Machine Design
A bolt of M 24 × 2 means that

The pitch of the thread is 24 mm and depth is 2 mm
The nominal diameter of bolt is 24 mm and pitch is 2 mm
The effective diameter of bolt is 24 mm and there are 2 threads per cm
Cross-sectional area of the threads is 24 mm²

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Machine Design
The fatigue stress concentration factor is defined as

The product of the endurance limits with and without stress concentration
The ratio of endurance limit without stress concentration to the endurance limit with stress concentration
All of these
The ratio of endurance limit with stress concentration to the endurance limit without stress concentration

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