10/29/2012 Lecture 24: Fatigue Failure Theories I BAEN 375 Design Fundamentals of Agricultural Machines and Structures Mechanisms of Fatigue Failure • Fatigue Failure always begins at a crack • All parts contain discontinuities – Microscopic (< 0.010 inch) – Macroscopic • Fatigue cracks start at a notch or other stress concentration • Dynamically loaded parts must be designed to minimize stress concentration 1 10/29/2012 Mechanisms of Fatigue Failure • There are three stages of fatigue failure: – Crack initiation (short duration) – Crack propagation (most of part’s life) – Sudden fracture due to unstable crack growth (instantaneous) Mechanisms of Fatigue Failure • Crack Initiation – Assume • Common ductile material • Regions of stress concentrations (voids, inclusions) • Time‐varying stress including tensile (positive) component – Local yielding occurs at stress concentrations (where yield strength is exceeded), leading to • Distortion • Slip bands (intense deformation due to shear) along crystal boundaries • Microscopic cracks over time 2 10/29/2012 Mechanisms of Fatigue Failure • Crack propagation – Once microcracks form • Larger stress concentrations exist • Each tensile cycle opens the crack slightly, causing – More material to yield – Reducing stress concentration – Slight crack growth – Cracks grow along planes normal to maximum tensile stress – Crack growth rate is low but significant over many cycles Mechanisms of Fatigue Failure • Crack propagation – Corrosion • Causes crack growth under static loads (“stress corrosion”) • Increases the rate of crack growth under dynamic loads (“corrosion fatigue”) 3 10/29/2012 Mechanisms of Fatigue Failure • Fracture – When the crack becomes large enough • The stress intensity factor (K) equals the material’s fracture toughness (Kc) • Sudden failure occurs 4.