Random Loading Conditions Research Articles

The stability loss of the rotor-slide bearings system under random load variations

The article presents how the system equipped with a rotor supported by slide bearings loses stability, operating under random load conditions. The computer simulations were applied for the analysis of the rotating machinery (using in-house developed codes). The MESWIR software is presented herein that can be used to assess dynamic performance of machines of this type. A special algorithm for the randomisation of loads was implemented and discussed in detail on the basis of a representative example. The rotor model was subjected to randomly-generated transverse forces during its operation. It exhibited unstable behaviour manifesting itself in the form of oil whirl or oil whip. The system operation was analysed both under constant and random load.

Adaptive operational modal identification for slow linear time-varying structures based on frozen-in coefficient method and limited memory recursive principal component analysis

Abstract To adaptively identify the transient modal parameters for linear weakly damped structures with slow time-varying characteristics under unmeasured stationary random ambient loads, this paper proposes a novel operational modal analysis (OMA) method based on the frozen-in coefficient method and limited memory recursive principal component analysis (LMRPCA). In the modal coordinate, the random vibration response signals of mechanical weakly damped structures can be decomposed into the inner product of modal shapes and modal responses, from which the natural frequencies and damping ratios can be well acquired by single-degree-of-freedom (SDOF) identification approach such as FFT. Hence, for the OMA method based on principal component analysis (PCA), it becomes very crucial to examine the relation between the transformational matrix and the modal shapes matrix, to find the association between the principal components (PCs) matrix and the modal responses matrix, and to turn the operational modal parameter identification problem into PCA of the stationary random vibration response signals of weakly damped mechanical structures. Based on the theory of “time-freezing”, the method of frozen-in coefficient, and the assumption of “short time invariant” and “quasistationary”, the non-stationary random response signals of the weakly damped and slow linear time-varying structures (LTV) can approximately be seen as the stationary random response time series of weakly damped and linear time invariant structures (LTI) in a short interval. Thus, the adaptive identification of time-varying operational modal parameters is turned into decompositing the PCs of stationary random vibration response signals subsection of weakly damped mechanical structures after choosing an appropriate limited memory window. Finally, a three-degree-of-freedom (DOF) structure with weakly damped and slow time-varying mass is presented to illustrate this method of identification. Results show that the LMRPCA algorithm, which is robustness to Gauss measurement noise disturbances, can well identify the transient modal parameters (transient natural frequencies & transient modal shapes) for weakly damped and slow LTV structures online only from non-stationary random response signals.

Evaluation and visualization of multiaxial fatigue behavior under random non-proportional loading condition

In cyclic multiaxial stress/strain condition under nonproportional loading in which principal direction of stress/strain are changed in a cycle, it becomes difficult to analyze stress/strain ranges because of complexity of multiaxial stress/strain states depending on time in cycles. In order to evaluate stress/strain simply and suitably under non-proportional loading, Itoh and Sakane have proposed a method called as IS-method and a strain parameter for life evaluation under non-proportional loading ??NP. In the method, 6-components of stress/strain are converted to an equivalent stress/strain indicating the amplitude and the direction of principal stress/strain as a function of time as well as an intensity of loading nonproportionality fNP. Based on IS-method, the authors also have developed a tool which enables to analyze multiaxial stress/strain condition with the nonproportionality of loading history and evaluate failure life under nonproportional multiaxial loading. The tool indicates the analyzed results on monitor and users can understand visually not only variation of the stress/strain conditions but also non-proportionality during the cycle, which helps the design of material strength.

Fatigue damage assessment under random and variable amplitude multiaxial loading conditions in structural steels

Fatigue damage assessment of multiaxial random loadings is a complex issue and a subject of actual interest in mechanical design. In this work, the performance of the stress scale factor (SSF) criterion is evaluated under variable amplitude loading conditions, and damage accumulation approaches. This evaluation is performed by taking into account two types of loading spectra, namely the loading block spectra (where the loading pattern is well identified and repeated until rupture), and the random loading spectra (where the stochastic behaviour of the axial and shear loading components do not allow a direct identification of the loading pattern). Moreover, the validity of the hypothesis in which the SSF damage map remains valid for any high strength steel under variable amplitude loading conditions is also inspected by analysing fatigue life correlation of the 1050 QT steel and the 304L stainless steel under a multiaxial loading block.

Research on the Novel Non-Contact Power Supply System Tuning Methodology based on Fuzzy Neural Network Theory under the Random Load Condition

In this paper, we combine the literature review and technique reports to conduct research on the novel non-contact power supply system tuning methodology based on fuzzy neural network theory under the random load condition. Power supply by noncontact way could increase the flexibility of electrical equipment, eliminates the traditional power supply methods of safe hidden trouble and makes up for the traditional power supply methods because of the existence of electrical connections of the lack of divination. The background harmonic separation of qualitative method is detecting system side and user side common connection point for the size of the harmonic distortion effect. The genetic algorithm with adaptive system, cellular automata, chaos theory and artificial intelligence is considered for the next decade computing technology has a significant impact on the key technology which is adopted by our research to optimize the proposed system. Mathematical analysis and dynamic load change can lead to circuit reactance, so system will deviate from the resonance state. The experimental simulation proves the feasibility and robustness of our proposed methodology. In the near future, more related research will be conducted to optimize and modify the current method.

Reliability-based design optimization applied to structures submitted to random fatigue loads

The reliability-based design optimization (RBDO) aims to find the most balanced design through a compromise between cost and safety when uncertainties affecting the system are considered. This strategy involves the evaluation of probabilistic constraints performed by the reliability analysis, leading to an expensive computational effort in order to solve the problem. For this reason, it is not trivial to take into account the high cycle fatigue in a RBDO method. This work deals with this problem, considering stationary Gaussian random load processes and the Sines multiaxial fatigue criterion. The efficiency is achieved through spectral tools related to the fatigue analysis. Therefore, the probabilistic constraints of the fatigue life in the RBDO problem are transformed in the frequency domain. The limit state function of the reliability analysis is defined by the mechanical component endurance submitted to random loads. The numerical application demonstrates the interest and the effectiveness of our proposal.

Random non-proportional fatigue tests with planar tri-axial fatigue testing machine

Complex stresses, which occur on the mechanical surfaces of transport machinery in service, bring a drastic degradation in fatigue life. However, it is hard to reproduce such complex stress states for evaluating the fatigue life with conventional multiaxial fatigue machines. We have developed a fatigue testing machine that enables reproduction of such complex stresses. The testing machine can reproduce arbitrary in-plane stress states by applying three independent loads to the test specimen using actuators which apply loads in the 0, 45, and 90 degree directions. The reproduction was tested with complex stress data obtained from the actual operation of transport machinery. As a result, it was found that the reproduced stress corresponded to the measured stress with an error range of less than 10 %. Then, we made a comparison between measured fatigue lives under random non-proportional loading conditions and predicted fatigue lives. It was found that predicted fatigue lives with ?cr, stress on critical plane, were over a factor of 10 against measured fatigue lives. On the other hand, predicted fatigue lives with ?ma, stress in consideration of a non-proportional level evaluated by using amplitude and direction of principal stress, were within a factor of 3 against measured fatigue lives. KEYWORDS. Fatigue; Non-proportional; Random; Plane stress; Fatigue life prediction; Testing machine.

Study of in-plane Fatigue Failure and Life Prediction of Weave Composites under Constant and Variable Amplitude Loading

The fatigue behaviour of two-dimensional weave composites under constant and variable amplitude loading was discussed in this article. The theories of life prediction methods were represented, including cycle counting, S–N formulation, constant life diagrams (CLD), damage accumulation rule and stress analysis. Special focuses on CLDs were studied through the interpolation method known as the kriging model. Fatigue tests under constant amplitude loading were carried out utilising the servo-hydraulic Shimadzu testing machine. Tension–tension, tension–compression and compression–compression loading patterns were investigated in fatigue testing. The macro-failure mode was obviously different between tensile and compressive loading. Micro-failure surfaces were observed in micro-mechanism with scanning electron microscopy (SEM). The Basquin equation and the linear Palmgren–Miner (P–M) rule were employed to characterize the fatigue properties under random loading conditions. A new constitutive relation was introduced which took into account the tensile-compressive asymmetry of composites. Finally, the benchmarking for the stochastic loading case was executed using proposed methodology and variable amplitude fatigue experiments. Through the contrast between prediction and the experimental data, it was inferred that the proposed prediction method under any loading conditions (stress ratios and ranges) gave an accurate life prediction.

An equivalence between generalized Maxwell model and fractional Zener model

Both classic rheological models and fractional derivative models have been widely adopted to model the viscoelastic behaviors of materials. In this work, we present a detailed comparison of the performance of the generalized Maxwell model and fractional Zener model. We first describe a method to determine the parameters of the generalized Maxwell model from the fractional Zener model based on the equivalence of complex modulus in the frequency domain of the two models. The two models are then applied to investigating the stress response under constant strain rate, stress relaxation, cyclic and random loading conditions. The simulation results of the two models show excellent quantitatively equivalence. This finding can provide insight into choosing the more suitable model for specific conditions.

嵐モデルに基づいた疲労センサによる船体構造の疲労寿命推定手法の高度化

The characteristics of Fatigue Damage Sensors (FDS) that are in use for fatigue life estimation of monitoring structural welding members in ship structures are discussed in order to improve prediction accuracy of estimated fatigue life exposed to random wave loads such as storms under various loading conditions peculiar to ship structures. In our past research, coefficients, which named load effect factors (LEFs) affected by loading histories against constant amplitude loading were introduced for the estimation procedure using FDSs and confirmed by several series of fatigue testing under simplified storm-type loading patterns. In this paper, it is ensured that the abovementioned LEFs are suitable for application to actual random wave load conditions on oceans by numerical simulation using load histories based on a storm model in consideration of variable of sea areas and seasons. Additionally, it is found that the characteristics of FDS under random wave load superimposed whipping vibration show the suitability for the modified Miner rule.

Estimations of fatigue life and variability under random loading in aluminum Al-2024T351 using strip yield models from NASGRO

This paper is concerned with the application of the strip yield models implemented in NASGRO to estimate fatigue crack growth under random loading. The two different strip yield model options (constant constraint-loss (CCL) and variable constraint-loss (VCL)) implemented in the software are considered and compared. In addition, three crack grow rate relations, obtained from constant amplitude data, the NASGRO material database and the literature, were also considered and compared. The capacity of the models to estimate the fatigue life and variability is analyzed by comparing simulated results with experimental data of fatigue crack growth under different stationary Gaussian random load processes on compact, C(T), specimens of aluminum alloy 2024-T351. The analysis performed showed that both model options provide very similar good fatigue life predictions. The estimated fatigue lives are within ±13% of the test lives with the best option. The variability of the results due to the randomness of the load is also analyzed. In this case, the variable constraint-loss option provides better estimation than the constant constraint-loss option.

Real-time prognosis of random loaded structures via Bayesian filtering: A preliminary discussion

Particle filters are effective tools for the monitoring of damage propagation phenomena. However, a common hypothesis of particle filters for damage prognosis is the constant-amplitude fatigue loading affecting the damage growth. This work constitutes a preliminary analysis of the performance of particle filtering in case of random load, relaxing the hypothesis of constant-amplitude fatigue. Two case studies referring to stationary random loads are introduced: the first concerning a narrow-band stress history, while the second focusing on a wide-band stress spectrum. A solution for each case study is provided and validated using numerical simulations of fatigue cracks in a metallic plate.

<b>The modeling and simulation of thermal based modified solid oxide fuel cell (SOFC) for grid-connected systems

This paper presents a thermal based modified dynamic model of a Solid Oxide Fuel Cell (SOFC) for grid-connected systems. The proposed fuel cell model involves ohmic, activation and concentration voltage losses, thermal dynamics, methanol reformer, fuel utilization factor and power limiting module. A power conditioning unit (PCU), which consists of a DC-DC boost converter and a DC-AC voltage-source inverter (VSI), their controller, transformer and filter, is designed for grid-connected systems. The voltage-source inverter with six Insulated Gate Bipolar Transistor (IGBT) switches inverts the DC voltage that comes from the converter into a sinusoidal voltage synchronized with the grid. The simulations and modeling of the system are developed on Matlab/Simulink environment. The performance of SOFC with converter is examined under step and random load conditions. The simulation results show that the designed boost converter for the proposed thermal based modified SOFC model has fairly followed different DC load variations. Finally, the AC bus of 400 Volt and 50 Hz is connected to a single-machine infinite bus (SMIB) through a transmission line. The real and reactive power managements of the inverter are analyzed by an infinite bus system. Thus, the desired nominal values are properly obtained by means of the inverter controller.

Hybrid Fuzzy Bang–Bang Mode Controller With Switching Function for Electric Motor Drive Applications

This paper proposes a real-time hybrid fuzzy bang–bang (BB) controller with switching function, in order to implement a full nonlinear control law. The controller consists of a fuzzy-BB controller in parallel with a BB controller while the system is in operation. In order to circumvent the shortcomings of fuzzy-BB and BB-based controllers, a hybrid fuzzy-BB controller is proposed, in which the output can be either the outputs of the two, i.e., the fuzzy-BB or BB units that are being switched as per the absolute error signal. Normally, the BB controller operates in general; the fuzzy-BB controller is activated when the drive system experiences oscillatory behavior. The choice between the fuzzy-BB and BB controllers is based on a set of simple if–then rules; the overshoot and the oscillatory behavior have to be detected by the absolute errors. Trajectory tracking of nonlinear motor drives in position mode serves as a test bed for the control law, and a real-time experiment allows for the assessment of its practical applicability. The hybrid fuzzy-BB controller has been implemented on hardware with a dSPACE digital signal processor, and it is observed that the performance of the controlled brushless dc motor drive with the hybrid fuzzy-BB controller is assuring. Noticeably, experimental results demonstrate that the proposed controller is capable of significantly cultivating the motor performance under random disturbance and different loading conditions.

Fatigue Life Prediction for Caliper Guide Pin under Random Vibrational Loading

A fatigue life prediction tool was developed for caliper guide pins under random vibrational loading. The Pie-Slice model was designed to provide detailed information about the failure location, orientation, and damage magnitude. A component test fixture was developed to determine the strain-life curve for a given guide pin design. Statistical analysis was conducted to insure the repeatability of the failure mode and the robustness of the setup. Weibull analysis was performed to the measured guide pin strain-life in order to insure that the developed strain-life data to insure that developed strain-life curve will account for all the manufacturing process variations, from a component, assembly, and a system level to a certain level of reliability and confidence. Rainflow cycle count was used to bin the damaging and non-damaging cycles based on their stain level. Fatigue life calculation was performed using the Smith-Watson-Topper strain-life approach. The predictive tool was able to accurately estimate the cumulative fatigue damage for guide pins under random loading conditions. The Pie-Slice model was also able to predict the failure location and orientation of a crack, as well as the damage magnitude. Both tools were validated using a pre-designed random block-load sequence at constant amplitude..

A New Criterion for Evaluating Multiaxial Fatigue Damage under Multiaxial Random Loading Conditions

Generally, mechanical components or structures are subjected to random and a three-dimensional stress state; there are very few field loading paths which can be experimentally fully simulated in laboratory. Loading path parameters such as load sequence, stress level or proportionality/non-proportionality presences are unknown variables with unknown levels under random loading conditions which are impossible to modulate in laboratory because the load spectra is unknown. The load spectrum depends on numerous factors such as environmental, mechanical or user behavior. At design stages the fatigue life estimation is based on typical loading paths or typical loading spectra, however that assumption may be very different from the usage regime. From here it can be concluded that the random multiaxial fatigue issue is of utmost importance to monitoring the in-field damage accumulation. This work presents a proposal to estimate the accumulated damage resulted from multiaxial random loadings based on the SSF equivalent stress and SSF virtual cycle counting concept.

LQG Control of Along-Wind Response of a Tall Building with an ATMD

Modern tall buildings use lighter construction materials that have high strength and less stiffness and are more flexible. Although this results in the improvement of structural safety, excessive wind-induced excitations could lead to occupant discomfort. The optimal control law of a linear quadratic Gaussian (LQG) controller with an active tuned mass damper (ATMD) is used for reducing the along-wind response of a tall building. ATMD consists of a second mass with optimum parameters for tuning frequency and damping ratio of the tuned mass damper (TMD), under the stationary random load, was used. A fluctuating along-wind load, acting on a tall building, was treated as a stationary Gaussian white noise and was simulated numerically, in the time domain, using the along-wind load spectra proposed by G. Solari in 1993. Using this simulated wind load, it was possible to calculate the along-wind responses of a tall building (with and without the ATMD), using an LQG controller. Comparing the RMS (root mean square) response revealed that the numerically simulated along-wind responses, without ATMD, are a good approximation to the closed form response, and that the reduced responses with ATMD and LQG controller were estimated by varying the values of control design parameters.

Оценка надежности валов по критерию усталостной прочности при случайном режиме нагружения

Оценка надежности валов по критерию усталостной прочности при случайном режиме нагружения

3-Dimensional observation of the interior fracture mechanism and establishment of cumulative fatigue damage evaluation on spot welded joints using 590 MPa-class steel

To investigate the fatigue fracture mechanism in spot welded joints using a 590MPa-class base metal, fatigue tests were conducted under constant loading conditions. In this study, three dimensional observation was made on the propagation behavior of fatigue cracks initiating at the edge of the slit between sheets. Moreover, an evaluation method of the fatigue life was proposed for random loading conditions. Since the mean load obviously affected the fatigue life of the spot welded joints, the proposed evaluation method was applied to account for this load effect. The proposed evaluation method satisfied the application range in the automobile industry. Therefore, it is thought that the proposed method is suitable for practical applications.

Fatigue Life Calculation in Conditions of Wide Spectrum Random Loadings – The Experimental Verification of a Calculation Algorithm on the Example of 41Cr4 Steel

Precision of fatigue life calculations of structural elements in programmed loading conditions is connected with proper elaboration of loading spectrum and assumption of a proper fatigue characteristic. On the base of literature data and own research there has been elaborated an algorithm for fatigue life calculations in random loading conditions with wide spectrum. Calculations were performed with the usage of chosen mathematical models of two-parametric fatigue characteristics. Results calculated with accordance to the described procedure were validated with experimental test results of specimens made of 41Cr4 steel with a method of programmed fatigue life tests.