Ultimate Strength Limit Research Articles

Effects of load rate, heating rate, and creep on the ultimate strength and rupture of A416 7-wire strand at elevated temperature

Tensile testing was performed on ASTM A416 cold-drawn 7-wire strands under varying combinations of applied loading and high temperature in three phases: “steady-state” tests, for which increasing load is applied to a specimen at steady-state temperature; “transient” tests, for which the ambient specimen is initially loaded to a constant target tension value and the temperature is then increased until the specimen fractures; and “creep” tests, for which the specimen is subjected to a constant target combination of temperature and tensile loading for a prolonged period of time, during which it accumulates creep strains until fracturing. Temperature-induced decreases in ultimate strength and proportional limit are relatively consistent compared to the existing literature for a wide range of cold-drawn strands fabricated under different governing standards. Steady-state tests produced slightly more thermally induced strength loss compared to transient tests, which suggests that design-basis predictions that are founded on steady-state test results can be used to conservatively predict the thermally induced strength loss in strands that are increasingly heated during fire exposure. Strands loaded to a stress utilization ratio beyond 70 % of their thermally dependent ultimate strength can experience a significant increase in creep strain under prolonged thermal exposure.

Quasi-static tests and numerical simulations of ductile seismic behavior for scoured bridge pile group foundations considering pile uplift

Scour is the main hazard for coastal bridges. Compared to the fully embedded pile group foundation before scour, the scoured pile group foundation is more prone to the inelastic damage and pile uplift when subjected to earthquakes. However, the seismic behavior of the scoured bridge pile group considering both the ductile and uplift behavior of piles is yet to be documented. This study aims to reveal the ductile seismic behavior of the scoured pile group foundation considering the pile uplift behavior. To this end, quasi-static tests were performed on two reinforced concrete (RC) 3 × 2 pile group foundation specimens embedded in homogeneous sand with and without pile uplift. The lateral loads were separately applied to the cap center of the pile group and the top of the pier to reproduce the pile uplift effect on the foundation. The experimental result of these two specimens was comprehensively compared and discussed. Furthermore, the finite element models for the quasi-static tests were developed and validated. The experimental result indicates that the pile group without uplift shows ductile properties, whereas that with uplift exhibits both ductile and self-centering behavior. The uplift of piles reduces the lateral strength and residual lateral deformation of the ductile foundation. Moreover, the pile group with the pile uplift has a more considerable displacement ductility at the first yielding limit state of belowground piles but a smaller displacement ductility at the peak lateral strength and ultimate residual strength limit states. The findings of this study provide insights into the seismic design/assessment of scoured bridges in coastal regions.

Mechanical properties of 2100 MPa parallel wire strands under and after elevated temperature

In this study, steady-state tensile tests were conducted at elevated temperatures and after cooling from elevated temperatures on 2100 MPa galvanized parallel wire strands (19 × 5.4 mm) used in long-span suspension bridges. Data collection was done using a charge-coupled device camera (CCDC) system. The results from tests at elevated temperatures indicate that the mechanical properties of parallel wire strands deteriorated gradually with increasing temperature. This degradation trend was well described by logistic curves. The results show that the mechanical properties after elevated temperature decreased significantly when the temperature exceeded 350 °C. By comparing with existing research, it was found that the higher the strength grade, the more severe the strength degradation after elevated temperature. During heating and cooling stages, moderate tension led to beneficial effects on residual strength of the specimens. This applies to temperatures below 400 °C. A linear-exponential-linear (L-E-L) stress-strain model was proposed to establish stress-strain equations for parallel wire strands at and after elevated temperatures. Degradation equations were also obtained for elastic modulus, yield strength, ultimate strength and proportional limit. These findings can be utilized to analyze fire resistance performance of cable systems in large-span bridges and assess post-fire damage.

The strength and elastic property of PLA + graphite composites: experimental and theoretical analyses

Background. The combination of additive technologies with reinforced materials opens up new vistas for creating lightweight and durable products having unique characteristics. Implementing these technologies into the production requires effective evaluation methods of the ultimate limit state of such products. Objective. The article deals with the mechanical properties of samples, manufactured by the method of surfacing FDM with two polylactide-based thermoplastic threads: PLA-Cg+ with 5% layered graphite filling and PLA-CCF with 10% carbon fiber filling. Methods. The impact of 3D printing process parameters, such as print orientation and layer thickness, on specimens' mechanical characteristics under conditions of tension and compression, has been experimentally researched. Results. It is shown that both print orientation and layer thickness substantially influence specimens' mechanical properties of both materials. A comparative analysis of experimental data with calculations by failure criteria has been carried out: Tsai-Hill, Tsai-Wu, Hoffman, Mises, and maximum stresses. Conclusions. The results of the tests proved that there is a significant influence of the studied parameters of the printing process on the mechanical characteristics of PLA + graphite specimens under both tension and compression. For the most part, samples with a smaller thickness have both a higher ultimate strength (limit of proportionality) and a greater relative elongation. It is defined that the best concurrence between experimental and calculated data for both materials can be achieved through using the generalized von Mises criterion.

Compressive behaviour of circular steel tube confined reinforced concrete (STCRC) columns considering shrinkage and creep

Thin-walled steel tube confined reinforced concrete (STCRC) columns are increasingly used in high-rise and heavily loaded structures. One concern is whether concrete shrinkage affects the load resistance of STCRC columns, as the confinement to the concrete may become less effective when the concrete shrinks. This paper presents ultimate tests on STCRC columns after subjected to 490-day-sustained-loading or 490-day-shrinkage. The compression tests are conducted on specimens with different steel ratios, reinforcement ratios, concrete strengths and steel-concrete interfacial bonding conditions, or with long-term load first applied at different concrete ages. The sustained load history and the shrinkage effects on the short-term axial stiffness, ultimate strength, and peak strain of STCRC columns are investigated. The development of confinement effects and the tube's axial stiffness contribution after shrinkage and creep history are compared against those test results without experiencing shrinkage or long-term load history. A non-linear finite element model is finally developed for the ultimate response prediction of STCRC columns with the consideration of the creep and shrinkage history, and its accuracy is compared against the test data. The results show that concrete shrinkage and creep significantly affect the occurrence and development of the confinement effects, but not the confining pressure at the peak load. The axial stiffness and peak strain of STCRC columns are sensitive to time effects, while the ultimate strength is not. Under service loading, the tube's contribution to the axial stiffness of STCRC columns is negligible due to concrete shrinkage, while at the ultimate strength limit state condition, the column compression resistance is barely influenced by shrinkage and creep effects. The proposed FE model achieves reasonable accuracy in evaluating the ultimate responses of STCRC columns considering the shrinkage and creep effects.

Experimental Study on the Fracture Toughness of Bamboo Scrimber.

In the past decade, bamboo scrimber has developed rapidly in the field of building materials due to its excellent mechanical properties, such as high toughness and high tensile strength. However, when the applied stress exceeds the ultimate strength limit of bamboo scrimber, cracks occur, which affects the performance of bamboo scrimber in structural applications. Due to the propensity of cracks to propagate, it reduces the load-bearing capacity of the bamboo scrimber material. Therefore, research on the fracture toughness of bamboo scrimber contributes to determining the material's load-bearing capacity and failure mechanisms, enabling its widespread application in engineering failure analysis. The fracture toughness of bamboo scrimber was studied via the single-edge notched beam (SENB) experiment and compact compression (CC) method. Nine groups of longitudinal and transverse samples were selected for experimental investigation. The fracture toughness of longitudinal bamboo scrimber under tensile and compressive loadings was 3.59 MPa·m1/2 and 2.39 MPa·m1/2, respectively. In addition, the fracture toughness of transverse bamboo scrimber under tensile and compressive conditions was 0.38 MPa·m1/2 and 1.79 MPa·m1/2, respectively. The results show that, for this material, there was a significant distinction between longitudinal and transverse. Subsequently, three-point bending tests and simulations were studied. The results show that the failure mode and the force-displacement curve of the numerical simulation were highly consistent compared with the experimental results. It could verify the correctness of the test parameters. Finally, the flexural strength of bamboo scrimber was calculated to be as high as 143.16 MPa. This paper provides data accumulation for the numerical simulation of bamboo scrimber, which can further promote the development of bamboo scrimber parameters in all aspects of the application.

Designing Lightweight Stadium Roofing Structures Based on Advanced Analysis Methods

The current structural engineering practical standards are unable to offer an universal structural design standard for long-spanning lightweight stadium roofing structures. As such, the design procedure of a particular stadium roof is not replicable to another. This research aims to present a novel design procedure for lightweight stadium roofing structures considering the Lakhwiya stadium the Optus Stadium and the CommBank Stadium as experimental cases. Using the finite element analysis (FEA) software Strand7, the cases will be modelled and analysed. Varying load cases and combinations such as ultimate strength (ULS) and serviceability limit states (SLS) based on the Australian Standard AS1170.0:2002 will be calculated and subsequently applied. Linear static analysis will then be undertaken where critical members will be identified within the model. Based on this, preliminary member sizing and design feasibility checks will be conducted in order to ensure structural stability and compliance to the Australian Steel Structure code AS4100:2020. A linear buckling analysis is also conducted based on the selected sizes from the initial stage to determine critical loads. Advanced analysis including non-linear buckling computation is comprehensively managed. Some of the crucial parameters such as maximum displacement, maximum/minimum principal stresses, critical buckling loads, as well as load factors are examined. The main novelty of this study is to determine a clear road map to design stadium roofing systems subjected to a combination of different types of the loading.

ANALYSIS OF THE STATISTICAL DATA GENERATED BY AN ADAPTIVE STRETCH FORMING PROCESS

: Constant industrial processes improvements represent a fundamental step in the evolution of efficient processing. Due to physical or financial limits, there is a limit to how the mechanical or electronic side can be optimised. A solution for improving industrial processes can come in the form of complex machine algorithms that analyse the process in real-time and decide, with each step, what is optimal. To put this statement into practise, we have designed and implemented a fully operational self-adaptive stretch forming process controlled with the help of a dedicated statistical analysis algorithm. The foundation of the algorithm lies in the deformation theory of metals. In its simplest form, it can be summarised that if a sheet of metal is stretched, its length will increase as the force acting upon it increases until the ultimate tensile strength limit is reached; after this limit, failure occurs. Therefore, the algorithm analyses the material strain controlling the bi-axial nature of the stretch forming process by constantly adjusting for axial force and die speed. It does this through complex computer-vision image analysis techniques for strain measurement and stretching pressure readings as input data. The readings are analysed using the ANOVA method, providing R-squared and p-values for stretching pressure and die speed. The decisions that the algorithm takes are based on the statistical analysis of its previous decision, aiming to improve the overall process R-squared. The overall results are validated by measuring the obtained stretched parts’ deviation to the die shape. Therefore, the measurements were taken using a GOM 3D measuring system. This paper aims to explain the methodology of the algorithm using how the measurements are taken, how the statical analysis generated decisions for controlling the industrial equipment, and to analyse the statistical data generated by the self-adaptive stretch forming algorithm for the experimental study by comparing the decision it takes for each for the 20 processed 1050 aluminium alloys blanks. The results indicate the ideal succession of decisions and which path should be taken to improve the decision-making for both elastic and plastic domains.

Implication of bridge resilience design and lessons from negative examples

AbstractBridge resilience is a newly proposed bridge design criterion that involves robustness, redundancy and reparability targeting on the rapidity of functionality restoration after suffering extreme actions and long-term durability deterioration. It stipulates a lower probability of reaching the ultimate limit state or strength limit state, which have been only partly involved in bridge design codes around the world. In AASHTO LRFD Bridge Design Specifications and Eurocodes, there are some design principles related to bridge resilience. Yet, it is also necessary to give more requirements for structural ductility and collapse resistance when the actual load exceeds the load combination in the code. This paper focuses on the resilience-based principles for bridge design, and exposes some problematic bridge structural systems and details, such as bridges most likely to overturning, steel bridges with fracture critical members, arch bridges with suspended desk, Morandi cable-stayed bridges, poor details for seismic vulnerability, etc. Whereas overturning is one of the worst anti-resilience scenarios, the resilience design against bridge overturning is highlighted through a detailed discussion including the calculation methods of anti-overturning factor, overturning stability of curved bridges, reasonable disposition of supports, and anti-overturning countermeasures.

Design considerations in the strengthening of composite lap joints using metal z-pins

Through-thickness reinforcements are an effective technique to improve the structural properties of bonded fibre-polymer composite lap joints. However, there is a lack of understanding of the effectiveness of this technique for internal step-lap joints. This study examines the strengthening effects of z-pins on different types of composite step-lap joints. Co-cured single, quadruple and multi-step lap joints made of carbon-epoxy laminate were reinforced with steel z-pins. The results reveal that z-pins were highly effective at increasing the ultimate strength (+20%) and elongation limit (+250%) of the single lap joint under tensile loading by suppressing catastrophic failure through plastic shear deformation following fracture of the co-bonded interface. However, the z-pins were less effective at improving the strength (+7.5%) and maximum strain (+18%) for the quadruple lap joint and did not provide any strengthening for the multi-step lap joint. The mutually exclusive relationship between the number of bonded interfaces and the z-pin strengthening efficacy is discussed.

Understanding influence of micro pores on strengths of LMDed AlSi10Mg material using a direct method based statistical multiscale framework

For mechanical components composed of AlSi10Mg material and produced by laser melting deposition (LMD), micro pores introduced during the additive manufacturing (AM) process significantly deteriorates the material performance. In order to evaluate how characteristics such as the porosity level, the shape and the spatial distribution of pores influence the ultimate strength and endurance limit of the material, a direct method based statistical framework is presented in this paper. Within this framework, a random sampling strategy is developed to bridge heterogeneities associated with two sub-scales, namely the scale of micro pores and the scale of material constituents. Through statistical analyses of results calculated from many 2.5D and 3D representative volume element (RVE) samples, the study shows that micro pores have more severe impact on endurance limit than on ultimate strength: With an averaged porosity degree of 0.71%, the former case drops for more than 20.0% while the latter case is almost unchanged. Meanwhile, compared to shape, spatial distribution of pores has a more significant influence on the endurance limit. The analysis in this paper shows that the established approach can be a viable means for studying similar AM materials and guiding reliability design of AM structures.

Numerical simulation of wave-induced hydroelastic response and flow-induced vibration of a twin-tube submerged floating tunnel

The Norwegian Public Road Administration is planning to upgrade Coastal Highway E39 by replacing ferry connections with floating bridges or submerged floating tunnels (SFTs). This study considers a potential pontoon-supported curved SFT designed for crossing Sognefjorden at a submergence of 12 m. It consists of two identical tubes with a diameter of 12.6 m each in a tandem configuration and with a length of approximately 4 km. The natural frequencies of the low-order modes are well within the energy content in the spectra of the second-order difference-frequency wave excitation forces and the vortex shedding-induced forces. In this paper, numerical simulation of wave-induced hydroelastic response and flow-induced vibrations of the twin-tube SFT is performed. Long- and short-crested waves, the first and second order wave loads, are considered. A time-domain approach to simulate crossflow vortex-induced vibration (VIV) and VIV-amplified inline drag forces, partly based on the coefficients obtained experimentally, is established and applied. The focus is on extreme conditions – relating to ultimate strength limit states. The second-order wave load substantially affects the lateral motion and lateral bending moment, as expected. The short-crested waves influence the response in both the lateral and vertical directions by exciting asymmetric eigenmodes. In strong flow conditions, once VIV is excited, the standard deviation of the vertical motion (of about 30% of the diameter) and the bending moment about the horizontal axis is more that an order of magnitude larger than that induced by the wave loads. The simulation of the wave- and flow-induced load effects provides a good reference for the design of SFTs.

Ultrahigh-strength steels at elevated temperatures

The main purpose of this research is to bring deeper understanding about the use of compression loaded ultrahigh-strength steel components at elevated operating temperatures. Another reason for this research is lack of design guidelines concerning ultrahigh-strength steels. This paper provides elevated temperature tensile test data for S700 and S960 ultrahigh-strength steels and discusses the implications of the data from the point of view of structural engineering based on Eurocode 3 design procedures. The experimental part of this paper consists of the tensile testing of two ultrahigh-strength steels grades at temperatures between room temperature and 1000 °C. The models for predicting the temperature reduction factors of Young's modulus, yield strength, ultimate strength and proportional limit are also proposed based on the test results. The obtained proportional limit values together with calculations exhibited that the capacities against buckling of S700 and S960 steels at elevated temperatures are weaker than assumed in Eurocode 3 and Tetmajer's theory for these steel grades. This is mainly due to the lower proportional limit values at room temperature than expected. However, the detected proportional limits do not decrease as fast as Eurocode 3 assumes while the operating temperature increases.

Fatigue behaviour of a spring: a case study

A durability design case study concerning a U spring is presented. For a given geometry of the component, the question was to predict the maximum permissible displacement per cycle whilst ensuring a service life of at least 260 kcycles. The stiffness of the spring was evaluated using the finite elements software SolidWorks. As an approximation a linear model was used in the finite elements analysis, which was also compared with an analytical treatment based on Castigliano’s theorem. Using a given safety factor, a high cycle fatigue analysis was performed assuming the use of a steel with 650 MPa ultimate tensile strength. The spring is either loaded by an imposed displacement, or it is load free,i.e.load ratio (minimum load / maximum load per cycle), isR= -1. An algorithm based on finite fatigue life prediction techniques, the Basquin and Goodman diagrams, and the relationship between ultimate tensile strength and fatigue limit was used and will be briefly presented and discussed. Also, the stress analysis helped to highlight the need for some minor geometry changes. The outcome of the study was the evaluation of the maximum permissible displacement per cycle, as required in the real life case addressed.

Implementation of ENTROPY-ARAS decision making methodology in the selection of best engineering materials

The primary objective of this present research work emphasizes the importance of selecting the best suitable material for engineering applications among a group of seven alternatives on the basis of six important criteria i.e. bending fatigue limit, core hardness, cost, surface hardness, ultimate tensile strength and surface fatigue limit by analyzing through multi-criteria decision making (MCDM) techniques. Here, Entropy method is adopted for the criteria weightages calculation and Additive Ratio Assessment (ARAS) method is used to select the best option and to propose the preference ranking order of the alternatives. From, the analysis, it is found that cast alloy steel is the best option followed by cast iron and carburized steels, whereas hardened alloy steel is the worst choice among the group. Moreover, the previous researchers’ analysis are reconstructed and modified alternative rankings are proposed.

Influence of Support Conditions on Single-Span Profiled Sheet Metal Strength Through Nonlinear FEM Analysis

Profiled sheets are widely used in modern steel structures, either as cladding or as casing in composite structures. Strength calculation of such structures is a crucial design issue, and it represents a complex task because one must deal with thin-walled structures that have complicated cross-section shape. Manufacturers of these structural elements provide data about its strength, mostly based on experimental testing. However, catalogues provide little data about the supporting conditions at the ends that may have influence on structural strength. In this research, the finite element method analysis with geometrical and material nonlinearity and contact analysis in the support zones was applied for the strength calculation of a profiled steel sheet with very high depth. The analysis encompassed determination of the ultimate limit strength and serviceability limit strength with limited deflection. All analyses were conducted for surface load applied over the area of one sheet. Thereat, influence of several factors related to support conditions was considered: friction on the supports, support width, and number of fasteners. Failure mechanisms were also analysed. Obtained results were compared with other sources and methods, like analytical solution, manufacturer’s software, and catalogue data. Results of the research showed that behaviour and strength of those elements depend to the great extent on end support detailing, which cannot be encompassed by classical calculation methods and linear analysis. Significance of the support conditions was proven, and attention was paid on absence of such data in published catalogues.

Mathematical and computer modeling of a new endoprosthesis with a carbon-carbon composite for interbody fusion of a lumbar spine

Endoprostheses for interbody fusion should have not only biocompatibility and high strength characteristics, but also good visualization with additional research methods (CT, MRI). One such material is carbon. Aim: using mathematical modeling (the finite element method) to develop a model of the interbody prosthesis from a carbon-carbon composite and evaluate the stress-strain state of the vertebral body-implant system.Methods: models created in SolidWorks software. 18 design schemes with endoprostheses from a carbon-carbon composite were developed and studied. Bending load was carried out by angular displacements (2°), compressive — by applying a force of 500 N to the upper cubic element. Additional symmetry conditions were also simulated.Results: a parametric model of the biomechanical system of the lumbar spine was constructed. The smallest equivalent stresses were obtained in the design scheme M17, and the largest — in M13. In the bone elements of the systems, they did not exceed the ultimate strength limits for cortical (160 MPa) and cancellous (18–22 MPa) bones. Contact pressure and displacement are determined for all design schemes.Conclusions: as a result of the numerical values and distribution fields of the components of the stress-strain state in the elements of the studied systems, it was found that the use of the proposed endoprosthesis made of a carbon-carbon composite, the geometric parameters of which correspond to the design scheme M17, is effective for achieving additional stabilization in the system «vertebral body – implant».

Fragility analysis of an ageing monopile offshore wind turbine subjected to simultaneous wind and seismic load

The loads induced as a result of seismic activities may jeopardize the serviceability of offshore wind turbines or may even lead the structure to reach the ultimate strength. The maximum load-carrying capacity of a support structure can be estimated by performing a structural assessment which accounts for the nonlinear effects arising from the material and geometry. The present work aims to analyze the fragility of a 5 MW monopile offshore wind turbine structure subjected to seismic activities accounting for soil interactions and time-variant structural degradation. The offshore wind turbine structure is subjected to different ground motions with different intensity. The nonlinear full transient dynamic structural analysis is carried out based on the finite element method, and the nonlinear monopile structural response during the different seismic activities is discussed. Finally, the fragility curves associated with the serviceability limit state design and the ultimate strength limit state are developed.

P.243 Experimental evaluation of the atomoxetine influence on the cognitive functions in rats

The Norwegian Public Road Administration is planning to upgrade Coastal Highway E39 by replacing ferry connections with floating bridges or submerged floating tunnels (SFTs). This study considers a potential pontoon-supported curved SFT designed for crossing Sognefjorden at a submergence of 12 m. It consists of two identical tubes with a diameter of 12.6 m each in a tandem configuration and with a length of approximately 4 km. The natural frequencies of the low-order modes are well within the energy content in the spectra of the second-order difference-frequency wave excitation forces and the vortex shedding-induced forces. In this paper, numerical simulation of wave-induced hydroelastic response and flow-induced vibrations of the twin-tube SFT is performed. Long- and short-crested waves, the first and second order wave loads, are considered. A time-domain approach to simulate crossflow vortex-induced vibration (VIV) and VIV-amplified inline drag forces, partly based on the coefficients obtained experimentally, is established and applied. The focus is on extreme conditions – relating to ultimate strength limit states. The second-order wave load substantially affects the lateral motion and lateral bending moment, as expected. The short-crested waves influence the response in both the lateral and vertical directions by exciting asymmetric eigenmodes. In strong flow conditions, once VIV is excited, the standard deviation of the vertical motion (of about 30% of the diameter) and the bending moment about the horizontal axis is more that an order of magnitude larger than that induced by the wave loads. The simulation of the wave- and flow-induced load effects provides a good reference for the design of SFTs.

Studying the Structure of Products Made of Titanium Carbide Hardened by the Impregnation Method

The paper presents the results of studying the structure of products made of titanium carbide hardened by the impregnation method. As a bonding alloy a high-temperature alloy of the Fe-Cr-Ni-Co system was used. Samples of different porosity were impregnated with that alloy, then their structure was investigated by the methods of mercury porosimetry and electron microscopy. The data of the parameters of porosity and some mechanical properties are given. It was found that there is a correlation between the average size of open pores and the tensile strength and the ultimate strength limit. It is shown that this relationship is logical if to consider the resulting structure as a composite material: the matrix of titanium carbide which pores are filled with a metal bond. It is shown at what parameters of the porous structure the impregnation process proceeds most fully.