Strength of materials (simple bending theory): In the common beam assumptions, “the material is isotropic” means which of the following?

Introduction / Context:
Classical bending theory relies on simplifying assumptions about material behavior and geometry. One key assumption is isotropy, which impacts how stress and strain relate in different directions when a beam bends.

Given Data / Assumptions:

• Homogeneous, linearly elastic material.
• Plane sections remain plane and normal to the neutral axis after bending.
• Small deformations and a straight prismatic member.

Concept / Approach:
Isotropic materials have identical mechanical properties in all directions at a point. Practically, constants such as Young’s modulus E and shear modulus G are direction-independent. This ensures that the linear stress distribution from bending is valid irrespective of orientation, provided other assumptions hold.

Step-by-Step Solution:

Verification / Alternative check:
Compare with orthotropic materials (e.g., composites); E differs by direction, violating isotropy and changing bending response.

Why Other Options Are Wrong:

• Normal stress constant in all directions: not a definition of isotropy and not true in bending.
• Normal stress varies linearly: that is a bending result, not the definition of isotropy.
• Elastic constants vary linearly: contradicts isotropy; that would be inhomogeneity.

Common Pitfalls:
Equating isotropy with homogeneity; they are related but distinct—homogeneity means properties are the same at every point, isotropy that they are direction independent at a point.

Final Answer:
Elastic constants are the same in all directions