Numerical study of fatigue damage under random loading using Rainflow cycle counting

PurposeThe purpose of this scientific work is to simulate the fatigue damage under random loading, taking into account the mean stress effect on fatigue lifetime and using the Rainflow counting technique to assess the fatigue damage by the Ansys software. The used material is aluminum alloy 6082-T6. A comparison with literature results has confirmed this investigation in this paper.Design/methodology/approachThe study of fatigue under random loading is based on the same concepts as constant loading with the addition of damage summation. The proportion of damage caused by a stress cycle depends not only on the alternating stress but also on the mean stress.FindingsAnalysis of the fatigue damage shows that the number of relative damage due to each cycle.Originality/valueThis paper aims to simulate the fatigue damage under random loading for aluminum alloys.

Effects of Damaged Fiber Ropes on the Performance of a Hybrid Taut-Wire Mooring System

In this study, effects of damage levels of fiber ropes on the performance of a hybrid taut-wire mooring system are investigated. The analysis is performed using a numerical floating production storage and offloading (FPSO) model with a hybrid mooring system installed in 3000 m of water depth. An in-depth study was conducted using the numerical model, the dynamic stiffness equation of damaged fiber ropes, the time-domain dynamic theory, the rainflow cycle counting method, and the linear damage accumulation rule of Palmgren-Miner. Results indicate that, in a mooring line with an increasing damage level, the maximum tension decreases, while the offset of the FPSO increases. Particularly, when a windward mooring line failure occurs, in addition to the significant increase in the offset of the FPSO, the maximum tension, tension range, and annual fatigue damage levels of the remaining lines adjacent to the failed also increase significantly. The present work can be of great benefit to the evaluation of the offset of the floating platform, the tension response, and the service life of the hybrid mooring systems.

Comparative Study of Ramp-Rate Control Algorithms for PV with Energy Storage Systems

The high variability of solar irradiance, originated by moving clouds, causes fluctuations in Photovoltaic (PV) power generation, and can negatively impact the grid stability. For this reason, grid codes have incorporated ramp-rate limitations for the injected PV power. Energy Storage Systems (ESS) coordinated by ramp-rate (RR) control algorithms are often applied for mitigating these power fluctuations to the grid. These algorithms generate a power reference to the ESS that opposes the PV fluctuations, reducing them to an acceptable value. Despite their common use, few performance comparisons between the different methods have been presented, especially from a battery status perspective. This is highly important, as different smoothing methods may require the battery to operate at different regimes (i.e., number of cycles and cycles deepness), which directly relates to the battery lifetime performance. This paper intends to fill this gap by analyzing the different methods under the same irradiance profile, and evaluating their capability to limit the RR and maintain the battery State of Charge (SOC) at the end of the day. Moreover, an analysis into the ESS capacity requirements for each of the methods is quantified. Finally, an analysis of the battery cycles and its deepness is performed based on the well-established rainflow cycle counting method.

Uncertainty Quantification of Riser Fatigue Damage due to VIV Using a Distributed Wake Oscillator Model

This study explores the use of polynomial chaos expansion (PCE) to quantify the uncertainty in accumulated fatigue damage in a top-tensioned riser (TTR) due to vortex-induced vibration (VIV). Time-domain simulations of the response of the selected riser are carried out using a distributed wake oscillator model and fatigue damage is computed using rainflow cycle-counting. The uncertainty in damage prediction that results from variability in parameters involved in the wake oscillator model is the focus of this study. The efficiency and accuracy resulting from use of the PCE model is demonstrated by comparison against Monte Carlo simulations (MCS). Specifically, starting with the use of two random variables (the cylinder maximum amplitude and a ratio of the vortex-shedding frequency to the natural frequency) and then by introducing one other random variable (the current velocity), the propagation of uncertainty from the wake oscillator model inputs to fatigue damage is studied. Numerical studies demonstrate the versatility of the PCE-based approach in such uncertainty quantification applications.

A Combined Creep and Fatigue Damage Estimation Tool for Power-Plant Monitoring

A user-friendly creep-fatigue damage calculation tool is developed in Visual Basic for Applications (VBA) with the familiar Microsoft Excel® user interface for power plant operators. Operational pressure and temperatures (steam and pipe exterior) are directly input and automatically converted to stress-time histories based on the summation of thermally- and mechanically-induced stresses. The stress history is automatically analysed and segregated into periods of sustained stress levels (creep range) and periods of fluctuating stress (fatigue range). Total damage is determined by summing the creep damage fraction (via Larson-Miller equation and Robinson’s rule) and fatigue damage fraction (via a rainflow cycle counting subroutine, the Smith-Watson-Topper fatigue parameter and Miner’s rule). Pre-existing damage fraction can be incorporated into the calculations. The remaining life estimates based on repetition of the load profile are outputted for the user. Finally, the estimated damage and remaining life are compared to that determined via the ASME, EN and TRD codes, where applicable.

The Use of Spectral Method for Fatigue Life Assessment for Non-Gaussian Random Loads

The well-known problem with the fatigue lifetime assessment of non-Gaussian loading signals with the use of spectral method has been presented in the paper. A correction factors that transform the non-Gaussian signal into an equivalent Gaussian signal proposed by Bracessi et al. (2009) has been used for the purpose of lifetime calculations together with Palmgren-Miner Hypothesis. The calculations have been performed for the 10HNAP steel under random non-Gaussian load with four dominating frequencies. The signal has been generated on the test stand SHM250 for random tension-compression tests. The results with zero and non-zero mean stresses have been used to calculate the fatigue life with the frequency domain method based on Dirlik’s model and with a time domain method with the use of the rainflow cycle counting algorithm. The obtained calculation results have been compared with experimental results.

CONSTANT AMPLITUDE SPECTRUM OF THREE COACHES TRAIN AND CYCLIC COUNTING ON PRESTRESSED CONCRETE SLEEPERS (PCS)

The raw strain data collected from Keretapi Tanah Melayu Berhad (KTMB) railway are in variable amplitude. This paper discovers how the variable amplitude data can be changed to the constant amplitude data. It is found that the raw strain data is not suitable for fatigue and strength testing on Prestressed Concrete Sleepers (PCS). Apart from that, the most suitable method in determining the numbers of cycles is Rainflow Cycle Counting Method. Through rainflow cycle counting method, the number of cycles is determined. The numbers of cycles are used to simplify the laboratory test such as fatigue and strength test for the PCS. The frequencies of dynamic loading test on the PCS are set based the numbers of cycles. The constant strain data are also converted into constant loading data using the relationship of stress-strain and loading-stress. Constant amplitude loading will again simplify the laboratory testing. The goal is to show that the designs used in PCS are appropriate based on current loading demand. Then, a comparison of constant amplitude data is made between different numbers of coaches and freight train. The maximum data from the comparison shows that the higher loadings are obtained from freight train.

CONVERTING CONSTANT AMPLITUDE LOADING OF PRESTRESSED CONCRETE SLEEPER (PCS) FROM VARIABLE AMPLITUDE

Prestressed Concrete Sleepers are structures that support railway system and absorb variable loading from the train that pass along the rail. In this study, variable amplitude loading from PCS taken from site measurement was converted into constant amplitude loading. Therefore the number of cycles and frequency are also important to be determined. There are various method for counting the number of cycles and the best adoptive method was Rainflow Cycle Counting. Once the number of cycles is obtained as well as the constant amplitude stress in a series of stress block, the prediction of fatigue life of PCS can be analyzed by further research in the laboratory work. However the limitation of this study is only to obtain the constant amplitude loading of PCS and also counting the number of cycles in various series of stress blocks.

Influence of Semi-Circular Cracks on Threaded Connection Fatigue by Means of Kitagawa-Takahashi Diagram and El Haddad Equation

Evaluation of semi-circular surface crack influence on threaded connection fatigue behavior, made of 42CrMo4 heat treatable steel, was carried out. Crack diameters were defined as 0.02, 0.05, 0.1, and 0.15 mm. Influence of semi-circular surface cracks was investigated by means of Kitagawa–Takahashi diagram and El Haddad equation. Assessments were done for survival probability of 99% on detailed FE model with normal metric thread profile and preload force at 70% of force at bolt yield point. The most critical location on threaded connections usually are thread roots which contain a very high notch effect. In order to accurately consider multiaxial stress field in thread root, multiaxial fatigue stress criterion based on a critical plane theory for fatigue assessment, was used. Mean stress influence was taken into account by means of Haigh diagram. Variable amplitude loading history of low-high (Lo-Hi) sequence spectrum was analyzed with the numerical algorithm of Rainflow cycle counting in the time domain. Fatigue damage was calculated according to the modified Palmgren-Miner linear damage accumulation hypothesis.