Explain the listed term related to the design of machine elements subjected to the variable loads: I. Notch sensitivity II. Endurance limit III. Surface finish factor IV. Size factor V. Stress concentration factor

Subject: Machine Design -I

Topic: Machine Design consideration

Difficulty: High

I. Notch sensitivity:

• Notch sensitivity is defined as the susceptibility of a material to succumb to the damaging effects of stress raising notches in fatigue loading.
• Notch sensitivity is denoted by ‘q’

II. Endurance limit:

• It is defined as maximum value of the completely reversed bending stress which a polished standard specimen can withstand without failure, for infinite number of cycles (usually 107 cycles).

III. Surface finish factor:

• When a machine member is subjected to variable loads, the endurance limit of the material for that member depends upon the surface conditions. Fig. shows the values of surface finish factor for the various surface conditions and ultimate tensile strength.

• When the surface finish factor is known, then the endurance limit for the material of the machine member may be obtained by multiplying the endurance limit and the surface finish factor.
• The surface finish factor for non-ferrous metals may be taken as unity.

IV. Size factor:

• The rotating beam specimen is small with 7.5 mm diameter. A little consideration will show that if the size of the standard specimen as shown in Fig. is increased, then the endurance limit of the material will decrease. This is due to the fact that a longer specimen will have more defects than a smaller one.

V. Stress concentration factor:

• It is defined as the ratio of the maximum stress in a member (at a notch) to the nominal stress at the same section based upon net area.
• Stress concentration factor is denoted by ‘Kt’