Elastic-plastic Curve Research Articles

Experimental investigation on seismic performance of cold-formed steel strap-braced stud walls under lateral cyclic and vertical loading

Cold-formed steel (CFS) construction offers several benefits over traditional construction, including being lightweight, sustainable, recyclable, quick to build, insect and rot-resistant, and suitable for computer-aided design and manufacture. Despite extensive research on CFS elements, there are still gaps in our understanding of the behaviour of CFS structural systems, particularly their response to severe earthquake events. This experimental program aims to provide a better understanding on the seismic behaviour of strap-braced stud-walls as a lateral force resisting system for residential medium-rise buildings. An innovative test rig is developed that is capable of applying a constant vertical load during lateral cyclic testing, replicating the loading conditions expected during an earthquake event in such cases. For the first time, seven full-scale tests are conducted on CFS walls under vertical and horizontal loading using standard sections in New Zealand. The vertical capacity, lateral stiffness, ultimate strength, ductility, and failure mechanisms of the specimens are investigated. Subsequently, a modified detailing is suggested, which allows CFS walls to exhibit considerably better seismic behaviour compared to the systems with conventional detailing. It is shown that by adopting the modified detailing, CFS walls exhibit an ideal elastic-plastic curve with negligible degradation in successive cycles and a high ductility factor exceeding 5. The results of this study should prove useful for more efficient design of CFS strap-braced stud walls for mid-rise buildings.

Experimental study on the effects of triangular groove inclination angles on the mechanical behavior of sand–concrete interfaces

The mechanical properties of pile–soil interfaces directly affect the bearing capacity of pile foundations. In this study, the effects of triangular groove inclination angles on the mechanical properties of sand–concrete interfaces were investigated. Random rough concrete surfaces were obtained through slurry intrusion testing, and the roughness of concrete surfaces was evaluated based on the sand cone method. According to the roughness of random concrete surfaces, five regular concrete surfaces with different triangular groove inclination angles were constructed. A series of direct shear tests between (i) sand and random concrete surface, (ii) sand and regular concrete surface, as well as (iii) sand and smooth concrete surface were performed. Moreover, a direct shear test of natural sand was conducted. Lastly, a shear strength model considering the triangular groove inclination angle of sand–concrete interfaces was established, and the shear failure mechanism of interfaces was discussed. The results firstly indicated that the shear stress-displacement curves of random rough concrete surfaces were ideal elastic-plastic curves; and the regular rough concrete surfaces were found to incur strain softening under a normal stress of 300 kPa. Secondly, the shear strength of sand–concrete interfaces increased with the increase in groove inclination angle of concrete interfaces. Thirdly, sand was found to achieve the highest shear strength, while sand–smooth concrete interfaces proved to attain the lowest. Lastly, the internal friction angle value of random concrete interfaces was shown to range between the values of concrete surfaces with triangular groove inclination angles of 70° and 90°.

Experimental Study on the Damage Characteristics and Fracture Behaviour of Rock-like Materials with Weak Interlayer Zones

To study the effect of the dip angle of the weak interlayer zone (WIZ) on the damage characteristics and fracture behaviours of rock-like samples, a series of uniaxial compression tests were conducted, accompanied by acoustic emission (AE) tests and the digital speckle correlation method (DSCM). The experimental results indicate that the stress—strain curves before the peak can be divided into plastic—elastic—plastic and elastic—plastic curves, and the stress—strain curves after the peak can be divided into plastic failure, transitional failure and brittle failure curves. The relationship between the peak stress and peak strain and WIZ angle can be well described by two linear functions. As the angle increased, the specimen gradually changes from tensile failure to shear failure, and the number of cracks and falling fragments decreases gradually. In addition, it becomes difficult to predict rock mass failure by AE when the WIZ angle is close to 60°. The AE counts decrease with an increasing angle, reach a minimum at 60°, and then increase. Furthermore, the WIZ plays a key role in the deformation of upper hard rock.

FINITE ELEMENT ANALYSIS OF GUSSET PLATE CONNECTION DESIGN FOR COLD-FORMED STEEL FRAMES

The application of cold-formed steel sections has been extended from secondary members to primary structural members in recent years. This increases the use of gusset plate connections in cold-formed steel since it is a common connection used in steel structures. However, current design codes on connection design do not have a comprehensive method to consider the effects due to the buckling of the thin cold-formed steel sections. Therefore, it is important to develop a more accurate model to predict the capacity of cold-formed steel connections. This paper aims to propose an equation for gusset plate beam-to-column connection using finite element models. Finite element models have been developed and compared with existing test results. The failure mode and ultimate strength of the numerical models are similar to the experimental results. The validated finite element models are then used to study the effects of gusset plate thickness, effects of cold-formed steel section depth and thickness, and the effects of bolt size and spacings. Elastic and plastic stiffnesses are obtained from the developed models. The connection behavior followed a typical elastic-plastic curve according to the connection ductility and failure mode. An empirical model is developed from the finite element models to predict the joint behavior of gusset plate beam-to-column connection for cold-formed steel structures. AS 4600 may have underestimated the initial stiffness of the connection.

Calculating the Mechanical Properties of DX56 Steel by the Spherical Indentation Method

The mechanical properties of metal materials are studied by indentation method in this paper. The elastic-plastic curve of indentation experiment is obtained by fitting the representative stress and strain points, and the theoretical elastic-plastic curve is described by the power index equation of three parameters. After optimal coincidence of two curves, the yield strength, strain hardening index and elastic modulus of the material are obtained. Taking DX56 steel as an example, which has good deep drawing property for automobile, the mechanical properties of the steel are tested by indentation method and compared with the results of uniaxial tensile test. The error of tensile properties is small, so the indentation method can be used to detect the mechanical properties of metal materials.

Experimental investigation and numerical analysis for concrete-CLT connections

The mechanical characteristics of concrete to cross laminated timber (CLT) connections plays an essential role in a hybrid structure. This study mainly deals with an experimental and numerical study on the load-slip behaviour of concrete-CLT connections under cyclic loading, where six specimens with two different loading types (i.e., parallel and perpendicular to the grain of CLT) were executed. The failure modes of all specimens were the bolts shear off at the location of the interface between concrete and CLT, associated with the cracking and splitting of CLT panels depending on loading directions. Then the numerical model whose nonlinearity is concentrated on the spring element is created. To calibrate the model parameters, the procedure of parameter estimation using unscented Kalman filter (UKF) is employed to identify the parameters of a hysteresis model. Two assessment methods, including the equivalent energy elastic-plastic curve assessment and an energy-based damage assessment, are used to comprehensively evaluate the estimated result. The model estimate is consistent with the experimental result, demonstrating the identified parameters having good accuracy. The calibrated parameters of a hysteresis model provide a good database when such type of connections are incorporated into a finite element model of a structural system.

Dynamic mechanical characteristics and failure mode of serpentine under a three-dimensional high static load and frequent dynamic disturbance.

The improved split Hopkinson pressure bar (SHPB) was used to study the dynamic mechanical properties and failure characteristics of surrounding rock in deep rock mass engineering that is under high stress and affected by blasting excavation and other dynamic disturbances. In a three-dimensional high static load and frequent dynamic disturbance test, the preload high axial pressure and confining pressure are used to simulate the high crustal stress of deep rock, and the effect of small disturbances on the rock is simulated by the low impact load. The results show that there are two types of dynamic stress-strain curves of deep rock: an elastic-plastic curve and plastic-elastic-plastic curve. The curves consists of five parts: the compaction stage, micro-crack steady development stage, micro-crack unstable propagation stage, fatigue damage stage, and fatigue failure stage. Reductive phenomena of constringent strain after dynamic peak stress appear because of the different degrees of rock damage. Moreover, these phenomena include two conditions, namely, whether rebound occurs or not. The impact resistance of rock is strongest when the ratio of the confining pressure to axial pressure is optimal, and the dynamic average strength of rock and accumulative impact times decrease with the increase of the preloaded axial compression and increase with the increase of the preloaded confining pressure. Both the dynamic deformation modulus and dynamic peak stress decrease with the increase of the accumulative impact time, while the maximum strain and the dynamic peak strain increase. The corresponding rebound strain as a whole first increases and then decreases with the increasing impact times. For deep rock, tensile failure and single-bevel plane shear failure are the main failure modes, and pull-compression mixed friction failure is the auxiliary failure mode. Additionally, the lumpiness of broken rock decreases with the increase of the preloaded axial compression and increases with the increase of the preloaded confining pressure.

A comparative study on characteristics of nanostructured Al–Zn–Mg/3 wt% Al2O3 composites synthesized through solid-state sintering and semi-solid thermomechanical processing

Solid-state sintering and thermomechanical processing (TMP) were used to synthesize nanostructured Al–Zn–Mg/3 wt% Al2O3 composite. After synthesizing the composite power by mechanical alloying, the solid-state sintering was carried out which consisted of cold compression at 150 MPa and sintering at 550 °C for 90 min. Moreover, the TMP was performed including repeating the following procedure three times: cold compression at 154 MPa and the liquid phase sintering (LPS) at 600 °C. The samples were characterized using the x-ray diffraction (XRD) analysis and a scanning electron microscope (SEM). Moreover, a transmission electron microscope was used for the characterization of the composite powder and one of the bulk samples after performing the TMP. In addition, compression testing and microhardness measurement were performed on the samples to evaluate the mechanical properties. The results showed a more uniform distribution of alumina particles and lower porosity in the thermomechanical processed samples compared with those of the solid-state sintered one. In addition, the fully linear elastic compressive stress-strain curve of the solid-state sintered composite changed to an elastic-plastic curve owing to the effect of the TMP.

Strengthening of mild steel struts using CFRP sheets subjected to uniform axial compression

The behaviour of Carbon Fibre Reinforced Polymers (CFRP) reinforced mild steel isolated plate (strut) under uniform axial compression is not well understood because there is a lack of experimental research. This paper presents experimental results for a series of struts reinforced with CFRP sheets subjected to uniform axial compression. The main parameters examined were the thickness of the steel plates, and the location and the number of the CFRP sheets. Three different thicknesses of steel plates were examined, namely, 2, 3 and 4mm. The testing program included 21 struts with three different width-to-thickness ratios of 50, 66.7, and 100. The CFRP sheets were adhesively bonded where the fibres were aligned in the longitudinal direction parallel to the direction of the applied force.The results showed that the debonding failure strain in the compression face occurred in the range of 0.3–0.6% for all the composite specimens. It was found that the amount of permanent damage in the composite strut identified by the residual displacement is significantly less than that of the bare strut. Experimental results showed a global increase in strength, energy absorption and ductility for all test specimens where the greatest percentage increases were 452%, 259% and 107%, respectively. The 2 and 3mm composite specimens showed greater percentage increases than the 4mm composite specimens due to the relative thickness of the carbon fibre sheets verses steel. All experimental specimens failed by overall buckling, achieving maximum capacity at the end of their elastic range showing that the specimens were successfully bonded and acting as composites within such range. An existing rigid plastic theory considers initial imperfections (defects) was modified and used to predict the response of the struts. Experimental and theoretical curves showed consistency where initial defects controlled the elastic-inelastic behaviour and the load bearing capacity of the strut. In general, specimens with initial defects produced a smooth elastic-plastic curve, while specimens with little to no initial defects showed a sharp peak at the buckling load. The rigid plastic theory well predicted the response of the bare specimens whereas it reasonably predicted the plastic collapse curve of the composite struts.

Cross-Laminated Timber Shear Connections with Double-Angled Self-Tapping Screw Assemblies

AbstractThe research presented in this paper examines the shear resistance performance of self-tapping screws (STS) in three-ply cross-laminated timber (CLT) panels. Specifically, the feasibility of using innovative STS assemblies with double inclination of fasteners was investigated for the shear connection of CLT panels. The specimens (1.5×1.5 m) were subjected to quasi-static and reversed-cyclic loading. The tests were set up to approximate pure shear loading, with three-panel CLT assemblies connected with STS. The resulting load-displacement and hysteretic curves were used to determine an equivalent energy elastic-plastic curve to estimate assembly capacity, yield load, yield displacement, ductility ratio, stiffness, and damping. Excellent structural performance in terms of capacity and stiffness was obtained while still providing the required ductility for the system to be used in seismic applications. The average static and cyclic yield loads were 6.0 kN/screw and 5.9 kN/screw, respectively. Aver...

Application of Computational-Experimental Methods for Designing Optimized Semitrailer Axle Supports

Computational and experimental methods were applied to the design and optimization of a semitrailer axle support subjected to fatigue loads. Numerical results based on the finite element method (FEM) were correlated with extensometric tests to assess the accuracy of the computational method. This paper is focused on the “minimum radius manoeuvre.” This situation represents the highly critical load case occurring in a semitrailer operation where the tractor vehicle pulls the semitrailer's kingpin at approximately 90° with respect to its longitudinal axis, and high stress and strain phenomena take place in the axle supports’ structure. Loads and boundary conditions that correspond to this load case were first adjusted by means of experimental tests and could be later applied to each semitrailer axle support in the numerical model. In this analysis, the stress-strain elastic-plastic curves of the base material, the welding, and the HAZ have been incorporated to the numerical models. Fatigue S- N curves combined with the maximum Von Mises equivalent stresses obtained in the computational analysis provided a maximum number of cycles that the semitrailer axle support could reach in case of the minimum radius manoeuvre being applied to the vehicle in a repeated manner. The initial design could then be optimized to improve its fatigue life.

Performance of Different Connections for a SFGP-RC Prototype Panel

The aim of this research regards the evaluation of mechanical performance of a prototype panel made by steel fiber reinforced concrete, SFRC, on the top, and fiber reinforced pultruded sandwich panel, GFRP, on the bottom, subjected to combined moment-shear actions through four-bending test. Two different mechanical solutions were used for the connection of the panels. A first steel connection previously designed and a second one with resin applied uniformly on the surface of GFRP panel. The SFRC-G panel involves the analysis of the weakness of GFRP material due to its very low deformability, the risk of the local instability and the elastic brittle behaviour till the collapse, while steel is obviously characterized by elastic-plastic curve. However in the test proposed the ultimate limit state (SLU) involves first of all the loss of bond strength between materials. The panels length/thickness ratio has been previously designed to give prominence to flexural-shear combined actions and in verifying the connections capacity.

Numerical Simulation on Failure Process of Rubber-Sleeved Headed Stud Shear Connector

Rubber-sleeved headed stud shear connector is flexible shear connector used in steel-concrete composite structures. In this work, nonlinear finite element model has been developed to simulate the failure process of the shear connector under shear loading. The stress distribution, deformation, crack propagation and failure mode were analyzed. The material nonlinearities of rubber, headed stud, concrete were considered in the material model. The rubber was assumed as a perfect material with no defect, and a modified reduced polynomial form of strain energy including an energy limiter and a new constant was introduced into the user material subroutine VUANISOHYPER-INV of ABAQUS software. Damaged plasticity model was used to model the concrete material. A tri-linear elastic-plastic curve was used in stud material model. Comparing the results obtained from the finite element analysis with those from push-out test, good agreement is highlighted in the capacity, ductility and failure mode of rubber-sleeved headed stud shear connector.