Question

The design stress for a component subjected to a completely reversible load, is found by applying the factor of safety to:

• A. Yield Strength
• B. Ultimate Strength
• C. Buckling Strength
• D. Endurance Strength

Hint:

For static loads, design stress is typically based on yield strength or ultimate tensile strength, depending on the application (whether yielding is acceptable or fracture must be avoided). However, for cyclic loads leading to fatigue, endurance strength is the critical material property.

Answer 1

The Correct Option is D

Step 1: Understand the concept of completely reversible load.
A completely reversible load is a cyclic load that varies symmetrically between equal positive and negative values (e.g., \( +\sigma_{max} \) to \( -\sigma_{max} \)). This type of loading leads to fatigue failure, even if the maximum stress is below the yield strength of the material. Step 2: Define relevant material strengths.
Yield Strength (\( S_y \)): The stress at which a material begins to deform plastically.
Ultimate Tensile Strength (\( S_{ut} \)): The maximum stress a material can withstand before fracturing under a tensile load.
Buckling Strength: The critical stress at which a structural member under compression will suddenly buckle. This is relevant for slender columns.
Endurance Strength (\( S_e \) or \( S_n \)): The maximum stress that a material can withstand for an infinite number of load cycles under fatigue loading. For ferrous materials, this limit often becomes constant after a large number of cycles.
Step 3: Determine the appropriate strength for fatigue loading.
When a component is subjected to a completely reversible load, the primary concern is fatigue failure. Fatigue failure occurs due to repeated cycles of stress, and the relevant material property that governs fatigue life under such loading is the endurance strength. Step 4: Apply the factor of safety.
The design stress (or allowable stress) for a component subjected to fatigue loading is determined by dividing the endurance strength of the material by a suitable factor of safety (\( FS \)): $$\text{Design Stress} = \frac{\text{Endurance Strength}}{FS}$$ Therefore, the design stress for a component subjected to a completely reversible load is found by applying the factor of safety to the endurance strength.