Stress concentration has been discussed in earlier lessons. However, it is important to realize that stress concentration affects the fatigue strength of machine parts severely and therefore it is extremely important that this effect be considered in designing machine parts subjected to fatigue loading. This is done by using fatigue stress concentration factor defined as
The notch sensitivity ‘q’ for fatigue loading can now be defined in terms of Kf and the theoretical stress concentration factor Kt and this is given by
The value of q is different for different materials and this normally lies between 0 to 0.7. The index is small for ductile materials and it increases as the ductility decreases. Notch sensitivities of some common materials are given in table22.214.171.124 .
126.96.36.199T- Notch sensitivity of some common engineering materials.
Notch sensitivity index q can also be defined as
where, is called the Nubert’s constant that depends on materials and their heat treatments. A typical variation of q against notch radius r is shown in figure188.8.131.52 .
184.108.40.206F- Variation of notch sensitivity with notch radius for steel and aluminium alloy with different ultimate tensile strengths
Fatigue cracks can start at all forms of surface discontinuity and this may include surface imperfections due to machining marks also. Surface roughness is therefore an important factor and it is found that fatigue strength for a regular surface is relatively low since the surface undulations would act as stress raisers.
It is, however, impractical to produce very smooth surfaces at a higher machining cost.
Another important surface effect is due to the surface layers which may be extremely thin and stressed either in tension or in compression. For example, grinding process often leaves surface layers highly stressed in tension. Since fatigue cracks are due to tensile stress and they propagate under these conditions and the formation of layers stressed in tension must be avoided. There are several methods of introducing pre-stressed surface layer in compression and they include shot blasting, peening, tumbling or cold working by rolling. Carburized and nitrided parts also have a compressive layer which imparts fatigue strength to such components. Many coating techniques have evolved to remedy the surface effects in fatigue strength reductions.