Dynamic Cyclic Loading Research Articles

Seismic Response of Multistory Buildings with Double Telescoping Self-Centering Energy-Dissipative Brace (DT-SCED)

Telescopic Self-Centering braces are one of the very successful examples of Self-Centering braces which perform well in seismic loading. In this study, a new example of Telescopic Self-Centering brace is introduced, which has superior features over other telescopic braces. These include: high axial load capacity, use of shorter cables in brace construction, simplicity of construction, use of separate cables for compressive and traction modes, less fatigue in cyclic loads and, allowing for more dynamic loading cycles. In this paper, a sample was designed with an axial force capacity of 300kN. Modeling of behavior (DT-SCED) was accurately expressed using numerical relationships. nonlinear incremental stiffness analysis method was also used to calculate the hysteresis brace behavior. The cyclic load test was applied to this brace and the result showed complete Self-Centering behavior. Then, a sample building with the double telescoping Self-Centering Energy-Dissipative Brace (DT-SCED) was subjected Single direction pushover analyses and the results are compared with the sample buildings with the Self-Centering Energy-Dissipative Bracing (SCED) and the Telescoping Self-Centering Energy-Dissipative Bracing (T-SCED). The results of the analysis and comparing with other samples confirm the seismic superiority of performance of the DT-SCED brace over other samples.

New method for monitoring the residual life of high pressure hoses

The presented method is designed to monitoring the residual life of hydraulic drive elements. In this work, as an element of the hydraulic drive, high-pressure hoses are used, which are used in hydraulic drives of technological equipment of transport and technological machines. As a diagnostic parameter that characterizes the technical condition of the high-pressure hose, the frequency of natural oscillations of the pipeline with the liquid is adopted. Experimentally, the dependence of the natural oscillation frequency of the high-pressure hose on the dynamic loading cycles of the harmonic type was obtained, which were used to determine the probability of failure of pipelines and to find a strategy for their replacement. The probability of failure of high-pressure hoses for different periods of operation is determined on the basis of the obtained experimental dependences of the natural vibration frequency of pipelines on their operating time. Using the obtained dependences of the probability of failure of high-pressure hoses, it is possible to determine the possible residual life of pipelines and the need for their replacement. This method can be used to determine the probability of failure of other elements of the hydraulic drive, such as hydraulic cylinders, hydraulic valves, pumps, and others.

On the physical basis for the creep of ice: the high temperature regime

AbstractThis work quantifies the increased temperature sensitivity of the constitutive behavior of ice with proximity to the melting point in terms of dislocation mechanics. An analysis of quasistatic and dynamic cyclic loading data for several ice types leads to the conclusion that high temperature (e.g. T ≥ −8°C) behavior is the result of a thermally induced increase in the number of mobile dislocations rather than an increase in the activation energy of dislocation glide or the introduction of a new deformation mechanism. The relationship between dislocation density and temperature is quantified and the model is shown to adequately represent the published minimum creep rate vs stress data for isotropic granular freshwater ice for −48 ≤ T ≤ −0.01°C.

Experimental analysis of performance degradation of 3-cell PEMFC stack under dynamic load cycle

A vehicular fuel cell is dynamically operated at the demand of the driver, so that the durability of the fuel cell quickly deteriorates. This study analyzes the durability of a 3-cell short stack under normal vehicle operation. An acceleration test is scheduled with operation temperatures of 55 °C and 70 °C at 50% relative humidity for 300 h. The dynamic load cycle (DLC) conditions are a repetition of the New European Driving Cycle (NEDC), which can allow a short stack to run on the vehicle operating load. At 100-hour intervals, recovery procedures are conducted to understand the order of performance retrieval. Significant stack degradation is observed at 75 °C operation for 300 h. Results show that the recovery protocol can return the performance of the fuel cell at a low and a middle current density regime, but it is hard to recover the performance at a very high current density regime. Performance recovery is very effective for lower temperature operation (55 °C), but the recovery procedures only returned about 4% of the performance at 300 h and 75 °C.

Influence of Geocell Reinforcement on Bearing Capacity of Low-Volume Roads

This study examined the working principles of the geocell reinforcement in low-volume roads through laboratory testing and finite-element modeling (FEM). A steel box of 1.5 m × 1.2 m × 0.9 m (5 ft. × 4 ft. × 3 ft.) was fabricated to accommodate multiple pavement layers. A quarter of the laboratory test box was modeled using commercially available FEM software. A low modulus base and subgrade materials were selected, and the base layer was reinforced with 152 mm (6 in.) high geocell to evaluate the benefits of geocell reinforcement. A dynamic cyclic load of 551 kPa (80 psi) was applied for a set number of cycles (20,000 in the laboratory and 100 during computer simulations). The laboratory test setup was instrumented to record the responses of the material under dynamic cyclic loading. Since the transducers generated substantial data points along with associated signal noise, a set of procedures were incorporated to minimize the electronic noise and reduce the data size. The laboratory test results indicated that the geocell-reinforced sections experienced lower vertical stresses imparted on top of the subgrade nearly by 30% in comparison to unreinforced sections. The vertical pressure distribution beneath the geocell layer suggests that the reinforcement is acting like a combination of flexible and rigid pavement. The geocell-reinforced layer performed well even with an increase in stresses from 689 kPa (100 psi) to 827 kPa (120 psi). Although similar hoop strains trends were observed, the hoop strains estimated from FEM were different than the ones measured in the laboratory.

Degradation studies of ceria-based solid oxide fuel cells at intermediate temperature under various load conditions

Despite several disadvantages with the electronic conduction of ceria, researchers report the high performance of ceria-based solid oxide fuel cells (SOFCs) at intermediate temperatures with advanced cathode materials. However, it appears the long-term stability of ceria-based SOFCs in the anode-supported cell configuration have still not been proved experimentally at practical operating conditions. In particular, the chemical expansion of ceria electrolytes can cause potentially irreversible failure of SOFCs under large PO2 gradients. Hence, we investigate the degradation behavior of NiO-Nd0.1Ce0.9O2-δ anode-supported cells with Nd0.1Ce0.9O2-δ electrolyte in dynamic electrical modes, load trip (0.7–0 A cm−2), load cycle (0.2–0.12 A cm−2), and constant load operation (0.2 A cm−2), at 650 °C. We identify the specific degradation phenomenon of ceria-based cells by analyzing the electrochemical characteristics in post-mortem investigations after durability testing under various electrical load conditions. Additionally, we suggest a possible operation strategy to mitigate the performance degradation of cells under various electrical stresses through understanding of the main degradation mechanism.

Fracture failure analysis of elliptical hole bored granodiorite rocks under impact loads

The aim of the present study is to evaluate the dynamic failure process of granodiorite rock including an elliptical hole. A series of experimental tests including both static and dynamic loads at four different loading rates was conducted on the samples. Dynamic cyclic loading testing was also applied to evaluate the fatigue behaviour of the rocks. The failure process was clearly recorded by a high-speed camera. The distribution of dynamic stress and energy density around the elliptical hole was numerically investigated using LS-DYNA. Thin-section surveying and scanning electron microscope (SEM) analysis were also used to evaluate response of different minerals during failure process and show the microcrack evolution in this type of rock. The results indicate that an elliptical hole in granodiorite rock leads to obvious stress concentration and energy accumulation which can cause considerable decreases in the strength parameters. Moreover, the dynamic strength and elastic modulus exponentially increased with an increase in the loading rate. The high-speed camera results indicate that the visible cracking and severity of the rock failure also increased with an increase in the loading rate. During cyclic Split Hopkinson pressure bar (SHPB) tests with decreasing striker velocity, the fracture nature was transformed from shear to tensile. Cleavage planes controlled the fracture evolution, which was detected in both the thin-section and SEM analyses.

Long term study of directly hybridized proton exchange membrane fuel cell and supercapacitors for transport applications with lower hydrogen losses

Fuel cells (FC) and supercapacitors (SC) directly hybridized allow instantaneous response to peak energy demands in transportation applications. Under urban driving cycle, Fuel Cell Dynamic Load Cycling (FC_DLC), gases are fed with a minimum gas flow rate, corresponding to a safety limit current density (iSL), inducing hydrogen oversupply. To reduce this oversupply, the present work focuses on iSL impact on the FC performance and durability. Two iSL conditions (0.2 and 0.05 A cm−2), were investigated under long-term FC_DLC runs with or without hybridization. At low iSL, hybridization allows 8% reduction in hydrogen oversupply; in addition, non-hybridized FC lifetime was found two-time shorter than that of hybridized FC. Erosion of the microporous layer of the gas diffusion layer (GDL) evidenced by scanning electron microscopy at the cathode, results in poorer water management, in consistence with the sudden voltage drops observed and the high increase in through-plane gas permeability in the GDL.

Durability of solid oxide electrolysis stack under dynamic load cycling for syngas production

A 6-cell solid oxide electrolysis stack was tested under H2O + CO2 co-electrolysis conditions. The cells used in the stack consisted of a nickel-yttria stabilized zirconia (Ni-YSZ) fuel electrode, YSZ electrolyte and lanthanum strontium cobaltite-gadolinium doped ceria (LSC-GDC) composite oxygen electrode. The aim of this study was to investigate the stack durability when operated under dynamic load conditions simulating a wind energy powered SOEC stack for synthesis gas production. The degradation of the stack was observed to be less than 1%/1000 h in terms of area specific resistance during the 1000 h operation. Detailed electrochemical analysis revealed a constant ohmic resistance, indicating intact contact in the stack. Only minor degradation was observed, mainly due to the fuel electrode process. The overall stack voltage degradation rate was 0.8%/1000 h.

An anisotropic bounding surface elastoplastic constitutive model for frozen sulfate saline silty clay under cyclic loading

To study the mechanical properties of frozen saline soil under cyclic loading, the cyclic tri-axial compressive experiments of 36 samples are performed for frozen saline silty clay with 0.0, 0.5, 1.5 and 2.5% of Na2SO4 contents at the temperature of −6 °C, respectively. The experimental results illustrate that the effect of the salt contents and confining pressures on the strength and deformation properties of frozen sulfate saline soils under cyclic loading is significant. To understand the deformation characteristics of frozen sulfate saline silty clay (this kind of saline soil), a bounding surface elastoplastic constitutive model is presented based on the test results and bounding surface plasticity theory. The proposed model includes the effects of salt contents and confining pressures on dynamic characteristics, therefore, it can suitably describe the dynamic stress-strain response of frozen saline soil under cyclic dynamic loading. This proposed model includes: (1) Critical stress ratio calculation formula, considering the effect of salt contents and confining pressures, is proposed. Critical stress ratio and bounding surface shape parameter varying with confining pressures are introduced into plastic potential surface function and bounding surface function to describe pressure melting and crushing characteristics of frozen soils under high confining pressures. (2) The influence of the initial anisotropy and the induced anisotropy by dynamic loads on bounding surface of rotation hardening is considered. A non-associated flow rule together with a mixed hardening rule is used to describe the plastic flow rule and hardening rule of soils during cyclic loading, respectively. (3) A radial mapping rule with mobile mapping origin is employed to simulate elastoplastic properties of soils during cyclic loading. By comparing its simulated results with the test results of cyclic triaxial compression tests, it is found that the proposed model in this paper can well predict deformation characteristics of this kind of saline soil under cyclic loading.

Impact of the testing protocol on the mechanical characterization of small diameter electrospun vascular grafts

AimFor the proper function of small diameter vascular grafts their mechanical properties are essential. A variety of testing methods and protocols exists to measure tensile strength, compliance and viscoelastic material behavior. In this study the impact of the measurement protocol in hoop tensile tests on the measured compliance and tensile strength was investigated. MethodsVascular grafts made out of two different materials, a thermoplastic polyurethane (PUR) and polylactid acid (PLLA), with three different wall thicknesses were produced by electrospinning. Samples were tested with a measurement protocol that allowed the comparison of dynamic sample loading to a common quasistatic tensile test. Influence of measurement temperature, preconditioning cycles and the influence of a high number of loading cycles was also investigated. Compliance and tensile strength were evaluated and compared between the different samples and the different load cases. ResultsIn all samples a significant difference in the measured compliance was seen between an unloaded sample and a sample that was already in a preloaded state. For example in the PUR group with 100 μm wall thickness at 37 °C, the first compliance was 32.6 ± 9.6%/100 mmHg, which reduced to 15.4 ± 2.9%/100 mmHg at preloaded state. The PLLA group showed 7.5 ± 4.3%/100 mmHg vs. 0.94 ± 0.11%/100 mmHg respectively. The measurements showed the importance of dynamic testing, as the samples viscoelastic behavior had a considerable influence on the measured compliance. The quasistatic ultimate tensile test alone was not able to predict the sample's in vivo compliance. The measurement temperature had a significant influence on tensile strength and compliance. Both, the number of preconditioning cycles and the high number of loading cycles had a minor influence on the sample's compliance. ConclusionWith a quasistatic tensile tests alone, overestimated compliance values are measured in viscoelastic electrospun vascular samples, therefore dynamic loading cycles are required. Measurements at 37 °C are mandatory, as temperature has a significant influence on the mechanical properties.

Fracture Resistance of Zirconia Oral Implants In Vitro: A Systematic Review and Meta-Analysis.

Various protocols are available to preclinically assess the fracture resistance of zirconia oral implants. The objective of the present review was to determine the impact of different treatments (dynamic loading, hydrothermal aging) and implant features (e.g., material, design or manufacturing) on the fracture resistance of zirconia implants. An electronic screening of two databases (MEDLINE/Pubmed, Embase) was performed. Investigations including > 5 screw-shaped implants providing information to calculate the bending moment at the time point of static loading to fracture were considered. Data was extracted and meta-analyses were conducted using multilevel mixed-effects generalized linear models (GLMs). The Šidák method was used to correct for multiple testing. The initial search resulted in 1864 articles, and finally 19 investigations loading 731 zirconia implants to fracture were analyzed. In general, fracture resistance was affected by the implant design (1-piece > 2-piece, p = 0.004), material (alumina-toughened zirconia/ATZ > yttria-stabilized tetragonal zirconia polycrystal/Y-TZP, p = 0.002) and abutment preparation (untouched > modified/grinded, p < 0.001). In case of 2-piece implants, the amount of dynamic loading cycles prior to static loading (p < 0.001) or anatomical crown supply (p < 0.001) negatively affected the outcome. No impact was found for hydrothermal aging. Heterogeneous findings of the present review highlight the importance of thoroughly and individually evaluating the fracture resistance of every zirconia implant system prior to market release.

Physico-mechanical Behaviors of Granite Under Coupled Static and Dynamic Cyclic Loadings

The stress distribution around constructions in deep rock can be modified by dynamic disturbances, such as earthquakes. For this, we proposed a new loading method applied to granite sample under coupled static and dynamic cyclic loading (CSDCL) condition. The variations in P-wave velocity, gas permeability, and mechanical properties of granite before and after CSDCL are revealed. It shows that with increasing axial static stress, the dynamic cyclic loading amplitude and cycle number, P-wave velocity, uniaxial compressive strength (UCS) and elastic modulus decrease, whilst the permeability and Poisson’s ratio increase. It seems that variations in those parameters have a close relation with the exciting frequency. In this case, CSDCL is applied for crack development along the axial direction in the rock samples, and subsequently results in degradation of mechanical properties, delay of P-wave propagation, and increase of the transport paths. The results show that the permeability and Poisson’s ratio are likely to be more sensitive to the CSDCL, in particular under the axial static stress. The empirical relationships of the P-wave velocity with permeability, UCS, and elastic modulus are established for a pragmatic monitoring purpose. From design point of view, we have established the relationships of the disturbance factors to damage variable. Then the correlations of the damage variable to permeability, UCS, and elastic modulus are analyzed. The testing results in this context could facilitate our understanding of rock stability upon excavation subjected to dynamic disturbances.

Analysis of the initial stage of fatigue wear in heterostructure materials under contact cyclic loading

Introduction. The process of formation of fatigue defects in metal alloys with different structural morphology is considered. The work objective is to develop a computational tool for determining the moment of the defect nucleation under cyclic loading.Materials and Methods. A physical model is built, calculation expressions are presented. The physical model is based on the theory of dislocations. It is shown that a structure factor is particularly important in the process of fracture nucleus origination under dynamic cyclic loading. Depending on the structure and properties of the material, as well as on the nature of the loads, the critical fatigue defect develops in the form of cracks, pores or micro-crater wear.Research Results. A numerical experiment was performed to determine the moment of nucleation of the critical-size defect in iron-base alloys under the drop hypervelocity impacts. Comparative data of calculations and bench tests for droplet impingement erosion of steels and alloys with the structure of ferrite, austenite, sorbitol and martensite are presented. The efficiency of the nucleation stage during the incubation period of erosive wear of the materials studied was evaluated.Discussion and Conclusions. There are no strict instrumental methods for determining the duration of the nucleation stage; therefore, it is recommended to use the proposed analytical model. In addition, the work performed gave a significant application result, i.e. it showed that the focused design of the material structure can significantly increase the wear resistance.

Dynamic and static load performance of dental biomaterial systems with conical implant-abutment connections.

The stability of the implant-abutment interface is an important factor that influences load distribution on the marginal bone. In this study, three dental implants with the same connection were subjected to different dynamic loading cycles. The fracture strengths and the horizontal compatibility of implants were assessed. Eighty four implant specimens were embedded in a polyacetal cylinder as simulated bone loss of 3 mm from the implant platform. Three of the implants were used to determine the endurance limit. The other specimens were subdivided into four subgroups (n = 6): three for dynamic + static loading, and one for static loading (control group). The tests were performed by applying a compression load. The dynamic loading experiments included three different cycles with endurance upper limit loads at a frequency of 10 Hz. The differences between the fracture strength values of the implant brands were found to be statistically significant. However, there were no meaningful differences between the fracture strength values of implants of the same brand. The specimens of the DTI implant system had the lowest strength (647.9 ± 41.5N) and the SEM analysis indicated that the Implantium implant system had the shortest horizontal gaps. There was a negative correlation between the fracture strengths and size of the microgaps. The importance of these in vitro results needs to be validated by clinical trials because the loads in the mouth can be applied from various angles.

PREDICTION OF OPERATION LIFE EXTENSION OF HEAT POWER EQUIPMENT

Based on the analysis of known researches, it is revealed that the quantity and accumulation rate of cyclic thermal and dynamic loads in the transient modes of normal operation conditions, when violating normal operation conditions and in accident conditions (except for the nuclear reactor vessel) are the key factors of prediction of operation life extension for a heat power equipment (heat exchangers, pumps, armature). The method for predictive estimation of terms of operation life extension of a heat power equipment depending on stress amplitudes in transient and accident conditions, quantity and accumulation rate of cyclic loads, strength metal parameters of a heat power equipment vessels (except for a reactor vessel) is provided. The method is implemented on the example of steam generators of WWERs and using operational data of South-Ukraine-1 (by 2010). Admissible accumulation rate of cyclic loads during operation life extension by 30, 40 and 50 years is a result. The results define insufficient substantiation of nuclear power plant operation in the “maneuverable” modes with a variable reactor power. In this case, the quantity of cyclic equipment loads increases dramatically, and terms of safe operation are limited. The developed method and the obtained results of prediction of operation life extension of heat power equipment can be used for industry programs to extend the operation of Ukrainian nuclear power plants, as well as to improve the regulatory documents governing the conditions and requirements for acceptable safe extension of the operation life of heat power equipment of nuclear and thermal power enterprises. Further improvement of the method proposed in the work for predicting operation life extension of heat power equipment can be based on the development of methods for analyzing the reliability of heat power equipment and databases on operation disturbances.The materials of the presented work are used in the educational process for the training, retraining and advanced training of specialists in the energy industry.

Development of failure criterion and fatigue model to characterize the fatigue resistance of asphalt binders under controlled-stress time sweep tests

The fatigue resistance of asphalt binders is of great significance to the design and maintenance of asphalt pavements. This study aims to develop a fatigue model that is independent of material types for asphalt binders subjected to controlled-stress time sweep tests. The ratio of dissipated energy change (RDEC) method and the normalized stiffness ratio (NSR) method are first evaluated with regard to this objective. It is demonstrated that both methods fail to provide a fatigue model that is independent of material types. A new failure criterion is then developed through analysis of the three phases of the dynamic shear modulus versus load cycles graph. The fatigue life is defined as the load cycle corresponding to the second inflection point on such a graph. In accordance with the plateau value concept utilized in the RDEC method, a new fatigue-related parameter is defined as the RDEC value at the fatigue life determined by the NSR method. Combine the fatigue life with the new fatigue-related parameter in the form of power function yields a new fatigue model, which is then fitted to the data points of all tested asphalt binders. It is found that all of the data points are located at or close to the best-fit line with an R2 value of 0.978. Therefore, this new fatigue model has the potential of being material type independent.

Biomechanical comparison of polylactide-based versus titanium miniplates in mandible reconstruction in vitro

ObjectivesEvaluation of the mechanical integrity and reliability of polylactide-based miniplates for osseous free flap fixation at the mandible in an experimental study setup of a mandible reconstruction model. Material and methods1.0mm titanium miniplates (group TI) (MatrixMandible, DePuy Synthes, Umkirch, Germany) and 1.5mm polylactide miniplates (group PL) (Inion CPS, Inion Oy, Tampere, Finland) were used to fix a polyurethane (PU) fibula segment to a PU mandible reconstruction model using monocortical non-locking screws. Mastication was simulated via unilateral cyclic dynamic loading at 1Hz with increasing loads (+ 0.15N/cycle, Bionix, MTS, USA). A 3D optical tracking system (Aramis, GOM, Braunschweig, Germany) was used to determine interosteotomy movements (IOM). ResultsIOM were higher in the polylactide group (distal: P=0.001, mesial: P=0.001). Differences in mean stiffness (titanium: 478±68N/mm; polylactide: 425±38N/mm, P=0.240) and mean force at a vertical displacement of 1.0mm (titanium: 201.6±87.1N; polylactide: 141.3±29.9N, P=0.159) were not significant. ConclusionsThe results of this study suggest that polylactide-based miniplates provide reduced mechanical integrity and higher interosteotomy movements in comparison to titanium miniplates in vitro. Indications for clinical use of polylactide-based miniplates in mandible reconstruction have to be placed critically. Future studies will focus on clinical complications of polylactide-based plates in risk patients.

Local impact of load cycling on degradation in polymer electrolyte fuel cells

Degradation mechanisms occurring during proton exchange membrane fuel cells (PEMFC) operation critically depend on the applied electrical load profile. In this work, durability tests were performed using different load cycling ranges. The tests include refresh procedures in order to recover temporary performance losses. Operando current density distribution measurements as well as electrochemical characterization techniques were applied to obtain essential data for understanding degradation behavior of the components of the membrane electrode assembly (MEA). The analysis is supported by scanning electron microscopy (SEM) and energy dispersive x-ray diffraction (EDX) analysis of MEA components. Dynamic load cycling has proven to lead to lower performance losses as compared to constant load operation. Moreover, reversible performance losses were associated with an increased heterogeneity of the current density distribution along the flow field. Thereby, areas with high local current density exhibit particularly strong Pt band formation in the membrane linked with a thinning of the cathode catalyst layer.

Study on liquefaction mechanism of saturated sand considering stress redistribution

Most of the saturated soil in actual condition is anisotropically consolidated, and can liquefy during minor earthquake. Based on the analysis of stress state in actual soil, the present paper considers that the stress redistributed during loading process, which result in the liquefaction. However, in present conventional element tests, the liquefaction in anisotropically consolidated saturated sands (ACS) cannot be induced under dynamic cyclic loading without stress reversal. It contributes to the uncertainty about the liquefaction resistance of soil and the development model of excess pore water pressure (PWP). A brief theoretical interpretation within the theory of limit state of soils is given to explain why the liquefaction in ACS could not be induced using the conventional element test under dynamic cyclic load with low or moderate amplitude, and the validity of the theoretical analysis is verified by experiments. The stress redistribution is realized by confining the lateral deformation based on the conventional test, and the liquefaction in ACS under minor cyclic load is realized. Furthermore, a new concept called “continued liquefaction” is proposed. Apart from “transient liquefaction”, the PWP, the total confining pressure and the total vertical stress are always at the same level, and the mean effective stress of sands is always zero in the “continued liquefaction” stage. Finally, a suggestion that the development model of PWP should be established on the basis of the ratio of the PWP to the initial vertical stress is recommended.