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Chapter 11
Impact, Fatigue and Wear

Excerpt

The three principal damage mechanisms resulting from flow-induced vibrations are impact (which may result in fatigue wear), fatigue and wear. Because turbulence-induced vibration is random, the zero-to-peak vibration amplitudes can occasionally exceed several times the computed rms response. Thus, the fact that the nearest component is three times the rms vibration amplitude away does not guarantee that impacting between the two components will not occur. The number of impacts between two vibrating components over a given time period can be estimated based on the theory of probability, assuming that the vibration amplitudes follow a Gaussian distribution.

Likewise, since the zero-to-peak amplitude of vibration of a structure excited by flow turbulence can, over a long enough period of time, exceed arbitrarily large values, there is no endurance limit in random vibration. Given a long enough time, any structure excited by any random force will theoretically fail by fatigue. The cumulative fatigue usage can again be calculated based on the probabilistic theory. Since in cumulative fatigue analysis, the usage factor is computed based on the absolute value of the zero-topeak vibration amplitudes, which follow the Rayleigh distribution function if the ± vibration amplitudes follow the Gaussian distribution, the Rayleigh probability distribution function must be used in computing the cumulative fatigue usage factor of a component excited by turbulent flow. From the ASME fatigue curves (which are based on the 0-topeak vibration amplitudes), corresponding fatigue curves based on rms vibration amplitudes had been derived for several types of materials. These are given in Figures 11.6 to 11.10.

Compare with fatigue usage calculations, wear analysis due to flow-induced vibration is orders of magnitude more complex. This is because the wear mechanisms are not only dependent on the dynamics of the structures, but also on the material and the ambient conditions. Generally, there are three major types of wear mechanisms: Impact wear is that caused by moderate to fairly large vibration amplitudes, with resulting high impact forces that can cause surface fatigue and rapid failure of the structure. Blevins (1984) proposed the simple equation

$srms=c(E4Mefn2maxD3)1∕5$
to estimate the rms surface stress of a heat exchanger tube impacting its support, with the contact stress parameter c obtained from tests (These are given in Figure 11.13). Blevins postulated that if the computed stress is below the endurance limit, then impact wear is not a concern. However, if it exceeds the endurance limit, then rapid wear of the material can be expected.

• Summary
• Nomenclature
• 11.1 Introduction
• 11.2 Impacts due to Turbulence-Induced Vibration
• Example 11.1
• 11.3 Cumulative Fatigue Usage due to Turbulence-Induced Vibration
• Crandall's Method
• Cumulative Fatigue Usage by Numerical Integration
• Fatigue Curves based on RMS Stress
• 11.4 Wear due to Flow-Induced Vibration
• Impact Wear
• Example 11.2
• Sliding Wear
• Example 11.3
• Fretting Wear
• Fretting Wear Coefficient
• 11.5 Fretting Wear and the Dynamics of a Loosely Supported Tube
• Connors' Approximate Method for Fretting Wear
• Example 11.4
• The Energy Method for Fretting Wear Estimate
• Example 11.5
• References
Topics: Fatigue, Wear
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