Loading Direction Research Articles

Bidirectionally graded honeycombs under quasi-static loading: Experimental and numerical study

Density-graded cellular solids possess tailorable mechanical properties through variable localized densities, made possible with design freedom offered by additive manufacturing. In this paper, a novel design strategy is proposed to generate bidirectionally graded honeycombs where the gradient direction is both parallel and perpendicular to the loading direction. A gradient function is developed to design three density gradient honeycombs having three different thickness gradients. The graded honeycombs along with their uniform density regular honeycomb counterparts of similar relative density are manufactured using material extrusion process. In-plane compression tests are carried out to perform a comparative study of the honeycombs. Unlike the regular honeycombs, graded honeycombs show layer-by-layer deformation, in addition to cell-wise collapse in lateral direction. Graded honeycombs show better energy absorption characteristics in low and high energy compressions, and high strain regime. Also, graded honeycombs have similar or higher specific energy absorption, densification strain, mean crushing force, and peak crushing force. The compressive responses of the honeycombs are simulated with finite element analysis and the simulation results agree well with the experimental results. The results substantiate the significance of thickness gradient in controlling the density gradation, and effectively tailoring the load-bearing capacity, deformation behavior, and energy absorption characteristics of cellular structures.

Analysis of tensile failure mechanism in intact, foliated, and cracked rocks using distinct element method: Influence of anisotropy

A thorough numerical analysis was conducted to understand the failure mechanisms and behaviour in intact rocks, foliated rocks and rocks with pre-existing cracks using the Brazilian tensile strength tests. The dominant anisotropy in rocks, especially foliated rocks like phyllites, slates and schists, as well as sedimentary rocks like sandstone, limestones and shale, leads to complex failure mechanism. The presence of cracks leads to anisotropy causing different mechanisms of failure and variation in tensile strengths of rock mass. Numerical simulations were conducted based on experimental studies of foliated phyllite and dolomitic limestone from the Tejam Group, Lesser Himalaya. The variations in failure behaviour and strength of foliated rocks were studied for samples at different foliation angle using the particle distinct element model. The simulation results show that with increase in foliation angle to the loading direction (from 0° to 90°), the peak tensile strength of rocks remains almost same for specimens with θ = 0° to 30° and then increases linearly till 90°. The influence of cracks as anisotropy was also studied by changing crack length (from 5 mm to 30 mm) and angle (from 0° to 90°). The development of failure patterns in rocks with pre-existing cracks leads to a better understanding of the failure modes and makes it possible to interpret failure behavior, pattern, and strength parameters. With increase in crack length, the tensile strength of rocks decreases. Overall, this study depicts the influence of anisotropy and microparameters on failure modes and behavior in Brazilian tensile strength tests on foliated and pre-cracked rocks.

The texture evolution near the shear band and corresponding impact on the microvoid nucleation and crack propagation of Ti6242s alloys under high strain rates

This study investigates the texture evolution near the adiabatic shear bands (ASBs) and its impact of on the adiabatic shear failure mechanism, focusing on microvoid nucleation and crack propagation in Ti6242s alloy with an equiaxed α microstructure under high strain rates, using a split Hopkinson pressure bar (SHPB) for the experiments. To achieve this, the initial microstructure was used to enable the texture with the c-axis perpendicular to the loading direction. After compression, the initial texture transformed into two main texture components through different mechanisms. One texture component, aligned parallel to the loading direction, is strongly associated with the {10 1‾ 2} tensile twins, which were widely distributed near the ASBs with their c-axis are 85° to the parent grains in the initial microstructure. The other texture component, with its c-axis perpendicular to the ASB, is formed by the basal slip activation during adiabatic shear deformation. Besides, it should be noted that twinning resulted in the formation of hard grains (with the c-axis of α-grains parallel to the loading direction), increasing the number of hard-soft grain boundaries. A key finding of this paper is the presence of numerous microvoids near the ASB mainly initiated at soft-hard grain boundaries (approximately 65 %), attributed to stress concentration at the boundaries of soft-hard grains, making these boundaries potential nucleation sites for microvoids. Additionally, basal slip bands in the equiaxed α grains provide rapid channels, accelerating the crack propagation rate. This study provides new insights into refining the adiabatic shear failure mechanism under dynamic loading.

Design of improved multi-cell L-shaped CFST columns under compression and bending

In recent years, there has been a notable surge in proposals for various forms of special-shaped concrete-filled steel tubular (CFST) columns. Despite various methods developed by researchers for determining the bearing capacity of multi-cell special-shaped CFST columns under different loading conditions, these methods remain lack unified. In this study, FE models were developed to simulate performances of improved multi-cell L-shaped CFST (ML-CFST) column under compression and bending. Parametric analyses have been performed to evaluate impact of various factors, including steel yield strength fy, compressive strength of concrete fcu, steel thickness t, loading direction θ, web height c, and axial load ratio n, on performances of ML-CFST columns. The findings of study indicate that the unidirectional flexural capacity of ML-CFST member exhibits a nearly linear relationship with the variables of t, fy, and c, with a minimal impact observed from fcu on flexural capacity. The width of extended section b, fcu, fy, and t have a certain effect on flexural capacity at any angle, particularly fy, t, and b. While fcu, fy, and t exert some influence on N/Nu-M/Mu correlation curves, their influences are marginal compared to the loading direction, which is the predominant influencing factor. Additionally, Mx0/Mπ/4-My0/M3π/4 curves gradually protrude outward with the increase in n. The effects of t, fy, and fcu on the shape of Mx0/Mπ/4-My0/M3π/4 curve were relatively insignificant. Simplified unidirectional flexural capacity calculation models were proposed based on stress analysis of cross-section under ultimate states. Given that flexural capacities estimated by simplified calculation model are slightly conservative, it is recommended to increase the flexural capacities by 10 %. Furthermore, an elliptical equation was formulated to predict the flexural capacity at any angle, with the predictions being slightly conservative. Based on ultimate equilibrium theory, a simplified calculation method was presented to predict unidirectional eccentric bearing capacities of ML-CFST columns. Through regression analysis, a simplified calculation method expressed as polar coordinate form for biaxial eccentric bearing capacity was established, with calculated values aligning well with FE results.

Influence of texture on anomalous yielding behavior of thermomechanically processed nickel-based superalloy 720Li

Precipitation-strengthened Ni-base superalloys are extensively used for high-temperature load-bearing applications due to their unique combination of yield stress anomaly (YSA), excellent creep, fatigue, and corrosion resistance. However, in line with the previous reported results, it is shown in the present study that 720Li alloy does not exhibit YSA during uniaxial deformation. The investigation explores the role of anti-phase boundary (APB) energy, Zener ratio and break away stress of Kear-Wilsdrof (KW) lock on YSA behavior. It is shown that in spite of meeting all the energetic requirements necessary for YSA (viz. energy for octahedral to cubic cross slip along with high break away shear stress, i.e. ∼ 80 MPa); the thermo-mechanically processed 720Li alloy does not exhibit YSA. In support of the observed absence of YSA, a plausible mechanism is provided for the first time through this work. This unusual absence of YSA has been attributed to the crystallographic texture of the alloy (specifically, the loading direction (LD) being parallel to [11¯1]), which prevents the formation of KW-locks in the first place. The [11¯1] texturing has been shown to rather lead to direct cubic plane slip i.e. {001} <110>, thereby paving way for suppression of KW-locks (and hence YSA). In order to validate this postulate, the study also examines a vacuum induction melt and cast Ni75(Al11, Ti14), L12 compound. The presence of YSA in the L12 compound under conditions of LD not being parallel to [11¯1], confirms that texturing impacts the occurrence of YSA.

A flexible yield criterion for strength modeling from biaxial compression to biaxial tension

Accurate strength modeling from equi-biaxial tension (EBT) to equi-biaxial compression (EBC) is critical for the plastic behavior prediction covering the wide-range of stress triaxiality encountered in sheet metal forming. To date, however, few yield criteria are available that can precisely model the initial yield and hardening behavior under six typical stress states between EBC and EBT, simultaneously. Furthermore, there is still a lack of a unified yield criterion for accurate strength modeling across various stress state ranges. To address the issues, a theoretical framework for constructing yield criteria dependent on stress states is provided and a new analytically described isotropic yield criterion is presented in this study. The flexibility in terms of the yield locus and application range is thoroughly explored to make the new yield criterion general. Subsequently, the isotropic yield criterion is extended into an analytically described anisotropic-asymmetric yield criterion. Furthermore, the extended yield criterion is applied to capture the initial yield behavior of DP980, AA5754-O, and AZ31 sheets, and the strain hardening behavior of QP1180 sheets at various stress states ranging from EBC to EBT along different loading directions. The predicted results from the extended criterion agree well with the corresponding experimental findings. The applications demonstrate that the proposed anisotropic-asymmetric yield criterion can effectively model the initial yield and hardening behavior of HCP, BCC, and FCC metal sheets under EBT, EBC, uniaxial tension (UT), plane strain tension (PST), shear (SH), and uniaxial compression (UC) in an analytical way.

Predicting the structural behaviour and failure mechanism of hybrid steel-grout connectors for cross-laminated timber panels using high-fidelity finite-elements modelling

In this paper, a symbiotic experimental and numerical approach is used to determine the structural performance parameters and analyse the failure behaviour of a novel high-performance hybrid steel-grout connector for cross-laminated timber (CLT) panels. The connector is composed of a steel rod, which is embedded into a 3-ply CLT panel with the aid of a thick layer of epoxy-based grout. Material tests on wood from the two lumber grades present in the CLT panels, and on the epoxy-based grout were conducted to provide an insight into their mechanical behaviour. Forty-two monotonic push-out shear tests were conducted on connectors, varying the grout layer diameter thrice and the loading direction twice. A finite-elements (F.E.) model for the connector was developed in the F.E. software ANSYS and calibrated based on experimental observations on materials and connectors. The tested connectors proved to be strong and stiff in both loading directions, with a maximum slip modulus of 111 kN/mm, yield resistance of 177 kN, and maximum resistance of 192 kN. The diameter of the grout was found to significantly influence all three performance parameters, and much so when the connector was loaded in the direction perpendicular to the CLT face layers. The developed F.E model was able to capture the slip modulus, yield resistance, and maximum resistance with an accuracy above 95 %. The F.E. model also sheds light on the structural performance, yield mechanism, and failure propagation for connectors.

Development of improved finite element formulations for pile group behavior analysis under cyclic loading

AbstractThe effect of cyclic loading is an essential factor leading to progressive soil strength degradation. Therefore, a comprehensive analysis of the pile‐soil system behavior under cyclic loading is required to ensure the stability of pile group. There is room for improvement in the inherent constraint of the conventional numerical model in terms of approximating the soil resistance distribution along the pile by point loads at element nodes, necessitating a specific element that integrates considerations of pile group effect and cyclic loading within a unified framework. This study aims to develop a newly specific type of element for efficiently predicting nonlinear behavior within the pile‐soil system, addressing simulations involving nonlinear pile‐soil interaction, pile group effect, and cyclic loading. Modified element formulations based on soil stiffness matrices and soil resistance vectors specifically address pile group effect and consider parameters that influence pile behavior under cyclic lateral loading. The numerical solution procedure with Newton‐Raphson iteration allows the calculation of pile responses in geometric and material nonlinear analyses. The validation of the proposed method includes several examples, comparing it with existing numerical solutions and experimental tests of single piles and pile groups under cyclic loading. These comparisons further support the consistency of the proposed method with measured data and validate its accuracy in considering group effect and cyclic loading. The parametric study illustrates the ability of the proposed method to capture cyclic loading parameters while considering the influence of the number and magnitude of load cycles, the cyclic load direction, and the installation methods.

Internal Short Circuit and Dynamic Response of Large-Format Prismatic Lithium-Ion Battery Under Mechanical Abuse

Abstract Prismatic lithium-ion batteries (LIBs) are becoming the most prevalent battery type in electric vehicles, and their mechanical safety is garnering increased attention. Understanding the mechanical response and internal short circuit (ISC) of prismatic LIBs during dynamic impact is important for enhancing the safety and reliability of electric vehicles. Thanks to the pioneer's works on the cylindrical and pouch LIB, prismatic LIB can draw on relevant experimental and numerical modeling methods. However, there is still a lack of research on the dynamic effects of prismatic LIB in various loading directions. To address this disparity, the current research utilizes quasi-static and dynamic impact experiments on prismatic LIBs as a foundation. First, the mechanical response of a sizable prismatic LIB under quasi-static conditions and the dynamic effects are examined when subjected to mechanical abuse from various loading directions. Second, an anisotropic finite element model that considers dynamic strain rates are developed, enabling it to accurately represent the mechanical response to both quasi-static and dynamic impact loads. At last, we performed an analysis of ISC occurring under dynamic loading conditions combining the experimental and simulated results. The experimental results as well as the established model can provide reference for the safe design, application, and analysis of prismatic LIBs.

Fabrication and in-plane compressive collapse of CFRP honeycomb metamaterials

A pin-slot positioning method was proposed to fabricate carbon fiber reinforced polymer (CFRP) metamaterials of hexagonal and zero Poisson’s ratio semi-re-entrant honeycombs with the same mass of monolayer continuous twill-woven carbon fiber/epoxy prepregs. The in-plane compressive collapse of CFRP honeycombs was explored through experiments and finite element (FE) simulations. Furthermore, analytical models were developed to predict the modulus and initial collapse stress of the hexagonal and semi-re-entrant honeycombs. Good agreement is achieved between analytical predictions, FE simulations and experimental results. It is shown that the initial collapse of CFRP honeycombs is by the bending of cell walls. Once initial collapse has been attained, both hexagonal and semi-re-entrant honeycombs have a stress softening. The collapse and failure modes of CFRP honeycombs strongly depend on cellular configurations and loading directions.

Systematic analysis of constitutive models of brain tissue materials based on compression tests

It's crucial to understand the biomechanical properties of brain tissue to comprehend the potential mechanisms of traumatic brain injury. This study, distinct from homogeneous models, integrates axonal coupling in both axial and transverse compressive experiments within a continuum mechanics framework to capture its intricate mechanical behaviors. Fresh porcine brains underwent unconfined compression at strain rates of 0.001/s and 0.1/s to 0.3 strain, allowing for a comprehensive statistical analysis of the directional, regional, and strain-rate-dependent mechanical properties of brain tissue. The established constitutive model, fitted to experimental data, delineates material parameters providing intuitive insights into the stiffness of gray/white matter isotropic matrices and neural fibers. Additionally, it predicts the mechanical performance of white matter matrix and axonal fibers under compressive loading. Results reveal that gray matter is insensitive to loading direction, exhibiting insignificant stiffness variations within regions. White matter, however, displays no significant differences in mechanical properties under axial and transverse loading, with an overall higher average stress than gray matter and a more pronounced strain-rate effect. Stress-strain curves indicate that, under axial compression, white matter axons primarily resist the load before transitioning to a matrix-dominated response. Under transverse loading, axonal fibers exhibit weaker resistance to lateral pressure. The mechanical behavior of brain tissue is highly dependent on loading rate, region, direction, and peak strain. This study, by combining experimentation with phenomenological modeling, elucidates certain phenomena, contributing valuable insights for the development of precise computational models.

True triaxial modeling test of high-sidewall underground caverns subjected to dynamic disturbances

Seismicity resulting from the near- or in-field fault activation significantly affects the stability of large-scale underground caverns that are operating under high-stress conditions. A comprehensive scientific assessment of the operational safety of such caverns requires an in-depth understanding of the response characteristics of the rock mass subjected to dynamic disturbances. To address this issue, we conducted true triaxial modeling tests and dynamic numerical simulations on large underground caverns to investigate the impact of static stress levels, dynamic load parameters, and input directions on the response characteristics of the surrounding rock mass. The findings reveal that: (1) When subjected to identical incident stress waves and static loads, the surrounding rock mass exhibits the greatest stress response during horizontal incidence. When the incident direction is fixed, the mechanical response is more pronounced at the cavern wall parallel to the direction of dynamic loading. (2) A high initial static stress level specifically enhances the impact of dynamic loading. (3) The response of the surrounding rock mass is directly linked to the amplitude of the incident stress wave. High amplitude results in tensile damage in regions experiencing tensile stress concentration under static loading and shear damage in regions experiencing compressive stress concentration. These results have significant implications for the evaluation and prevention of dynamic disasters in the surrounding rock of underground caverns experiencing dynamic disturbances.

A generalised framework for modelling anisotropic creep-ageing deformation and strength evolution of 2xxx aluminium alloys

The 2xxx aluminium alloys are extensively applied in the aerospace industry due to their lightweight and balanced performance characteristics. However, a comprehensive method for modelling both the anisotropic creep deformation and strengthening behaviour in creep age forming (CAF) for 2xxx aluminium alloys remains lacking. This paper presents a generalised framework for establishing constitutive models capable of describing the anisotropic creep deformation coupled with the microstructure and material strength evolutions during creep-ageing of both the original and the pre-deformed 2xxx series Al alloys. This framework extends the rolling direction-based material model to anisotropic scenarios at varying angles between the loading and rolling directions, by employing the non-uniform rational B-splines (NURBS). The details about the anisotropic model calibration and numerical simulation implementation are demonstrated. The feasibility of this method was verified by its application to various 2xxx series aluminium alloys with or without pre-deformation, through constitutive modelling and numerical simulation, with satisfactory agreements between prediction and experimental data. For the first time, the proposed framework provides a generalised routine for establishing anisotropic creep-ageing models for various 2xxx aluminium alloys.

Unveiling mechanical response and deformation mechanism of extruded WE43 magnesium alloy under high-speed impact

To clarify the mechanical behavior and deformation mechanism of rare earth magnesium (Mg) alloy WE43 under extreme service loads, high-speed impact tests under various deformation temperatures and loading paths were conducted using a split Hopkinson pressure bar. The flow stress along extrusion direction (ED) and extrusion radial direction (ERD) decreases apparently with deformation temperature. Compared with conventional Mg alloys, it exhibits a slight anisotropy and an unusual C-shaped characteristic. Cellular dislocation, mechanical twin and fine grain that occur after high-speed impact deformation are insensitive to the loading direction, but strongly dependent on the deformation temperature, especially superimposed with adiabatic temperature rise. As a result, dynamic recrystallization (DRX) occurs even at an ambient temperature of 25 °C. Double twinning and prismatic slip or pyramidal slip are the dominant deformation mechanisms at 25 °C. These twins induce mechanical cutting refinement to form some fine-grained structures, accompanied by a small number of fine grains by twinning induced DRX. In contrast, the deformation at 250 °C is mainly controlled by prismatic slip and pyramidal slip, accompanied by various types of twinning in early deformation stage. Compared with 25 °C, more fine-grained microstructures are formed at 150 and 250 °C through a synergy mechanism of twinning induced mechanical cutting and twinning induced DRX.

Manually directional splitting of in-situ intact igneous rocks into large sheets

This paper presents a directional large-area rock fracturing method. The method had distinctive features compared with other common fracturing methods. The area of the fracturing surface could reach 10–500 m2. The fractured rock was sheet-like in shape, with a thickness of 6–8 cm. The main fracturing tools and procedures used were described in the paper. This paper analysed the reason for controllable and directional (also mode-I) rupturing in rock from the view of fracture mechanics. Counter-intuitively, the fracturing surface of the rock sheet had an angle (approximately 25°) to the loading direction (i.e., the orientation of the maximum principal compressive stress). The rupture behavior was controlled by the relationship between the load and the geometric boundary of the rock. It is found that the fracturing surface can suddenly and rapidly propagate after a certain strike by calculating the energies of the rock sheet. The striking energy could be converted into elastic strain energy, which accumulates in a very-slightly bent rock sheet step by step until exceeds the bearing limit of rock sheet. Most of the stored elastic strain energy was subsequently released in the form of splitting energy, leading to rock fracturing. This study provides insights into the occurrence of tectonic earthquakes.

Effect of directional loading on mechanical performance of HDPE warp knitted mesh fabric

Effect of directional loading on mechanical performance of HDPE warp knitted mesh fabric

Using fractal voids to understand Mode I compressive fracture in brittle materials – A two-dimensional analysis

Unlike cases of direct or indirect tensile loading, analysis of Mode I fracture in cases of compressive loading must necessarily consider two-dimensional flaw geometries. The consequent analytical complications have resulted in little theoretical evolution towards understanding Mode I fracture in compressive loading. Researchers have recently observed that the linear elastic (LE) stress fields surrounding two-dimensional flaws in compression appear to have indicative value regarding Mode I fracture, but no rational framework has yet been proposed for their interpretation. Herein it is demonstrated that by idealizing void surfaces as fractal, and using what will be called the “small flaw assumption,” LE stress fields can be interpreted to obtain significant insight regarding brittle Mode I compressive fracture. Using fractal voids, a rational and consistent explanation for the satisfaction of the stress and energy criteria at the surface of two-dimensional flaws is presented. The discussion resolves many typical theoretical issues in the literature in a manner consistent with fundamental Griffith theory, and accounts for size and shape effects. The framework further enables the computationally efficient prediction of peak KI values associated with the propagation of line cracks from two-dimensional flaws from a brittle medium subject to macroscopic uniaxial compression via LE stress fields.

Neural network based rYld2004 anisotropic hardening model under non-associated flow rule for BCC and FCC metals

This paper extends the reduced Yld2004 (rYld2004) function to present the anisotropic hardening behavior for body-centered cubic and face-centered cubic metals under the proportional loading conditions based on neural network. The parameters of the rYld2004 anisotropic hardening model (AH_rYld2004) are determined by the uniaxial tensile yield stresses along 0°, 15°, 30°, 45°, 60°, 75° and 90° from the rolling direction as well as equibiaxial tension. The evolution of anisotropic parameters are described by the back propagation neural network optimized by ant colony optimization algorithm. The predicted data by AH_rYld2004 and some common anisotropic models are compared with the experimental results to verify the precision of the AH_rYld2004 in characterizing anisotropic hardening. The comparison proves that the AH_rYld2004 precisely characterize the anisotropic evolution with increasing plastic deformation for AA 3003-O and QP980. Simultaneously, the AH_rYld2004 function based on neural network is used to accurately simulate of circular cup deep drawing for AA 3003-O and uniaxial tension for QP980. The results indicate that the AH_rYld2004 model is capable to accurately represent the plastic anisotropic evolution for uniaxial tension along seven loading directions and equibiaxial tension.

Crack propagation behavior of different zones in weldment under creep-fatigue loadings

In this paper, crack propagation behaviors in a weldment are simulated under cyclic loadings using crystal plasticity finite element coupled with the XFEM. The result shows that the cracks in heat affected zone grow faster than that in the welded zone. Short crack propagation dominated by dislocation activity takes approximately 4 to 5 grains, followed by long crack propagation which is unrelated to dislocation activity and the growth direction kept an angle to the loading direction (45°-51°). Experimentally observed deflections within grains and at grain boundaries, retardation at grain boundaries are captured by the simulation.

Experimental behavior and fracture prediction of a novel high-strength and high-toughness steel subjected to tension and shear loading tests

In this study, laboratory testing and numerical simulation methods are used to investigate the mechanical behavior and perform fracture prediction of a novel high-strength and high-toughness steel with a negative Poisson's ratio (NPR) effect under combined tensile-shear loading conditions. A test device capable of meeting different tensile-shear combination test angles is designed and manufactured, wherein the mechanical experiments on the NPR (Negative Poisson's Ratio) steel specimens are carried out at various testing angles. Q235 steel and MG400 steel are used as experimental control groups. The results show that the mechanical deformation of NPR steel is significantly better than that of Q235 steel and MG400 steel. Its tensile-shear test curve has no yield plateau and it has quasi-ideal elastic-plastic mechanical properties. The loading direction gradually changes from tension-dominated to shear-dominated as the tension-shear angle increases, and the strength and deformation of the specimens show a decreasing trend. Based on the laboratory test results, a finite element numerical model of NPR steel is established. A series of numerical simulations are carried out under the conditions of different tension and shear angles and the average stress triaxiality and fracture strain data are obtained. The fracture data of NPR steel are fitted using the Johnson-Cook fracture criterion, and the Johnson-Cook fracture parameters under the tensile-shear test conditions of NPR steel are thus obtained. The numerical simulation verifies that the fracture model can accurately predict the tensile-shear fracture behavior of NPR steel.