Non-uniform Strain Distribution Research Articles

Bond performance and damage assessment of self-sensing steel–fiber composite bar with geopolymer concrete

This study employed a novel steel–fiber composite bar (SFCB) enhanced with Distributed Optical Fiber Sensors (DOFS) technology to reveal the local bonding and damage mechanisms between composite reinforcement and seawater sea sand geopolymer concrete. By embedding DOFS within the steel core of the SFCB reinforcement, the local bond stress between the reinforcement and geopolymer concrete was monitored. The continuous measurement provided by DOFS revealed a non-uniform strain distribution along the length of the SFCB reinforcement and an uneven distribution of bond stress within the bonded section. Based on test results, a modified bond–slip model was utilized to predict the bond–slip relationship between the SFCB reinforcement and the geopolymer concrete, taking into account the influence of surface treatment on the descending branch of the bond–slip curve. Additionally, a damage model was employed to evaluate the extent of damage at the bond interface between the SFCB reinforcement and the geopolymer concrete under different load levels. Finally, the modified bond–slip model was also used to propose the anchorage length for SFCB reinforcements, which was compared with ACI, CSA, and JSCE codes.

Investigation of the influence of non-uniform strain zone on the crack propagation of PMMA material based on 2D-DIC

Hole defects can lead to non-uniform strain distribution under the impact load, thereby influencing crack propagation behavior. In this paper, 2D-DIC technology was employed to examine the effects of holes of varying sizes and loading rates on crack dynamics in PMMA materials, aiming to elucidate detailed knowledge into the characteristics of crack propagation under complex strain fields. Through DIC analyses, the dynamic evolution of strain fields around the crack tip and hole periphery could be precisely captured, enabling tracking of crack propagation behavior including crack propagation velocity, crack propagation path, and crack deflection angle. It is concluded that the non-uniform strain zones generated by holes exert both inhibitory and attracting effects on crack growth. The influence of non-uniform strain zones on crack propagation increases with the elevation of loading rate and hole size. However, as the loading rate increases, the kinetic energy of the crack itself also increases, necessitating sufficiently large hole sizes to effectively influence crack propagation. Overall, this study provides a detailed experimental explanation of the effects of holes on cracks, which will aid engineers in maximizing the positive impact of holes on material performance and their application in the design of microstructure materials.

Linear strain gradient-regulated bifurcation of circular bilayer plates

Bilayer structures with controllable self-folding capability have found applications in a variety of cutting-edge fields such as flexible electrics, wearable devices and soft robotics. The folding of bilayer structures occurs when the mismatch strain between the two layers exceeds the bifurcation threshold, resulting in a deformation transition from an axisymmetric to a folded state. Previous efforts have predominantly focused on bilayer structures with uniform and/or anisotropic strain distributions. However, the role of non-uniform in-plane strain distributions in regulating the bifurcation of bilayer structures has not been fully understood. In this study, the effects of linear in-plane strain gradients on the bifurcation of circular bilayer plates, both with and without geometric mismatch, are systematically investigated by combining theoretical analysis, finite element simulations and experiments. Our results reveal that both the mismatch strain gradient and the geometric mismatch between the two layers play crucial roles in regulating bifurcation. Notably, linear mismatch strain gradients with larger strain at the center delay bifurcation, while those with larger strain along the edge promote bifurcation. This work offers new insights into the design of controllable self-folding bilayer structures, which is of great significance for advanced applications.

Axial compression performance of concrete-filled steel tubular columns with damaged BFRP jackets

Concrete-filled basalt fiber reinforced polymer (BFRP) -steel composite tube (CFBCT) columns have become a highly anticipated composite structure. The lateral confinement provided by BFRP can effectively suppress the possible local buckling of the steel tube, thereby achieving the goal of optimizing axial compression performance. With the widespread application of CFBCT in engineering practice, the BFRP outer layer is likely to be affected by factors such as construction accidents or connection design, resulting in slotting and drilling damage to the BFRP outer layer. Therefore, the mechanical properties of CFBCT subjected to local damage are critical. This article examined the effects of slotting and drilling the BFRP outer layer on the axial compression performance. The non-uniform strain distribution at the damaged site was analyzed using digital image correlation (DIC) testing technology. The results show that the local damage can lead to a decrease in the bearing capacity and ductility. Compared with drilling damage, slotting has a more significant impact on axial behavior. Due to the local damage, the ultimate stress decrease by 8.1 % and 12.4 % respectively. A new computational method was introduced to predict the quantitative impact of local damage on CFBCT.

Integration of 3D printing with acoustic-assisted assembly of nanomaterials for tunable strain sensors

This study reports a novel methodology for the development of tunable strain sensors by leveraging 3D printing for structural designs and nanomaterial assembly for functional layer coating. We utilized material extrusion (MEX) to print sensor substrates using composite ink of polydimethylsiloxane (PDMS) and silica nanoparticles. MEX allows us to create a non-uniform strain distribution of the sensor substrate by designing alternating wide and thin strips with different mechanical properties along the stretching direction. Then, we assembled nanometer-thick graphene flakes on the surface of the substrate using an acoustic-assisted dip-coating method to construct strain sensors. The graphene network on the narrow strips will experience large deformation leading to significantly increased resistance. By fixing the width of wide strips and tailoring the width ratio of the wide strip over narrow strips (r), the gauge factor can be controlled from 8.53 (r = 1:1) to 33.15 (r = 16:1). Also, by printing the narrow strips with softer PDMS, the sensitivity of the sensor can be further increased. The research pioneers the integration of 3D printing and nanomaterial assembly for strain sensors. More importantly, it paves the way for the generic design of flexible electronics and other hybrid systems of polymer and nanomaterials.

Damage evolution of coal with a strong bursting liability and 3D measurement method for elastic deformation energy distribution

When coal with a strong bursting liability is disturbed by a load, catastrophic instability disasters and rockbursts can easily occur. Loaded coal with a strong bursting liability undergoes complex damage evolution and failure behavior due to the intricate interplay among loads, gangue, fractures, and the coal matrix. Intuitively quantifying and characterizing the complex discontinuous structural features, deformation fields, and energy fields within coal is the foundation for the scientific analysis and prevention of disaster mechanisms caused by rockbursts. Therefore, studying the distribution and evolutionary processes of deformation fields and energy fields caused by the evolution of discontinuous structures within coal is highly important for the prevention and control of coal mine rockburst disasters and the protection of coal mine safety during mining operations. In this study, a series of X-ray computed tomography (X-CT) tests were conducted on coal specimens with varying gangue contents under uniaxial compressive loading. The X-CT images clearly illustrated the internal damage evolution of coal with a strong bursting liability, revealing fracture propagation, shear bands, and localized failure under varying loading conditions. Image segmentation was employed to quantitatively analyze the development of damage in coal with a strong bursting liability, revealing the influence patterns of gangue content on the prepeak deformation localization and strength of coal. Additionally, using digital volume correlation, the 3D volumetric displacement and strain of loaded impact-prone coal were determined based on the changes in natural speckle images. The nonuniform distribution of strain was mainly caused by the nonuniform distribution of gangue and pore fractures at the microscopic scale, which explains the localized deformation in coal materials. Importantly, this study provides new full-field strain tensor data and a novel method for measuring the elastic deformation energy of the 3D deformation and cracking processes of loaded coal.

Spatial reconstruction, microstructure-based modeling of compressive deformation behavior, and prediction of mechanical properties in lightweight Al-based entropy alloys

Lightweight Al-based entropy alloys are newly emerged materials with promising potential for structural applications; however, their modeling and strengthening prediction require further exploration. In this study, the spatial distributions of various phases in three Al-based entropy alloys were investigated. A closely interconnected intermetallic compound (IC) network comprising Al2Cu and Al45Cr7 phases was identified. Finite element (FE) modeling was conducted based on the reconstructed three-dimensional microstructure to simulate compressive deformation behavior. The IC network served as the main stress bearer during deformation. Thin sections in the Al2Cu network were the weak sites where stress concentration and damage first occurred. However, the breakage of this limited region contributes to relatively coordinated deformation and extended plasticity. The breakage of large particles accounts for the final alloy failure. The results of the FE model were compared with the experimentally measured stress–strain behavior and mechanical properties, showing very good agreement. Given the non-uniform distribution of strain and stress during deformation, a strengthening model, merging the Voigt and Reuss models, was also developed to predict the mechanical properties of Al-based entropy alloys with the aim of facilitating Al-based entropy alloy development.

A highly sensitive stretchable constraint fiber mat sensor with tailored near-zero Poisson’s ratio for very early stage of battery over charge/overdischarge monitoring

Battery state monitoring is crucial for safe operation and efficient management of lithium-ion battery (LIB) systems, but early stage of timely and accurate diagnosis remains challenging. Herein, we propose to utilize a highly sensitive stretchable strain sensor to in-situ monitor the slight strain variation of cells during charge and discharge, through which the state of the battery can be accurately reflected. The high sensing capability is realized by tailoring the Poisson’s ratio of low-modulus conductive fiber mats through a facile constraint manner. The non-uniform strain distribution together with the near-zero Poisson’s ratio synergistically result in the superior sensing performance, including ultralow strain detection limit (0.006%), high strain sensitivity particularly under small strains (gauge factor of 116.1 between 0 and 2% strain), and high strain resolution under large deformations. When being practically utilized for LIB monitoring, the constraint fiber mat sensor can produce instant resistance response reflecting the cell capacity, and offer timely warning signal once the cell is overcharged or overdischarged. The developed stretchable strain sensor possesses comprehensive advantages of low mechanical constraint, tailorable small size, easy integration and conformality to cells, and little temperature interference, which would serve as a promising candidate for future battery state analysis and safety monitoring.

Liquid Metal-Based Sensor With Nonuniform Strain Distribution for Wearable Applications

Liquid Metal-Based Sensor With Nonuniform Strain Distribution for Wearable Applications

Accurate monitoring of coal pillar deformation based on distributed optical fiber

In many structural deformation monitoring applications, determination of the response length of embedded optical fibers is difficult due to the use of strain integration method, which increases the measurement errors. For systematic investigation of the deformation law of coal pillars under mining action, distributed fiber optic technology was used herein to analyze its response length. This study discusses how non-uniform strain distribution can increase the accuracy error of fiber optic displacement measurement, and verifies the effectiveness of theoretical analysis by using coupled FEM–DEM numerical simulation method. Furthermore, the relationship between the Brillouin gain spectrum characteristics of different sampling points and the non-uniformity of strain distribution was explained. To reduce accuracy errors, a new method was proposed herein for determining the response length of optical fibers. The feasibility of this method was verified through indoor experiments. Next, this method was applied to the deformation monitoring of coal pillars in a section of Huojitu Well. By using the proposed method, the deformation of coal pillars can be accurately monitored.

Visualization of the Magnetic Domain Structure in FeBO3 and Fe0.94Ga0.06BO3 Single Crystals by Synchrotron X-Ray Topography

The method and experimental cell for the X-ray diffraction diagnostic of crystalline materials exposed to magnetic fields have been described. It has been shown that the application of even weak magnetic fields (about several Oersteds) allows one to significantly reduce the diffraction contrast on X-ray topograms of FeBO3 and Fe0.94Ga0.06BO3 single crystals, which is due to a nonuniform distribution of magnetostriction strains, and thereby to improve the degree of their structural perfection. The magnetic domain structure of Fe1 –xGaxBO3 solid solution single crystals has been visualized. It has been found that small concentrations of diamagnetic Ga ions do not significantly change the configuration of domain walls in FeBO3-based crystals.

Effect of thermomechanical processing on microstructure evolution and mechanical properties of metastable β Ti–5Al–5V–5Mo–3Cr alloy

Two distinct thermomechanical processing (TMP) schedules are applied to as-cast Ti-5553 alloy pancakes, to optimize its microstructure and mechanical performance. While TMP-I involves 40 % thickness reduction via forging, TMP-II employs rolling with 20 % thickness reduction. Typical bimodal microstructures with nearly similar sizes, distributions, and morphologies for the primary αp grains and colonies of secondary αs and β are noted to generate for both TMP-I and TMP-II. Certainly, rolling for only half the extent of that used for forging, appears ideal and economical for obtaining the targeted microstructure. Interestingly, similar and improved localized hardness values with respect to as-cast alloy, are obtained for bimodal microstructure, generated through both the TMP schedules. However, indentation size effect is noted to be nominally lower for the rolled alloy, thereby proving its suitability for large-scale application. Global behavior assessed through tensile tests also shows that bimodal microstructures obtained through forging or rolling leads to higher strengths, toughness and ductility, as compared to as-cast counterparts. This is primarily owing to non-uniform strain distribution in the as-cast alloy, resulting to crack initiation and failure, as revealed through digital image correlation-based study. Overall, TMP employing rolling with reduced deformation in particular appears beneficial for the mechanical property improvement of the alloy.

Meso-scale deformation and strength mobilization mechanisms of EPS composite soil

Expanded polystyrene (EPS) composite soil is composed of two solid constituents (cemented soil and EPS beads), which together with the inter-constituent interfaces form a unique mesoscopic structure. The macroscopic behavior of EPS composite soil has been widely investigated so far, but the focus has seldom been put on the meso-scale mechanisms behind the macroscopic deformation features and strength mobilization processes. In this study, systematic experiments and refined numerical simulations are utilized to examine the meso-scale mechanical responses of EPS composite soil under triaxial compression to reveal the associated mechanisms. Here, meso-scale is a scale smaller than a sample scale but larger than an EPS bead scale. First, the mechanical behavior of cemented soil, EPS material and their interface is obtained in experiments; their constitutive models are formulated, and the model parameters are calibrated based on experimental results. Then, experiments and refined numerical simulations of triaxial compression on EPS composite soil are performed. It is found that the rich macroscopic mechanical behavior of EPS composite soil has a mechanical source (contrast in mechanical behavior between EPS and cemented soil) and a geometric source (non-uniform distribution of EPS beads). The mechanical source leads to highly non-uniform stress and strain distributions within an EPS composite soil. The geometric source causes strain localization in zones with local concentration of EPS beads, leading to three macroscopic deformation modes, i.e., overall shear localization, local lateral expansion, and overall uniform expansion. In strength mobilization, cemented soil matrix first mobilizes its strength because it takes most of the load and contributes much to deformation at the early stage of triaxial compression. EPS beads are later progressively involved in the overall load bearing and deformation of the EPS composite soil sample. EPS beads make more contribution to stress than to strain due to its soft nature.

Equal-strength beam design of acoustic wave accelerometers

Abstract Surface acoustic wave (SAW) based accelerometers have received significant attention due to their digital output, low cost, mass production and easy implementation of wireless passive function. However, conventionally rectangular cantilever-beam based SAW accelerometers often have non-uniform strains generated along the beams, which cause emergence of parasitic wave modes and measurement errors. In this paper, a simulation platform was developed to analyze and optimize designs of SAW accelerometers and variable-thickness and equal-strength beams were designed to solve the critical issue of non-uniform strain distribution along the beam. Frequency responses of SAW accelerometers under the acceleration were successfully obtained using the simulation platform, with the visualized strain/stress distribution and particle displacement field. The accuracy of this simulation platform was verified using the experimental result reported in literature. A highly sensitive and equal-strength beam SAW accelerometer was achieved with a sensitivity up to 1.40 kHz g−1, a linearity coefficient of ∼1, and a measurement range of 0∼15 g. Furthermore, a high-G accelerometer was designed, with the capability of enduring large shocks up to 11,500 g and a sensitivity of 6.96 Hz g−1.

Monitoring corrosion-induced concrete cracking adjacent to the steel-concrete interface

Substantial research effort has been devoted on linking corrosion-induced cracking of concrete with the internal corrosion damage level. Still, numerical models of the corrosion and cracking process require internal parameters, that cannot be directly evaluated from experimental data. Therefore, this study provides a novel experimental method for monitoring the effects of steel corrosion adjacent to the steel-concrete interface. This non-destructive method is suited for small-scale laboratory-made specimen, and was designed to provide missing information required for subsequent calibration of numerical models. Hollow steel bars were cast into concrete and subjected to accelerated corrosion using the impressed current technique. The deformations of the hollow steel bars were measured using distributed strain sensing in an optical fibre, attached to the inner surface of the hollow steel bars. After the corrosion period, X-ray Computed Tomography scans were performed to evaluate concrete cracking and corrosion level. The results reveal a non-uniform distribution of strain around the perimeter of the steel, indicating a non-uniform radial stress distribution. The non-uniformity correlated very well with the position of the corrosion-induced cracks; with extension hoop strains in the steel at the location of these cracks and contraction hoop strains in between. Further, the corrosion level varied around the perimeter, with higher values near cracks. The combination of non-destructive monitoring techniques used in this study on small-scale laboratory-made specimens show great potential to reveal new insights on how the corrosion pattern, corrosion-induced cracking of the concrete cover and stress (indirectly measured through the strain in the steel) interact throughout the corrosion process.

Experimental investigation of rotational behavior of pseudo-ductile adhesive angle joints exhibiting variable strain rate

Pseudo-ductile behavior in composite structural frames can be achieved by enabling their beam-column joints to exhibit nonlinear structural response due to progressive damage, by using pseudo-ductile adhesives. This study investigates the static behavior of a novel pseudo-ductile bonded-bolted angle joint with a bolt at the geometric centroid of the joint. A pseudo-ductile elastomer adhesive is used which is notably sensitive to the strain rate. The bolt ensures structural integrity and produces pure torsional moments in the adhesive planes. This configuration induces nonuniform strain rates in the adhesive layers in the radial direction, i.e., from the center outwards. The experimental results reveal that under the nonuniform shear strain distribution, the initial stiffness, yield rotation, and maximum torsion exhibit an increasing trend with strain rate, while maximum rotation decreases. An analytical model was developed using stress-strain relations obtained from single-lap joints with uniform stress distribution to predict the angle joint responses. The analytical model overestimates the initial joint stiffness due to the nonuniform stress distribution but shows good agreement for strength and post-yield behavior. Lastly, the angle joint exhibits the lowest ductility ratios compared to linear lap joints, showing that the pseudo-ductile adhesive capacity may not be fully utilized in the angle configuration.

Matrix‐Driving Stretchable Structural Colors in Multi‐Pixels Operation Using Chiral Liquid Crystal Elastomers

AbstractStretchable structural color technologies are garnering significant attention in photonics for various applications owing to its stretchable flexibility and ability to control the photonic wavelength of light. However, stretchable photonic media, such as chiral liquid crystal elastomers (CLCEs), are subjected to uncontrollable nonuniform strain distribution and position‐dependent color change during the stretching‐assisted color tuning process. Therefore, controlling the colors of the mechanochromic structures is limited to just single‐unit operation level and practically desirable matrix driving mechanochromic devices for multi‐pixel applications have not been realized yet. The study reports position‐independent and multi‐pixel‐driven uniform stretchable structural color switching realized through row and column matrix switching operations by electrically stretchable CLCEs. The uniform strain‐induced color control mechanism and operations of a multi‐arrayed matrix‐driven stretchable color structure are theoretically and experimentally investigated. This breakthrough of a uniform and multi‐pixel color switching of a seven‐segment matrix driving stretchable CLCEs will be a technological leap into full‐scale use of mechanochromic structural colors in practical applications.

Compressive behavior of brick masonry columns confined with composites embedded in the horizontal mortar joint

The application of composite materials embedded in the horizontal mortar joints to strengthen masonry structures with insufficient load-bearing capacity has shown beneficial effects in terms of improving mechanical behavior and preserving the aesthetics of the building. To investigate the effectiveness of this strengthening technique, concentric axial compressive tests were performed on eight masonry columns, including two unconfined columns, five specimens confined by carbon fiber reinforced polymer (CFRP) plates, and one specimen confined by engineered cementitious composite (ECC). The failure mode, strength, deformability, and energy dissipation capacity of the specimens were investigated. The effective tensile strain of the CFRP plates was also discussed. The results showed that the CFRP-confined masonry columns exhibited brittle failure, which was characterized by the debonding failure of the CFRP caused by the sudden fracture of the epoxy resin, while the failure of the ECC-confined masonry column exhibited significant ductility, the reason for which was that the propagation of vertical cracks was delayed due to the strain-hardening characteristics of the ECC. The reinforcement technique was effective for masonry columns in obtaining better compressive performance. The peak strength and energy dissipation of the confined columns were increased by a maximum of 86.31% and 115.71%, respectively. A non-uniform distribution of the effective tensile strain of the CFRP plates along the perimeter was observed. In addition, a simple strength model for masonry columns confined by embedded material in the horizontal mortar joints was developed based on the best-fit analysis of a database assembled from the existing literature and the current study. The predictive capabilities of the proposed model and an existing model for the compressive strength of confined masonry columns were analyzed.

Modelling of ultrasonic vibration-assisted forming considering the distribution of ultrasonic field with structure deformation

Ultrasonic vibration-assisted (UVA) forming is typically used to promote the forming quality of complex structures due to the acoustic-plastic effect. Ultrasonic vibration (UV) is always applied via the contact of tools with the structure surface; thus a distribution of ultrasonic field exists owing to the propagation and attenuation of ultrasonic waves, which may vary with structural deformation. This makes the prediction and control of ultrasonic effect extremely difficult in achieving high forming quality. To develop a new coupling model considering the interaction between ultrasonic field and deformation field, UVA tensile experiments of superalloy sheet were performed first. The experimental results demonstrated that UV had a local effect on the tensile specimen, resulting in non-uniform distribution of temperature, strain and dislocation density. During the modelling, the intensity of ultrasonic field was quantified by acoustic energy density, which can be calculated using vibration velocity. A dislocation-based constitutive relation was also established, in which ultrasonic-related functions were introduced to reflect the effect of ultrasonic field on structural deformation. Then, a sequential coupling modelling method was proposed. Through the cyclic calculation of ultrasonic field and deformation field, the simulation of UVA forming considering the distribution of ultrasonic field with structure deformation was realised. With the coupling model, the non-uniform distribution of deformation accompanied by a non-uniform ultrasonic field during UVA forming was revealed. When applied to a complex W-bending process, the targeted softening effect on the W-bending specimen was precisely predicted. The coupling model is expected to lay a foundation for the optimisation and control of the UVA forming process.

Weld Effective Lengths for Axially Loaded Transverse Plate-to-CHS Connections

Large-scale experimental tests were conducted to evaluate the strength of partial joint penetration (PJP) groove welds in axially loaded transverse plate-to-circular hollow section (CHS) X-connections. Six connections, comprising 12 joints, were designed to investigate the influence of branch-to-chord width ratio and chord slenderness on weld strength. Nonuniform strain distributions were observed in the branches adjacent to the joints, indicating that the welds were only partially effective. Results from the experimental tests were used to evaluate design guidance for PJP welds in the American steel design standard, AISC 360-16, and a weld effective length equation is proposed for axially loaded plate-to-CHS connections.