Nearly every published S/N curve was measured under steady, repeatable loading. Real structures never see that. Introduction The S/N curve — stress amplitude versus cycles to failure — is the foundation of classical fatigue analysis. It is intuitive, experimentally well-established, and embedded in virtually every… Read more
Introduction A previous VibrationData post examined cold dwell fatigue in titanium — the phenomenon in which simply holding a moderate stress for one to two minutes causes far more damage per cycle than the same stress applied and immediately removed. The mechanism is driven by… Read more
Introduction Most engineers have an intuitive feel for fatigue: apply a stress repeatedly, and eventually the material cracks — even if each individual load is well below the static yield strength. Dwell fatigue adds a further, less intuitive twist. Dwell fatigue occurs when a sustained… Read more
Introduction Shock and vibration environments are almost always characterized in terms of velocity. The shock response spectrum (SRS) is fundamentally a pseudo-velocity construct. Random vibration specifications are integrated to give RMS velocity as a check on response severity. Field data from accelerometers is routinely converted… Read more
Introduction A question that comes up regularly in fatigue testing and vibration qualification programs is whether the frequency of cyclic loading — or the associated strain rate — influences the fatigue life of a component. The honest answer is: it depends very much on the… Read more
Image reference: https://www.sciencedirect.com/science/article/abs/pii/S0020740323003296 Introduction Classical fatigue analysis is built on stress. The stress-life (S-N) approach correlates nominal stress amplitude to cycles to failure; the strain-life ($\varepsilon$-N) approach handles the plasticity at notch roots through the Coffin-Manson equation; linear elastic fracture mechanics (LEFM) characterizes crack growth… Read more
As NASA’s Artemis program and international space agencies transition from short-duration sorties to a sustained human presence on the Moon, mission planners and structural engineers must confront a mechanical hazard that received little attention during the Apollo era: lunar seismicity. The Moon is not a… Read more
A recurring question in random vibration analysis is: How do we determine the relative displacement between two points when the structure is subjected to a Power Spectral Density (PSD) excitation? This quantity is often far more critical than the absolute response. Connector pin engagement, cable… Read more
Image reference: https://onlinelibrary.wiley.com/doi/full/10.1002/adem.202300394 What Is Anodizing? Anodizing is widely used to improve corrosion resistance and surface durability in aluminum and other light metals. However, under cyclic loading, anodized coatings can play a surprising, and sometimes detrimental, role in fatigue performance. This post walks through why… Read more
Rolls Royce Engine Blade-off Test Video Link 1 Video Link 2 The engine blade-off test is performed to make sure that the engine can survive a fan. compressor or turbine blade breaking off within the engine, without fragments being ejected through the outside… Read more