Reverse Loading Research Articles

Experimental and numerical investigations on adaptive stiffness double friction pendulum systems for seismic protection of bridges

This study examines the design, development, and characterization of double friction pendulum systems with variable sliding surface geometry. The proposed isolator has a spherical surface with a small radius within an inner sliding range while different geometric functions are used to describe the sliding surface in the outer range. As a result of variable curvature used as sliding surface, the isolators exhibit an adaptive stiffness behavior with increasing displacement amplitude. First, the analytical equations that describe the characteristics of the proposed isolators were derived. Then, a large-scale prototype adaptive isolator that employs a logarithmic function for the sliding surface geometry in the outer range was fabricated. The response of the prototype isolator was characterized under increasing amplitude reversible cyclic loads considering two different levels of vertical isolator loads. A high-fidelity finite element model was developed to capture the response of the prototype isolator. The results obtained from the experimental tests and numerical simulations were used to verify the analytical equations derived for the proposed isolators. Next, a typical multi-span bridge structure was modeled with adaptive stiffness isolators and its response under bi-directional ground motions records were analyzed. In contrast to a conventional double friction pendulum bearing, the suggested isolators yield significantly reduced base shear forces while exhibiting greater isolator displacements. However, the proposed isolators retain the capability to effectively curtail excessive displacements, ensuring the prevention of collapse.

Studies on high-temperature fatigue behaviour and mechanism for conventionally cast and directed energy deposited forms of Alloy 625

The aim of the present work is to study the low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behaviour of two product forms of Alloy 625. Experiments were performed under total strain control mode using triangular and trapezoidal waveforms under complete reverse loading with R = −1, with a total strain amplitude of ±0.25 %, ±0.4 % and ±0.6 % employing a constant strain rate of 3 × 10−3 s−1 over a temperature range of 298 to 973 K. The additive manufacturing process using the directed energy deposition (DED) technique resulted in an improved fatigue life as compared with the conventional casting (CCA) route. Microstructural analysis using electron back scatter diffraction indicated that an alternating strain got generated within the microstructure and the damage was observed as a strain incompatibility within the partitioned regions of fine and neighbouring coarser grains. The preferred columnar growth occurred in the 〈001〉 direction. Inter-dendritic decohesions were attributed to the Mo, Nb and Ti enrichments. While at ambient temperature, an inter-dendritic de-cohesion and strain incompatibility was the cause for final failure, the occurrence of additional time dependent phenomena such as dynamic strain ageing (DSA), oxidation and creep triggered an earlier fatigue failure at elevated temperatures.

Damage and fracture behavior under non-proportional biaxial reverse loading in ductile metals: Experiments and material modeling

This paper deals with a modified anisotropic stress-state-dependent plastic-damage continuum model incorporating combined hardening and softening rules to predict the plastic, damage, and fracture behavior of ductile metals under monotonic and reverse loading conditions. In the experimental part, a newly designed biaxially loaded cruciform flat HC-specimen of aluminum alloy EN AW 6082-T6 was subjected to biaxial non-proportional loading to generate a wide range of stress triaxialities during experiments. Thus, it is enabled to perform shear monotonic and reverse loading superimposed by various preloads without unloading processes. The experimental data reveal the Bauschinger effect, the strength-differential (SD) effect, and the change in the hardening ratio after shear reverse loading. This is captured by a Drucker–Prager-type yield criterion with an extended Voce isotropic hardening and modified Chaboche’s kinematic hardening rule. In addition, the damage surface is assumed to be translated in the direction of the rate of the damage strain. The numerically predicted global force–displacement curves and local strain fields are verified in terms of the digital image correlation (DIC) technique. Moreover, the scanning electron microscopy (SEM) is used to examine the fracture surfaces and to validate the proposed damage constitutive model.

Asymptotic numerical method for cyclic elasto-plasticity and its application to a steam turbine rotor

Flexible operation of the conventional power plants is necessary for the integration of renewables in the energy mix. Steam turbine rotors are one of the most affected components due to the daily thermal load cycling requirements. In order to accurately study the effect of cycling on fatigue, it is necessary to solve cyclic 3D elasto-plasticity problems for a large number of cycles. The established methods are iterative in nature to arrive at the current yield surface for each elasto-plastic step and are computationally extensive. In this paper, we propose the non-iterative Asymptotic Numerical Method (ANM) based solution technique for 3D cyclic elasto-plasticity problems for the first time by a novel regularized Kuhn-Tucker condition. Regularization techniques used in previous works take care of the partial cycle only, i.e., elastic loading, elastic-plastic transition, elasto-plastic flow and elastic unloading for simple linear work hardening models of plasticity. In the present work, the proposed method extends the applicability of ANM by incorporating elastic-plastic transition and elasto-plasticity in reverse loading, second unloading and repeated loading for any number of cycles so that the computationally efficient fatigue analysis can be carried out. In addition, the extension also includes the application of ANM to non-linear kinematic hardening through the Chaboche plasticity model combined with isotropic hardening. For these advancements, additional regularizations are proposed and implemented in a Finite Element Code. The obtained results are then compared to the solutions obtained from the conventional Newton Raphson (NR) solution technique. The proposed ANM is applied to a steam turbine rotor undergoing a typical thermal load cycling and the cyclic elasto-plastic response is compared with that obtained using the conventional Newton Raphson solution technique.

Effect of diagonal bamboo strips on seismic behavior of sprayed composite mortar-original bamboo composite walls: An experimental study

In the context of China's pursuit of carbon neutrality and carbon peaking, there is a growing demand for green buildings and environmentally-friendly construction materials. In this regard, bamboo structures offer significant potential for development. One promising option is the sprayed composite mortar-original bamboo composite structure system, which exhibits good load-bearing capacity, insulation, and durability. To evaluate the impact of diagonal bamboo strips on the seismic performance of sprayed composite mortar-original bamboo composite walls, as well as analyze the failure modes and hysteretic behavior under low-cycle reverse horizontal loading, a quasi-static test study was conducted on two full-scale walls. One wall incorporated diagonal bamboo strips on both sides of the bamboo studs, while the other wall did not. The results of the study indicate that the presence of diagonal bamboo strips in sprayed composite mortar-original bamboo composite walls enhances their stiffness and bearing capacity, but potentially reduces their ductility and energy dissipation capacity. Compared to other low-rise building walls, sprayed composite mortar-original bamboo composite walls demonstrate higher bearing capacity, and stiffness, as well as good ductility and energy dissipation capacity.

From ductile damage to unilateral contact via a point-wise implicit discontinuity

Ductile damage models and cohesive laws incorporate the material plasticity entailing the growth of irrecoverable deformations even after complete failure. This unrealistic growth remains concealed until the unilateral effects arising from the crack closure emerge. We address this issue by proposing a new strategy to cope with the entire process of failure, from the very inception in the form of diffuse damage to the final stage, i.e. the emergence of sharp cracks. To this end, we introduce a new strain field, termed discontinuity strain, to the conventional additive strain decomposition to account for discontinuities in a continuous sense so that the standard principle of virtual work applies. We treat this strain field similar to a strong discontinuity, yet without introducing new kinematic variables and nonlinear boundary conditions. In this paper, we demonstrate the effectiveness of this new strategy at a simple ductile damage constitutive model. The model uses a scalar damage index to control the degradation process. The discontinuity strain field is injected into the strain decomposition if this damage index exceeds a certain threshold. The threshold corresponds to the limit at which the induced imperfections merge and form a discrete crack. With three-point bending tests under pure mode I and mixed-mode conditions, we demonstrate that this augmentation does not show the early crack closure artifact which is wrongly predicted by plastic damage formulations at load reversal. We also use the concrete damaged plasticity model provided in Abaqus commercial finite element program for our comparison. Lastly, a high-intensity low-cycle fatigue test demonstrates the unilateral effects resulting from the complete closure of the induced crack.

Characterization of ductile damage and fracture behavior under shear reverse loading conditions

AbstractThis paper deals with experimental and numerical analysis of the ductile damage and fracture behavior undergoing biaxial non‐proportional shear reverse loading conditions. In the numerical analysis, an anisotropic cyclic plastic‐damage constitutive model is considered to characterize the damage and fracture behavior. Additionally, the change in the hardening rate is observed after shear reverse loading. Thus, a newly modified non‐hardening strain region method is introduced to capture the change in the hardening ratio between isotropic and kinematic hardening. Furthermore, the damage surface is assumed to translate in the direction of the damage strain increment. Hence, a softening law based on the damage strain is proposed. The modified constitutive model predicts macroscopic force‐displacement and microscopic evolution of plastic and damage behaviors under shear reverse loading with a tensile preload. Through digital image correlation technique and scanning electron microscopy, numerical results can be validated by experimental ones. Furthermore, scanning electron microscopy pictures also reveal different damage and fracture mechanisms on the micro‐level.

Elastoplastic Analysis of Metallic Parts Employing a Meshless Method.

The use of finite element method-based approaches has been popular in studying the elastoplastic behavior of metal parts. However, there has been a growing demand for meshless methods. In response, researchers have developed a meshless solution for 2D elastoplastic evaluation of metal components. This approach obtains the locally symmetric weak form of the governing elastoplastic integral equations at each node throughout the problem area and boundary. The elastoplastic constitutive relationships consider a small deformation rate independent associative flow theory applicable to isotropic hardening materials. The proposed solution algorithm can handle loading, unloading, and reverse loading. Numerical results were computed using Gaussian and spline weight functions, and the presented meshless solution proved to be robust and accurate for conducting the elastoplastic investigation of metallic parts. Furthermore, the Gaussian weight function was found to be more robust than the spline weight function. In conclusion, this paper presents a reliable meshless solution for elastoplastic analysis and highlights the advantages of using Gaussian weight functions.

The influence of reverse loading sequence on the behaviour of beam-column joints

The formation of plastic joints in the reinforced beam-to-column joint is influenced by a number of factors. The behaviour and location of this joint is of major importance since it defines the overall performance of the rigid framework when subjected to loading. Mandated by the majority of codes and standards, the strong-column weak-beam philosophy is used and recognized by the formation of this joint in the beam. This study looked into the effect of reverse loading to the behaviour of plastic joints as compared to a monotonic loading protocol. The study analysed the load-deformation behaviour and ultimate load carrying capacity while additional information is collected through visual observation on the failure and cracking pattern. The study concluded that the reverse loading negatively influenced the load carrying capacity, but negligibly affected the overall behaviour of the joint.

Textile Reinforced Mortar/masonry joints under reverse cyclic in-plane shear

This study investigates for the first time the effect of reverse cyclic loading on the shear bond capacity of Textile Reinforced Mortar (TRM)/masonry joints. The single-lap/single-prism set-up was adopted for monotonic tests, whereas a modified version thereof was developed for reverse cyclic tests making use of a single piston. Specimens comprised wallettes reinforced with glass fiber TRM overlays of various bond lengths (BL = 250–50 mm, step = 50 mm). Reverse cyclic loading: (i) bore no effect on the joints’ failure modes; (ii) resulted in a reduction of the shear bond strength by <20%.

Acoustic emission source location method for glass fibre reinforced plastics based on virtual loading focusing enhancement technology

ABSTRACT Because of the inhomogeneity and anisotropy of glass fibre reinforced plastics (GFRP), it is always a challenge to detect the real-time dynamic quality during manufacturing and service. In this paper, a virtual loading focusing enhancement technology (VLFET) based on time reversal (TR) theory is proposed to accurately locate the acoustic emission (AE) source in the GFRP. First, the focusing enhancement theoretical model with virtual reverse loading is deduced according to the TR theory. Then, after measuring the optimal detection range and the average acoustic velocity, the sensor array is positioned on the GFRP specimen. The excitation signals applied to simulate the AE source at some positions are produced by using lead breaking, and the detection signals are received by the sensor array. Finally, the signals received by the sensor array are processed according to the theoretical model, and the fluctuation image in the monitoring area is reconstructed with fluctuation amplitude of each pixel to evaluate the AE source position. The experimental results demonstrate that the excitation sources can be accurately evaluated with the VLFET. The location error with the proposed method is less than 3.1%, higher than that from the current AE instrument. The proposed technology can also be applied to the AE evaluation of other anisotropic materials.

Mechanical Model of Exposed-Dowel Half-Through Tenon Joints in Chinese Traditional Timber Structures Subjected to Bending

ABSTRACT The exposed-dowel half-through tenon (EHT) joint is a common and important connection configuration in southwest China. However, deformation and looseness in the EHT joint constitute the primary causes of failure in the joint connection performance. In this study, a mechanical model of the joints under bending was established and analyzed. The geometric, physical, and equilibrium equations were constructed, the bending moment-rotation relationship was obtained, and the bending moment-rotation envelope curve and the flexural capacity were calculated. A low cycle reciprocating experiment of the EHT joint was conducted to measure the moment-rotation hysteretic curve and envelope curve of the joints. A comparison of the envelope curves calculated using the model and test curves was conducted, and the results showed that the calculation curve was in good agreement with the test curve. The ultimate bending capacity errors of the joint in the forward loading stage and reverse loading stages were 11.2 and 5.8%, respectively. A parametric analysis of the joint using the established mechanical model showed that increasing the diameter of the wooden pillar and the width of the tenon greatly improved the flexural capacity of the joints while increasing the height of the small tenon and reducing that of the joints during the reverse loading stage. These research results provided a reference for the new construction and restoration of the Drum Towers in southwest China.

Microstructural evolution and mechanical behaviors of the third–generation automobile QP980 steel under continuous tension and compression loads

As one of the representatives of the third–generation automobile steel, QP980 is a high–quality material for the improvement of automobile lightweight and crash safety. In this paper, tension and compression tests are conducted for the QP980 steel based on a self–designed testing platform. Microstructural evolution and mechanical behavior under monotonic and continuous tension/compression loads are investigated. The results show that the martensitic transformation in QP980 steel is more easily excited in the monotonic tension state than in the monotonic compression state. In the tension–compression–tension (T–C–T) and compression–tension–compression (C–T–C) modes, the Bauschinger effect of QP980 depends on the pre-strain amount and the pre–strain mode. The Bauschinger effect is stronger at a higher pre–strain. It is also stronger in T–C–T than in C–T–C because the transformation of retained austenite after T–C–T is greater than that after C–T–C. By analyzing the hardening and softening behaviors in the forward, reverse and reforward loading branches of T–C–T and C–T–C, the hardening ability during reverse and reforward loading branches is higher than that during forward loading. Permanent softening occurs between reverse loading and forward loading of T–C–T instead of C–T–C. The research result of this paper is favorable for understanding the relation between mechanical behavior and microstructural evolution of QP980 steel under the condition of complex loads, and promote its large–scale application.

Development of combined hardening model for spring-back simulation of DP600 in multi-stage sheet metal forming

High strength steel DP600 is widely used in automobile industry due to its excellent mechanical property compared to conventional steel, but spring-back issue is considered to be the major challenge of DP600 in forming process, as the spring-back is triggered by the release of complicated internal stress. In multi-stage sheet metal forming, the sheet may be subject to several cycles of loading and reverse loading, Bauschinger effect greatly influence the strain stress behaviour. A well designed material model can be used to accurately predict and compensate spring-back in forming process. A combined hardening model especially designed for multi-stage forming is proposed in this paper, and the iterated compensation method could be applied by using LS-DYNA to get an optimized result. An experimental testing facility was developed for calibration of this model with cyclic tension and compression strain stress curve. The compensation method is applied in the numerical simulation of a multi-stage manufacturing process of an A-pillar, the result demonstrates the feasibility of the combined hardening model.

Investigation on microstructure evolution of hybrid manufactured TC17 titanium alloy during cyclic deformation

In this work, laser direct energy deposition technology was used to produce hybrid manufactured TC17 titanium alloy with wrought substrate. The cyclic deformation behavior in the total strain range of 1.5%, 2.0%, 2.5%, and 3.0% at ambient temperature and dislocation morphology was investigated. The experimental results show that the three specimens represent cyclic softening when the total strain range is large than 1.5%. All the hybrid manufactured specimens are fractured in the deposition zone and the size of the deformated α phase decreases with increased strain. Slip transfer between α phase and β matrix and a small amount of dislocation tanglement in β matrix was observed in the specimen with strain of 1.5% and cyclic hardening. The dislocation density in β phase increased abruptly when the strain range is 2.5%, which promotes cyclic softening. The heterogeneous deformation between the high and low density dislocation zones formed in the α phase of the cyclic softening samples is the cause of the deformation and fracture of the α phase. The tension-compression asymmetry under forward and reverse loading revealed the Bauschinger effect in specimens with total strain exceeding 1.5%. Dislocation tanglement promoted cyclic softening by increasing the local back stress, at the same time, the back stress helped easier dislocation movement during reverse loading. Meanwhile, dislocation rearrangement and annihilation reduced the friction stress and further accelerate cyclic softening.

Seismic behavior of welded high-performance steel beam-to-column joints with different reinforced configurations

Extensive experimental and analytical investigations have evaluated the hysteresis behavior of welded flange-bolted web beam-to-column joints made of high-strength steels subjected to simulated earthquakes. However, the seismic behavior of newly developed high-performance seismic structural steels, which exhibit a combination of high strength and high ductility at the joint level, has been less attentively scrutinized. In this study, four welded beam-to-column joints were designed with standard or reinforced configurations. The reinforced joints included tapered haunch and transition plate connections. Hysteresis tests were conducted on these joints under low-cycle reversal loading to evaluate their seismic performance. The results indicated that the failure modes of these joints were primarily influenced by the welded heat-affected zones. Last, numerical models were developed and validated. The analysis focused on assessing their ductility coefficient, load-bearing capacity, stiffness degradation, and energy dissipation. The reinforced connections proved to be more effective in improving the initial stiffness, load-carrying capacity, and energy dissipation of the joints compared to the standard joints with the same beam and column sections. The initial stiffness of tapered haunch and transition plate reinforced joints were found to be 29.9% and 12.7% higher than that of the standard joint without reinforced configuration. And the equivalent viscous damping coefficient of the standard joint was 23% lower than the transition plate reinforced joint. However, the reinforced joints exhibited a slight reduction in ductility coefficient compared to the standard joints, with decreases of 11% and 2% for the tapered haunch and transition plate reinforced joints, respectively.

Experimental assessment of retrofitted damaged mortarless dry stacked interlocking masonry walls

The aim of this research study is to experimentally investigate the effectiveness of ferrocement overlay on the seismic capacity of damaged mortarless dry stacked masonry constructed with self-interlocking compressed earth blocks. For the achievement of the aim, two full scale damaged walls were retrofitted with ferrocement overlay technique. To study the effects of openings, one wall was provided with a window, while the other one was a solid wall. Retrofitting materials consist of 18-gauge galvanized steel welded wire mesh having opening size of 12.5 × 12.5 mm (0.5" × 0.5″) and 1:4 conventional cement-sand mortar. The walls were tested with displacement-controlled quasi static cyclic loading protocols. The seismic performance of the retrofitted walls under a combination of vertical axial load and lateral reverse cyclic loading was compared to the specimens before retrofitting. Damage patterns and different seismic parameters such as force deformation curve, energy dissipation, stiffness degradation and performance levels were assessed. The results show that the seismic capacity of damaged mortarless dry stacked interlocking masonry can be restored and enhanced as the overall response of the structure is significantly improved.

Seismic performance evaluation of plastered cellular lightweight concrete (CLC) block masonry walls

The current research presents a novel and sustainable load-bearing system utilizing cellular lightweight concrete block masonry walls. These blocks, known for their eco-friendly properties and increasing popularity in the construction industry, have been studied extensively for their physical and mechanical characteristics. However, this study aims to expand upon previous research by examining the seismic performance of these walls in a seismically active region, where cellular lightweight concrete block usage is emerging. The study includes the construction and testing of multiple masonry prisms, wallets, and full-scale walls using a quasi-static reverse cyclic loading protocol. The behavior of the walls is analyzed and compared in terms of various parameters such as force–deformation curve, energy dissipation, stiffness degradation, deformation ductility factor, response modification factor, and seismic performance levels, as well as rocking, in-plane sliding, and out-of-plane movement. The results indicate that the use of confining elements significantly improves the lateral load capacity, elastic stiffness, and displacement ductility factor of the confined masonry wall in comparison to an unreinforced masonry wall by 102%, 66.67%, and 5.3%, respectively. Overall, the study concludes that the inclusion of confining elements enhances the seismic performance of the confined masonry wall under lateral loading.

Building a model of the impact grinding mechanism in a tumbling mill based on data visualization

A mathematical model was built based on data visualization for the impact grinding mechanism in a tumbling mill, which is mainly implemented during coarse grinding. The determination of impulse interaction parameters is problematic due to the difficulty of modeling and the complexity of hardware analysis of the behavior of intramill loading. Conceptually, it was envisaged to identify the relative dynamic parameters of the impact action as components of the model, which are criteria for the similarity of the loading movement and the grinding process. Impact power was taken as an analog of grinding performance. The initial characteristic of the impact was considered to be the averaged vertical component of the speed of loading movement in the flight zone at the boundary of contact with the shear layer. The formalization of the model revealed the effect on the performance of the mass fraction of the flight zone and the reversibility of loading. The method of numerical modeling was applied, based on experimental visualization of the behavior of granular loading in the cross section of a rotating chamber. The influence of the rotation speed on the performance at a chamber filling degree of 0.45 and a relative particle size of a milling load of 0.0104 was estimated by experimental simulation. The maximum productivity value was found at the relative speed of rotation ψω=1–1.05. A rational condition for impact grinding at ψω=0.75–0.9 has been established. The test proved the effectiveness of using visualization to evaluate dynamic loading interaction analogs. Verification of modeling results was implemented by comparison with the data of the technical standard. The use of similarity criteria unifies approaches to modeling different mechanisms of destruction. The model built makes it possible to predict the rational parameters of the grinding processes by impact, crushing, and abrasion

Quantitative diagnosis of loose piston rod threads in reciprocating compressors for hydrogen storage and transport

Fracture of the piston rod yields catastrophic accidents in reciprocating hydrogen compressors, resulting in costly and time-consuming maintenance and developing the latest challenge to hydrogen safety and economy. The fracture-prone section is mostly located at the connecting thread of the crosshead or of the piston, mainly due to thread loosening and hereby insufficient preload followed by fatigue and fracture. Here, a novel quantitative diagnosis method for thread loosening of hydrogen reciprocating compressor piston rods was proposed. First, a finite element model of a single-throw reciprocating compressor was established to analyse the dynamic behaviour of the operating posture and load pattern at the crosshead and piston rod connecting thread. Further, the impact events triggered by crosshead displacement and rod load reversal events were identified to enhance the interpretability of the signal, thereby obtaining a loose-sensitive frequency band of 2–15 kHz and specific phase angle features. Next, the multi-band feature spectrum was formed using variational modal decomposition and data dimensionality reduction based on the Hilbert transform, which demonstrates satisfactory performance in the feature reservation as well as spectrum compression of acceleration signals in both the time and frequency domain. The proposed methodology was validated by experiments with various levels of thread loosening. The result indicated the multi-band feature spectrum feature was effective in quantitative loosening diagnosis with accurate and stable classification.