Step Load Research Articles

Small Punch Test to Estimate the Threshold Stress in Aggressive Environments by Incremental Step Loading

The present work is a relevant advance in the validation of the incremental step loading technique (ASTM F1624 standard) when applied to Small Punch tests (SPT) for the threshold load determination of medium- and high-strength steels in aggressive environments, as a novel alternative to conventional time-consuming tests under constant load. It completes previous works by the authors on this topic, extending a methodology to estimate the threshold stress from SPT tests in aggressive environments, covering the whole range of hardness marked by ASTM F1624 as the main goal. This is achieved by calibrating a model of the material’s hardness by the use of a coefficient in function of it. For this purpose, four medium- and high-strength steels of 33, 35, 50 and 60 HRC (Hardness Rockwell C) are exposed to three different cathodic polarization hydrogen embrittlement environments of 1, 5 and 10 mA/cm2 in 1N H2SO4 acid electrolyte connected to a platinum anode. Threshold stresses in these circumstances are obtained by uniaxial specimens following ASTM F1624 and compared to their homologous threshold loads obtained by Small Punch tests according to the authors’ original methodology proposal. Finally, the aforementioned model, consisting of a correlation based on composing an elastic and a plastic part, is calibrated for a hardness ranging 33–60 HRC, this being the main original contribution of this work; the elastic part is dependent just on the elastic-to-plastic transition SPT load, while the plastic part is ruled by a material hardness-dependent coefficient. This technique supposes an advance in engineering tools, due to its applicability in situations of material shortage, such as in-service components, welded joints, local areas, complex geometries, small thicknesses, etc., often present in aerospace, automotive or oil–gas, among others.

Using surrogate models to accelerate load step methods for nonlinear finite element problems in hyperelasticity

Using surrogate models to accelerate load step methods for nonlinear finite element problems in hyperelasticity

3D Printing of Biocompatible and Antibacterial Silica-Silk-Chitosan-Based Hybrid Aerogel Scaffolds Loaded with Propolis.

The aim of this study is to design a therapeutic enhanced three-dimensional (3D) silk fibroin (SF)-based scaffold containing propolis (Ps)-loaded chitosan (CH) nanocarriers. To this aim, we initially synthesized a hybrid gel-based ink by a synergistic sol-gel and self-assembly approach and then processed the resulting gels by microextrusion-based 3D printing followed by supercritical drying to obtain 3D hybrid aerogel scaffolds. Ps was utilized to enhance the final scaffold's bactericidal efficacy and cell responsiveness. For the synthesis of the scaffold, two Ps loading methods (in preprint and postprinting steps) were investigated in order to optimize the Ps drug quantities in the scaffold and maximize the antibacterial properties of scaffold. In the postprinting Ps loading step, the hybrid silica-oxidized SF (SFO)-CH hydrogel ink was 3D printed into a construct with an interconnected porous structure, and then, Ps was loaded into the printed construct. In the preprint loading method, PS was incorporated into the SF and a hydrolyzed silane solution prior to gelation. The morphological studies demonstrate that the addition of Ps encapsulated CH nanoparticles (NPs) into the hydrogel solution improved the porosity of the developed scaffolds. The rheological analysis of the designed gel ink with and without Ps loading and the release kinetics were studied. The antimicrobial results show that the Ps-loaded scaffolds in the postprinting step exhibited superior antibacterial activity against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) strains compared to a preprinted Ps-loaded scaffold. Direct and indirect in vitro cytotoxicity tests also confirmed the designed Ps-loaded scaffold biocompatibility toward a mouse fibroblast (L929) cell line. We demonstrated that the scaffold formulated by propolis-loaded chitosan NPs can enhance the migration and proliferation of L929 fibroblast cells. The obtained results prove the promise of the designed 3D printed silica-SFO-CH-Ps scaffolds as a potent 3D scaffold to mediate tissue regeneration but also as an antibacterial highly porous matrix to support wound healing.

Observer-based decoupling control of air flow and pressure in fuel cell systems

This paper investigates the control of the air supply system for marine proton exchange membrane fuel cells. A mathematical model of the air supply system is established, and a sliding mode diagonal decoupling controller based on a state observer is proposed. The control effectiveness is validated through experiments and simulations. A decoupling control strategy for the air supply system of marine fuel cells is presented. First, the load input current of the fuel cell system is selected according to the load characteristics of the target vessel, ensuring that the load current meets the power level of the fuel cell system model. Second, the impact of the cathode pressure on the fuel cell system is analyzed. An observer-based measurement method is proposed to address the measurement issue of cathode pressure. On this basis, a decoupling control strategy combining a diagonal decoupling matrix and sliding mode variable structure control theory is proposed. Finally, the effectiveness of the decoupling controller is validated through Matlab/Simulink. The results show the proposed decoupling controller exhibits superior performance to existing controllers with environmental disturbances, continuous time-varying loads, and step loads. The maximum relative error is 2.95%, the average relative error is 0.022%, and the longest adjustment time is 0.5 s. These results indicate that the control performance of the proposed decoupling controller is superior to that of diagonal PI controllers and sliding mode controllers, thereby validating the superiority of the sliding mode decoupling controller. This article provides a useful method for the development and application of control technology for the air supply system of marine fuel cells.

Evaluation of Concrete Fatigue Damage with Elastic Wave-based Measurement Techniques

Fatigue damage is a progressive and permanent change in the internal material structure due to cyclic loading. The cyclic loads can be due to wind, waves, temperature variation, and traffic loads. Each cyclic load application introduces micro-damages that accumulate to induce failure. Detecting these micro-damages requires sensitive measuring techniques. Hence this research compares the sensitivity of different measuring techniques applied to fatigue experiments of concrete. The applied NDT are the passive acoustic emission technique (AET) and active ultrasonic measurements. In addition, linear variable differential transformers (LVDTs) are applied as a reference measurement. Accordingly in this research, cylindrical concrete samples are subjected to monotonic and cyclic Brazilian splitting tests. The cylinders are 100 mm in diameter and 150 mm in length. The cyclic load is applied to these samples with a loading frequency of 5 Hz. During the tests, the damage evolution is monitored with AET, ultrasonic measurements and LVDTs. There are eight AE sensors attached to the sample. The two LVDTs are positioned at the center of the sample's front and back side. Besides passively monitoring the acoustic emissions, the AET sensors are also used to do active ultrasonic measurements. The research reports the comparison conducted on the deformations and cracks measured with the LVDTs, the damage growth identified with the AET, and the change in the ultrasonic pulse velocity measurements. In addition, the study also presents the damage analysis capacity of the methods across different types of loading: monotonic, constant amplitude, and stepped fatigue loading. The comparison shows that combination of AET and ultrasonic velocity measurement is a sensitive indicator for damage growth.

Effects of the excavation of deep foundation pits on an adjacent double-curved arch bridge

The excavation of deep foundation pits can cause variations in the displacement and stress fields of surrounding soils, which hence induces adverse effects on adjacent structures. This study presents a two-stage method to quantify the impact of the excavation of a deep foundation pit on the adjacent double-curved arch bridge in the historical city of Nanjing, Southeastern China. The entire process of the foundation pit excavation was simulated and the induced deformation of the arch foot was obtained in the first stage by hardening soil model with small-strain stiffness. Then, the obtained deformation of the arch foot was applied to the bridge structure as a displacement boundary in the second stage to calculate the internal forces and deformations of the double-curved arch bridge structure. The tensile strength of concrete is taken as the limit value of the tensile stress of the double-curved arch bridge. The limit values of arch foot displacement under four evaluation conditions are obtained by step loading calculation. The present results provide construction guidance and safety warning for the process of foundation pit excavation adjacent to double-curved arch bridges for historical preservation.

Analysis of the load‐bearing characteristics of hydraulic supports for top‐coal caving under the impact of coal and gangue

AbstractTo study the effect of different sizes and speeds of coal gangue falling on hydraulic supports for top‐coal caving discharge hydraulic support on the bearing characteristics of the hydraulic support, the ADAMS hydraulic support rigid‐flexible coupling analysis model was established. HyperMesh is used to flexibly process the hydraulic support, and the column and jack are equivalently replaced by spring damping system. By applying step loads of different sizes and speeds to simulate the impact force generated when the coal gangue falls on different positions of the cover beam, the dynamic response characteristics of the column, balance jack and other articulation points are obtained. Analyses show that: when the impact load is closer to the roof beam, the articulation point of the roof beam‐cover beam and the balancing jack are easy to be damaged. When the same impact load is applied to the cover beam, the faster the load is applied, the hydraulic support column and balancing jack may not be able to absorb and disperse the pressure effectively in time, resulting in the articulation points being subjected to a larger impact force, which is easy to cause damage or destruction. When different sizes and speeds of impact loads are applied to the cover beam, the latter has a greater impact on the dynamic response of the front and rear connecting rod articulation points and the cover beam‐roof beam articulation point than the former, and the hydraulic bracket will be more prone to vibration, deformation, and stress concentration, thus affecting the stability and safety of the bracket. The article is of guiding significance for designing and optimizing the structure and parameters of the hydraulic support, and improving the support performance and stability of the hydraulic support in the comprehensive release working face.

Output speed control for hydro-mechanical continuously variable transmission of tractor.

In order to improve the speed stability of tractors equipped with hydro-mechanical continuously variable transmission (HMCVT) during field operations, the speed regulation characteristics, torque characteristics, and power diversion characteristics of HMCVT are analyzed. And a comprehensive control strategy for HMCVT output speed is proposed by analyzing the factors affecting the fluctuation of output speed, which combines engine speed control and displacement ratio control of hydraulic speed control system. This strategy adopts fuzzy control to regulate the engine speed to maintain the stability of engine operation. Moreover, adopting feed forward compensation control for speed compensation enhances the anti-interference ability of the hydraulic speed control system. And combined with model predictive control, adjust the swash plate swing angle of the variable pump to reduce the fluctuation of HMCVT output speed. By establishing joint simulation model, the effectiveness of the control strategy was verified using tractor acceleration mode and step load disturbance mode. The results show that the strategy can reduce the fluctuation rate of engine speed and impact degree, shorten the adjustment time, and improve the stability of tractor operating speed.

Microcracking evolution and clustering fractal characteristics in coal failure under multi step and cyclic loading

During the deep mining process, coal mass encounter intricate geo-environmental stress, such as periodic weighting loading and repeatedly excavation unloading–reloading cycles, which weakens coal’s mechanical integrity and predisposing it to severe coalburst accidents. To investigate the microcracking damage mechanisms and predictive indicators in coal failure under in-situ stress analogs, the multistage step and cyclic loading experiments are conducted on cubic coal specimens. Acoustic emission (AE) technology is employed to track the spatiotemporal-energy evolution of stress-induced damages and discern the microcracking nature through AF/RA assessments, and the power-law scaling relation of AE activity near the catastrophic failure of coal is investigated. Then the clustering fractal structures of microcracking events in the stressed coal are quantified across temporal, spatial and energetic domains, utilizing correlation integral methodologies and b-value derivations from magnitude-frequency relation. Findings indicate that irrespective of the loading mode (step or cyclic), escalating stress triggers an intensification of irreversible fatigue deformations. AE characteristic parameters manifest a gradual rise, culminating in a precipitous peak coinciding with the critical failure point. This escalation adheres to a power-law correlation between AE occurrence frequency and time to failure, observable in the immediate pre-failure seconds, reflecting a universal attribute of coal fracture. Prior to ultimate failure, a marked increase in shear microcracks is discernible, despite tensile-dominated cracks (constituting about 80 % of total microcracks) prevailing as inferred from the variation of AF/RA values, aligning with an inferred “X” conjugate wedge splitting pattern from AE event density and energy mapping. The microcracking events in the loaded coal exhibit a clustering fractal structure that spans across temporal, spatial, and energetic (or magnitude) domains. Notably, the temporal fractal dimension, spatial fractal dimension, and b-value (i.e., a parameter characterized the energetic fractal dimension) all follow a parallel decrease pattern as the loading stress escalates, with a pronounced diminution becoming especially evident as the specimen approaches its catastrophic failure threshold. This insight offers fresh perspectives for predicting rock/coal dynamic disasters, emphasizing the necessity of concurrently monitoring the shift from diffuse microcracking to localized failure across time, space and energy domains. These research findings contribute to a deeper understanding of microcracking damage evolution and failure mechanism of loaded coal, and provide a foundational basis for early warning of rock failure such as the coalburst disasters.

A Stress‐Driven Double‐Phase–Field Framework for Tensile Fracturing Processes in Transversely Isotropic Rocks

ABSTRACTWe present a double‐phase–field framework for tensile fracturing processes in transversely isotropic rocks. Two distinct phase‐field variables are introduced to represent smeared approximations of tensile fractures along the weak bedding planes and through the anisotropic rock matrix, respectively. Driving forces that control fracture propagation in the phase‐field framework are constructed as a stress‐based formula with a recently developed tensile failure criterion that distinguishes the two failure modes in transversely isotropic rocks. For numerical implementation, we adopt a staggered integration scheme and decouple the governing equations so that the displacement field and phase‐field variables can be updated in sequence for a given loading step. The finite element formulation of the proposed framework is introduced in detail in this paper and is implemented in an in‐house finite element code. The numerical implementation is then validated by reproducing the uniaxial tension test results of Lyons sandstone. After that, we conduct simulations on a pre‐notched square plate loaded in tension to demonstrate the features of the proposed framework. Finally, we conduct simulations of three‐point bending tests of Pengshui shale and show that the proposed model can reproduce the force–displacement curves and failure patterns of specimens with different bedding plane orientations observed in laboratory experiments.

An Innovative Technique for the Strengthening of RC Columns and Their Connections with Beams Using C-FRP ROPES

The application of the innovative C-FRP ropes for the strengthening of reinforced concrete columns is experimentally examined. Two real-scale specimens with the same geometrical characteristics and the same steel reinforcements were constructed for the needs of this investigation. The primary objective of the study is to investigate the efficacy of the use of C-FRP ropes as externally mounted reinforcement for the strengthening of deficient external columns. In this direction, (a) C-FRP ropes are applied as longitudinal reinforcement of the column for the increase in the flexural strength, (b) C-FRP ropes are applied as external confining stirrups in the critical end parts of the column for the improvement of the concrete strength and the development of local element ductility, and finally (c) C-FRP ropes are applied as external stirrups in the form of diagonal X-shaped reinforcement for the increase in the capacity of the part of the column connected with the beam (joint panel). Both specimens are tested under the same cyclic loading procedure that comprises seven steps and each step includes three full loading cycles. The maximum loads of the strengthened specimen at the three loading cycles of the seventh step were 40%, 72% and 87% higher than the corresponding ones of the unstrengthened specimen. On the other hand, the measured shear deformations of the joint panel of the pilot (unstrengthened) specimen at the sixth and the seventh steps were 43% and 44% higher than the corresponding ones of the strengthened specimen. In general, it is concluded that the strengthened column exhibited improved hysteretic response and the whole behavior was apparently improved compared to the pilot specimen without strengthening in terms of maximum loads per loading step, dissipated energy, and shear deformations of the joint panel. In particular, it is stressed that the measured shear deformations of the joint panel and strain gauge measurements have substantiated that the column and the connection panel of the strengthened specimen remain almost intact, whereas damage and eventually failure have been located in the column and the joint panel of the pilot specimen. Additionally, it is emphasized that the C-FRP ropes can easily be applied in structures with complex configuration without any geometrical restraints.

Insight into the mechanisms and in-plane reaction heterogeneity of the dynamic response of proton exchange membrane fuel cells

The voltage undershoot is one of the main dynamic problems of proton exchange membrane fuel cells (PEMFCs) under dynamic loadings. However, its mechanisms remain poorly understood due to its complex and coupled influencing factors. Besides, PEMFCs with a large active area are more prone to local dynamic failures for their pronounced in-plane (I-P) heterogeneity, while few studies have focused on the I-P heterogeneity of the dynamic response of large-area fuel cells. In this work, the mechanisms and the I-P heterogeneity of the voltage undershoot of a 320 cm2 PEMFC are thoroughly studied based on an in-situ monitoring method and a voltage loss decoupling method. Results show that the voltage overshoot is primarily driven by the ohmic loss (OL) overshoot and the concentration polarization (CP) overshoot. Results also prove that the OL overshoot is mainly caused by the instantaneous water shortage in the proton exchange membrane (PEM) which depends on the initial water content in the PEM. Besides, the CP overshoot is due to the instantaneous reactant shortage at the catalyst, and it is much sensitive to the change in the oxygen diffusion coefficient of the ionomer caused by the change in its water content. Meanwhile, both OL overshoot and CP overshoot demonstrate significant I-P heterogeneities under poor dynamic operating conditions, especially in cases of high load step and low inlet air humidity. Finally, the influences of operating conditions on the voltage undershoot and its I-P heterogeneity are discussed, and suggestions for improving the dynamic performance of PEMFCs are proposed.

Comparative Analysis and Performance Evaluation of Dual Active Bridge Converter Using Different Modulation Techniques

The development in the renewable energy systems and the necessity of the enhanced grid that includes the conventional energy sources and the renewable energy sources and storage systems have realized the importance of power electronics conversion systems. These interfaces are crucial for enhancing the efficiency as well as the control in bi-directional power flow. The use of HEVs as a means of preserving the future supply of fossil fuels, as well as the need for improving the efficiency of the power electronics interfaces required for efficient power management between the two energy sources of the vehicle, is discussed. Furthermore, the improvements in the Uninterruptible Power Supply (UPS) systems and regenerative power systems also require sophisticated power electronic conversion systems. To address these issues of modern technologies, the Dual Active Bridge (DAB) converter comes in as a potential solution. In this paper, several modulation techniques employed in DAB converters are discussed and compared in detail in order to find the best approach to control the converter’s performance in the whole range of its operation. The paper also presents an enhanced modulation strategy applied to DAB converters and built with FPGAs and MPC to minimize the power loss. This proposed optimization technique is more general and easier than the current ones. The performance of the converter is assessed in terms of efficiency, current stress, and the power of backflow, in addition to the proposed optimization and modulation techniques. The article also looks at the closed-loop performance under step load fluctuations and circuit performance as well as efficiency.

Finite element analysis of bolted joints under torsional loads

The presence of service torque, which is generated after tightening, directly affects the mechanical properties of bolted joint. In this work, the mechanical behavior of bolted joints under torsional loads is investigated using finite element analysis (FEA). Models with two plates and multiple plates are established separately to investigate the mechanical properties of different kinds of bolted joints. Three analysis steps involving the entire process from tightening to torsional loading are adopted, namely tightening step, initial loss step and loading step. Torsional loads in two directions along the thread axis are considered and defined as loosening torsional load and tightening torsional load. The clamping force, as well as the friction torques of various contact surfaces, of the bolted joint are extracted in all the analysis steps. Different scenarios of friction torque combinations for bolted joints are obtained by means of adjusting the friction coefficient of the corresponding contact surfaces. On this basis, the mechanical properties of bolted joints and the corresponding torque thresholds are extracted on various scenarios. FEA results showed that how the service torque makes the directional characteristic of the mechanical properties of bolted joints under torsional loads. Moreover, the service reliability of bolted joint can be further improved based on the directional characteristics and the extracted torque thresholds.

COMPARACIÓN DEL RENDIMIENTO DE VARIOS ENFOQUES DE SOFT COMPUTING EN EL CONTROL DE LA FRECUENCIA DE CARGA PARA SISTEMAS ELÉCTRICOS INTERCONECTADOS QUE COMBINAN FUENTES TÉRMICAS-HIDRÁULICAS Y TÉRMICAS-DIÉSEL

Interconnected power systems offer a solution for meeting the increasing demand for reliable and efficient electricity supply. However, maintaining stability in such networks, especially in the face of frequency fluctuations, presents a significant challenge. Effective load frequency control (LFC) optimization is vital for ensuring stability by mitigating the impact of these fluctuations resulting from various disturbances. This study introduces PID LFC optimization strategies applied to a thermal-hydro and thermal-diesel interconnected power system. Six optimization methods, including ACO, PSO, GA, TLBO, DE, and CIO, are explored using four cost functions. Random step load perturbations are applied to test the response consistency of the LFCs. Initial findings indicate that recently developed CIO outperforms other algorithms in achieving desired outcomes while maintaining competitive computational expenses. Specifically, the CIO-ITAE optimized LFC demonstrates improved resilience in addressing challenges in the interconnected power system. Keywords: Load Frequency Control, LFC, Ant Colony Optimization (ACO), Differential Evolution (DE), Teaching Learning based Optimization (TLBO),Particle Swarm Optimization (PSO), Genetic Algorithm (GA) and Cohort Intelligence Optimization (CIO).

OPTIMAL LOAD FREQUENCY CONTROL OF INTERLINKED NONLINEAR POWER SYSTEM INTEGRATED WITH SMES-TCSC AND HVDC

In this article Multi-Area Multi-Source Interconnected Power System (MAMS-IPS) incorporated with an Automatic Voltage Regulator (AVR) in both the areas is considered for Load Frequency Control (LFC) study. Nonlinearity such as Governor Rate Constraints (GRC), Governor Dead Band (GDB), and Boiler are introduced with the thermal reheat generating unit. The effect of energy storage device (SMES) and fact device (TCSC) on the dynamic responses of IPS is also tested. PD-PID controller is suggested along with PID/PI-PD controller for minimizing the Automatic Control Error (ACE). The parameters of the proposed controller are optimized through a nature-inspired algorithm (PSO). PD-PID is superior then other suggested PID/PI-PD controllers. Objective functions ITAE, IAE, ISE, and ITSE are taken into consideration for finding the optimized values of the suggested controller as well as of TCSC and SMES devices. A step load of 10% is applied in both IPS areas. The rigidness of the suggested controller is verified by varying the loading condition of the IPS. A 10% increment in step load form is introduced for each area. The effect of renewable energy sources such as solar and a wind unit is also taken into consideration in this article. Varying step input is applied to this renewable unit to absorb its effect on the output responses of MAMS-IPS. The superiority of the PD-PID controller is verified by comparing its result with the recently published optimal controllers results described in the literature. SMES helps in improving the dynamic responses of the MAMS-IPS. Based on performance indices and Settling-time (ST) results of proposed controllers are compared with each other. For commenting on the stability of the introduced Power system (PS) eigenvalue analysis is also conducted. MATLAB version 2018 @ simulation software is used for simulation and for creating .m files

Viscoelastic-adhesive modeling of hybrid adhesively bonded double-lap joints: analytical and numerical studies

ABSTRACT There are a limited number of studies examining the viscoelastic and viscoplastic behavior of adhesive joints. If there is more than one adhesive in the overlap region, the problem becomes more complex and viscoelastic modeling of the overlap region becomes difficult. There is no study in the literature that covers such a bonding (overlap) scheme and examines the viscoelastic behavior of the double lap joint (DLJ). In this study, the effect of viscoelasticity of polymeric adhesives on shear stress and strain distributions in the hybrid-adhesive layer of a DLJ is studied. Maxwell-Wiechert (MW) viscoelastic material model is used to define the viscoelastic behavior of the adhesives. Analytical and finite element (FE) results are compared for both mono- and hybrid-adhesive joints under the step load. Good matches are observed between the analytical and numerical results. Four different hybrid-adhesive joints with different combinations of the adherend thicknesses are investigated by considering only the analytical method. In order to investigate the effect of the loading type on the viscoelastic behavior, step and ramp loads were applied to the joint separately. It is observed that applying the ramp load to the hybrid-adhesive joints affects the shear stresses occurred at the ends of the overlap more than the mono-adhesive joints.

A multi-objective co-optimization method of controller parameters for the overall system of small pressurized water reactor

Small pressurized water reactors (SPWRs) normally have complicated and varying working conditions, and become an important direction for nuclear energy development. The overall system of SPWR, including the reactor coolant average temperature control system, the feedwater control system of the once-through steam generator (OTSG), the steam turbine speed control system and the steam dump control system, is used to guarantee the safe and stable operation. The overall system of SPWR is designed with multiple PI controllers. To accomplish intelligent self-tuning of PI controller parameters, reduce the dependence on human engineering experience, and improve the control performance of the overall system, a multi-objective co-optimization method(MMOCO) with PI controller parameters for overall system of SPWR is proposed. Firstly, based on the coordinated control theory and PI control algorithm, the overall system of SPWR is established. Then, a MMOCO for the overall system of SPWR is designed with the Non-Dominated Sorting Genetic Algorithm-II (NSGA-II). Finally, to compare the performance of the controller parameters derived from the MMOCO and the controller parameters obtained via the engineering tuning method, step load decrease transients are selected. The results show that the ability to control the overall system of SPWR is effectively improved after applying the MMOCO.

Discrete mechanical model for nonlinear dynamical analysis of planar guyed towers considering the unilateral contact of cables

Towers are widely used for power line transmission, wind power plants, TV and radio broadcasting, and telecommunications. To enhance their stability, cables are often employed to anchor these towers to the ground. In this study, we investigate the nonlinear static and dynamic responses of a planar guyed tower in which the unilateral constraints on the cables are considered. A representative discrete mechanical model with two degrees of freedom is developed to simulate the central mast of the tower, and the cables are modeled as unilateral springs with linear stiffness. The nonlinear equilibrium equations are derived using an energy approach that incorporates the dissipative forces, total potential, and kinetic energies into the Euler-Lagrange equations. Unilateral cable contact is directly included in the nonlinear equilibrium equation for the guyed tower, allowing for numerical analysis without the need to evaluate the contact point at each time or load step. Several numerical strategies are employed to obtain nonlinear static equilibrium paths, bifurcation diagrams, phase portraits, and Poincaré sections. Our analyses provide novel results for the influence of unilateral cable contact in nonlinear static and dynamic analysis, evaluating the effects of unilateral contact and prestressing on the results. A parametric analysis reveals that cable contact affects nonlinear oscillations, bifurcation, and stability. Our numerical results indicate that unilateral cable contact introduces less structural stiffness compared to bilateral contact, thereby significantly affecting the static and dynamic stability of a planar guyed tower. This is evidenced by a decrease in the static limit load and alterations in the bifurcation diagrams, where unilateral contact destroys the trivial solutions, leading to periodic and quasi-periodic solutions at low levels of vertical load.

Large‐scale elasto‐plastic topology optimization

AbstractThis work presents large‐scale elasto‐plastic topology optimization for design of structures with maximized energy absorption and tailored mechanical response. The implementation uses parallel computations to address multi million element three‐dimensional problems. Design updates are generated using the gradient‐based method of moving asymptotes and the material is modeled using small strain, nonlinear isotropic hardening wherein the coaxiality between the plastic strain rate and stress is exploited. This formulation renders an efficient state solve and we demonstrate that the adjoint sensitivity scheme resembles that of the state update. Furthermore, the KKT condition is enforced directly into the path dependent adjoint sensitivity analysis which eliminates the need of monitoring the elasto‐plastic switches when calculating the gradients and provides a straight forward framework for elasto‐plastic topology optimization. Numerical examples show that structures discretized using several millions degrees of freedom and loaded in multiple load steps can be designed within a reasonable time frame.