Failure Analysis & Durability Improvement

Stress Concentration Factor

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The stress concentration factor (usually designated K_t in formulas) is the ratio of peak stress to nominal stress. The stress concentration is the result of a component's geometry. Consider the smooth, flat plate on the left with a cross sectional area of 1 square inch. If one end is held fixed and 100 pounds of load applied on the other, the stress is 100 psi everywhere. If a hole is drilled through the plate as shown on the right, the stress field changes and is no longer uniform. Stress will concentrate on the surface of the hole (Point A in the figure). For a hole in an infinite plate, the stress concentration factor is 3. For finite plates, the factor is slightly higher. This means that the the stress at point A is over 300 psi. Fatigue cracks generally originate in regions where there is a high stress concentration factor. Therefore steps should be taken to minimize a stress concentration factor if it is in a life limiting location. One way to decrease the stress concentration factor of the panel on the right is to replace the round hole with an elliptical hole that has its major axis in the vertical direction.

Uniformly Loaded Panel	Panel With Stress Concentrator

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Failure Analysis & Durability Improvement

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