Answer & Solution
Answer: Option
C
Solution:
Endurance Limit: The endurance limit (also called fatigue limit) is the stress level below which a material can endure an essentially infinite number of stress cycles without failure.
Endurance Limit in Flexure (Se): This is the endurance limit when the material is subjected to cyclic bending or flexural stresses.
Endurance Limit in Shear (Ses): This is the endurance limit when the material is subjected to cyclic shear stresses.
Ratio of Shear to Flexure Endurance: For most ductile materials, experimental results and standard design handbooks (like Shigley's Mechanical Engineering Design) indicate:
Ses ≈ 0.5 × Se
This ratio is based on fatigue failure theories and verified through material testing.
Why 0.5 is Accepted:
1. Fatigue Failure Theories: Theories such as the Maximum Shear Stress Theory and the Distortion Energy Theory are used to estimate fatigue life under different stress types. These theories show that shear stresses are generally more damaging than normal stresses at the same level.
2. Empirical Observations: For many steels and ductile materials, actual fatigue tests show that the shear endurance limit is around 50% of the flexural endurance limit.
3. Conservative Design Practice: Machine design often relies on conservative approximations to ensure safety. A 0.5 ratio provides a reliable and safe basis for fatigue design calculations.
Why Not 0.55? Although 0.55 may be found in some very specific materials or under certain test conditions, it is not the standard accepted value in textbooks or exams. Using 0.5 gives a safer and widely accepted estimate.
Conclusion: The accepted and commonly used value for the ratio of endurance limit in shear to flexure is 0.5, which corresponds to Option C.
