Asymmetric Loading Conditions Research Articles

Crack propagation kinetics under asymmetric cyclic loading for 316LN SS in high-temperature solution

Crack propagation rates (CPRs, or da/dN) of 316LN stainless steel (SS) in high-temperature solution under asymmetric cyclic loading conditions are experimentally determined, focusing primarily on the effect of load-rise time on crack propagation. The increment in crack propagation per cyclic block mainly occurs during the load-rise period. When the stress intensity factor range (ΔK) and load ratio (R) are kept similar, a faster increase in the stress intensity factor (dK/dt) during the load-rise period results in a lower average crack propagation rate (Vave-tr) during that phase. Secondary cracks and fatigue striations are observed on the fracture surface. The CPRs show a higher environmental enhancement factor (Fen) as the load-rise time increases.

On the cyclic elastoplastic shakedown behavior of an auxetic metamaterial: An experimental, numerical, and analytical study

This article presents the first experimental, numerical, and analytical study of the elastoplastic shakedown response of an auxetic metamaterial structure that elucidates interactions between auxeticity and maximum shakedown loading capacity. The study aims to determine the safe elastoplastic shakedown limit of perforated auxetic aluminum sheet structures (AA5083-TO) with fixed void fraction (16.4%) under ambient cyclic asymmetric uniaxial loading conditions. The motivation is that shakedown-based designs can be used to expand the feasible design space under cyclic loading conditions compared to conventional yield-limited designs. Finite element analyses with calibrated hardening models are used to develop Bree load-interaction diagrams that are experimentally validated. It is found that shakedown occurs at stress levels up to almost four times the elastic limit of the structure for a fixed allowable equivalent strain level near three percent. This shakedown multiplier is also sensitive to the extent of auxeticity in the structure and a parametric study and analytical model are used to identify underlying mechanisms and a potential maximum condition.

An enhanced multiaxial low-cycle fatigue life model

To improve the accuracy of predicting multiaxial fatigue life, an enhanced multiaxial low-cycle fatigue life model is proposed based on the FS model. This model introduces a correction parameter for the stress-related term to consider the influence of normal stress and shear stress on the critical plane. The feasibility of the model is investigated, and it is validated using fatigue test data from eight different materials. The results indicate that the proposed model is applicable for both symmetric and asymmetric loading conditions under constant amplitude loading, with highly accurate fatigue life prediction results.

Unified AI-Based Predictive Models for the Ultimate Capacity of Multi-Planar Gapped KK Steel Pipe Joints

The multi-planar steel pipe joints are widely used in communication towers, industrial structures, and offshore platforms. The current design formulas consider this joint as a uniplanar joint and account for the multi-planar effect using empirical correction factors. Recent studies deal with this multi-planar joint as a 3D joint but considering certain loading conditions. Hence, the aim of this research is to develop more general AI-based predictive models for the ultimate capacity of multi-planar gapped KK steel pipe joints, considering both symmetric and asymmetric loading conditions. Three AI techniques were applied to a database of previously published works. These techniques are “Genetic Programming” (GP), “Artificial Neural Network” (ANN), and “Evolutionary Polynomial Regression” (EPR). The prediction accuracies of the developed AI models were compared against two previously published formulas. The results indicated that the developed AI models are much more accurate than the previously published formulas. Also, the results showed that both the ANN and EPR models have almost the same level of accuracy (about 92%), but the EPR model has the advantage of presenting a closed-form equation that could be implemented either manually or using software. Doi: 10.28991/CEJ-SP2024-010-07 Full Text: PDF

Asymmetric two-dimensional poroelastic analysis of heterogeneous smart discs considering hygrothermal loading

The objective of this study is to examine through the finite element method the impacts of employing two-dimensional (2D) properties grading on the behaviors of piezoelectric sensors/actuators due to their significance. These structures are modeled as variable thickness discs and are assumed to be exposed to simultaneous asymmetric loading conditions (thermal, hygral, electrical, and mechanical loads). Results demonstrate that the 2D grading is superior compared to the conventional one-dimensional grading for reducing the von Mises stress. The reduction reached, for instance, 8.3% allowing further increase of loading and/or safety. Also, the change of the displacement is varied between −4.5% and 31% depending on the considered grading pattern, and thus can be controlled to a desired value which helps to improve the sensing and actuating functionality for certain requirements. The study also examines the effects of porosity, and the results are compared to nonporous discs. Reductions of the maximum compressive tangential stress and von Mises stress reached 79.75% and 33.6%, respectively at specific porosity distribution patterns and volume fractions. These findings have significant implications for the manufacturing of smart structures, enabling them to operate in severe working conditions with better sensing and actuating capabilities than previously possible.

The cyclic response behavior, failure mechanism, and life prediction model of 9% Cr‐based steel under different strain ratios in the low cyclic fatigue regime at 430°C

AbstractDuring steam turbine rotor operation, the startup and showdown processes can cause asymmetric loading stress, leading to fatigue damage. To research the cyclic deformation behavior and fatigue failure mechanisms of 9% Cr‐based steel under asymmetric loading conditions, the strain ratios including −1, 0, and 0.1 are introduced into the low cyclic fatigue tests at 430°C. The results show the cyclic softening behavior is observed under three strain ratios at 430°C. As the strain ratio increases, the crack initiation sources, plastic strain, and consumed energy present an increasing trend. Comparing the microstructure morphology at different strain ratios, the coarsening lath and carbides affect fatigue crack initiation. Besides, the grain rotation and dynamic recrystallization can be observed, resulting in the cyclic softening behavior during the fatigue test. Finally, a new fatigue life prediction model is proposed at 430°C.

Dynamic analysis of segmental linings of shield tunnels using a state space method and its application in physical test interpretation

Based on the state space method and D'Alembert's principle, a new analytical method is established to study the dynamic response of circular segmental linings of shield tunnels. The tunnel lining segments are considered to be curved Euler–Bernoulli beams, the joints connecting the segments are modeled by a series of distributed springs, and the lining–surrounding soil interaction is represented by distributed reaction springs with viscous dampers. With the state space method, the governing equations can be concisely expressed in matrix form, and a separate variable method in conjunction with a frequency scanning strategy is employed to obtain the natural frequencies and corresponding modal shapes of the free vibration of the linings. The orthogonality of the vibration modes of the linings is demonstrated using a symplectic inner product, and the response of the linings under arbitrary dynamic loads is solved using the modal superposition method. Numerical and model experimental examples are presented to verify the efficiency and robustness of the proposed method. We prove that the proposed method can solve the dynamic response of segmental linings by considering the effect of joint and asymmetric loading conditions with high efficiency and accuracy.

Mixed-mode fracture of compacted tailing soils. II: Crack properties from full-field displacement fields

This paper deals with the acquisition of the crack evolution and propagation behaviors of the compacted tailing soils from full-field displacement fields by incorporating two-dimensional optical technique-digital image correlation (2D DIC). Meanwhile, the fracture properties, such as mode I+II fracture toughness were also measured and compared to the results obtained in Part I. Cracking properties, such as strain energy rate, cracking initiation angle, and size of critical distance were investigated from the perspective of full-field displacement field and compared to the classic fracture mechanics. It is more intuitive to recognize the mixed mode fracture properties of the compacted tailing soils under asymmetric loading conditions by using optical techniques in full-field displacement fields. This work largely enriched the database of the fracturing properties of compacted tailings soils.

Research on the spatial effect of foundation pit under asymmetric loads

Abstract: This research describes the evolution of the spatial effects of foundation pits considering internal support and external loads. Based on the existing concept of “plane strain ratio”, the term “plane strain ratio considering maximum surface settlement” is proposed to characterize the spatial effects of an asymmetric foundation pit. A series of finite element model calculations were carried out using the Nanchang Aixi Lake foundation pit, including 1) the calculation of simulated actual conditions, 2) the calculation of simulated full symmetric load, and 3) the calculation of simulated asymmetric load. The results indicate that for the symmetric condition at 20 kPa and below, the spatial effect range increases as the load increases. For the symmetric condition above 20 kPa, the load has a negligible impact on the spatial effect range. On the side with a larger load under asymmetric loading conditions, the spatial effect of the working condition below 30 kPa is smaller than the corresponding symmetric load. On the side with a smaller load, the spatial effect of the working condition above 80 kPa increases compared with that of the corresponding symmetrical load. Given and verified are the modified fitting equations that take into account the influence range of spatial effect on both sides of the foundation pit under symmetrical and asymmetrical loads.

Triple-Active-Bridge Converter With Automatic Voltage Balancing for Bipolar DC Distribution

The bipolar dc grid outperforms the conventional unipolar dc distribution in terms of quality, flexibility, efficiency, reliability, transmission capacity, and safety. Despite many advantages, it has a voltage imbalance problem. Unbalanced and asymmetrical load conditions are the major contributors to this imbalance. This article proposes the triple-active-bridge converter with an automatic voltage balancing capability for bipolar dc distribution that eliminates the need for a dedicated voltage balancing controller and any additional active/passive components. Furthermore, along with the bidirectional power flow feature of a converter, the proposed method utilizes the magnetic integration of a converter's existing inductors. This voltage balancing coupled inductor offers the required inductance for power transfer and zero-voltage-switching, over and above provides effective voltage balancing without increasing the magnetic volume of a converter. The proposed method does not require an individual output voltage sensing and moreover; it decreases the complexity by reducing the system from two control variables to one. A 5-kW prototype is tested under an extreme-unbalanced load condition during the steady-state and as well as during transients. The analysis and experimental results are presented to validate the performance.

Generalized simple model for predicting the modulus degradation and strain accumulation of clay subject to long-term undrained cyclic loading

Modulus degradations and strain accumulations of clays under repeated environmental loads are important factors in the design of offshore foundations. In this study, a simple model is proposed for predicting the modulus degradation and strain accumulation of clays subject to long-term cyclic loading. The development of the proposed model is based on a comprehensive database, including a series of undrained cyclic triaxial tests on the reconstituted kaolin clays. The concept of fatigue failure under cyclic loading is adopted to develop the model. Several influence factors, such as the initial stress ratio, initial confining stress, and overconsolidation ratio, are considered in the prediction model. The main feature of the proposed model is to predict the modulus degradation and strain accumulation simultaneously. This process is typically achieved separately by different models. At the isotropic condition or the symmetric (two-way) loading condition, the model tends to predict the modulus degradation. At the anisotropic condition or the asymmetric (one-way) loading condition, the model tends to predict the strain accumulation. The model predicts the modulus degradation and strain accumulation at the intermedium conditions defined by the initial stress ratio. This physical-based model can be easily calibrated with new test results or extended with additional influence factors (e.g., loading frequency).

Phase current estimation based shunt active power compensation

Nonlinear and unbalanced loads introduce distortions to the power network in the form of harmonics and asymmetric load conditions. Through the rising penetration of residential renewable plants, energy is fed back into the grid. By shaping the output of those power converters the distortions can be mitigated. The technique of shunt active power filtering allows the instantaneous compensation of a dedicated nonlinear load, by measuring its distorted consumption. The goal of this article is to present an extension to this method which allows to compensate for a given area utilizing the advanced metering infrastructure in a smart grid. Given a compensation unit, instantaneous voltage measurements are collected from neighboring consumers. By utilizing the model of the distribution line, a linear Kalman filter allows the estimation of the phase currents. These values replace the dedicated nonlinear load measurements, and by compensating for line currents, the power quality of a given transformer area is improved. Analyzing the model based approach against real world like scenarios and elaborating the robustness shows the potential for real world applicability.

Analytical solution of micropolar functionally graded materials (FGM) hollow cylinder under thermal asymmetric load (r, θ)

AbstractThis paper focuses on the analysis of two‐dimensional steady‐state thermal stresses based on micropolar theory in hollow and solid cylinders which are made of functionally graded materials (FGM) under asymmetric loading condition (r, θ) with the general thermal and mechanical boundary conditions along the inside and outside surfaces. The heat conduction and Navier equations were solved using the separation of variables and complex Fourier series for micropolar theory. Finally, the exact numerical results for both micropolar and classical solutions are illustrated and discussed using two examples: In Example 1, based on the micropolar theory, when thermal tensions are exerted on the inside surface of the cylinder, some of the strain energy is used for the object rotation; therefore, considering this condition, the calculated values resulted from the micropolar theory are smaller than those of the classic theory. In Example 2, by exerting normal tension on the inside surface of the cylinder, the rotation created is very small; therefore, the results of the micropolar and classic theories are similar. In addition to FGM materials, this solution is applicable to minerals and organic matter multiphase fiber and particulate composites, soil, rock, concrete, and various granular materials [59].

Computational model of endochondral ossification: Simulating growth of a long bone.

Mechanical loading is a crucial factor in joint and bone development. Using a computational model, we investigated the role of mechanics on cartilage growth rate, ossification of the secondary center, formation of the growth plate, and overall bone shape. A computational algorithm was developed and implemented into finite element models to simulate the endochondral ossification for symmetric and asymmetric motion in a generic diarthrodial joint. Under asymmetric loading condition the secondary center ossifies asymmetrically leaning toward the external load and results in tilted growth plate. Also the mechanics seems to have greater influence in the early onset of the ossification of the secondary center rather than later progression of the center. While previous models have simulated select stages of skeletal development, our model can simulate growth and ossification during the entirety of post-natal development. Such computational models of skeletal development may provide insight into specific loading conditions that cause bone and joint deformities, and the required timing for rehabilitative repair.

Distinction of Asymmetric Load From Eccentricity or Synchronous Electric Generator Winding Fault Using Search Coil Measurements

Forces acting on the stator teeth of the 400 kVA, a four-pole synchronous generator was analyzed for asymmetric electric load conditions. The measurement of the flux density was performed with the measuring coils installed on the stator teeth and calculations were done by the finite-element method. The measured data were processed to determine the radial forces of the stator teeth through the Maxwell stress tensor integration algorithm. The processed data were used for the detection of asymmetric load condition. The methodology for the distinction of the asymmetric load from the eccentricity or winding fault is presented. The parameters of the measuring coil sensors and their adequate position can be determined with finite-element calculation before performing sensor installation on the machine.

Variations of stress field and stone fracture produced at different lateral locations in a shockwave lithotripter field

During clinical procedures, the lithotripter shock wave (LSW) that is incident on the stone and resultant stress field is often asymmetric due to the respiratory motion of the patient. The variations of the LSW-stone interaction and associated fracture pattern were investigated by photoelastic imaging, phantom experiments, and three-dimensional fluid-solid interaction modeling at different lateral locations in a lithotripter field. In contrast to a T-shaped fracture pattern often observed in the posterior region of the disk-shaped stone under symmetric loading, the fracture pattern gradually transitioned to a tilted L-shape under asymmetric loading conditions. Moreover, the model simulations revealed the generation of surface acoustic waves (SAWs), i.e., a leaky Rayleigh wave on the anterior boundary and Scholte wave on the posterior boundary of the stone. The propagation of SAWs on the stone boundary is accompanied by a progressive transition of the LSW reflection pattern from regular to von Neumann and to weak von Neumann reflection near the glancing incidence and, concomitantly, the development and growth of a Mach stem, swirling around the stone boundary. The maximum tensile stress and stress integral were produced by SAWs on the stone boundary under asymmetric loading conditions, which drove the initiation and extension of surface cracks into the bulk of the stone that is confirmed by micro-computed tomography analysis.

Topology Optimization Design and Experimental Research of a 3D-Printed Metal Aerospace Bracket Considering Fatigue Performance

In the aerospace industry, spacecraft often serve in harsh operating environments, so the design of ultra-lightweight and high-performance structures is a major requirement in aerospace structure design. In this article, a lightweight aerospace bracket considering fatigue performance was designed by topology optimization and manufactured by 3D-printing. Considering the requirements of assembly with a fixture for fatigue testing and avoiding stress concentration, a reconstructed model was presented by CAD software before manufacturing. To improve the fatigue performance of the structure, this article proposes the design idea of abstracting the practiced working condition of the bracket subjected to cycle loads in the vertical direction via a multiple load-case topology optimization problem by minimizing compliance under a variety of asymmetric extreme loading conditions. Parameter sweeping was used to improve the computational efficiency. The mass of the new bracket was reduced by 37% compared to the original structure. Both numerical simulation and the fatigue test were implemented to support the validity of the new bracket. This work indicates that the integration of the proposed topology optimization design method and additive manufacturing can be a powerful tool for the design of lightweight structures considering fatigue performance.

Transient deformation and curvature evolution during the snap-through of a bistable laminate under asymmetric point load

Bistable carbon fiber composites, whose bistability arises from having asymmetric fiber layouts in different layers, have shown immense potential for using in shape morphing and adaptive structure applications. While many studies in this field focus on these composite laminates' external shapes at the two stable states, their snap-through behavior of shifting from one stable shape to the other remains a critical aspect to be investigated in complete detail. Moreover, symmetric loading conditions have been extensively studied based on the classical lamination theory, but the asymmetric loading conditions received far less attention. Therefore, this study examines an asymmetric, localized point load on a [0∘/90∘] bistable laminate and its complex transient deformation during the snap-through. Finite element simulation and experiment results reveal three uniquely different snap-through behaviors — two-step snap, one-step snap, and no snap — depending on the point load location. The localized initiation and propagation of a “curvature inversion zone,” calculated from finite element and digital image correlation results, are directly related to these snap-through characteristics. This study also explored the feasibility of using an extended analytical model of classical lamination theory to qualitatively reproduce the above findings. This model compares three polynomial functions of different orders to approximate the out-of-plane laminate displacement field. This study's results can offer valuable insights into the fundamental mechanics of snap-through behaviors and the actuation designs for the bistable composites for different loading scenarios.

Poroelastic response of a functionally graded hollow cylinder under an asymmetric loading condition

This paper aims to develop semi-analytical results of the poromechanical fields in a hollow cylinder subjected to an asymmetric loading condition. The poroelastic properties of the hollow cylinder vary in the radial direction. The heterogeneous hollow cylinder is approximated by a multilayer structure in which each cylindrical layer is assumed to be homogeneous. The analytical results of the poromechanical field are first established in the transformed Laplace space, and then the ones in time space are obtained by the inverse Laplace transformation. They are perfectly in coherence with existent results in the literature that were developed for the homogeneous and the heterogeneous cases at the steady state regime. Sensitivity analysis shows a significant effect of the radial variations of the poroelastic properties on the response of the hollow cylinder.

Carrying Asymmetric Loads While Walking on a Treadmill Interferes with Lower Limb Coordination

The purpose of this study was to investigate the effect of different load carriage modes on coordinative patterns in the lower extremities during walking. Twenty-five university students walked on a treadmill at their preferred pace under three different load conditions: symmetric load (5% of body mass in messenger bags on each shoulder hanging vertically and against the hips), asymmetric load 1 (10% of body mass in a messenger bag on one shoulder hanging vertically against the ipsilateral hip), and asymmetric load 2 (10% of body mass in a messenger bag on one shoulder with the bag draped across the trunk to the contralateral hip). Altered thigh-shank and shank-foot couplings were found for the loaded side during the stance of gait when comparing the asymmetric 1 and 2 to the symmetric load. In addition, thigh-thigh coupling was changed during gait when comparing the asymmetric load 2 and symmetric load. However, we did not find any significant differences in intralimb and interlimb couplings between the two different asymmetric load conditions. The results suggest potential benefits when carrying symmetrical loads in order to decrease abnormal limb coordination in daily activities. Thus, it may be advisable to distribute load more symmetrically to avoid abnormal gait.