogenic treatment

Ti–6Al–4V alloy was subjected to cryogenic treatment at −196 °C followed by single and multiple treatment cycles. Compression tests revealed that, despite a decrease in yield strength, cryogenically treated samples (especially by multiple cryogenic treatment) exhibited noteworthy improvements, with a 10.2% failure strain and 9.4% rise in compressive strength compared to the as-received state. The cryogenically treated samples were further refined and exhibited a reduced β-phase volume fraction, showed higher dislocation density and microstrain in X-ray diffraction results. Remarkably, the surface hardness was significantly improved by the cryogenic treatment, leading to enhanced wear resistance with lower coefficients of friction. This was attributed to the formation of smaller wear debris and a decrease in abrasive wear under sliding conditions. Electron backscatter diffraction (EBSD) findings demonstrate that the friction-induced localized deformation layers exhibit the high density of low angle grain boundaries (LAGBs) with deformation-induced sub-grains. The synergistic improvement of wear resistance and mechanical properties is ascribed to the intricate microstructural alterations induced during cryogenic treatment, particularly accentuated in multiple cryogenic treatment. These findings highlight the efficacy of cryogenic treatment in improving both mechanical and wear properties of Ti–6Al–4V alloy.

Flame visualization and spectral analysis of combustion instability in a premixed methane/air-fueled micro-combustor

The present study experimentally explores the combustion instability characteristics of premixed methane/air flames in a planar micro-combustor by utilizing the high-speed camera and the noise data acquisition system. It aims to identify the sound pressure fluctuation of unstable flames and shed light on the relationship between flame structure and thermoacoustic instability under micro-scale conditions. Four types of flame structures, i.e., flame with repetitive extinction and ignition (FREI), weak flame, steady flame, and oscillating flame, are experimentally observed by varying operating parameters. Among them, the weak flame at low flow velocities and the oscillating flame at high flow velocities form thermoacoustic oscillation. For weak flames, the collision with the wall and the local deformation of the flame result in high-frequency harmonics. Varying the inlet velocity from 0.14 m/s to 0.15 m/s leads to the pressure fluctuation of the weak flame being reduced by 10 Pa, and the dominant frequency being changed from 125 Hz to 167 Hz. As for oscillating flame, the variation of the inlet velocity plays a decisive role in the sound pressure fluctuation. At the equivalence ratio of 0.7, an increase of 0.01 m/s in the inlet velocity reduces the pressure fluctuation by 4 Pa.

Microplasticity induced damping of aluminum foam

Aluminum foam's high dampening properties provide substantial potential for reducing mechanical vibration and noise in lightweight applications. However, inadequate understanding of the damping mechanism cannot provide sufficient theoretical guidance to improve the damping performances. In this study, the DMA (Dynamic Mechanical Analysis) tests results were compared with the numerical simulations to examine the correlation between local plastic deformation and the aluminum foam's damping performances. The 3D geometric modeling of aluminum foam was imported into ABAQUS/Explicit. The loss factors derived from the plastic energy dissipation of the numerical simulations are well consistent with the DMA tests. The simulation showed that the cell walls produces localized plastic deformation at the strains well below the elastic limit of engineering point view. Meanwhile, the plateau borders remain elastic, and thereby maintain the overall elasticity of aluminum foams. Aluminum foam's damping properties are tailorable by altering the pore size and local solid materials thickness. Under vibration loading, the aluminum foam sandwich offers higher damping performances due to greater local plastic deformation within the overall elastic range of the components.

Local mechanical properties of dissimilar metal TIG welded joints of CoCrFeMnNi high entropy alloy and AISI 304 austenitic steel

Multiple principal element alloys encompass the well-known high entropy alloys (HEA). The alloy system represents a new class of materials consisting of at least three alloying elements, each containing 5 to 35 at.%. Thus, this alloying concept differs fundamentally from conventional materials such as steel or nickel alloys. For this purpose, the alloying elements are specifically selected, the microstructures are adjusted in a single-phase and, in some cases, multi-phase manner. In particular, conflicting goals, such as the trade-off between strength and ductility in conventional steels, are overcome. However, in the last 20 years, the focus has been on material synthesis. With the increase in available material quantities, the focus is now on processing issues such as joining and welding processes. The weldability of HEAs has received very little attention so far. The experience with dissimilar metal welds is completely lacking but is essential for the application of these materials in combination with conventional materials. The present study presents comprehensive experimental results on the weldability of an equimolar CoCrFeMnNi-HEA in cold-rolled and heat-treated condition, which was joined by tungsten inert gas welding to an austenitic steel AISI 304. The mechanical properties of the dissimilar metal welds were characterized by cross-weld tensile samples, whereas the local deformation in the weld of the different welding zones was measured by digital image correlation. In accordance with the respective initial HEA condition (cold-rolled vs. heat-treated), the local strain behavior was divergent and influenced the global mechanical properties of both DMW types. Nonetheless, the experiments provided proof in principle of the weldability for dissimilar joints of the CoCrFeMnNi-HEA welded to conventional materials like austenitic stainless steels ensuring a corresponding capability for mechanical loading. This allows further considerations on the application of these innovative materials.

Hydrodynamic analysis of a novel box-type floating marine airport

Very Large Floating Structure (VLFS) plays a crucial role in mitigating land resource scarcity in developed coastal areas. This paper introduces an innovative VLFS concept consisting of numerous concrete box-type floats interconnected by steel tie rods, designed for a 4000-m-long floating marine airport. Based on three-dimensional potential flow theory, two numerical models for the VLFS are established: the single-float model and the multi-float model interconnected by joints, where each float treated as rigid. The research explores the impact of wave direction, wave period, and real sea conditions on the hydrodynamic response of the VLFS. Additionally, this paper focus on the discrepancies in dynamic characteristics between single-float and muti-float, and investigate the special response principles of the muti-float model that closely resemble the actual hydrodynamic features of VLFS. The findings show that local deformation in the VLFS results in elevated wave loads on the structure, posing a significant challenge in designing the VLFS mooring system. Higher structural stiffness correlates with decreased mean and pulsation values of mooring cable tension. Incident waves at 0° induce local deformation on the wave-facing side of the VLFS; however, the deformation rapidly dissipates, exerting minimal impact on the floats supporting the airport runway—thus, creating an indirect wave-absorbing mechanism. When waves incident at 90°, the VLFS bends, adversely impacting the operations of marine airports and causing uneven stresses on the connectors, making them susceptible to fracture and damage. Hence, it is advisable to install marine airports parallel to prevailing wave directions. Furthermore, the hydroelastic behaviour of VLFS contribute to alleviating internal stresses within the structure. The focus of future research lies in balancing the impact of VLFS hydroelasticity on both system motion performance and structural internal stresses. The VLFS concept presented in this study performs effectively under various real sea conditions. The rationality and feasibility of the proposed VLFS concept are substantiated. The study lays the foundation and presents ideas for the future development of marine space.

Feasibility Study of Material Deformation and Similarity of Spatial Characteristics of Standard Coal Rocks

The comparison between similar materials and original coal rock is the basis for similar simulation experiments in coal mines. The differences in mechanical properties, acoustic characteristics, and damage laws between similar materials and the original coal rock are of great significance for similar simulation research, to reveal objective laws. First, materials similar to coal rock with similar theoretical ratios were taken as the object of research, and the sand–cement ratio, the carbon paste ratio, and the water content were determined by multivariate linear regression to accurately match the ratios. Second, by using acoustic emission and digital scattering technology to explore the acoustic law, deformation characteristics, and spatial feature similarities of the materials similar to coal rock, the acoustic emission evolution law of the original rock was found to be the same as that of the similar materials. Digital scattering was able to describe the localization of strain in the similar materials, and the correlation between the overall deformation and the local deformation was explored. This indicates that materials similar to coal rock can effectively simulate the deformation of actual coal rocks. Lastly, these materials were found to allow effectively simulating the deformation characteristics and spatial properties of actual coal rock, which provides an important experimental means and method for similar research in the field of coal rock engineering.

Impact of local plastic deformation on thermo-magnetic instability of high-J c Nb3Sn strand

The future fusion reactor devices built by Institute of Plasma Physics, Chinese Academy of Science will be a compact burning plasma facility to fill the gap between ITER and the China Fusion Engineering Test Reactor, with the mission of studying the deuterium–tritium plasma in steady-state operation. The winding-package (WP) of its toroidal field coil is graded into high-field WP and low-field WP, and the high critical current density (J c) Nb3Sn strand will be applied to the high-field WP based on the current design. The thermo-magnetic instability is the main issue in the application of high-J c Nb3Sn strand, which may induce premature quenching in the low field regions. This issue can be improved by reducing the effective filament diameter (d eff) to suppress flux jumps and increasing the residual resistivity ratio (RRR) to enhance the release of heat generated by flux jumps. However, in the conductor manufacturing process, local plastic deformations (indentation) of strands can impact the sub-element layout and degrade the RRR of the strand, which would increase again the risk on instability. In this study, magnetization measurements and V–I tests were performed on samples with different indentation depths. The hysteresis loss and d eff of the indented sample was obtained from the test results. The impact of local plastic deformation on the thermo-magnetic instability of two types of Nb3Sn strand was confirmed by cross-sectional metallographic observation and quantitative microstructure analysis. It was shown that flux jumps were suppressed at indentation depths below 0.3 mm. Further indentation increase leads to severe flux jumps.

Local Instability and Deformation Control of Steel Roof of Factory Building in a Smart City

Abstract The traditional control methods of instability and deformation of steel structure neglect the analysis of its natural vibration characteristics, which lead to the poor effect of hysteretic vibration of steel structure nodes, and the stability optimization of steel structure is not ideal. Therefore, a new method to control the local instability and deformation of the steel roof is proposed. According to the steel structure composition of the upper door rigid frame and the connection mode of the frame beam and column, the roof steel structure characteristics of the workshop are analyzed. Multi-order natural frequency method is used to obtain the cumulative modal mass contribution rate of steel structure and analyze the natural vibration characteristics of steel structure. Based on the characteristics of natural vibration and element transfer matrix, the local instability control method of roof steel structure is designed. Simulation of strong vibration, using space analysis of tall-building with wall-elements (SATWE) software analysis of strong vibration under elastic load state, to obtain the external deformation of steel structure, under simulation of vibration deformation control model. Taking a chemical plant as an example, a finite element model was established to simulate the vibration wave, and the numerical simulation results of plastic deformation resistance of steel structure were analyzed. The experimental results show that the proposed method can effectively enhance the stability of steel structure and reduce its deformation degree.

Analysis of Fine Fault Electrothermal Characteristics of Converter Transformer Reduced-Scale Model

Converter transformer is the key equipment in UHVDC transmission. If a local overheating fault occurs, it will inevitably form a local hot spot on the core, winding or other structural parts. Among these faults, multipoint grounding and interlaminar short circuit faults account for 30–50% of core accidents. The continuous overheating of 150–250 °C will cause ablation on the silicon steel sheet, which will destroy the insulation material and reduce the insulation performance. In severe cases, it will cause thermal expansion, resulting in local deformation or displacement of the core. Considering the scale of size and temperature parameters, the scale model of converter transformer is established based on the principle of constant magnetic flux density. By using the homogenization theory, the scaled model under multipoint grounding and interlaminar short circuit fault is simulated by electromagnetic heat. First, the single-phase four-column model of the core is established according to the scaled principle, and the core is refined. Secondly, taking the refined model as the research object, the magnetic thermal coupling simulation and analysis are carried out under multi-point grounding and interlaminar short circuit fault, and the magnetic density, eddy current loss and temperature distribution on each lamination are obtained. Finally, the correctness of the simulation is verified by the one-dimensional eddy current loss analytical equation, which provides a reference for the local overheating problem of converter transformers.

The effect of composite materials interfacial discontinuities on the impact safety of future composite rail vehicles

AbstractIn contrast to single‐phase materials the manufacturing process of composites creates multiscale interfaces among constituent materials and between laminar multilayers. Targeting the interfaces in these composite structures, this paper presents a perspective study on the impact safety of potential future composite rail vehicles. The aim is to conceptually explore the key role of interfacial discontinuities of composite material structures as they are a critical issue affecting the impact performance of future composite rail vehicles. Following a theoretical description, the issues are addressed in two parts. First, composite materials are characteristically analyzed from the perspective of their interfacial discontinuities within the materials and between the laminate multilayers to identify their influence. Second, the structural conditions required for crashworthiness are determined in relation to the dual requirements of global stability and local deformability for efficient energy absorption. The key findings are: (1) The interfacial discontinuities of the material phases and the designed structural assemblies need to be tailored for crashworthiness performance and (2) Global stability and locally deformability are the key dual requirements for the energy absorbing progressive deformations that are essential for application of composite for crashworthiness of rail vehicles. The research conceptually explores a key issue of the impact mechanics of composite structures from the perspective of impact safety of composite rail vehicles.

First-principles study on electronic, mechanical, and optical properties of pressure-induced vanadium-based perovskite KVO3

Vanadium-based compounds exhibit a variety of excellent characteristics due to their variable oxidation states. In this study, Density Functional Theory (DFT) is performed to investigate the structural, electronic, mechanical, and optical properties of KVO3 under high pressure. The thermodynamical and mechanical stabilities under high pressure are probed by the negative value of formation enthalpy and Born-mechanical stability criteria, respectively. KVO3 displays a nearly metallic nature in both pressurized and unpressurized systems owing to its remarkably narrow band gap. Furthermore, a positive pressure coefficient of the band gap is revealed from electronic properties. A lower effective mass value for charge carriers indicates KVO3's suitability for optoelectronic applications. The mechanical properties indicate that increasing pressure results in a slight increase in elastic constants, elastic moduli, hardness, and machinability index. Additionally, it has been disclosed that it is capable of enduring localized plastic deformation and fracturing under intense pressure in a prospective application. The calculations of the complex dielectric function, absorption, reflectivity, conductivity, refractive index, and loss function for KVO3 with and without pressure reveal a substantial behavior change. The absorption edge exhibits a significant blue shift with the applied pressure. The high peaks in the UV region suggest that it can be used in UV detectors and anti-reflection coatings. The refractive index shows promising application in waveguides.

Incompatible Deformations in Hyperelastic Plates

The design of thin-walled structures is commonly based on the solutions of linear boundary-value problems, formulated within well-developed theories for elastic plates and shells. However, in modern appliances, especially in MEMS design, it is necessary to take into account non-linear mechanical effects that become decisive for flexible elements. Among the substantial non-linear effects that significantly change the deformation properties of thin plates are the effects of residual stresses caused by the incompatibility of deformations, which inevitably arise during the manufacture of ultrathin elements. The development of new methods of mathematical modeling of residual stresses and incompatible finite deformations in plates is the subject of this paper. To this end, the local unloading hypothesis is used. This makes it possible to define smooth fields of local deformations (inverse implant field) for the mathematical formalization of incompatibility. The main outcomes are field equations, natural boundary conditions and conservation laws, derived from the least action principle and variational symmetries taking account of the implant field. The derivations are carried out in the framework of elasticity theory for simple materials and, in addition, within Cosserat’s theory of a two-dimensional continuum. As illustrative examples, the distributions of incompatible deformations in a circular plate are considered.

Buckling mediated by mobile localized elastic excitations.

Experiments reveal that structural transitions in thin sheets are mediated by the passage of transient and stable mobile localized elastic excitations. These "crumples" or "d-cones" nucleate, propagate, interact, annihilate, and escape. Much of the dynamics occurs on millisecond time scales. Nucleation sites correspond to regions where generators of the ideal unstretched surface converge. Additional stable intermediate states illustrate two forms of quasistatic inter-crumple interaction through ridges or valleys. These interactions create pairs from which extended patterns may be constructed in larger specimens. The onset of localized transient deformation with increasing sheet size is correlated with a characteristic stable crumple size, whose measured scaling with thickness is consistent with prior theory and experiment for localized elastic features in thin sheets. We offer a new theoretical justification of this scaling.

Effect of aspect ratio on mechanical anisotropy of lattice structures

Triply periodic minimal surface (TPMS) lattice structures with controllable mechanical properties and porous architecture are promising candidates for lightweight and energy-absorbing applications. In parametric structural design, the research on structural geometric characteristics, such as aspect ratio (R) and surface curvature, has mainly focused on fluid and heat transfer considerations. However, their impact on multi-directional mechanical properties has yet to be thoroughly investigated. This study, combining representative volume elements (RVE) based on periodic boundary conditions and theoretical surface morphological characteristics, investigates the mechanical properties and deformation behavior of lattice structures with different aspect ratios in multiple directions. The results indicate that the aspect ratio highly influences the deformation behavior of the lattice structure in different directions. The local curvature and force state of the lattice structure can be adjusted by aspect ratio to reduce local stresses and deformations in different loading directions. Compared with the simulation results of representative volume elements, the structural stress concentration and failure position can be predicted by the Gaussian curvature distribution. In addition, the elastic modulus of the structures in the [100] and [001] directions can also be adjusted by aspect ratio. Through experimental verification in the [001] direction, the structure with an aspect ratio of 2 can greatly enhance the elastic modulus (121%∼440%), maximum stress (10%∼183%), and energy absorption (33%∼85%). The significance of this work is to improve the understanding of the influence of geometric features on the mechanical properties and deformation behavior of lattice structures, which provides a new design approach for triply periodic minimal surface lattice structures in applications of impact protection and bone scaffold.

Investigation of tensile deformation behavior of pure titanium at 20 K

The cryogenic tensile properties and deformation behaviors of pure titanium at liquid hydrogen temperature (20 K) were investigated. Compared with 298 K, high ultimate tensile strength (UTS) of 978 MPa and the excellent elongation-to-fracture (El) of 48 % were achieved. A large number of {101‾2}<1‾011 > and {112‾2}<112‾3‾ > deformation twins released local stress concentration and lead to high work hardening rate, which endow pure titanium with excellent strength and ductility. In addition, the inhibition of multiple necking of pure titanium at 20 K is due to a large number of deformation twins released the stress concentration and produced a strong dynamic hardening effect, which relieved the local plastic deformation. And the serrated stress-strain curve is caused by intermittent activation of twining and dislocation slip. Finally, the twins are thinner at 20 K are attributed to the more obvious stress concentration. The cryogenic deformation mechanism and fracture behavior are explained from two aspects of dislocation accumulation and thermal physics.