Geometric stress concentration factor Kt v/s Fatigue Stress Concentration factor Kf
Geometric stress concentration factors can be used to estimate the stress amplification in the vicinity of a geometric discontinuity. Geometric Stress Concentration Factor (Kt) is defined as
The existence of irregularities or discontinuities, such as holes, grooves, or notches, in a part increase the magnitude of stresses significantly in the immediate vicinity of the discontinuity. Fatigue failure mostly originates from such places. Hence its effect must be accounted and normally a fatigue stress-concentration factor Kf
is applied when designing against fatigue, even if the materials behavior is ductile.
Stress concentration factor need not be used with ductile materials when they are subjected to only static loads, because (local) yielding will relieve the stress concentration. However under fatigue loading, the response of material may not be adequate to nullify the effect and hence has to be accounted. The factor Kf commonly called a fatigue stress concentration factor is used for this. Normally, this factor is used to indicate the increase in the stress; hence this factor is defined in the following manner.
Fatigue stress concentration factor (Kf) can be defined as
NOTCH SENSITIVITY (q): The notch sensitivity of a material is a measure of how sensitive a material is to notches or geometric discontinuities.
The values of q are between zero and unity. It is evident that if q=0, then Kf =1, and the material has no sensitivity to notches at all. On the other hand if q=1, then Kf = Kt, and the material has full notch sensitivity. In analysis or design work, find Kt first, from geometry of the part. Then select or specify the material, find q, and solve for Kf from the equation
· Kf is normally used in fatigue calculations but is sometimes used with static stresses.
· Convenient to think of Kf as a stress concentration factor reduced from Kt because of lessened sensitivity to notches.
· If notch sensitivity data is not available, it is conservative to use Kt in fatigue calculations.