Tensile and shear capacity of metals

[LT Judd]

TL;DR Summary

What is the underlying reason for the difference in tensile stress capacity and shear stress capacity in metals?

Steel and other metals data sheets and mill specs most commonly quote some tensile strength metric, like Proof, Yield or Ultimate Tensile Stress. Less common is the value for shear strength. Often as rule of thumb the allowable shear stress is taken as half the allowable tensile stress but, when you do find actual data, its often more like 70%. Example Source: https://www.engineersedge.com/materials/material_tensile_shear_and_yield_strength_15798.htm.
My question is what the underlying theoretical reason is why metals are weaker in shear than in tension, and what is the practical reason why the actual difference is less than the theoretical.

 

[Lnewqban]

The reason could be the reduced energy or work that is required for the type of deformation that the metal cubic crystal structure suffers under shear load.

Copied from:
https://engineeringlibrary.org/reference/properties-of-metals-doe-handbook

"When metal experiences strain, its volume remains constant. Therefore, if volume remains constant as the dimension changes on one axis, then the dimensions of at least one other axis must change also. If one dimension increases, another must decrease."

 

[256bits]

Summary: What is the underlying reason for the difference in tensile stress capacity and shear stress capacity in metals?

and what is the practical reason why the actual difference is less than the theoretical.

I don't see in your post anything that backs up that statement.