High Frequency Loading Research Articles

Coupled finite element analysis of the dynamics of poroelastic media considering the relative fluid acceleration

AbstractThis paper mainly discusses the dynamics of poroelastic media using the finite element analysis based on the u‐v‐p full formulation, where , , and p denote the solid displacement, relative fluid velocity with respect to solid velocity, and pore fluid pressure. It incorporates the effect of relative fluid acceleration with respect to solid acceleration on soil dynamic response. The ‐‐p formulation is first verified through the comparison with the analytical solution. After that, the response of one‐dimensional saturated soil column and two‐dimensional partially saturated soil layer subjected to vertical loading with different combinations of soil permeability and loading frequency are investigated using the analyses with the ‐‐p and ‐p formulations, based on which the effect of relative pore fluid motion is discussed and the differences in the soil response between two kinds of analysis approaches are examined. Results reveal that the rapid fluid flow has a pronounced effect on soil dynamics and the soil with a greater degree of saturation is more sensitive to the increase of loading frequency and soil permeability. The soil dynamic response can basically be divided into two main categories that represent insignificant and significant flow motion depending on loading frequency and soil permeability. Furthermore, despite vertical loading, horizontal soil response can also be affected by the large relative pore fluid flow when loading frequency and soil permeability are large. In addition, the simplified formulation can still be applicable to predict dynamics of poroelastic media in cases with high loading frequency and low soil permeability.

Numerical analysis of plastic deformation mechanisms in polycrystalline copper under cyclic loading with different frequencies

The frequency of cyclic loading has a strong effect on the fatigue lifetime of materials. High loading frequencies allow reaching very high cycle fatigue regimes. The effects of cyclic loading with different frequencies are studied using the finite element method within the framework of crystal plasticity. The numerical study represents copper polycrystal that undergoes tension–compression loading with frequencies 80 Hz and 20 kHz respectively. The spatial distributions of stresses, strains, and plastic slip are used for estimation of volume fraction, morphology, and plasticity mechanism of persistent slip bands. The results show that at 80 Hz cyclic loading, plasticity in persistent slip band cells is dominated by the creation and annihilation of dislocations while at 20 kHz cyclic loading, plasticity is driven by the back-and-forth movement of dislocations inside the persistent slip band cells. The numerical study shows a correlation between amplitude and frequency of loading with spatial distribution and morphology of persistent slip bands that can be related to the fatigue lifetime.

Experimental study on the influence of cyclic loading frequency on liquefaction characteristics of saturated sand

A series of undrained cyclic torsional shear tests was conducted to investigate the effect of cyclic loading frequency on the liquefaction characteristics of saturated sand using a hollow cylinder apparatus. The test results show that the dilative and contractive tendencies of various saturated sands are not only related to the physical properties of the sand, but are also affected by loading frequency. Under low-frequency loading, the saturated sand has a dilative behaviour, excess pore water pressure fluctuates after initial liquefaction and the soil maintains the ability to resist liquefaction to some extent after the initial liquefaction. The liquefaction mode in terms of the stress–strain relationship generally performs as the ‘cyclic mobility’. However, under high-frequency loading, the saturated sand has a contractive behaviour, and excess pore water pressure generally remains stable after the initial liquefaction. The liquefaction mode in terms of stress–strain relationship generally exhibits as ‘cyclic instability’. The deformation caused by low-frequency loading is significantly larger compared with that caused by high-frequency loading. At higher loading frequencies, the phase transformation stress ratio increases with the increase of loading frequency, and gradually approaches the failure stress ratio.

Evaluation of an experimental kelp farm’s structural behavior using regression modelling and response amplitude operators derived from in situ measurements

An experimental kelp farming system for exposed ocean conditions was designed, deployed, planted with kelp and instrumented for evaluation of its dynamic response to ocean waves, tides, and currents. The farm featured a novel “lattice” mooring design and anchor lines and cultivation lines (horizontal lines used as kelp growth substrate) made of fiberglass rods. The farm was deployed at a site in Saco Bay, Maine with 13 m (MLLW) water depth. There the farm was exposed to waves with heights up to 5.9 m. Anchor line tension, tide and wave height time series were gathered and processed into response amplitude operators (RAOs) and least squared error linear regression models enabling recognition of meaningful patterns between the forcing factors and the mooring response. Mean mooring line tensions were shown to increase nonlinearly with tide. Anchor line tension response amplitudes were shown to exhibit high sensitivity to both low and high frequency wave forcing. Numerical free-release test simulations suggested natural frequencies in heave of 0.91 Hz, indicating that tension response sensitivities at high frequency could be the result of resonance. Low frequency tension response disproportionate to the low frequency wave forcing could be explained by wave forcing on kelp cultivation arrays modulated by wave group envelopes. Instances of high magnitude, potentially damaging peak tensions, deemed shock loads, were prevalent in most load cases. Anchor line tension dynamics including RAO and shock load magnitudes were shown to be sensitive to mooring stiffness (ratio of tension to resulting elongation) and, in some cases, significant wave amplitude. Patterns of anchor line response indicated that additional mooring elasticity or geometric compliance and use of floatation with less sensitivity to high frequency waves could help avoid the cause of and costly consequences of amplified high frequency loading and high amplitude shock loading. RAOs and regression model results also indicated a subdued response in frequencies associated with ocean swell waves, suggesting desirable performance in waves most dominant in extreme storm events. With the proposed improvements, the farm system design suggests merit as a robust and durable macroalgae biomass production platform.

Hybrid self-centering braces with NiTi-SMA U-shaped and frequency-dependent viscoelastic dampers for structural and nonstructural damage control

This study proposes a novel hybrid self-centering brace (HSB) with structural and nonstructural damage-control capabilities for the seismic resilience of building structures. NiTi-based shape memory alloy (SMA) U-shaped dampers and frequency-dependent viscoelastic dampers are included in HSBs to provide self-centering capacity for avoiding structural damage and velocity-proportional damping for nonstructural damage control, respectively. This study includes two major parts, i.e. component-level mechanical behaviour and system-level dynamic response evaluation of HSBs. The working principle and desired nonlinear behaviour of the HSB were first presented. Then, the preliminary design procedures were developed. After validating the efficiency of the modelling of SMA U-shaped and viscoelastic dampers using prior test results, comprehensive 3D finite element model simulations were performed to study the hysteretic responses of HSBs under cyclic loadings, where the influences of loading frequency, loading amplitude and viscoelastic damper’s contribution were considered. Analysis results demonstrated that the designed HSB can attain the desired deformation mechanism and nonlinear hysteretic behaviour during dynamic cyclic loadings. Higher loading frequency and amplitude and more contributions of the viscoelastic damper can introduce higher loading and energy-dissipation capacity. Subsequent dynamic analyses of the single-degree-of-freedom systems were conducted to assess the dynamic performance and advantages of HSBs. Results showed that compared with traditional self-centering systems without velocity-proportional damping, the systems with HSBs can achieve zero residual deformations, lower base shear demands and smaller absolute acceleration responses, confirming the promising potential of HSBs in developing seismic resilient building structures by reducing structural and nonstructural damage.

DEM analysis of the dynamic characteristics of QH-E lunar soil simulant under cyclic triaxial tests

In this paper, cyclic triaxial tests are performed by using discrete element method (DEM) to investigate the dynamic behaviors of QH-E lunar soil simulant. The influences of number of cycles, loading frequency, waveform, cyclic stress ratio (CSR) and confining pressure on the dynamic behaviors of lunar soil simulant are discussed. The results indicate that the axial strain and dynamic modulus first increase and then tend to be stable with increasing number of cycles. The dynamic modulus increases with increasing the loading frequency, while the axial strain decreases. The damping ratio is not particularly sensitive to the confining pressure and CSR, it only decreases slightly with increasing confining pressure or decreasing CSR. Under the same conditions, rectangular wave dynamic load can produce a larger axial strain and a smaller dynamic modulus than those of sine and triangular waves, and a higher loading frequency will lead to a higher dynamic modulus and a lower axial strain, these characteristics are consistent with other lunar soil simulants and sandy soils tested in the experiment. However, the effect of confining pressure on the dynamic modulus depends on the magnitude of the CSR. For the lower CSR (CSR<0.6), the dynamic modulus increases with increasing confining pressure. While for the higher CSR(CSR>0.6), the dynamic modulus decreases significantly with increasing confining pressure, which demonstrates the unique dynamic characteristics that differs from other lunar soil simulants and sandy soils, and shows the importance of considering the magnitude of the CSR under cyclic triaxial tests.

Study on Shear Slip Characteristics of Sandstone Plane Joints under Normal Dynamic Load Disturbance

Abstract Rock joints are susceptible to slip instability due to dynamic load disturbances such as blasting, earthquakes, and fracturation. A series of direct shear tests under the dynamic load were conducted on sandstone plane joints using the RDS-200xl. The work investigated the effects of normal static loads and normal dynamic-load frequencies and amplitudes on plane joints. Besides, the following items were proposed, that is, the peak-to-valley response rate, shear velocity vibration dominant frequency, shear-stress reduction coefficient, and discrete element numerical simulation method for plane-joint direct shear tests. The results were as follows: (1) The normal dynamic load frequency played a role in attenuating the shear stress amplitude with a threshold value of 0.5 Hz. (2) The shear velocity of the plane joint was completely controlled by the high normal dynamic load frequency. Their vibrational dominant frequencies were identical. (3) The amplitude of shear stress increased, and the median stress decreased with the increased normal dynamic load amplitude. The reduction-coefficient equation for sandstone plane joints was proposed to evaluate the shear stress under the normal dynamic load disturbance. (4) The shear-stress hysteresis phenomenon existed in the plane joints under the normal dynamic load, which required excessive shear displacements to reach peak shear strength. The peak shear displacement increased with the increased normal static load. Numerical simulations and indoor tests showed that high- and low-shear-velocity regions were the main reason for shear-stress hysteresis. The findings are conducive to revealing the shear destabilization mechanism of rock joints under dynamic load disturbance.

Simulation of a High-Frequency Loading Machine for Railguns Utilizing Residual Electric Energy

Simulation of a High-Frequency Loading Machine for Railguns Utilizing Residual Electric Energy

Optimization of Piezoresistive Response of Elastomeric Porous Structures Based on Carbon-Based Hybrid Fillers Created by Selective Laser Sintering.

Recently, piezoresistive sensors made by 3D printing have gained considerable interest in the field of wearable electronics due to their ultralight nature, high compressibility, robustness, and excellent electromechanical properties. In this work, building on previous results on the Selective Laser Sintering (SLS) of porous systems based on thermoplastic polyurethane (TPU) and graphene (GE)/carbon nanotubes (MWCNT) as carbon conductive fillers, the effect of variables such as thickness, diameter, and porosity of 3D printed disks is thoroughly studied with the aim of optimizing their piezoresistive performance. The resulting system is a disk with a diameter of 13 mm and a thickness of 0.3 mm endowed with optimal reproducibility, sensitivity, and linearity of the electrical signal. Dynamic compressive strength tests conducted on the proposed 3D printed sensors reveal a linear piezoresistive response in the range of 0.1-2 N compressive load. In addition, the optimized system is characterized at a high load frequency (2 Hz), and the stability and sensitivity of the electrical signal are evaluated. Finally, an application test demonstrates the ability of this system to be used as a real-time wearable pressure sensor for applications in prosthetics, consumer products, and personalized health-monitoring systems.

Resonance-based testing facility for fatigue tests of different axially loaded structural elements

This work presents a new resonance-based testing facility for axially loaded structural elements for performing efficient fatigue tests. Two different configurations make it possible to test specimens either under compressive or under tensile loads. In both cases, a mean load is applied. In contrast to conventionally used servo-hydraulic actuators, synchronised imbalance motors with low energy input were used, which allow to generate cyclic loads with high loading frequencies. By setting the excitation frequencies of the motors close to the natural frequency of the system resonant behaviour occurs, which results in a dynamic amplification. Thus, input forces and test duration can be reduced. As a verification of the developed testing facility, two tests, one in each configuration, were carried out and analysed.

A new experimental setup to investigate the cyclic response of soft soils under induced earthquakes

Energy technologies, which work by extracting or injecting fluids in the ground, such as geothermal energy systems or underground liquefied gas storage, may induce seismic events, see e.g., [1]. In the Netherlands, induced earthquakes are continuously recorded from the Groningen gas field, with the largest magnitude ever recorded of ML 3.6 at Huizinge. Even though the magnitude of these events is not high, compared to natural earthquakes, damage to the built environment is still caused because of the shallow depth of the events and site amplification, especially where soft soils are encountered [2]. Proper quantification of the induced seismic risk requires better understanding of the response of soft soils to these repeated short events, covering a range of frequencies from 1 to about 20 Hz. This motivated the development of a new advanced dynamic equipment to experimentally investigate the coupled response of soft organic clays and peats from the typical deltaic areas of the Netherlands.&#x0D; Direct simple shear (DSS) apparatuses are preferred usually to investigate the soil behaviour under cyclic and dynamic loading. Among them, a number of multi-directional DSS setups have been developed to investigate the soil behaviour under multidirectional loading [3, 4, 5, 6, 7, 8]. Applying multi-directional loading to soil specimens in the laboratory is a keystone for elucidating the cyclic and dynamic soil response, as several studies have shown that the cyclic and post-cyclic response of soils is affected by multiple loading directions [6, 9, 10, 11, 12]. However, traditional DSS devices have a number of shortcomings, which are inherited by multi-directional DSS devices. The main deficiency of the DSS device is that the shear stress acting on the lateral side of the specimen cannot be controlled, and hence, a homogeneous stress state cannot be achieved, in spite of the common assumptions. Lateral stresses cannot be measured either in traditional setups, which leaves a knowledge gap on the stress state and the stress path of the sample.&#x0D; In addition, the majority of laboratory element tests are performed by imposing “slow” undrained cyclic loads, to try to guarantee uniform water pressure distribution within the sample, for the sake of interpretation and modelling. However, seismic events encompass much higher loading frequencies than typically available, with loading rate effects playing a key role in the response of soft soils such as organic clays and peats. In order to fully understand the cyclic behaviour of soft soils, “fast” cyclic tests are crucial.&#x0D; The innovative multidirectional shear device, developed in the section of Geoengineering at TU Delft (Cyclic-Dynamic shear simulator for Organic Soft Soils, CYC-DOSS), was designed to overcome some limitations of previous equipment. The underlying idea is to abandon the homogenous stress-strain state assumption and monitor the response with local sensors, which allows conditioning a numerical back-analysis of the test data. The new device shown in Figure 1 is characterised by (1) servo-hydraulic control; (2) multi-directional loading in 3 axes; (3) bender elements to measure both P-wave and S-wave velocity; (4) fully controlled cell pressure and back-pressure; and (5) possibility to reproduce the full acceleration time history of seismic events. The device is capable to apply loading frequencies up to 25Hz and a wide variety of multidirectional cyclic loading patterns. The apparatus is equipped with advanced sensors, also developed at TU Delft, including local pressure, displacements, and accelerations devices. The sensors are installed to reduce a priori assumptions on the soil response, better interpret the experimental results as a small-scale physical model and further investigate in depth the soil response under a variety of cyclic loading histories. The experimental information from the setup will be used to develop and calibrate an advanced bounding surface constitutive model for soft organic soils.

Stochastic generation of residential load profiles with realistic variability based on wavelet-decomposed smart meter data

Residential smart meter data with high time resolution are integral to many data-driven applications, ranging from hosting capacity studies to R&D activities of private enterprises. However, privacy legislation restricts public availability of large-scale datasets. Furthermore, existing datasets may suffer from imbalances in terms of underrepresented classes. To address these concerns, this study presents a novel decomposition–recombination approach for generating synthetic load profiles that exhibit realistic variability and demand peaks. High-frequency load profiles are decomposed into a low-frequency base load and high-frequency variability at the daily level through a discrete wavelet transformation. Components from different households are subsequently rescaled, shifted and recombined in a stochastic load profile generator to obtain new daily load profiles with high-fidelity behavior. The performance of this generator is evaluated through benchmarking, resulting in a mean average error of 0.09 kW on an average value of less than 3 kW for the daily peaks, whilst preserving their seasonality. The introduced load profile generator is validated as an alternative to privacy-sensitive residential smart meter data in a hosting capacity case study. The analysis focuses on the voltage drop caused by residential electric vehicle charging, considering both real and synthetic data. The synthetic data demonstrated voltage drops with a mean average error less than 0.2 V for the 10th and 90th percentile when benchmarked with respect to the real voltage level distribution. The introduced decomposition–recombination method is shown to accurately capture the high-frequency variability and peak behavior, and is suitable for practical applications at the daily level.

Optimization of High Power SMES for Naval Applications

The electrification of transport has been no exception for waterborne vessels. The reduction of emissions, especially dur-ing port maneuvering and operation, has accelerated the transi-tion from fossil fuel to electric propulsion. Moreover, military ves-sels, with specialized electric weapon systems, or civil vessels, like offshore wind tug boats, have high power systems that additionally contribute to a higher electrification of the ship grid. Energy stor-age systems are essential to meet the power load economically and to improve the system`s reliability and efficiency. Therefore, the use of high energy density storage systems, as chemical batteries, or hydrogen fuel cells, has experienced a significant increase in de-mand. However, during high frequency load fluctuations, high en-ergy density storage systems are not capable to actuate, since the discharge rate of these systems is physically limited, which may provoke: deterioration and reduction of the lifetime of the storage systems, voltage and frequency fluctuation of the ship grid. To overcome this limitation, this paper studies the use of a Supercon-ducting Magnetic Energy Storage (SMES) as a supporting energy storage device for the ship grid. The guidelines for dimensioning, in power and energy, the SMES are stablished. Furthermore, an optimization method is developed in order to compare different superconducting materials, and operating temperatures.

Estimation of Response of Composite Shells to Acoustic Excitation Using Sea and Its Experimental Verification

Shells made up of composite material are extensively used in aerospace structures as they offer excellent specific strength and stiffness properties over their metallic counterparts. These elements experience high frequency and high intensity acoustic loads during the launch. Response behaviour of structures in the high frequency region is analyzed in a Statistical Energy Analysis (SEA) framework. Though the methodology of estimating the response using SEA exists, its applications to composite cylindrical shells and how they compare with experimental results are seldom reported. Present work fills this gap. Acceleration and strain responses of a typical composite sandwich cylinder under diffused acoustic field are estimated in SEA framework. Modal densities and coupling loss factors are estimated using expressions. The dissipation loss factors are determined experimentally. Same cylinder is subjected to diffused acoustic field in reverberation chamber and the accelerations and strains are measured. The work presents how much the theoretically estimated responses compare with the experimentally obtained results. It is seen that the acceleration responses compare reasonably well, but needs improvement. The strain-velocity relations compare very well. Acceleration and strain bounds such that 99 % of all measurements are within are also predicted which could be used in the structural design.

Experimental and statistical investigation on the performance of asphalt overlays reinforced with geocomposite in controlling the reflective cracks under different loadings and temperatures

One of the easiest method to repair the old pavement is to apply an overlay on them. However, the deteriorated pavement affects the overlay performance by propagating the existing cracks upwards to the overlay due to the stress and strain concentration from the loading and temperature variations. Today, using the asphalt overlays reinforced with geosynthetic interlayers is one of the most effective methods to prevent the reflective cracking. This paper investigates the effects of the temperature, bending fatigue loading frequency and geocomposite tensile strength on the asphalt overlays performance experimentally. The crack initiation and its propagation rate were analyzed by the help of image processing technique and statistically. The obtained results revealed that the temperature has the greatest effect on the reflective cracking rate, so that by increasing the temperature from 20 to 40 °C in addition to increasing the crack growth rate and changing the vertical deformation under the load, the direction of crack formation changes, and besides the reflective cracks, the top-down cracks also appear. Furthermore at the higher loading frequency more than 10 Hz, even in samples reinforced with geocomposite, increasing the temperature from 0 to 20 and 40 °C will increase the crack growth rate between 5 and 30 times. The obtained coefficients of determination (R2) and Adjusted R2 are equal to 0.9936 and 0.9907, respectively, indicating the satisfactory prediction of the model compared to the real observations.

Thermoelastic Stress Analysis for composite laminates: A numerical investigation

A Finite Element model of the thermoelastic effect in orthotropic composite laminates subject to cyclic loading is developed. A meso-scale approach is employed to model the thermoelastic effect at the lamina level and the through-thickness heat transfer. The model proved itself capable of modelling the dependency of the thermoelastic response on the loading frequency, presence of a superficial resin-rich layer, material system, and layup. Results show that a steady state response is reached almost immediately, after only a few loading cycles, and that in some cases, even for very high loading frequencies, adiabatic conditions are never fully onset. Numerical predictions also confirm that the presence of a superficial resin-rich layer strongly affects the through-the-thickness heat transfer and the measured thermoelastic signal.

Dynamic Energy Dissipation Performance of Nonlinear Large-Deformation Viscoelastic Joint Damper under Strong Dynamic Load

Large-displacement corner deformation often occurs at the joints of towering structures and long-span space structures subjected to strong earthquake. In order to improve the energy dissipation capacity of structural joints, the Nitrile Butadiene Rubber/High Abrasion Furnace Black (NBR/HAF) high damping composite was fabricated by formulation optimization in this paper, where a nonlinear large-deformation viscoelastic joint damper was proposed. The damper consists of five layers of restrained steel plates and four layers of shear energy dissipating material, which can reach 60[Formula: see text]mm shear displacement. In this paper, dynamic mechanical performance tests and static mechanical tests were firstly conducted on the core energy dissipating media of the damper. The loss factor of NBR/HAF composite reached a peak of 1.51 at about 8.2°C, while its wide damping temperature range was 27°C. Second, the accuracy of the simulation method was verified by comparing the simulated and experimental hysteresis curves. Then, the refined numerical simulation of this damper was carried out using ABAQUS finite element software with the high damping material as its core energy dissipating media. Finally, the magnitude and type of energy dissipation of each part of the viscoelastic joint damper under different loads were investigated. It was found that the large-deformation viscoelastic joint damper based on the composite had good damping capacity. Its dynamic characteristics were affected by the displacement amplitude and excitation frequency, which exhibited hardening nonlinear characteristics. As the frequency and load amplitude increased, the peak displacement in the loading direction gradually decreased, whereas the total energy input to the damper and the energy dissipated by the viscoelastic material increased monotonically. The input energy was only dissipated by the viscoelastic material, and no plastic loss occurred in the steel plate during the entire loading stage. Under high frequency and large loads, the damper can also have good energy dissipation characteristics. Because of the negative strain energy caused by plastic dilatancy, the energy dissipated by the material was gradually greater than the input energy with the load increasing.

Rail Magnetic Flux Leakage Detection and Data Analysis Based on Double-Track Flaw Detection Vehicle

The rapid development of the railway industry has brought convenience to people’s lives. However, with the high speed, high frequency and heavy load characteristics of rail use, the safety of rail is seriously threatened. In this paper, a magnetic flux leakage testing (MFL) detection technology of rail based on a double-track flaw detection vehicle is introduced in detail, which can effectively detect the damage of rail top surface, which is the blind area of ultrasonic detection. The magnetic dipole model is used to analyze that the leakage magnetic field in the direction of Bx and Bz above the damage is related to the depth and width of the damage. The relationship between the depth of the damage and the leakage magnetic field is quantitatively studied for the damage with fixed width but varying depth. The finite element simulation tool is used to model and simulate the damage at different depths. After analyzing the different characteristic values, it is found that the peak value of magnetic leakage signal has a certain correlation with the depth of damage, and the natural logarithm function is fitted out—VBx = 0.1451ln(b) + 0.2705, VBz = 2.7787ln(b) + 0.0087. In order to verify the prediction function of the injury depth fitted by the simulation data, the human injury with different depths was processed and the dual-track flaw detector was used to carry out the experiment of high-speed detection environment. The peak-to-peak fitting of the magnetic leakage signals in the direction of Bx and Bz of the experimental results shows that the peak-to-peak variation rule is roughly in line with the natural logarithm function in the simulation. The correlation between the fitting results of the experimental data and the simulation fitting function is analyzed using the Pearson coefficient. The Pearson coefficient in the direction of Bx is ρx = 0.91386. The Pearson coefficient of the Bz direction is ρz = 0.98597, the peak-to-peak value of Bx and Bz direction is positively correlated with the depth of damage and the fitting effect of the Bz direction is better than that of the Bx direction.

Research on the Fatigue Properties of Rejuvenated Asphalt Prepared by Waste Cooking Oil Pre-Desulfurized Crumb Tire Rubber.

Fatigue cracking has hitherto been a crucial constraint on the development of reclaimed asphalt pavements attributed to the performance of rejuvenated asphalt binder. Therefore, it is extremely significant to evaluate the fatigue performance of rejuvenated asphalt precisely and objectively and to improve the fatigue life of rejuvenated asphalt binders. With preceding research in our group, this paper investigated the fatigue properties of waste rubber/oil (WRO) rejuvenated asphalt and universal rejuvenated asphalt by dynamic shear rheometer test (DSR). The applicability of common fatigue life evaluation indexes and the response to internal and external influences on the fatigue performance of rejuvenated asphalt were analyzed. It is demonstrated that Np20 corresponding to the mutagenesis of phase angle is physically significant and independent of the parameters including rejuvenator type, loading mode and loading level, which was recommended as the evaluation index for fatigue life of rejuvenated asphalt in this paper. The fatigue performance of both WRO and universal rejuvenated asphalt is found to decrease with loading frequency and loading level, but the fatigue life of WRO rejuvenated asphalt is comparatively superior to the latter, particularly at high loading frequencies and levels. Influenced by waste tire crumb rubber (WTCR), increasing the proportion of WTCR can improve the fatigue life of rejuvenated asphalt. When compared to other rejuvenated asphalt, RWRO@55 rejuvenated asphalt shows better fatigue performance and its fatigue life rebounds at high loading frequency. Consequently, the recommended mastic-oil ratio is 5:5. However, when the rheological recoverability compensation is considered, the fatigue lifetime evaluation of rejuvenated asphalt will be changed significantly, and therefore the fatigue performance evaluation of rejuvenated asphalt should consider the influence of rheological recoverability to develop a comprehensive evaluation system.

Fatigue behavior of DIN 1.4307 and DIN 1.4306 stainless steels under high frequency loading

Two stainless steels DIN 1.4307 and DIN 1.4306 were tested by an ultrasonic fatigue loading device under push-pull mode (stress ratio R = -1). Life-time curves were determined in the high and very high cycle fatigue regions. The time dependence of frequency was monitored and evaluated. At the beginning of the tests (up to 1 × 107 cycles), an increase of the frequency was observed, followed by a slow frequency decrease. It relates to changes of microstructure. No failure occurred in the region between 1 × 109 and 1 × 1010 cycles. The steel 1.4306 exhibits a higher fatigue life-time than the steel 1.4307. This is caused by the difference in the chemical composition of the two steels, namely by the higher nickel content in the 1.4306 steel.