Quasi-static Loading Protocol Research Articles

Micro-Mechanical Fracture Investigations on Grain Size Tailored Tungsten-Copper Nanocomposites

Tungsten-copper composites are used in harsh environments because of their superior material properties. This work addresses a tungsten-copper composite made of 20 wt.% copper, which was subjected to grain refinement by high-pressure torsion, whereby the deformation temperature was varied between room temperature and 400 °C to tailor the grain size. Deformation was performed up to microstructural saturation and verified by hardness measurement and scanning electron microscopy. From the refined nanostructured material, micro-cantilever bending beams with cross-sections spanning from 5 × 5 to 35 × 35 µm2 were cut to examine possible size effects and the grain size influence on the fracture behavior. Fracture experiments were performed in situ inside a scanning electron microscope by applying a quasi-static loading protocol with partial unloading steps. Inspection of the fracture surfaces showed that all cantilevers failed in an inter-crystalline fashion. Nevertheless, remaining coarser tungsten grains impacted the resultant fracture toughness and morphology. Cantilevers fabricated from the 400 °C specimen exhibited a fracture toughness of 220 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm $$\\end{document} 50 Jm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{\ ext{J}}}{{{\ ext{m}}}^{2}}$$\\end{document}. For the room temperature cantilevers, a fracture toughness of 410 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm $$\\end{document} 50 Jm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{\ ext{J}}}{{{\ ext{m}}}^{2}}$$\\end{document} was observed, which declined to 340 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm $$\\end{document} 30 Jm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{\ ext{J}}}{{{\ ext{m}}}^{2}}$$\\end{document} for cantilevers < 10 × 10 µm2, confirming a size effect. The increased fracture toughness is attributed to the delamination-like structures formed in the room temperature sample.

Experimental validation of a novel curved steel damper for steel frames with flexible beam-column joints

In this study, results of an experimental program are presented to validate the performance of a novel energy dissipation device (EDD) designed for use in moment-resisting steel frame structures with flexible beam-column connections. The EDD, composed of a curved steel plate, stiffeners, and flanges on both ends, was tested in laboratory conditions using a displacement control quasi-static loading protocol. Three half-scale steel frames were used, one serving as a control and the other two incorporating the device in both column-beam joints. The device was fabricated using two different materials, mild steel, and stainless steel, and the results were compared. The results of the experimental program revealed that the frames equipped with the energy dissipation device performed significantly better than the control frame. This was observed qualitatively, in terms of the amount of damage sustained by the frames, and quantitatively, through measurements of the frames' capacity and displacement ductility. Frames that were equipped with dampers showed an approximately 1.5-fold increase in displacement ductility and an 84–113% increase in cumulative energy dissipation compared to the control frame. Overall, this research highlights the potential of this novel energy dissipation device to enhance the seismic performance of moment-resistance steel frame structures.

Restoring seismic capacity of damaged dry stacked Self-Interlocking masonry structure through ferrocement overlay

Based on the increasing demand of the self-interlocking stabilized earth blocks in masonry industry and history of the earthquake losses, research study on strengthening and retrofitting techniques is needed for this novel masonry material. Therefore, in this research study, a damaged dry stacked self-interlocking block masonry structure was retrofitted with ferrocement overlay and tested under quasi-static loading protocol. The performance of the tested structure before and after retrofitting are compared in terms of different seismic parameters including force–deformation, hysteresis loops, energy dissipation, lateral stiffness, displacement ductility, response modifications factors, and performance levels. It was found that the seismic capacity of the damaged dry stacked self-interlocking masonry structure was restored and enhanced.

Biomechanical testing of different fixation techniques for intraoperative proximal femur fractures: a technical note

ABSTRACT Intraoperative proximal femoral fractures (IPFF) represent a rare but challenging complication of total hip arthroplasties. They usually occur as a longitudinal split. This pilot trial aimed to compare the biomechanical primary stability of different fixation techniques for IPFF. Standardised longitudinal medial split fractures of the proximal femur (type II, Modified Mallory Classification) were created in artificial osteoporotic and non-osteoporotic composite femora after implantation of a cementless femoral stem. Five different fixation techniques were compared: cerclage band, cerclage wiring with one or two wires, and lag screw fixation with one or two lag screws. A quasi-static loading protocol was applied and failure loads were evaluated. The observed median failure loads were 4192N (3982N – 5189N) for one cerclage band, 4450N (3577N – 4927N) for one cerclage wire, 5016N (4175N – 5685N) for two cerclage wires, 6085N (5000N – 8907N) for one lag screw, and 4774N (4509N – 8502N) for two lag screws. Due to the wide range of failure loads within the experimental groups, there were no observable differences between the groups. All fixation techniques provided sufficient primary stability in osteoporotic and non-osteoporotic composite bones. Further cadaveric studies with larger sample sizes may be needed to confirm the results presented here.

Seismic experimental assessment of beam-through beam-column connections for modular prefabricated steel moment frames

This study proposes a beam-through moment-resisting joint for H-section (wide-flange) beams and columns that can be prefabricated and easily assembled onsite for low-rise and mid-rise steel structures. Three groups (six in total) of beam-through beam-column connections are designed and tested under quasi-static loading protocols. The output parameters include joints' deformation, failure mechanism, hysteretic behavior, backbone curves, bearing capacity, ductility, energy dissipation capacity, joint rotational stiffness. The results demonstrate that the combination of through-beam continuity plate stiffeners, panel zone doubler plates, and reduced beam sections provide optimal seismic performance. A simplified analytical method is also proposed to calculate the initial rotational stiffness of the through-beam column joint.

Experimental investigation of reinforced concrete masonry shear walls with C-shaped masonry units boundary elements

There is a global drive to promote and optimize the design of high performance buildings at low cost and minimum environmental impact. Although reinforced masonry construction is known for its better fire protection, structural durability, and construction cost reduction, its use is hindered by a lack of understanding of its resistance to earthquake loads. This research aims to quantify the effect of influential parameters on the seismic performance of Reinforced Masonry Shear Walls (RMSW) with masonry boundary elements (MBEs). A new system/configuration for RMSW with C-shaped masonry units has been proposed. A new experimental test setup was designed and built to capture the response of the lower panel of RMSW in a 12-storey building subjected to quasi-static loading protocol. Test results showed that the proposed system could provide the lateral strength and ductility required to resist earthquake events and that the average measured compressive strain was almost four times the limit specified in CSA S304-14. The results also showed that single out-of-plane support at the loading beam level was inadequate, leading to out-of-plane failure. When additional out-of-supports were provided, the failure mode changed to web crushing and buckling of vertical web reinforcement.

Generalized Loading Protocols for Experimentally Simulating Multidirectional Earthquake Actions on Building Columns in Regions of Low-to-Moderate Seismicity

AbstractThis study aimed to quantitatively develop realistic quasi-static loading protocols for simulating bidirectional cyclic actions and axial load variation on building columns in a way that is...

Study on Quasi-Static Loading Protocols considering the Action Characteristics of Long-Period Ground Motions

Due to abundant low-frequency components of long-period ground motions (LPGMs), long-period structures are susceptible to severe damage. The corresponding time-history displacement responses have significant “large-displacement” and “long-duration” characteristics. These action characteristics essentially reflect the different loading paths imposed on structures of LPGMs from ordinary ground motions (OGMs). Hence, revealing the influence mechanism of the action characteristics on the seismic performance of structural components is the key to investigating the influence of LPGMs on the whole structure. This paper presents a kind of quasi-static loading protocol considering the action characteristics of LPGMs. Firstly, nonlinear time-history analyses on structural systems subjected to 50 selected representative LPGMs were conducted. Inelastic cycles and corresponding amplitudes of time-history displacement responses under LPGMs were statistically analyzed through the rainflow method. Then, considering two of the most significant factors, structural period and target ductility, a prediction model of cycle number and cycle amplitude was obtained by regression. On this basis, the quasi-static loading protocol considering the action characteristics of LPGMs was developed. Proposed protocols can be directly applied to experimental investigations on the seismic performance for structural components under LPGMs.

Experimental-numerical study on weakened HSS-to-HSS connections using HBS and RBS approaches

Hollow structural sections (HSS) have been used more often as columns because of their symmetrical geometry and admissible performance in bearing axial forces along with bending and torsional moments. The use of hollow sections as beams in moment-resisting frames would be beneficial and is worthy to assess. To this aim and considering the importance of developing plastic bending hinges of the special moment-resisting frames in a zone far from the column face, two methods for weakening beam sections are studied and compared in this paper: 1) Reduced beam section (RBS) and 2) Heat-treated beam section (HBS). For the experimental investigation, a furnace was first made to heat the desired zone of the beam. Then, a cyclic quasi-static loading protocol was applied to two full-scale beam-column connection specimens. The experimental specimens were made by box sections in which the beams had reduced or heat-treated sections. Moreover, for verification and parametric studies, finite element models were generated using ABAQUS. The columns of numerical models had box sections whereas rectangular and circular hollow structural sections were used as beams. Additionally, the seismic behavior of two beam-column connection models with box and pipe heat-treated beam sections was investigated under near-field loading protocol. Compared with RBS sections, both the results of experimental and numerical studies indicated the acceptable bending behavior of heat-treated hollow structural sections in terms of prevention of lateral-torsional buckling along with a stable hysteretic behavior.

Hysteretic behavior investigation of self-centering double-column rocking piers for seismic resilience

Seismic resilient structures with low structural damage and self-centering behavior after an earthquake are focus issues of current earthquake engineering. Unbonded pretensioned rocking columns offer advantages for accelerated bridge construction in seismic regions, which utilize column-uplifting mechanisms, high strength post-tensioning, and replaceable energy dissipation devices to enhance seismic performance and resilience. The experiment of three 1:3 scale unbonded, post-tensioned rocking bridge bents with different types of external replaceable energy dissipation devices subjected to quasi-static loading protocols, were carried out according to practical engineering demand in high-intensity earthquake regions. An improved analytical model was proposed for predicting the force-displacement relationship of the post-tensioned rocking bridge bent systems considering the neutral axis depth. Minimal physical damage was observed for the post-tensioned rocking bridge bent systems, which exhibit good energy dissipation and self-centering behavior. Especially, the external replaceable buckling-restrained plates dissipaters can be easily replaced if severely damaged subjected to higher than expected ground motions. The satisfactory analytical and experimental comparisons are presented as a validation of the reliability for the high-performance seismic-resistant bridge bent systems and the modelling techniques in this study.

Cyclic Loading for RC Bridge Columns Considering Subduction Megathrust Earthquakes

Current structural design philosophies rely on the inelastic capacity of structures for resisting seismic excitations. To assess such capacity, cyclic loading protocols have been used as a common practice in laboratory and numerical evaluations. The main objective was to obtain quasi-static loading protocols that reflect the increase in the inelastic demands of bridge columns subjected to long duration ground motions caused by subduction megathrust earthquakes. To study the demands imposed on the structural system, results from nonlinear time-history analyses considering numerous subduction ground motions imposed on structures with a wide range of structural periods and target ductilities were used. Because the number of inelastic cycles and the expected damage can be closely related, statistical analyses of the number of inelastic cycles and the cumulative inelastic demands were used to develop the loading protocols. Due to the observed dependence of the demand parameters on the structural period, different protocols were developed for short-, medium-, and long-period responses. The proposed cyclic deformation histories are more representative of the inelastic demands from subduction earthquakes; therefore, their application would improve the seismic assessment of bridge columns exposed to such hazards.

High-Strength Fiber-Reinforced Concrete Beam-Columns with High-Strength Steel

A series of reinforced concrete (RC) components with high-strength materials were tested under pure flexural monotonic and cyclic quasi-static loading protocols. The specimen materials included high-strength concrete (HSC), ultra-high-strength steel (UHSS), and steel fibers to create high-strength fiber-reinforced concrete (HSFRC) specimens. The replacement of conventional steel with UHSS increased the specimen’s ultimate flexural capacity more than 60%, but reduced its ductility. The addition of steel fibers to HSC specimens increased the peak strength by an additional 10%, and greatly reduced cracking and spalling of HSC specimens. However, the energy dissipation capacity of the specimens did not increase due to the presence of fibers.

Nonlinear phase field theory for fracture and twinning with analysis of simple shear

A phase field theory for coupled twinning and fracture in single crystal domains is developed. Distinct order parameters denote twinned and fractured domains, finite strains are addressed and elastic nonlinearity is included via a neo-Hookean strain energy potential. The governing equations and boundary conditions are derived; an incremental energy minimization approach is advocated for prediction of equilibrium microstructural morphologies under quasi-static loading protocols. Aspects of the theory are analysed in detail for a material element undergoing simple shear deformation. Exact analytical and/or one-dimensional numerical solutions are obtained in dimensionless form for stress states, stability criteria and order parameter profiles at localized fractures or twinning zones. For sufficient applied strain, the relative likelihood of localized twinning vs. localized fracture is found to depend only on the ratio of twin boundary surface energy to fracture surface energy. Predicted criteria for shear stress-driven fracture or twinning are often found to be in closer agreement with test data for several types of real crystals than those based on the concept of theoretical strength.

Performance Evaluation of Different Masonry Infill Walls with Structural Fuse Elements Based on In-Plane Cyclic Load Testing

This paper discusses the performance of a structural fuse concept developed for use as a seismic isolation system in the design and retrofit of masonry infill walls. An experimental program was developed and executed to study the behavior of the structural fuse system under cyclic loads, and to evaluate the performance of the system with various masonry materials. Cyclic tests were performed by applying displacement controlled loads at the first, second, and third stories of a two-bay, three-story steel test frame with brick infill walls; using a quasi-static loading protocol to create a first mode response in the structural system. A parametric study was also completed by replacing the brick infill panels with infill walls constructed of concrete masonry units and autoclaved aerated concrete blocks, and applying monotonically increasing, displacement controlled loads at the top story of the test frame.

Experimental and numerical developing of reduced length buckling-restrained braces

Buckling-restrained braced frames (BRBFs) have been widely used as an efficient seismic load resisting system in recent years mostly due to their symmetric and stable hysteretic behavior and significant energy dissipation capacity. However, buckling-restrained braces (BRBs) are heavier and more expensive in comparison to other concentric bracing systems. In order to facilitate the use of BRBs, the idea of reducing the length of the core and the encasings which can result in lighter and more replaceable BRBs is proposed and experimentally investigated in this paper. Two relatively similar all-steel reduced length BRBs (RLBRBs) are designed, detailed and constructed using a special debonding and stopper mechanism. The design and construction procedure is accomplished by paying special attention to low-cycle fatigue (LCF). The specimens were tested under the quasi static loading protocol, and withstood high axial strains of 4–5% without any global or local failure. The hysteretic responses of the specimens were stable and symmetric. Moreover, numerical models were developed and nonlinear cyclic analyses were performed to provide better insight into the core and encasing performance as well as application of the RLBRB in the brace configuration.

Characterization and prediction of rate-dependent flexibility in lumbar spine biomechanics at room and body temperature

Background contextThe soft tissues of the spine exhibit sensitivity to strain-rate and temperature, yet current knowledge of spine biomechanics is derived from cadaveric testing conducted at room temperature at very slow, quasi-static rates. PurposeThe primary objective of this study was to characterize the change in segmental flexibility of cadaveric lumbar spine segments with respect to multiple loading rates within the range of physiologic motion by using specimens at body or room temperature. The secondary objective was to develop a predictive model of spine flexibility across the voluntary range of loading rates. Study designThis in vitro study examines rate- and temperature-dependent viscoelasticity of the human lumbar cadaveric spine. MethodsRepeated flexibility tests were performed on 21 lumbar function spinal units (FSUs) in flexion-extension with the use of 11 distinct voluntary loading rates at body or room temperature. Furthermore, six lumbar FSUs were loaded in axial rotation, flexion-extension, and lateral bending at both body and room temperature via a stepwise, quasi-static loading protocol. All FSUs were also loaded using a control loading test with a continuous-speed loading-rate of 1-deg/sec. The viscoelastic torque-rotation response for each spinal segment was recorded. A predictive model was developed to accurately estimate spine segment flexibility at any voluntary loading rate based on measured flexibility at a single loading rate. ResultsStepwise loading exhibited the greatest segmental range of motion (ROM) in all loading directions. As loading rate increased, segmental ROM decreased, whereas segmental stiffness and hysteresis both increased; however, the neutral zone remained constant. Continuous-speed tests showed that segmental stiffness and hysteresis are dependent variables to ROM at voluntary loading rates in flexion-extension. To predict the torque-rotation response at different loading rates, the model requires knowledge of the segmental flexibility at a single rate and specified temperature, and a scaling parameter. A Bland-Altman analysis showed high coefficients of determination for the predictive model. ConclusionsThe present work demonstrates significant changes in spine segment flexibility as a result of loading rate and testing temperature. Loading rate effects can be accounted for using the predictive model, which accurately estimated ROM, neutral zone, stiffness, and hysteresis within the range of voluntary motion.

Seismic testing and performance of buckling-restrained bracing systems

This paper describes a subassemblage seismic test program performed on six buckling-restrained braces (BRBs). Two different brace core segment lengths and two different buckling-restraining mechanisms were examined. The applied loading histories included a qualifying quasi-static cyclic test with stepwise incremental displacement amplitudes and a dynamically applied seismic loading. A test was also carried out on a conventional bracing member for comparison purposes. The concrete-filled tube specimens exhibited satisfactory performance under the quasi-static loading protocol, regardless of the length of the core segment. Strain hardening and frictional responses resulted in brace axial forces significantly exceeding the core yield capacity. The steel BRB system exhibited good performance under the quasi-static and dynamic loading sequences, provided that the clearance between the brace core and the buckling-restrained mechanism was kept to a minimum. The dynamic loading protocol was less severe for low-cycle fatigue than the quasi-static loading, but higher strain rates resulted in amplified yield resistance. The conventional bracing member withstood the entire quasi-static loading history but exhibited limited energy-dissipation capacity compared with the concrete-filled BRBs.Key words: concentrically braced steel frames, bracing members, buckling, energy dissipation, friction, yielding, fracture, seismic.

Cyclic Testing of Built-Up Steel Shear Links for the New Bay Bridge

Tests were conducted on two prototype steel shear links for the main tower of the new San Francisco-Oakland Bay self-anchored suspension bridge to evaluate the link force and deformation capacities. The links were built-up wide-flange sections, designed to yield in shear. A quasi-static loading protocol was used to test the links in reverse curvature, simulating the expected seismic demand. The link capacities exceeded the predicted demands from the Safety Evaluation Earthquake. The specimens behaved in a ductile manner until small cracks initiated at the end of the vertical fillet welds connecting the intermediate stiffeners to the link web. As the cracks propagated further, brittle fracture of the web ensued. The maximum shear strength was nearly twice the expected yield shear strength, a significantly higher overstrength than current codes recognize. Alleviating the stress concentration on the vertical fillet welds of the intermediate web stiffeners is necessary to avoid brittle fracture and to increas...