Displacement-controlled Loading Research Articles

Uncoupled fracture model for E250, E350, and E450 grades of structural steel under monotonic loading

Forty-eight specimens with four notch specimen configurations have been extracted from three structural steel grades used in the Indian construction industry. The extracted specimen simulates stress states of ductile fracture initiation similar to the sacrificial element installed in seismic-resistant steel structures. All the specimens are monotonically tested under a displacement-controlled loading protocol. With the help of a universal testing machine and digital image correlation, the load versus notch elongation until the point of ductile fracture initiation has been captured for all the specimens. A validated numerical model is then developed to extract the ductile fracture initiation parameters, namely stress Triaxiality (T), Lode function (L - ξ or θ or Lp), and Equivalent Plastic Strain (PEEQ). The extracted T, L, and PEEQ are then used to construct a three-dimensional fracture locus for all three study grades. In addition, a new uncoupled ductile fracture model named the Exponential based Lode parameter Void Growth Model (ELVGM) is proposed to predict the point of ductile fracture initiation and is then compared with six well-established existing uncoupled ductile fracture models using eight error measures. The proposed ELVGM with two free parameters is observed to provide better prediction accuracy for both low and high-yield strength steel corresponding to the considered stress states.

Compressive stress–strain behaviour of masonry prisms made of low elastic modulus burnt clay bricks

Masonry, being a quasi-brittle material, displays mechanical properties and manifests non-linear behaviour, which is influenced by the specific attributes of its constituent units and mortar. The compressive stress–strain behaviour of masonry is a vital factor in the realistic analysis of masonry structures when subjected to ground deformations arising from construction activities such as excavation, underground tunnel construction, utility installations, and for evaluating their seismic vulnerability. In India, the commonly used masonry unit is burnt clay bricks, which are highly localized due to changes in the constituent material and manufacturing process. This leads to variations in the strength and stiffness of brick units from region to region. An experimental investigation was conducted using twenty-four brick specimens, seventy-two mortar cube specimens and forty-eight four-unit single-wythe masonry prism specimens, subjected to displacement-controlled uniaxial compressive load. The specimen displacements of brick units and masonry prisms were obtained using Digital Image Correlation analysis. The stiffness of the brick units was significantly lower than those reported in studies conducted in India or elsewhere. Statistical analysis has been performed, and relations between the respective compressive strength and elastic modulus of brick units, mortar and masonry prism specimens have been obtained. An equation has been proposed to estimate masonry prisms’ elastic modulus from its components. An analytical model has been proposed for predicting the compressive stress–strain curve of masonry prisms, incorporating the elastic modulus of masonry prisms, which can be predicted using the expression proposed in this study. This is a fundamental improvement over the expressions available in the literature.

Experimental assessment of retrofitted damaged mortarless dry stacked interlocking masonry walls

The aim of this research study is to experimentally investigate the effectiveness of ferrocement overlay on the seismic capacity of damaged mortarless dry stacked masonry constructed with self-interlocking compressed earth blocks. For the achievement of the aim, two full scale damaged walls were retrofitted with ferrocement overlay technique. To study the effects of openings, one wall was provided with a window, while the other one was a solid wall. Retrofitting materials consist of 18-gauge galvanized steel welded wire mesh having opening size of 12.5 × 12.5 mm (0.5" × 0.5″) and 1:4 conventional cement-sand mortar. The walls were tested with displacement-controlled quasi static cyclic loading protocols. The seismic performance of the retrofitted walls under a combination of vertical axial load and lateral reverse cyclic loading was compared to the specimens before retrofitting. Damage patterns and different seismic parameters such as force deformation curve, energy dissipation, stiffness degradation and performance levels were assessed. The results show that the seismic capacity of damaged mortarless dry stacked interlocking masonry can be restored and enhanced as the overall response of the structure is significantly improved.

Exploiting perforations to enhance the adhesion of 3D-printed lap shears

Soft polymeric adhesives capable of experiencing large deformations are used in many applications, including soft robotics, stretchable electronics, and tissue engineering. The growing use of these materials raises many questions regarding their ability to maintain adhesion under loads. This work aims to study the adhesion of soft adhesives to perforated substrates. To this end, we design lap shear-based specimens from the acrylic adhesive VHB 4910 and stiff 3D-printed perforated substrates. Substrates with four different void sizes are investigated and the lap shears are subjected to displacement-controlled and force-controlled loadings. Our experiments show that the shear response can be divided into two main regions. First, the adhesion area between the soft adhesive and the substrate is maintained such that the adhesive experiences shear. Next, a debonding process of the adhesive from the substrate begins and ends with the ultimate rupture of the elastomer. Comparison between displacement and force controlled loadings reveals that the latter is characterized by limited extensibility, since the soft adhesive cannot relax. As opposed to the standard lap shears, the perforations yield a highly heterogeneous stress state that enables one to increase adhesion strength and enhance the performance of lap shears in a variety of applications.

A hybrid-sandwiching system for buckling-restraining of reduced beam sections

The present study proposes a new system for preventing local buckling in reduced beam sections (RBSs). The proposed method is called the “hybrid-sandwiching system”. In this approach, high-strength grout sandwiches an RBS part of a beam with the help of steel strips and high-strength bolts. Two T-shape specimens representing a beam-to-column connection were tested under displacement-controlled cycling loading. One specimen includes the conventional RBS and the other includes the proposed hybrid-sandwiched RBS. The experimental results of the specimens showed that the specimen with hybrid-sandwiched RBS experienced a considerable rotation up to a drift angle of R = 7% without any buckling in the web and flanges. However, in the specimen with conventional RBS, the flanges were buckled, and consequently, a significant strength degradation was observed at a drift angle of R = 4%. The experimental results confirmed that the hybrid-sandwiching system not only provides great ductility for RBSs but also does not disturb the basic role of the RBSs in moving the location of the flexural plastic hinges away from the columns' faces. In addition to experimental investigations, the finite element models (FEMs) of the specimens were conducted and validated with experimentally-obtained results. A series of FEMs was also carried out to verify the influence of hybrid-sandwiching systems on RBSs with different sizes.

The influence of intervertebral disc overloading on nociceptor calcium flickering.

Mechanical overloading can trigger a degenerative-like cascade in an organ culture of intervertebral disc (IVD). Whether the overloaded IVD can influence the activation of nociceptors (i.e., the damage sensing neurons) remains unknown. The study aims to investigate the influence of overloaded IVD conditioned medium (CM) on the activation of nociceptors. In the static loading regime, force-controlled loading of 0.2 MPa for 20 h/day representing "long-term sitting and standing" was compared with a displacement-controlled loading maintaining original IVD height. In the dynamic loading regime, high-frequency-intensity loading representing degenerative "wear and tear" was compared with a lower-frequency-intensity loading. CM of differently loaded IVDs were collected to stimulate the primary bovine dorsal root ganglion (DRG) cultures. Calcium imaging (Fluo-4) and calcitonin gene-related peptide (CGRP) immunofluorescent labeling were jointly used to record the calcium flickering in CGRP(+) nociceptors. Force-controlled loading led to a higher IVD cell death compared to displacement-controlled loading. Both static and dynamic overloading (force-controlled and high-frequency-intensity loadings) elevated the frequency of calcium flickering in the subsurface space of CGRP(+) nociceptors compared to their mild loading counterparts. In the organ culture system, IVD overloading mediated an altered IVD-nociceptor communication suggesting a biological mechanism associated with discogenic pain.

Curing–dependent structural behavior of ultra-high-performance hybrid fiber-reinforced concrete beams

The application of ultra-high-performance fiber-reinforced concrete (UHP-FRC) is exceptionally versatile. However, a comprehensive understanding of UHP-FRC beam structural performance characteristics is lacking. The majority of research has concentrated on the behavior of structural members following standard water curing. This study investigated the impact of the curing regime on the structural performance of UHPC beams. The four different curing regimes used were standard, simulated hot arid weather, steam curing at various ages, and 5% sodium sulfate curing. Experimentally measured mechanical properties of UHP-FRC have been determined following ASTM guidelines, and the investigated beams have been tested under displacement-controlled four-point loading. Additionally, ACI 318 and FHWA-HIF codified formulas were employed in this study to predict the moment capacity of the tested beams. As a result of the various steam systems of varying ages, the load-carrying capacities of the beams were improved. By accelerating cement hydration, steam curing produces a stronger bond between fibers and the cementitious matrix, improving flexural strength. The control beam (under standard curing) showed a smaller maximum deflection, while all other beams showed similar deflection capacity. By accelerating the drying shrinkage, the beams under the 28-day standard with hot air curing were negatively impacted, with an average deflection value around 5% lower than control beams. The cured beams under simulated hot, arid conditions had the lowest flexural rigidity. A steam-cured beam had the highest moment-curvature performance, while a beam cured under simulated arid conditions had the lowest. Additionally, the ACI 318 design equation significantly underestimated beam maximum moments, while the FHWA approach marginally overestimated beam maximum moments with a predicted-to-tested average of 0.82.

Ratcheting assessment of pressurized elbow pipes using the visco-plastic Chen-Jiao-Kim hardening rule frameworks

In this study, the ratcheting responses of pressurized long and short radius 90° elbow pipes made of 304L austenitic stainless steel under displacement-controlled cyclic loading is evaluated mainly using the Chen-Jiao-Kim (CJK) kinematic hardening rule at room temperature. Visco-plastic flow rule and isotropic hardening are introduced into the CJK model to better reflect the time-dependent behavior and cyclic hardening characteristics of the 304L stainless steel material. The model is implemented employing the user material subroutine (UMAT) in the ABAQUS finite element software. The predictions show that the maximum ratcheting strain occurs at the flank of the long and short radius 90° elbow pipes. The influence of internal pressure, cyclic loading, and bending radius on the ratcheting responses of the elbow pipes is respectively investigated. It is found that the predictions of the CJK model are very nearly the same as the test data. In addition, the ratcheting responses of two long radius elbow pipe specimens under force-controlled cyclic bending are evaluated. Further, the C-TDF (The Committee of Three Dimensional Finite Element Stress Evaluation) combined with CJK model and Linear Matching Method (LMM) is respectively used to determine the shakedown and ratcheting boundaries of the pressurized long and short radius 90° elbow pipes under displacement-controlled and force-controlled bending. The comparison with the test data in this study shows that the shakedown and ratcheting boundaries determined by LMM, divides the shakedown and ratcheting zone well.

Mesoscopic modeling and simulation of tensile properties of cracked concrete using cohesive model

A random aggregate (RA) model consisting of mortar, aggregate, and the interfacial transition zone (ITZ) was established to investigate the effect of existing cracks on the tensile properties of concrete. Based on this, a method for establishing a geometric model of cracked concrete was proposed by inserting 0-depth cohesive elements into the RA model. The influence of crack length, width, and area on the tensile properties of concrete were studied using the established cracked concrete model. In addition, the key parameters of bond strength and fracture energy of the mortar and ITZ regions were tested. The cement is ordinary Portland cement with a strength grade of 42.5, and the sand is medium sand. The fracture energy tests were conducted using a displacement-controlled loading model with a loading rate of 0.4 mm/min, while the tensile strength tests were conducted using a stress-controlled loading method with a loading rate of 0.06 MPa/s. The feasibility and accuracy of the numerical method used were verified by comparing the results with those of a finite compression simulation. The numerical results show that as the crack length increases, the tensile strength and elastic modulus of concrete gradually decrease, but if the crack length is 0.47 h, the elastic modulus of concrete shows an abnormal increase. The range of the crack's influence on the tensile performance of concrete is between 50 mm and 100 mm. If the crack width is less than 0.55 mm, the tensile strength of concrete gradually decreases with the increase of crack width, and when the crack width is greater than 0.55 mm, the tensile strength stabilizes.

Evaluation of Ductility of Precast Column Foundation Connections

Objectives: To investigate and compare the ductility performance of precast column foundation connection using the method proposed by Park and ASCE 31-03. Methods: Strength and ductility are the most important parameters to resist earthquake forces for the overall safety of a structure. Evaluating ductility from experiments is a cumbersome process. From the experimentally obtained load-displacement envelope, the ductility was evaluated for four specimens of precast column foundation connection by the method proposed by Park and Paulay (1989) and ASCE 31-03. The results were also compared with monolithic specimen. Each of the specimen was subjected to displacement-controlled lateral cyclic loading. The maximum displacement was maintained at 40 mm due to laboratory constraints. Findings: From the evaluated results, ASCE 31- 03 gives better values of ductility than the method proposed by Park. The calculated ductility by ASCE 31-03 were 31.5 %, 41.07 %, 72.7%, 68.3% and 48.9 % higher for monolithic specimen, PCBJ, PC I, PC II and GS specimen respectively. Novelty: The present study reveals the best-suited method that can be applied to evaluate ductility for specimen subjected to lateral cyclic loading that has not been reported in earlier studies. Keywords: Ductility Ratio; Yield Displacement; Ultimate Displacement; Precast Connection; Load Displacement Envelope

Pull-out behavior of torque-controlled expansion anchors in HPSFRC under different installation conditions

To study the pull-out behavior of torque-controlled expansion(TCE) anchors on high-performance concrete(HPC) and high-performance fiber-reinforced concrete(HPSFRC) substrates, 28 pull-out tests are set up. Different installation conditions of anchors, including the embedment depth(Hef) and the preload torque(Tinst), and the types of substrates, including no-fiber HPC and HPSFRC with 1% fiber content, are considered in the test. The split tensile(ft) and compressive strengths(fcu) of concrete are measured to evaluate the influences of fibers on the properties of concrete. Pull-out tests are conducted TCE anchors installed in C70 HPC/HPSFRC cylinder specimens by the displacement-control loading method. From test results, the relationship between failure mode, load-displacement behavior, pullout load-bearing capacity(Nu) and installation factors (Hef, Tinst, etc.) are analyzed. The preload loss law of the fastener system within 24 h is monitored. It is found that the larger Tinst leads to a larger preload loss rate, and the loss in Tinst tends to be stable within 5–7 h. A modified model of Nu of TCE anchors in HPSFRC is proposed. The prediction accuracy of the modified model and the existing models (CCD model, Toth et al's model) is compared; both CCD and Toth et al's model underestimate Nu, which would lead to uneconomical over-design, and the prediction results of the modified model are in a good agreement with the experimental results.

Experimental & numerical investigation of all-steel assembled cruciform buckling-restrained braces

In this paper, an all-steel assembled cruciform buckling-restrained brace is proposed for the first time. It is mainly composed of 26 main components, including one external restraint steel tube, eight internal restraint steel tubes, one inner core, and 16 high-strength bolts for connection. Four BRB specimens were designed in the test stage, and all specimens were subjected to displacement-controlled axial cyclic load. As a result, hysteretic performance, energy dissipation, and ductility were summarized and analyzed for each specimen. In addition, 50 nonlinear finite element models are established using the large-scale commercial finite element software ABAQUS for numerical simulation analysis. The results of the experiment and numerical simulation show that: (1) A new type of all-steel assembled cruciform buckling-restrained braces (BRB) with stable hysteretic performance has been successfully designed, and a corresponding simplified but refined three-dimensional nonlinear finite element model that can be in good agreement with the test results has been established. (2) The traditional concept of restraining ratio has been disassembled into the concept of internal and external restraining ratios in this BRB configuration. (3) The concept of restraining length ratio of this BRB configuration and related design standards are proposed.

Sliding modes of fault activation under constant normal stiffness conditions

Fault activation has been the focus of research community for years. However, the studies of fault activation remain immature, such as the fault activation mode and its major factors under constant normal stiffness (CNS) conditions associated with large thickness of fault surrounding rock mass. In this study, the rock friction experiments were conducted to understand the fault activation modes under the CNS conditions. Two major parameters, i.e. the initial normal stress and loading rate, were considered and calibrated in the tests. To reveal the response mechanism of fault activation, the local strains near the fault plane were recorded, and the macroscopic stresses and displacements were analyzed. The testing results show that the effect of displacement-controlled loading rate is more pronounced under the CNS conditions than that under constant normal load (CNL) conditions. Both the normal and shear stresses drop suddenly when the stick-slip occurs. The decrease and increase of the normal stress are synchronous with the shear stress in the regular stick-slip scenario, but mismatch with the shear stress during the chaotic stick-slip process. The results are helpful for understanding the fault sliding mode and the prediction and prevention of fault slip.

A mixed-mode fatigue crack growth model for co-consolidated thermoplastic joints

In this work, a mixed-mode fatigue crack growth model for simulating interfacial fracture of co-consolidated thermoplastic joints was developed. The model is based on the cohesive zone modeling approach and was specifically designed to account for any I + II mode-mixity, requiring only pure mode I and II loading data as input. The fatigue crack growth rate is constantly updated using a function of the energy release rates and the computed mode-mixity ratio. A linear mode-mixity law is applied. Both force-controlled and displacement-controlled loads can be modeled in specimens ranging from coupon-scale sizes to larger structural elements. Implementation of the model was done for the low-melt polyaryletherketone (LM-PAEK) material by developing a user-defined subroutine in the LS-Dyna FE software. To feed the model, mode I and mode II fatigue tests were conducted using double-cantilever beam and end-notch flexure specimens, respectively, while to validate the model mixed-mode single lap shear (SLS) tests were conducted. The model reproduces accurately the mode I and II fatigue crack growth while it predicts also accurately the mixed-mode fatigue crack growth of the SLS specimens.

Performance-Based Seismic Design of Hybrid GFRP–Steel Reinforced Concrete Bridge Columns

Damage quantification in terms of engineering demand parameters (EDPs) is a critical element of the performance-based design (PBD) approach. A widely used EDP for reinforced-concrete (RC) bridge columns is the drift ratio. This study establishes drift ratio limit states, and corresponding strengths for hybrid GFRP–steel RC circular bridge columns. The adopted reinforcement layout in this study consists of two layers of reinforcement, exterior with GFRP and interior with steel. Such coupling between the two materials in concrete bridge columns improves their corrosion resistance while maintaining their stiffness and ductility. Here, a validated fiber-based model is utilized to predict global as well as local responses of hybrid RC columns under monotonic displacement-controlled loading. A full factorial analysis was first adopted to screen parameters potentially influencing drift ratio limit states and corresponding strengths for their significance. The resulting data were then fitted to mathematical expressions using machine learning-based symbolic regression. Lateral load–deformation responses predicted based on the proposed expressions were validated against existing data from the literature. A complete example demonstrating how the proposed expressions could be utilized to design a hybrid bridge column within the context of PBD is also presented.

Effect of the core bone engaged length on the BASHTI fixation strength, an in-vitro study on bovine tendons using identical-density surrogate bones

BackgroundBASHTI is an implant-less anterior cruciate ligament (ACL) reconstruction technique, which resolves the problems caused by implants such as interference screws. This study aims to investigate the effect of the drill bit and tendon’s diameter on the Core Bone Engaged Length (CBEL) and the fixation strength. CBEL is the length of core bone which has a full engagement with both tunnel and graft at the same time.Methods60 in-vitro tests were conducted for 6, 7, 8, and 9 mm tendon sizes with a 10 mm bone tunnel. In this study bovine tendons and dummy bone blocks were used to model the fixation. Drill bits were used to extract the core bone for securing the auto-graft. A three-stage tensile test including a force-controlled cyclical preloading of 10–50 N with a frequency of 0.1 Hz for 10 cycles, followed by the main force-controlled cyclical loading of 50–200 N with a frequency of 0.5 Hz for 150 cycles, and immediately a displacement-controlled single cycle pull-out load with a rate of 20 mm/min were carried out to discover the fixation strength of each sample.ResultsThe 6 mm group had the greatest CBEL. However, all cases in this group failed in loadings below 200 N, which is the minimum required strength after ACL reconstruction. The fixation strength of cases with more than 200 N fixation strength for 7, 8, and 9 mm tendon diameters were 275 ± 42, 330 ± 110, and 348 ± 93 N, respectively, showing insignificant difference between groups (P-value = 0.45). Nevertheless, CBELs for these groups were 16.6 ± 3.4, 9.6 ± 2.4, and 11.7 ± 3.8 mm, respectively, implying a significant increase in CBEL in the 7 mm group than that for 8 and 9 mm groups (P-value = 0.002 and 0.049, respectively).ConclusionResults showed that CBEL could assess the quality of BASHTI technique. However, CBEL was an inverse function of tendon compression, so it was not an independent parameter to determine BASHTI strength. Also, the CBEL of 7 mm group which fulfilled the 200 N threshold was higher than that of 8 and 9 mm groups, so its healing process speed may be higher, which is recommended for a future study in this field.

Effect of cyclic loading on the mobilization of unit base resistance of model piles jacked in sand

This paper reports the results of a series of monotonic compressive and cyclic load tests performed on a closed-ended jacked model pile installed in a half-cylindrical calibration chamber with image analysis capabilities. The monotonic compressive load tests were carried out before and after performing a displacement-controlled cyclic load test to determine the impact of cycling on unit base resistance. Digital images of the sand and the model pile were taken during cyclic loading and processed using the digital image correlation (DIC) technique to obtain the cumulative displacement and strain fields in the sand domain. The results show that the ultimate unit base resistance can drop significantly after cycling. The magnitude of the drop in ultimate unit base resistance depends on both the magnitude of the cyclic displacement amplitude and the number of cycles. However, the unit base resistance at plunging increases after large-displacement half-amplitude cycling. The increase in unit base resistance at plunging after cycling is linked to the flow of sand particles to the zone below the conical base, the occurrence of sand particle crushing, and the dilative behavior of the sand outside a bulb of crushed particles formed during cyclic loading. The processed DIC data show that for cyclic displacement half-amplitudes ∆wcyclic less than or equal to 0.25 mm (0.0007B), the cumulative radial and vertical displacements in the soil domain normalized by ∆wcyclic are negligible. Values of ∆wcyclic greater than 0.25 mm produce normalized displacements in the soil domain that increase as the cyclic displacement amplitude increases.

Dilatancy Effects on Surface Foundations on Dry Sand

This paper presents the results of finite-element analyses of rigid-surface strip foundations on dry cohesionless soil subjected to displacement-controlled vertical loading. A two-surface constitutive model with a non-associated flow rule, representing the full range of pressure-dependent drained shear strength and volume change behavior of dry sand, is employed to model the soil. A range of foundation widths and sand initial relative densities is covered. The results show that, for narrow, rigid-strip foundations, the conventional bearing-capacity calculation provides a reasonable indication of the load-carrying capacity of the foundation. However, there is no peak in the load-carrying capacity for wide foundations, and the load–deformation curve exhibits stiffening behavior, even at large settlements; this is particularly apparent at larger values of the initial relative density. Based on the results, it is observed that with the increase in vertical foundation loading, the vertical stiffness of the foundation increases with each loading increment, due to the tendency of the dense sand to become more dilatant with the increase in confining pressure. At large settlements, the state of the sand beneath the foundation approaches the critical state. Settlement vectors beneath the foundations on dense sand reveal that the displacements approximate one-dimensional compression.

Creep cavitation evolution in polycrystalline copper under conditions of stress relaxation

Creep in metals and alloys has been observed and studied extensively over the past century. Most studies are based on constant load or less frequently on constant stress conditions. Under certain stress regimes during the period of service, such as creep-fatigue (cyclic), and under displacement-controlled loading the stress can relax. This paper uses novel millimetre length-scale beam bend geometry test specimens with constant displacement to simulate stress relaxation and explore cavity nucleation and early-stage growth/closure in a model polycrystalline material, namely oxygen-free high-conductivity copper. The role of changing grain size over the range 43 μm–2350 μm has been explored. Power-law creep theoretical modelling and finite element analyses have been adopted to predict creep relaxation and explain cavity nucleation and early-stage growth/closure for the test conditions. The results are compared with the experimental observations. The overall experimental and modelling outcomes are considered with respect to the underlying creep damage mechanism.

Multiaxial Plastic Deformation of Zircaloy-4 Nuclear Fuel Cladding Tubes

The following work is motivated by the desire to devise an internal pressure test that can mimic a displacement-controlled loading scenario and demonstrate how to apply the multiaxial stress and strain data from the test to develop an elastic/plastic constitutive model for a thin-walled tubular component. This is achieved by conducting simultaneous measurements of tangential and axial strain during the pressure test and integrating these strain measures into a feedback loop with the pressure controller. It is shown how data from such a test can be used to develop a large mechanical property data set relevant to biaxial loading conditions. The data obtained have high confidence evidenced by their low variability and alignment with other literature studies. Additionally, data from these internal pressure tests combined with full-tube axial tensile tests allow for the derivation of the Hill anisotropic yield function. The developed Hill yield function is validated by comparing the plastic strain ratios from the full-tube tension tests and by comparing the predicted yield stress in the tangential direction with measured values from ring tension tests in a previous study.