• Images
• Videos
• Documents
• Books
• Articles

Vibration-fatigue-life estimation

Random load description

In a random process, the amplitude can not be described as a function of time, because of its probabilistic nature. However, certain statistical properties can be extracted from a signal sample, representing a realization of a random process, provided the latter is ergodic. An important characteristics for the field of vibration fatigue is the amplitude probability density function, that describes the statistical distribution of peak amplitudes. Ideally, the probability of cycle amplitudes, describing the load severity, could then be deduced directly. However, as this is not always possible, the sought-after probability is often estimated empirically.

Effects of structural dynamics

Random excitation of the structure produces different responses, depending on the natural dynamics of the structure in question. Different natural modes get excited and each greatly affects the stress distribution in material. The standard procedure is to calculate frequency response functions for the analyzed structure and then obtain the stress responses, based on given loading or excitation.³ By exciting different modes, the spread of vibration energy over a frequency range directly affects the durability of the structure. Thus the structural dynamics analysis is a key part of vibration-fatigue evaluation.

Vibration-fatigue methods

Calculation of damage intensity is straightforward once the cycle amplitude distribution is known. This distribution can be obtained from a time-history simply by counting cycles. To obtain it from the PSD another approach must be taken.

Various vibration-fatigue methods estimate damage intensity based on moments of the PSD, which characterize the statistical properties of the random process. The formulas for calculating such estimate are empirical (with very few exceptions) and are based on numerous simulations of random processes with known PSD. As a consequence, the accuracy of those methods varies, depending on analyzed response spectra, material parameters and the method itself - some are more accurate than others.⁴

The most commonly used method is the one developed by T. Dirlik in 1985.⁵ Recent research on frequency-domain methods of fatigue-life estimation⁶ compared well established methods and also recent ones; conclusion showed that the methods by Zhao and Baker, developed in 1992⁷ and by Benasciutti and Tovo, developed in 2004⁸ are also very suitable for vibration-fatigue analysis. For narrow-band approximation of random process analytical expression for damage intensity is given by Miles.⁹ There are some approaches with adaptation of narrow-band approximation; Wirsching and Light proposed the empirical correction factor in 1980¹⁰ and Benasciutti presented α0.75 in 2004.¹¹ In 2008, Gao and Moan published a spectral method that combines three narrow-band processes.¹² Implementation of those method is given in the Python open-source FLife¹³ package.

Applications

Vibration fatigue methods find use wherever the structure experiences loading, that is caused by a random process. These can be the forces that bumps on the road extort on the car chassis, the wind blowing on the wind turbine, waves hitting an offshore construction or a marine vessel. Such loads are first characterized statistically, by measurement and analysis. The data is then used in the product design process.¹⁴

The computational effectiveness of vibration-fatigue methods in contrast to the classical approach, enables their use in combination with FEM software packages, to evaluate fatigue after the loading is known and the dynamic analysis has been performed. Use of the vibration-fatigue methods is well-suited, as structural analysis is studied in the frequency-domain.

Common practice in the automotive industry is the use of accelerated vibration tests. During the test, a part or a product is exposed to vibration, that are in correlation with those expected during the service-life of the product. To shorten the testing time, the amplitudes are amplified. The excitation spectra used are broad-band and can be evaluated most effectively using vibration-fatigue methods.

See also

• Physics portal
• Chemistry portal

• Fatigue (material)
• Structural failure
• Vibration
• Structural dynamics
• Modal analysis
• Random vibration
• Rainflow-counting algorithm
• Seismic analysis
• Solder Fatigue

References

1. Nuno Manuel Mendes, Maia (1998). Theoretical and experimental modal analysis (Reprinted. ed.). Baldock: Research Studies Press. ISBN 0863802087. 0863802087 ↩

2. Sarkani, Loren D. Lutes, Shahram (2004). Random vibrations analysis of structural and mechanical systems ([Online-Ausg.] ed.). Amsterdam: Elsevier. ISBN 9780750677653.{{cite book}}: CS1 maint: multiple names: authors list (link) 9780750677653 ↩

3. Slavič, Janko; Boltežar, Miha; Mršnik, Matjaž; Česnik, Martin; Javh, Jaka (2020). Vibration Fatigue by Spectral Methods: From Structural Dynamics to Fatigue Damage – Theory and Experiments (1st ed.). Amsterdam, Netherlands: Elsevier. doi:10.1016/C2019-0-04580-3. ISBN 9780128221907. S2CID 243156155. 9780128221907 ↩

4. Mršnik, Matjaž; Slavič, Janko; Boltežar, Miha (31 July 2012). "Frequency-domain methods for a vibration-fatigue-life estimation - application to real data". International Journal of Fatigue. 47: 8–17. doi:10.1016/j.ijfatigue.2012.07.005. http://lab.fs.uni-lj.si/ladisk/?what=abstract&ID=75 ↩

5. Dirlik, Turan (1985). Application of computers in fatigue analysis (Ph.D.). University of Warwick. ↩

6. Mršnik, Matjaž; Slavič, Janko; Boltežar, Miha (31 July 2012). "Frequency-domain methods for a vibration-fatigue-life estimation - application to real data". International Journal of Fatigue. 47: 8–17. doi:10.1016/j.ijfatigue.2012.07.005. http://lab.fs.uni-lj.si/ladisk/?what=abstract&ID=75 ↩

7. Zhao, W; Baker, M (1 March 1992). "On the probability density function of rainflow stress range for stationary Gaussian processes". International Journal of Fatigue. 14 (2): 121–135. doi:10.1016/0142-1123(92)90088-T. /wiki/Doi_(identifier) ↩

8. Benasciutti, D; Tovo, R (1 August 2005). "Spectral methods for lifetime prediction under wide-band stationary random processes". International Journal of Fatigue. 27 (8): 867–877. doi:10.1016/j.ijfatigue.2004.10.007. /wiki/Doi_(identifier) ↩

9. Miles, John W. (1954). "On structural fatigue under random loading". Journal of the Aeronautical Sciences. 21 (11): 753–762. doi:10.2514/8.3199. /wiki/Doi_(identifier) ↩

10. Wirsching, Paul H.; Light, Mark C. (1980). "Fatigue under wide band random stresses". Journal of the Structural Division. 106 (7): 1593–1607. doi:10.1061/JSDEAG.0005477. /wiki/Doi_(identifier) ↩

11. Benasciutti, Denis; Tovo, Roberto (2004). Rainflow cycle distribution and fatigue damage in Gaussian random loadings (Report). Department of Engineering, University of Ferrara. ↩

12. Gao, Zhen; Moan, Torgeir (2008). "Frequency-domain fatigue analysis of wide-band stationary Gaussian processes using a trimodal spectral formulation". International Journal of Fatigue. 30 (10–11): 1944–1955. doi:10.1016/j.ijfatigue.2008.01.008. /wiki/Doi_(identifier) ↩

13. "FLife". GitHub. Retrieved 30 September 2020. https://github.com/ladisk/FLife ↩

14. Varoto, Kenneth G. McConnell, Paulo S. (2008). Vibration testing : theory and practice (2nd ed.). Hoboken, N.J.: John Wiley & Sons. ISBN 978-0-471-66651-6.{{cite book}}: CS1 maint: multiple names: authors list (link) 978-0-471-66651-6 ↩