The relevance of non-stationarities and non-Gaussianities in vibration fatigue

Abstract

In classical fatigue of materials, the frequency contents of dynamic loading are well below the natural frequencies of the observed structure or test specimen. However, when dealing with vibration fatigue the frequency contents of dynamic loading and structure's dynamic response overlap, resulting in amplified stress loads of the structure. For such cases, frequency counting methods are especially convenient. Gaussianity and stationarity assumptions are applied in frequency-domain methods for obtaining dynamic structure's response and frequency-domain methods for calculating damage accumulation rate. Since it is common in real environments for the structure to be excited with non-Gaussian and non-stationary loads, this study addresses the effects of such dynamic excitation to experimental time-to-failure of a structure. Initially, the influence of non-Gaussian stationary excitation is experimentally studied via excitation signals with equal power density spectrum and different values of kurtosis. Since no relevant changes of structure's time-to-failure were observed, the study focused on non-stationary excitation signals that are also inherently non-Gaussian. The non-stationarity of excitation was achieved by amplitude modulation and significantly shorter times-to-failure were observed when compared to experiments with stationary non-Gaussian excitation. Additionally, the structure's time-to-failure varied with the rate of the amplitude modulation. To oversee this phenomenon the presented study proposes a non-stationarity index which can be obtained from the excitation time history. The non-stationarity index was experimentally confirmed as a reliable estimator for severity of non-stationary excitation. The non-stationarity index is used to determine if the frequencydomain methods can safely be applied for time-to-failure calculation.

Fatigue Reliability Assessment of an Automobile Coil Spring under Random Strain Loads Using Probabilistic Technique

“…The probabilistic Coffin-Manson strain fatigue life model was obtained by substituting Equation (18) with (21) and then substituting it with Equation (19). Hence, the newly proposed probabilistic modeling equations for the Coffin-Manson, Morrow, and SWT strain fatigue life models can be modeled as follows:…”

“…The relationship of nonstationarities and non-Gaussianities was studied by Cesnik and Capponi in a vibration fatigue analysis. The Gaussianity and stationarity were important assumptions of the fatigue damage theory in the frequency domain approach [17,18]. Capponi indicated that different rates of amplitude-modulated nonstationary excitation have a shorter fatigue life than the stationary excitation level for the dynamic structure's response and dynamic loading.…”

A time-domain procedure for non-Gaussian stationary environmental testing using zero-memory nonlinear transformation

“…Recently, industry has increasingly recognized that most of the vibrations in the real world are non-Gaussian. Non-Gaussian random excitations contain more numerous bursts of high-excursion peaks in their signals and elicit a more significant stress response than the Gaussian case (Cesnik et al, 2018; Kihm et al, 2015; Palmieri et al, 2017). As a result, non-Gaussian vibration environments are widely adopted for fatigue test purposes.…”

Collection of experimental data for multiaxial fatigue criteria verification

“…[1][2][3][4][5][6][7][8] Furthermore, the relevance of many input factors has been widely verified, 9 such as the influence of temperature, 10,11 geometry, [12][13][14][15][16][17][18] chemical composition of the material, 19 and the influence of statistic parameters of excitation. [20][21][22][23] Most works dealing with fatigue consider loading resulting in a uniaxial stress state, whereas it is referred to as multiaxial fatigue when dealing with a combination of fatigue loads resulting in a combination of different stress components. A schematic illustration of multiaxial fatigue loading (tension and torsion) is shown in Figure 1, with the corresponding stress state and possibility of different phase angles between the different loads.…”

“…Numerical and experimental analyses have been performed in order to verify fatigue criteria in both time and frequency domain . Furthermore, the relevance of many input factors has been widely verified, such as the influence of temperature, geometry, chemical composition of the material, and the influence of statistic parameters of excitation . Most works dealing with fatigue consider loading resulting in a uniaxial stress state, whereas it is referred to as multiaxial fatigue when dealing with a combination of fatigue loads resulting in a combination of different stress components.…”