Axial Pre-compression Research Articles

On the dynamic failure mechanism of Vit-1 bulk metallic glass: Coupling effects of pre-made damage and strain rate

To clarify the two distinct effects and the underlying physical mechanisms of pre-made damage on the failure strength of bulk metallic glass (BMG), i.e. strengthening or weakening, the influence of axial pre-compression on the dynamic failure of Vit-1 BMG are investigated experimentally. Quasi-static and dynamic compression tests were conducted on pre-compressed BMG specimens for a wide range of nominal strain rates between 0.001 s−1 to 6000 s−1. The focus is on the coupled effects of pre-compression and strain rate on the dynamic failure stress. Two contrasting mechanisms were identified that influence how the pre-made shear bands affect the propagation of cracks under different strain rates. It will be shown that the dynamic failure stress exhibits a transition from strengthening to knockdown with increasing strain rate; and, that the extent of these strengthening and knockdown effects depends on the pre-compression strain level. Finally, a failure model that captures the effects of both strain rate and pre-made damage is proposed.

Assembled self-centering brace with replaceable friction damper for seismic resilience: Development and mechanical behavior research

A novel seismic resilient brace, integrating a replaceable friction damper mechanism with an assembled self-centering device of combination disc springs, has been developed and investigated through experimental research. This brace can be named as the Assembled Pre-pressed Disc-springs Self-centering Replaceable Energy Dissipation (APD-SRED) brace. The working principles, structural configurations, fabrication process and mechanical behaviors of this bracing system were discussed and analyzed firstly in detail. Subsequently, five brace specimens were designed incorporating different axial pre-compression forces within a combination disc springs system and varying friction forces in the damper device. Test results exhibit a stable and repeatable flag-shaped hysteretic responses with satisfactory energy dissipation, which can be implemented under the low cyclic reversed loading. Additionally, it can be noted that the excellent seismic resilience performance can be obtained by maintaining a relative balance between self-centering and energy dissipation characteristics. The stiffness and strength of this brace can withstand cyclical loading without degradation, making superiorities against sequential design earthquakes without the replacement or repair, and facilitating replacement of the friction energy dissipation components to meet different demands. Moreover, the proposed calculation model can be validated accurately predicting the mechanical behavior of the APD-SRED brace by considering friction effects in disc-springs. This validation was achieved through comparisons with experimental results, and several crucial design factors influencing the brace's behavior were analyzed and discussed for practical application in a parametric design. Finally, drawing upon a combination of experimental and analytical studies, this study ultimately presented practical design recommendations.

Determination of distal fibula fracture for prediction of ankle injuries

Objective This study quantified the local fracture tolerance of the distal fibula. The purpose of this data is to refine the understanding of ankle fracture tolerance in the population and enhance injury prediction by computational tools. Methods Fracture patterns of the fibula were obtained from the US National Highway Traffic Safety Administration (NHTSA) Crash Injury Research Engineering Network (CIREN). Of 143 cases of ankle injury, 120 included fibula fracture, many of which accompanied by radiology images. The most common fibula fracture type was a Weber C fracture, which was then used as the testing target to replicate with postmortem human surrogates. Isolated distal fibulae (male and female, across a wide anthropometric range) were subjected to quasi-static lateral-medial four-point bending superimposed on axial precompression. Results Of the 20 specimens tested, 17 fractured in compression and bending, two fractured in compression, bending, and shear, and one did not fracture upon the imposed displacement. Fractures occurring outside of the target testing span were treated as right-censored data points. Fracture patterns varied among specimens, with oblique fractures being most common, followed by segmental fractures. At failure, compressive force ranged from 77 N to 370 N and bending moment from 17 Nm to 47 Nm. Conclusions From the 19 fractured specimens, the variety of fracture patterns observed were generally consistent with the range of fibula fracture types and locations commonly observed in the field. While comparative studies with fibula specimens are largely absent from literature, comparison of this study to other long bones (radius and ulna) show that bending moment at fracture is similar. To the author’s knowledge, this is the first study to quantify the fracture tolerance of isolated distal fibulae under loading conditions representative of ankle injury mechanisms in motor vehicle collisions. By combining the results of this study with complementary results from parallel tibia-focused experiments (currently in-press), these results will aid development of tissue-level lower leg fracture prediction of human body models, thus enhancing prediction of ankle injury during safety assessment simulations. Specifically, the fracture tolerance information might generate an injury risk function to guide tissue-level injury prediction with human body models.

Parametric study of reinforced interlocking brick wall under cyclic loading

Mortarless interlocking brick walls are a new form of masonry structures that can be efficiently constructed. In this study, a detailed numerical model of an interlocking brick wall under quasi-static in-plane cyclic loading is generated and verified with laboratory test data. The influence of the vertical reinforcement placement methods, i.e., grouted vs. unbonded, and the influence of wall-to-brick size ratio are firstly investigated. Parametric studies are then carried out to quantify the influences of axial precompression level, shear span ratio and friction coefficient between bricks on the lateral load-carrying capacity, deformation capacity, and energy dissipation capability of the studied mortarless interlocking brick walls subjected to in-plane cyclic loading with analytical equations summarised for predicting the resistance performance of this type of mortarless interlocking brick walls under in-plane cyclic loading.

Steel tubes subjected to combined axial compression and bending

The bending response characteristics of ten tubes subjected to axial pre-compressive stresses ranging from 0 to 45 % of the tube wall yielding stress (σr) were investigated experimentally. The tubes had different diameter-to-thickness ratios of 42 and 56 and length-to-diameter ratios of 15 and 20. The results showed up to a 31 % reduction in the bending capacity of tubes when σr increased from 0 to 45 %. Experimental interaction diagrams for axial pre-compression and bending moment capacity were compared with the proposed relationships in different codes, with the results showing a substantial load-bearing capacity of tubes beyond the codes' predictions.

Geometric properties and imperfections influence on buckling behavior of Variable-Thickness steel cylindrical shells subjected to combined Loading: Experimental and numerical study

In this study, a total of five cylindrical shells with different stepwise wall thicknesses and varying geometric ratios (including Length-to-radius ratio (L/R) and radius-to-thickness ratio (R/t)) were subjected to the same axial pre-compression, followed by a uniform external pressure load, with the axial pre-compression value being constant. The primary objective of this study was to evaluate the buckling capacity, crustal cross-section deformation in the middle of each strake, and failure mechanism in the specimens. The results indicated that the shell with the lowest L/R and R/t ratios demonstrated the highest buckling capacity and exhibited more stability during failure.Furthermore, the experiments were conducted to verify the accuracy of nonlinear finite element analyses by employing ABAQUS FEA. Then, the simulated method was utilized to numerically investigate the effect of the geometric properties, pattern distribution thickness in the height of shells, the number of strakes, and geometric imperfection of the full-scale cylindrical shells. The findings revealed that the distribution of buckling waves and buckling capacity for cylindrical shells with variable thickness strongly depended on the thickened region and the coefficient of distribution thickness pattern in the height of shells. Additionally, in cylindrical shells with varying stepwise wall thickness, the sensitivity to imperfection increased with increasing axial pre-compression.

On the experiment and design of the Tachi-Miura polyhedron’s radial load-bearing capacity

Recent research has yet to note the structural performance of the radial direction. This paper experimentally verified the factors influencing the TMP’s radial load-bearing capacity and designed the new intermediate layer patterns for the radial extension. The methodology first assumed the axial motion of TMP as the approaching among sections. Based on the analysis, the relationship among the parameters was revealed according to the experiments, which marked the TMP as a high-performance structure. The results proved the positive correlation between the axial pre-compression and the radial load-bearing capacity where the folding angle has reached the value range of the self-locking state. The new intermediate layer patterns were correspondingly designed to substantiate the hypothesis regarding the specific parts. The overlapped triangular facets and the concave vertex have enhanced the radial load-bearing capacity. The combination of the relevant parameters and the spatial attributes expands the application scope of the TMP configuration.

Behaviour of reinforced mortarless interlocking brick wall under cyclic loading

Interlocking brick masonry has gained much attention due to its high construction efficiency and low labour skill requirement. Most designs of interlocking bricks only use interlocking keys to provide alignment for easy construction, and most previous studies of interlocking masonry structures concentrate on their static loading capacities. This study examines the behaviour of reinforced mortarless interlocking brick walls under cyclic loading. Interlocking brick wall made of a specific type of interlocking bricks with large keys that provide not only alignment in construction but also shear resistance is constructed and tested under in-plane cyclic loading. A detailed numerical model is generated and validated with the testing data, which is then used to assist the analysis of wall responses. The damage mode, hysteresis response and energy dissipation characteristics are analysed. The test results are compared with conventional masonry wall from literature to demonstrate the superior performance of interlocking masonry wall in resisting seismic loading and dissipating seismic energy. The influences of axial precompression and shear span-to-length ratio are investigated via numerical modelling.

Experimental and numerical studies of the shear resistance capacities of interlocking blocks

Interlocking bricks could improve construction efficiency, reduce labour cost, and provide better mechanical performance for masonry structures. Nevertheless, the shear properties of mortar-less interlocking bricks have not been systematically investigated which may impede their wide applications. In this study, the shear performance of a new type of interlocking brick is investigated in detail. Laboratory shear test is firstly conducted to study the damage and shear capacity of mortar-less (dry-stacked) interlocking bricks. Numerical model is then generated with consideration of contact imperfection and validated with test results. Intensive parametric studies are conducted to quantify the influences of material strength, axial pre-compression force, friction coefficients, and contact imperfection at brick interfaces on the shear response of interlocking prisms. The accuracy of existing methods for predicting the shear capacities of shear key by design standard and empirical formula are evaluated. Based on the numerical and laboratory results, an empirical design formula is proposed to predict the shear capacity of the interlocking brick.

Influence of different design parameters on the seismic performance of partially grouted masonry shear walls

In recent earthquakes in Chile (e.g., Maule’s 2010 earthquake), buildings built with bed-joint partially grouted reinforced masonry (BJ-PG-RM) shear walls made of multi-perforated clay bricks (MPCB) have not collapsed, although significant damages have been reported. Additionally, experimental data on the influence of the different design parameters that control its seismic behavior is scarce, and the available expressions for estimating their lateral resistance are inaccurate. To address these issues, nine full-scale BJ-PG-RM walls made of MPCB were tested under axial pre-compression and cyclic lateral loads. The study addressed the influence of the aspect (height-to-length) ratio, axial pre-compression, mortar compressive strength, mortar joints’ thickness, bricks’ height, and horizontal and vertical reinforcement ratio. The results were analyzed in terms of hysteretic response, damage evolution, seismic performance parameters (shear strength, equivalent viscous damping ratio, ductility, and lateral stiffness degradation). All designed walls failed in a diagonal tension failure mode. Besides, all studied variables influenced the stresses and crack patterns. Also, the shear strength increases when: (i) lower aspect ratio or joint thickness are used; and (ii) higher axial load ratio, horizontal reinforcement ratio, vertical reinforcement ratio, or mortar compressive strength are used. Moreover, when damage progresses, the lateral secant stiffness decays faster as a result of: (i) larger joint thickness or vertical reinforcement ratio; (ii) lower mortar compressive strength. Most walls exhibited an equivalent viscous damping ratio between 5% and 10% for an intermediate state of damage. However, the evaluated design parameters did not considerably affect the development of the viscous damping ratio as a function of the drift ratio. The displacement ductility of the walls ranged from 1.5 to 2.5, where the height of bricks and the vertical reinforcement ratio had positive effects, and the joint thickness, the mortar compressive strength, and the horizontal reinforcement ratio a negative effect. The lateral resistance of tested walls was also estimated with six expressions, but none of them provided accurate results.

Metallurgical assessment of large size tensioning components for the precompression structure of the ITER central solenoid

The precompression system of the ITER Central Solenoid (CS) consists of nine identical units designed to provide an axial precompression force of 210 MN at room temperature to the stack of the six independently energised modules of the CS. This load allows the modules to be maintained in contact with each other in the vertical direction during operation. Precompression is achieved by using tensioning components, by pretensioning at room temperature the tie plates of the precompression system. In addition, such components allow maintaining the necessary preload during the operation of the CS at cryogenic temperature. Tensioning components consist of large diameter tensioners (up to M190) and threaded rods (up to M110), as well as of different assemblies including multi-jackbolt tensioners, washers and jackbolts. They are manufactured from double aged forged or hot rolled Inconel 718, an age-hardenable, high strength nickel based superalloy. Stringent specification requirements apply to the threaded rods and the other components of the tensioning system. They were extensively characterised through measurements of metallurgical, physical, mechanical properties and non-destructive examinations. The paper provides a summary of the results of such wide campaign of evaluation based on testing heavy gauge products of a broad range of sizes and shapes, fully representative of the series production. The results are assessed against the requirements of the technical specification and applicable national and international standards.

Quasi-static testing of concrete masonry shear walls with different horizontal reinforcement schemes

Constructions built with partially grouted reinforced masonry (PG-RM) shear walls are common in several countries, and consequently, different construction solutions can be expected. In this context, one of the main differences detected is the type of horizontal steel reinforcement used. The most traditional options consist of bed-joint reinforcement and bond-beam reinforcement. Despite the advances in this research area, there is little experimental evidence to determine which of these horizontal reinforcement schemes exhibits a superior performance under seismic loads. Thus, the research described in this paper focused on assessing and comparing the seismic performance of PG-RM shear walls with different horizontal reinforcement options. For this purpose, four concrete masonry walls with an aspect ratio of 0.86 were tested undergoing constant axial pre-compression and quasi-static cyclic incremental lateral displacements. The tested walls had the same horizontal reinforcement ratio but different layouts (only bed joint reinforcement, only bond beam reinforcement, and a combination of them). The other design characteristics remained constant. The obtained results were analyzed in terms of the force-displacement curves and seismic performance parameters such as the maximum resistance, stiffness decay, energy dissipation, and equivalent viscous damping ratio. In addition, a comparative analysis of damage progression of the tested walls was carried out using the digital image correlation (DIC) technique.The tested walls experienced progressive deterioration of the lateral stiffness in proportion to drift increments regardless of the reinforcement scheme. Once achieved the lateral resistance, the degradation of the behavior accelerated, turning into a rather unpredictable response. Employing different horizontal reinforcement layout showed no influence on the lateral capacity of the walls when the same horizontal reinforcement ratio and material qualities are used. It was also observed that the distributed bed-joint reinforcement was better than the bond-beam reinforcement layout in controlling crack widths. Additionally, a combination of bond-beams and bed-joint reinforcement seems to be the most suitable reinforcement strategy based on hysteretic behavior, energy dissipation capacity, and ductility. The presented experimental evidence is promising, although further studies are required in order to promote its use in design codes and construction projects.

Near-surface-mounted retrofitting of damaged/undamaged adobe walls using steel bars: Analytical evaluation of experimental results

This study examines the results of an extensive experimental program pertaining to seismic strengthening of adobe walls using near-surface-mounted (NSM) steel bars. The experiments were conducted on seven adobe wall panels (1020×800×200mm) subjected to the combined action of vertical pre-compression stresses (0.1 and 0.3 MPa) and incremental lateral loading reversals. The parameters investigated included NSM reinforcement ratio, level of vertical stress, and pre-existing damage. The load–displacement curves obtained from pushover tests were employed in the seismic evaluations using a capacity spectrum approach called the N2 method. Based on the experimental and analytical findings, the adopted retrofitting technique was found capable of effectively enhancing the in-plane seismic performance of both the undamaged and damaged adobe walls in terms of lateral strength, displacement capacity, energy dissipation, and the tolerated peak ground acceleration. It was found that the retrofitting intervention has a more beneficial contribution to the displacement capacity, rather than lateral resistance, of adobe walls under high axial pre-compression loads. Finally, analytical models originally developed for masonry walls were modified for estimating the in-plane lateral resistance of reinforced/unreinforced adobe walls.

Experimental investigation into the effect of vertical loading on in-plane cyclic behavior of adobe walls

Adobe buildings are highly susceptible to lateral loads, which is responsible for their collapse during earthquakes in the past resulting in vast damages and immense human casualties. Nonetheless, a majority of such buildings in the seismic-prone regions worldwide, including those in the Iranian plateau, are in every-day use. Thus, close examination of their behavior under lateral loading is vital for developing appropriate retrofit techniques aimed at preventing their severe damage and the associated casualties during strong ground motions. Towards this, the present experimental investigation was designed and implemented to study the in-plane lateral behavior of adobe walls under different axial pre-compression loads. The experimental program consisted of 12 adobe wall panels, all 1000 mm long and 200 mm thick but of three different heights of 800, 900, and 1000 mm. Loading was applied in the form of an incremental lateral loading regime in the presence of four vertical pre-compression stresses of 0.1, 0.3, 0.5, and 0.7 MPa. The aim was to investigate the effects of vertical loading and wall height on the in-plane lateral responses of adobe walls. The results were compared in terms of hysteretic load-displacement response, lateral resistance, displacement capacity, ductility, stiffness degradation, and energy absorption. Another aspect of the study involves a detailed examination of the failure modes and general behavior of the specimens.

Wrinkling to crinkling transitions and curvature localization in a magnetoelastic film bonded to a non-magnetic substrate

This work studies experimentally and numerically the post-bifurcation response of a magnetorheological elastomer (MRE) film bonded to a soft non-magnetic (passive) substrate. The film-substrate system is subjected to a combination of an axial mechanical pre-compression and a transverse magnetic field. The non-trivial interaction of the two fields leads to a decrease of the critical magnetic field with applied pre-compression, while the observed wrinkling patterns evolve into crinkles, a bifurcation mode that is defined by the accompanied curvature localization and strong shearing of the side faces of the wrinkled geometry. Using a magneto-elastic variational formulation in a two-dimensional finite element numerical setting, we find that the crinkling is an intrinsic feature of magnetoelasticity and its presence is directly associated with the repulsive magnetic forces of the neighboring wrinkled-crinkled faces. As a result, the presence of the magnetic field prohibits the formation of creases and folds. In an effort to obtain a good quantitative agreement between the numerical and the experimental results, we also introduce an approximate way to model the friction of the lateral film-substrate faces. This analysis reveals the strong effects of friction upon the magneto-mechanical wrinkling modes.

Strength and stability in architectured spine-like segmented structures

Architectured and segmented material designs have recently emerged as a powerful approach to increasing the strength and toughness of brittle materials. Architectured materials are made of regular building blocks that can collectively slide, rotate, separate or interlock, providing a wealth of tunable mechanisms and properties. In this work we have used experiments and modeling to explore the mechanical response of idealized segmented systems made of a linear array of cubes subjected to axial pre-compression and to a transverse force. From simple tabletop experiments with playing dice with instrumented tests on 3D printed cubes and simple models, we highlight the effects of axial pre-compression, number of blocks, friction coefficient and surface morphology on strength, energy absorption (toughness) and stability (catastrophic vs. graceful failure). We identified two failure modes in this segmented system: a sliding mode where one or more blocks slide on one another, and a “hinging” mode where some interfaces open and rotate about hinge points. The failure mode transition between hinging and sliding was established, to assist the design of modern architectured structures and materials. Finally, we demonstrate that enriching the morphology of the cubes with curved interfaces (akin to the vertebrae in the spine of reptiles) delays hinging and improves stability.

Analysis of the Radial Stiffness of Rubber Bush Used in Dynamic Vibration Absorber Based on Artificial Neural Network

Rubber bush is used in dynamic vibration absorber as dissipating devices in damping boring bar. These devices actually have to support radial load in compression when chattering occurs. Mastering the behavior of the radial stiffness of the rubber bush implies an accurate understanding of dynamic vibration absorber. The behavior is, however, complex due to the changeable cross-sectional shape and boundary conditions of the rubber bush. By using artificial neural network, the radial stiffness can be predicted efficiently. According to the authors’ knowledge, simulations and tests on radial stiffness of the rubber bush under combined different cross-sectional shape and boundary conditions by using artificial neural network have not been performed yet. The purpose of this study is thus to find the law of radial stiffness of rubber bush under different cross-section shapes and axial pre-compression conditions. In order to achieve this aim, simulations and tests under different chamfering sizes and axial pre-compression by using artificial neural network were first carried out.

An assessment of axial pre-compression for the KSTAR central solenoid

The central solenoid (CS) of the KSTAR consists of four pairs of superconducting coils compressed axially by preloading structures. The axial pre-compression is needed to suppress the maximum attractive or repulsive force of the coils based on reference operation scenarios. All CS coils were assembled with eight sets of preloading and supporting structures under precise dimensional control. Only shells of preloading structures were heated to make 6mm gap between the preloading structure and coil assembly. The gap was, then, filled up by wedge movement so that axial pre-compression be applied after thermal contraction of shells. However measured pre-compression is about 55% of the design value. A detailed investigation has been carried out to find reasons for the preloading reduction by comparing strain measurement and structural analysis results. The minimum residual compression is estimated as 6.5MN. It is expected to have no serious problem during large plasma current H-mode discharge with consideration of the anti-slip design. A structural analysis model of KSTAR CS magnet has been developed based on the measured strains during operations and will give a guide line for future high performance and long pulse operations of KSTAR.

Experimental study on in-plane cyclic response of partially grouted reinforced concrete masonry shear walls

This article describes the experimental results of ten partially grouted reinforced concrete masonry shear walls (PG-RCMSW) that were subjected to reverse lateral in-plane cyclic loads. The variables analysed in this study were: aspect ratio, shear reinforcement ratio and level of axial pre-compression. The influence of each of these variables on different structural parameters such as degradation of stiffness, shear strength, displacement ductility, dissipation of energy, hysteretic damping and level of drift, was evaluated. In addition, the precision of certain analytical expressions reported in the literature to predict the maximum shear strength of walls was examined and contrasted with the experimental results obtained.The results showed that the evolution of the damage was propagated in a similar way in all the walls tested until reaching the level of maximum strength. From this point, the evolution and extension of the damage depended on the characteristics and loading conditions particular to each wall. Also, a strong interdependence of the variables studied was identified, which became evident in the evaluation of shear strength, dissipation of energy, hysteretic damping, and level of drift. Using a bilinear idealization, displacement ductility values between 2.85 and 7.94 were found to reflect the presence of a moderate level of ductility in the walls tested. The equivalent viscous damping ratio associated with a non-linear response was found to range from 5% to 11%, indicating a moderate level of energy dissipation before the peak load was reached. Finally, the comparison between the predictions of the analytical expressions from the literature and the experimental results showed that those expressions that incorporated some interdependence in their design variables did not possess an appropriate degree of confidence to be applied in assessing the shear strength of PG-RCMSW, while expressions proposed by some international codes seem to be more reliable and conservative.

Prediction of the shear strength of reinforced masonry walls using a large experimental database and artificial neural networks

This paper analyses the accuracy of a selection of expressions currently available to estimate the in-plane shear strength of reinforced masonry (RM) walls, including those presented in some international masonry codes. For this purpose, predictions of such expressions are compared with a set of experimental results reported in the literature. The experimental database includes specimens built with ceramic bricks and concrete blocks tested in partially and fully grouted conditions, which typically present a shear failure mode. Based on the experimental data collected and using artificial neural networks (ANN), this paper presents alternative expressions to the different existing methods to predict the in-plane shear strength of RM walls. The wall aspect ratio, the axial pre-compression level on the wall, the compressive strength of masonry, as well as the amount and spacing of vertical and horizontal reinforcement throughout the wall are taken into consideration as the input parameters for the proposed expressions. The results obtained show that ANN-based proposals give good predictions and in general fit the experimental results better than other calculation methods.