02-03-2013, 03:17 PM
Fatigue Failure Resulting from Variable Loading
Fatigue Failure.ppt (Size: 2.9 MB / Downloads: 58)
Characterizing Fluctuating Stresses
Fluctuating stresses in machinery often take the form of sinusoidal pattern because of the nature of the nature of some rotating machinery.
Other patterns some quite irregular do occur.
In periodic patterns exhibiting a single maximum and single minimum of force, the shape of the wave is not important.
The peaks on both sides (maximum, minimum) are important.
Fmax and Fmin in a cycle can be used to characterize the force pattern.
A steady component and an alternating component can be constructed as follows:
Possible ways of quantifying the problem:
Residual Stress Method
All stresses (both mean and alternating) are multiplied by the fatigue stress concentration factor Kf , and correction is made for yielding and resultant residual stresses if the calculated peak stress exceeds the material yield strength.
Nominal Mean Stress Method
In this method, stress concentration factor is applied only to alternating stress.
Reduction in mean stress from not multiplying it by Kf , might be about the same as the reduction in mean stress achieved with the residual stress method by taking yielding and residual stress into account.
Modified Goodman Diagram
It has midrange stress plotted along the abscissa and all other components of stress plotted on the ordinate, with tension in the positive direction.
The endurance limit, fatigue strength, or finite-life strength whichever applies, is plotted on the ordinate above and below the origin.
The midrange line is a 45o line from the origin to the tensile strength of the part.