10/29/2012

Lecture 24: 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

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Mechanisms of Fatigue Failure • There are three stages of fatigue failure: – Crack initiation (short duration) – Crack propagation (most of part’s life) – Sudden 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 strength is exceeded), leading to • Distortion • Slip bands (intense deformation due to shear) along crystal boundaries • Microscopic cracks over time

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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 – • Causes crack growth under static loads (“stress corrosion”) • Increases the rate of crack growth under dynamic loads (“corrosion fatigue”)

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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

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