Displacement-controlled Loading Research Articles

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.

Strain evaluation of axially loaded collateral ligaments: a comparison of digital image correlation and strain gauges

The response of soft tissue to loading can be obtained by strain assessment. Typically, strain can be measured using electrical resistance with strain gauges (SG), or optical sensors based on the digital image correlation (DIC), among others. These sensor systems are already established in other areas of technology. However, sensors have a limited range of applications in medical technology due to various challenges in handling human soft materials. The aim of this study was to compare directly attached foil-type SG and 3D-DIC to determine the strain of axially loaded human ligament structures. Therefore, the medial (MCL) and lateral (LCL) collateral ligaments of 18 human knee joints underwent cyclic displacement-controlled loading at a rate of 20 mm/min in two test trials. In the first trial, strain was recorded with the 3D-DIC system and the reference strain of the testing machine. In the second trial, strain was additionally measured with a directly attached SG. The results of the strain measurement with the 3D-DIC system did not differ significantly from the reference strain in the first trial. The strains assessed in the second trial between reference and SG, as well as between reference and 3D-DIC showed significant differences. This suggests that using an optical system based on the DIC with a given unrestricted view is an effective method to measure the superficial strain of human ligaments. In contrast, directly attached SGs provide only qualitative comparable results. Therefore, their scope on human ligaments is limited to the evaluation of changes under different conditions.

Tolerance of oxide wedging crack growth of manufacturing flaws in CANDU Zr-2.5Nb pressure tubes to cyclic operating stresses

In-service inspections of Zr-2.5Nb pressure tubes in CANDU nuclear reactors have detected inner diameter surface breaking flaw indications which are classified as manufacturing flaws. It is considered prudent to evaluate potential growth by means of a postulated oxide wedging crack growth mechanism. An oxide wedging crack growth model has previously been developed and applied to predict future flaw sizes. Fatigue crack growth due to cyclic operating stresses is calculated for these flaws, and this is added to the postulated oxide wedging crack growth for fitness-for-service evaluations. This established approach is overly conservative since fatigue crack growth cannot occur while the oxide wedging crack growth mechanism is active, due to the essentially displacement-controlled loading of the flaw by the oxide wedge, which causes the stress intensity factor for the flaw to be effectively constant under cyclic operating stresses. This paper addresses this conservatism by evaluating crack growth due to postulated oxide wedging, and demonstrating that fatigue crack growth is not possible under oxide wedge loading.

Experimental investigation on debonding behavior of Fe-SMA-to-steel joints

This work is the first systematic study on the static behavior of adhesively-bonded Fe-SMA-to-steel joints in applications adopting iron-based Shape Memory Alloys (SMAs). In order to provide a better understanding on the mechanical behavior of the adhesively bonded joint, an experimental campaign was established, involving 24 lap-shear tests in a displacement-controlled loading regime. The test series includes two types of Fe-SMAs (non-prestrained and prestrained), three types of adhesives (SikaDur 30, Araldite 2015, and SikaPower 1277), and three different thickness values (0.5, 1, and 2 mm) for the adhesive. A digital image correlation (DIC) technique was employed to measure the full-field displacement and strain, which were then used to infer the shear behavior. The mechanical behavior was analyzed on the basis of the experimentally derived load–displacement curves, the shear stress profiles along the bond line, and the bond–slip curves; three stages were observed during the loading process of a bonded joint: (i) a linear stage, (ii) a damage accumulation stage, and (iii) a debonding propagation stage. The test results indicate that a more ductile adhesive or a thicker adhesive layer possess a higher fracture energy, leading to a greater bond capacity. The results were also compared against those from lap-shear tests on carbon fiber reinforced polymer (CFRP) bonded joints. It is found that an Fe-SMA bond and a CFRP bond behave similarly when a linear adhesive is utilized; a nonlinear adhesive, however, results in significant mechanical differences between the two bonded joints, which merit individual analysis.

Load-carrying and energy-absorbing performance of honeycombs with different cross sections under cyclic loading

Honeycombs, characterized by advantages including large compressive deformation and controllable deformation, have been widely used in energy-absorbing components. The loading process of honeycombs is generally influenced by recovery after unloading in application environments. Expounding the load-carrying and energy-absorbing performance of honeycombs under cyclic loading is critical for the practical engineering application of honeycombs. Honeycombs with four different cross sections were selected to compare their deformation process and load-carrying performance under cyclic loading by conducting quasi-static compression tests and displacement-controlled cyclic loading and unloading tests. On this basis, influences of the cyclic loading on the energy-absorbing performance of honeycombs were revealed. It is shown that cyclic loading does not change the four-stage characteristics of the displacement-force curve of honeycombs while to some extent influences the load-carrying capacity of honeycombs. Under cyclic loading, the honeycombs have larger plateau force and plateau force efficiency and exhibit better stable load-carrying capacity and uniformity. Moreover, evaluation of the energy-absorbing performance of honeycombs based on the total energy absorption index has some limitations, which takes the energy absorption of structures in each stage into account. In view of this, the effective energy absorption was proposed to accurately evaluate the energy-absorbing performance of honeycombs. It is found that honeycombs exhibit better energy-absorbing performance under cyclic loading. The research achievements provide theoretical support for application of honeycombs in fields including mines and underground spaces.

Steel fiber reinforced concrete wall-slab joints subjected to cyclic loading

Study of steel fiber reinforced concrete (SFRC) as a construction material has increased significantly in recent years. This is attributed to the benefits that steel fibers provide to concrete, such as the increase of strength and deformation capacity. Since SFRC is an attractive alternative to conventionally reinforced walls when building low-rise earthquake-resistant houses, an experimental program of SFRC wall-slab joints was conducted. Quasi-static cyclic reversible loading was applied on two specimens reinforced with a steel fiber dosage of 30 kg/m3, with and without steel deformed bars. Additionally, one benchmark monolithic wall-slab joint, reinforced conventionally using deformed steel bars only, was tested for comparison purposes. A displacement-controlled loading protocol was applied until failure of the specimens was observed. The main objective of these tests was to better understand the load-deformation capacity of wall-slab joints reinforced with steel-fibers. The performance of the tested specimens was assessed in terms of evolution of cracking patterns, failure mode, hysteretic response and stiffness and strength degradation. Results showed that an adequate combination of steel fibers and deformed steel reinforcement can lead to a satisfactory behavior of wall-slab joints, equivalent to those reinforced solely with deformed steel bars; while using only steel fibers can lead to smaller resistance and deformation capacity (about 60% that of the benchmark specimen). For the three tested specimens, the failure mechanism was dominated by flexural deformation at the slabs, close to the wall heel, where damage tended to be concentrated. A significant pinching behavior was seen in the hysteretic curves of the specimens. Stiffness degradation of the wall-slab joints was similar for the three specimens and it was directly related to the inter-story drifts demanded on them. Significant reductions were seen, with values as low as 20 and 10% of the initial stiffness, for inter-storey drifts of 0.002 and 0.01, respectively. Strength degradation was significant when only fibers where used, and small when they were combined with deformed bars.

The Flexural Performance of BFRP-Reinforced UHPC Beams Compared to Steel and GFRP-Reinforced Beams

The performance of ultra-high-performance concrete (UHPC) reinforced with BFRP bars was investigated in this research study. To achieve the objectives of this study, a total of six UHPC beams were cast and tested for flexure, under displacement-controlled loading conditions. The performance of BFRP-reinforced beams was compared against GFRP and steel reinforced beams. All beams had a cross-section of 185 mm × 250 mm, and a total length of 2200 mm. The experimental results were presented and discussed in terms of cracking moments, cracking patterns, failure modes, flexural capacity, midspan deflection, as well as strains in concrete and reinforcement. Results showed that UHPC enhanced the flexural performance of BFRP-reinforced beams in terms of moment capacity, deflection response and cracking patterns. The experimental results were complimented with analytical results that were calculated using the ACI 440 and CAN/CSA S806 code provisions. It was found that moment predictions using relevant ACI equations are acceptable for under-reinforced beams, but were slightly unconservative for the over-reinforced beams.

Fatigue Threshold Analysis of Adhesives: Displacement Control vs. Load Control Strategy

The aim of this paper is to study the response of a two-part polyurethane-resin adhesive under quasi-static and fatigue loading conditions, to compare load control and displacement control approaches for the mode I fatigue threshold analysis of the tested material. To achieve this, double cantilever beam (DCB) joints were manufactured and tested. For the post-processing of the raw data, a compliance-based beam method was used. Out of this analysis, R curves and Paris law curves were obtained. Both approaches showed a similar Paris law slope meaning a low sensibility of the crack growth rate between them. As the displacement control load decreases gradually during the test, it can give more precise threshold energy than the load control technique.

Biomechanical influence of T1 tilt alteration on adjacent segments after anterior cervical fusion

Background: Anterior cervical fusion (ACF) has become a standard treatment approach to effectively alleviate symptoms in patients with cervical spondylotic myelopathy and radiculopathy. However, alteration of cervical sagittal alignment may accelerate degeneration at segments adjacent to the fusion and thereby compromise the surgical outcome. It remains unknown whether changes in T1 tilt, an important parameter of cervical sagittal alignment, may cause redistribution of biomechanical loading on adjacent segments after ACF surgery. Objective: The objective was to examine the effects of T1 tilt angles on biomechanical responses (i.e.range of motion (ROM) and intradiscal VonMises stress) of the cervical spine before and after ACF. Methods: C2-T1 FE models for pre- and postoperative C4-C6 fusion were constructed on the basis of our previous work. Varying T1 tilts of -10°, -5°, 0°, 5°, and 10° were modeled with an imposed flexion-extension rotation at the T1 inferior endplate for the C2-T1 models. The flexion-extension ROM and intradiscal VonMises stress of functional spinal units were compared between the pre- and postoperative C2-T1 FE models of different T1 tilts. Results: The spinal segments adjacent to ACF demonstrated higher ROM ratios after the operation regardless of T1 tilt. The segmental ROM ratio distribution was influenced as T1 tilt varied and loading conditions, which were more obvious during displacement-control loading of extension. Regardless of T1 tilt, intradiscal VonMises stress was greatly increased at the adjacent segments after the operation. As T1 tilt increased, intradiscal stress at C3-C4 decreased under 30° flexion and increased under 15° extension. The contrary trend was observed at the C6-C7 segment, where the intradiscal stress increased with the increasing T1 tilt under 30° flexion and decreased under 15° extension. Conclusion: T1 tilt change may change biomechanical loadings of cervical spine segments, especially of the adjacent segments after ACF. Extension may be more susceptible to T1 tilt change.

A phase-field model for interactive evolution of phase transformation and cracking in superelastic shape memory ceramics

This work presents a modified phase-field model for accurate coupling of phase transformation and cracking in shape memory ceramics. The existing phase-field models underestimate the elastic response at the beginning of the mechanical response. We modified the chemical free energy to control the rate of phase transformation and consequently obtain a physical elastic response before initiation of phase transformation. First, the forward and reverse martensitic phase transformation in a superelastic single crystal 3 mol% yttria-stabilized tetragonal zirconia is studied. Then, the interaction between phase transformation and fracture under displacement-controlled loading condition is investigated. The model predicts a realistic mechanical response and the experimentally observed microstructure and crack deflection due to the phase transformation. In addition, the model captures the reverse phase transformation and the stress drop due to the crack propagation.

A peridynamic approach to solving general discrete dislocation dynamics problems in plasticity and fracture: Part II. Applications

In Part II of this work we extend the method introduced in Part I to consider dislocation dynamic evolution through dislocations nucleation, glide, pile-up, and annihilation. The SP DDD-PD scheme is employed to investigate uniaxial tension in a single crystal and a polycrystal and verify its accuracy. The model is then used to simulate elastoplastic fracture by considering interactions between dislocations and crack growth. For Mode I elastoplastic fracture in a single crystal, we observe that the crack path is “attracted” towards regions of high density of gliding dislocations, leading to an undulating crack paths, as observed in experiments but never replicated by continuum-level computational models before. Tests on different sample sizes show how the proximity of constraints to the crack tip can lead to plastic hardening. Ductile-to-brittle transition happens naturally in this model when the crack, under Mode I displacement-controlled loading, approaches a free edge. A new way to calibrate the critical bond strain based on the material toughness or fracture energy is proposed. The present SP DDD-PD scheme can be used to investigate complicated elastoplastic fracture problems in which the interaction between dislocation motion and damage is critical.

Experimental, numerical, and theoretical crushing behaviour of an innovative auxetic structure fabricated through 3D printing

This paper aims to investigate the crushing behaviour and protective performance of an innovative auxetic structure, named the hourglass structure (HGS), under in-plane compressive loads. The HGS was fabricated with a 3D printing method through fused filament fabrication technique. The 3D printed HGS specimens were subjected to a displacement-controlled in-plane quasi-static compressive load. A 3D comprehensive numerical model was developed and calibrated with the experimental results. The calibrated numerical model was used for a parametric study to explore the effects of relative density, functional grading, and crushing velocity on the protective performance of the HGS. Furthermore, a theoretical model was proposed to correlate plateau stress of the HGS with geometric design parameters, effective Poisson’s ratio, and matrix material properties. Verified with a series of numerical analyses of different HGS designs, the proposed theoretical model predicted plateau stress with less than 10% relative error. In addition, experimental, numerical, and theoretical analyses uncovered load–deformation relationship, Poisson’s ratio, crushing mechanism, and energy absorption capacity of the novel auxetic. Overall, the HGS exhibited excellent features for protective engineering applications such as shock mitigation and blast energy absorption.

Experimental investigation of effect of changing gap size in buckling restrained braces

This experimental study investigates the effect of changing the debonding gap between the core and the mortar filler on the global stability of BRBs using different cross-section restrained members. A total of eight mortar-filled BRBs were produced using a rectangular cross-section steel core and two different restraining members, a circular hollow cross-section (BRB-C series) and a square hollow cross-section (BRB-S series). In choosing the gap sizes, the gap limit formulas in the literature were considered. Not exceeding the limits of the gap, 2.8 mm, 5.6 mm, 8.4 mm, and 11.2 mm gaps were left as the multiples for the radius of inertia of the rectangular cross-section core in the BRBs. All specimens were subjected to displacement-controlled axial cyclic loading. As a result, failure modes, hysteretic performance, energy dissipation, and ductility were summarized and analysed for each specimen. The results showed that the specimens had a better structural performance with a 2.8-mm gap size, which is the radius of inertia of the core. Furthermore, the local buckling modes throughout the core increased in number as the size of the gap decreased, thus raising the members' axial load capacity and energy dissipation. It was concluded that the best structural behaviour of the tested series was provided with the smallest gap of 2.8 mm, better behaviour in energy dissipation may be obtained with smaller gap sizes. However, in this case, it is more likely to increase buckling at the gusset plates and cause local deformations with the end of the restraining members.

Experimental and analytical investigations of steel shear buckling-inhibited energy dissipation devices

This study presents the development and cyclic performance assessment of steel buckling-inhibited shear yielding devices (St-BISYDs). These devices consisted of thin steel plates as the main energy dissipating elements and modular built-up restrainers as the buckling inhibition mechanism. St-BISYDs are designed to achieve a shear strain of at least 20% without buckling to maximize the energy dissipation potential by minimizing the heat-affected zone (HAZ) and using the bolted connections. Ten specimens with two geometric configurations, namely, hourglass and rectangular, steel plates were experimentally tested under quasi-static displacement-controlled cyclic loading. The proposed restrainers greatly enhanced the cyclic performance of St-BISYDs and helped the St-BISYDs to reach the target shear strain level without significant strength and stiffness degradation under cyclic loading. An analytical study is conducted to predict the force-deformation characteristics of St-BISYDs. Experimental results of the tested specimens are also compared with other similar energy dissipation devices. Further, nonlinear modeling parameters are proposed for the force-deformation response of St-BISYDs which can be directly used in the design practices.

Discrete element simulation on failure mechanical behavior of transversely isotropic shale under two kinds of unloading paths

Based on the discrete element method, a two-dimensional numerical model of shale with different bedding inclinations was established. Using this model, the macro-and meso-mechanical properties of bedded shale under different unloading paths and bedding inclination were studied. In this study, the damage mechanism of bedded shale during the unloading process is revealed. The simulation uses the displacement-controlled loading method, with a loading speed of 0.05 m∙s−1. The results indicate that the bedding inclination of the shale has a significant impact on both mechanical parameters and failure modes under unloading. The peak strength and the ultimate bearing capacity show a “U” shape trend under both unloading paths. Observations of maximum values are observed at β = 0° or β = 90° while observing the minimum values at β = 30°or β = 45°. The failure mode of shale specimens with β = 0° at lower initial confining pressure, the failure mode is splitting damage along the multiple bedding plans, which gradually changes to shear damage with increasing initial confining pressures crossing the weak surface. The failure mode of the specimens with β = 15–45° at different initial confining pressure is shear-slip damage along the bedding planes. And shale at β = 60–90° failure modes are all conjugate shear damage intersecting with the weak surface. The damage to the specimen caused by unloading and increasing the axial stress is greater than that caused by the unloading and constant axial strain. The effects of β = 0° and 90° on microcracking and stress distribution within the specimen are significantly smaller than for the other bedding inclinations.

A fully coupled 3D elastohydrodynamic model built with MITC element for static performance analysis of gas foil bearings

PurposeThis paper aims to present a 3D static performance analysis model for the gas foil journal bearing to provide better understanding of the gas foil journal bearing and extend the development of the calculation about the static performance.Design/methodology/approachThe foil bearing can be seen as a shell structure, and the mixed interpolation of tensorial components (MITC) element was used to build the shell model. The augmented Lagrange method was used to calculate the contact involving friction between foils and between the foil and the bearing sleeve. A displacement-controlled load scheme was used to calculate the deformation of the foils. A mapping operator was used to map the film pressure from the gas to the surface of the top foil.FindingsThis method provides high precision of calculation in the prediction of the static performance. The calculation results were compared with the experimental data, and they show good agreement. Meanwhile, the model can be applied in the prediction of the bearing performance in a broad range of working conditions.Originality/valueThis method extends the calculation of the gas foil journal bearing to a 3D scale and shows good agreement with the experimental data. Meanwhile, the present model has a good adaptability on the revolution speed and can be applied to the predictions in varied working conditions.

Finite-Time Stability of Polyhedral Sweeping Processes with Application to Elastoplastic Systems

We use the ideas of Adly-Attoych-Cabot [Adv. Mech. Math., 12, Springer, 2006] on finite-time stabilization of dry friction oscillators to establish a theorem on finite-time stabilization of differential inclusions with a moving polyhedral constraint (known as polyhedral sweeping processes) of the form $C+c(t).$ We then employ the ideas of Moreau [New variational techniques in mathematical physics, CIME, 1973] to apply our theorem to a system of elastoplastic springs with a displacement-controlled loading. We show that verifying the condition of the theorem ultimately leads to the following two problems: (i) identifying the active vertex ``A'' or the active face ``A'' of the polyhedron that the vector $c'(t)$ points at; (ii) computing the distance from $c'(t)$ to the normal cone to the polyhedron at ``A''. We provide a computational guide to implement steps (i)-(ii) in the case of an arbitrary elastoplastic system and apply the guide to a particular example. Due to the simplicity of the particular example, we can solve (i)-(ii) by the methods of linear algebra and minor combinatorics.

Experimental Investigation on Flexural Performance of Masonry Wallettes Strengthened with Cementitious Matrix Grid

A large experimental study was conducted at IIT Patna, India to evaluate the effectiveness of different types of cementitious matrix grids (CMGs) in improving the flexural performance of unreinforced masonry specimens. Four types of masonry wallettes: brick-lime mortar, brick-cement mortar, brick-mud mortar and autoclaved aerated concrete (AAC) block masonry were prepared in the laboratory. In this study, eleven different CMG comprising of glass fabric and steel wire meshes embedded in the five different grades of cementitious matrix were used to strengthen the masonry assemblages. The aim of this study is to understand the role of various parameters such as tensile strength of CMG, compressive strength of masonry and cementitious matrix in influencing the efficiency of the strengthening scheme. In total 130 specimens with failure plane-parallel and perpendicular to the bed joint were prepared and tested under quasi-static displacement control loading. Considering the ease of installation, the fabric was directly placed on the masonry wallette using mechanical anchors and then covered with a thick layer of cementitious matrix.Test results highlights that all strengthening schemes are effective and can significantly enhance the flexural moment capacity in the range of 2.5 - 63.0 times the flexural moment of the respective control specimens. These strengthening schemes effectively mitigate the brittle behaviour of masonry wallettes and improved the deformation capacity by 1.2 - 18.1 times when compared to the respective control specimens. The study also illustrated that the strength of cementitious matrix can play an important role in contributing to the strength and deformability of the masonry specimens strengthened with CMG. For low strength cementitious matrix, debonding failure was commonly observed, whereas, for high strength cementitious matrix, the failure/rupture of reinforcement was noticed. In addition, the shear failure of masonry or debonding failure of reinforcing mesh was observed for specimens in which CMG had higher percentage of reinforcement.

COMPARISON OF MRF AND CBF STRUCTURAL RESPONSE TO EARTQUAKE IN OFFICE BUILDING SURABAYA

Indonesia is a country that prone earthquakes. Therefore, knowledge of earthquake-resistant building construction is needed in reducing the risk of damage caused by earthquakes. There are several methods of earthquake-resistant construction planning, one of which is the pushover analysis method. Pushover is a nonlinear static analysis method in which the structure is subjected to gravity loading and displacement-controlled lateral loads which continue to increase through elastic and inelastic behavior until the final condition. One of the popular construction materials in structural planning is steel construction, which has a uniform structure, lightweight, strong, and easy to work with. In modeling earthquake-resistant structures, several popular models are the MRF and CBF models. MRF is a structural configuration model configured on beams that are firmly connected to columns. Based on a rigid beam-column connection, the moment frame cannot be moved laterally without bending the beam or column depending on the connection geometry. Results The case study on the comparison of the two structural models concluded that in the design of earthquake-resistant steel structures using the pushover method, it was found that the MRF structure had higher ductility than the CBF, namely 2.1:1.6.

Experimental study on two innovative ductile moment-resisting precast concrete beam-column connections

The use of precast concrete elements in construction leads to increased output quality, better product control, cost savings, and reduced construction time. The main problem of precast concrete structures is the improper performance of connections under lateral loads. In the present study, the aim is to improve the behavior of precast connections under cyclic loading. Two new beam-column precast connections with steel profiles as corbel with high-strength bolt fasteners are developed. Those connections have been designed to be used in intermediate moment frames in regions with high seismic risk. The advantage of these connections is the elimination of the need for remarkable concrete formwork, temporary supports, and low demand for in-situ concrete during construction and providing the possibility of construction progress on the floors in parallel. For studying the seismic performance of proposed connections, six specimens including four precast and two monolithic connections with different rebar ratios were made. The specimens were subjected to a horizontal displacement-control cyclic loading to obtain the load-displacement curves. The maximum lateral strength, ductility, strength degradation, stiffness reduction, and energy dissipation of precast connections were evaluated and compared with the in-situ connections. The ultimate strength and ductility of the precast connections were similar to the in-situ specimens, and proposed connections showed good cyclic behavior. The strength degradation and stiffness deterioration of precast connections were more than the in-situ connection, and the energy dissipation of them was less than monolithic connections. According to the analysis results, the performance of the proposed connections is acceptable and satisfies the criteria of the connections for use in areas with high seismicity.