this is about factors in fatigue stress

1. By Onkar A. Powar

2. Fatigue is a phenomenon associated with variable loading or more precisely
to cyclic stressing or straining of a material. Just as we human beings get
fatigue when a specific task is repeatedly performed, in a similar manner
metallic components subjected to variable loading get fatigue, which leads
to their premature failure under specific conditions.
Fatigue loading:
Fatigue loading is primarily the type of loading which causes cyclic variations
in the applied stress or strain on a component.Thus any variable loading is
basically a fatigue loading.
Introduction

3. • Change in the magnitude of applied load Example: punching or shearing
operations
• Change in direction of load application Example: a connecting rod
• Change in point of load application Example: a rotating shaft
Types of loading

4. A fatigue failure begins with a small crack; the initial crack may be so minute
and can not be detected.The crack usually develops at a point of localized
stress concentration like discontinuity in the material, such as a change in
cross section, a keyway or a hole.
Thus three stages are involved in fatigue failure namely
-Crack initiation
-Crack propagation
-Fracture
Initiation of fatigue failure

5. Endurance curve

6. (a) Surface Condition (ka): such as: polished, ground, machined, as-forged,
corroded, etc. Surface is perhaps the most important influence on
fatigue life
(b) Size (kb):This factor accounts for changes which occur when the actual
size of the part or the cross-section differs from that of the test
specimens
(c) Load (Kc):This factor accounts for differences in loading (bending, axial,
torsional) between the actual part and the test specimens
(d) Temperature (kd): This factor accounts for reductions in fatigue life which
occur when the operating temperature of the part differs from room
temperature (the testing temperature)
(e) Reliability (ke):This factor accounts for the scatter of test data. For
example, an 8% standard deviation in the test data requires a ke value of
0.868 for 95% reliability, and 0.753 for 99.9% reliability.
(f) Miscellaneous (Kf):This factor accounts for reductions from all other
effects, including residual stresses, corrosion, plating, metal spraying,
fretting, and others.
Factors to be considered

7. a) the most highly-stresses fibers are located at the surface (bending
fatigue
b) the intergranular flaws which precipitate tension failure are more
frequently found at the surface
c) ka = a*S^b
Surface Factor Ka

9. The size factor accounts for the variations in the size of the component
when compared to the test specimen.
The larger the size higher the probability of internal defects, hence lower the
fatigue strength. An empirical relation for the case of bending and torsion
can be expressed as given below
For large sizes, kb further reduces to 0.60 and lower Note that for axial
loading there is no size effect, therefore use kb = +1.0 in this case
Size factor Kb

10. • Axial Loading
Though there is no apparent size effect for specimens tested in
axial or push–pull fatigue, there is definite difference between the axial
fatigue limit and that in reserved bending.
• Torsional Loading
comparing the torsional endurance limit with the bending
endurance limit yielded a load factor for torsion of 0.577
Load Factor Kc

11. Temperature Factor Kd

12. the Soderberg’s criterion guards against yielding.The linear theories of
Figure can be placed in equation form: The equation for the Soderberg’s
criteria (line) is
Failure Criteria's