भंग के अधिकतम अपरूपण प्रतिबल सिद्धांत के अनुसार, एक वृत्ताकार शाफ़्ट में अधिकतम अनुज्ञेय मरोड़ी आघूर्ण 'T' है। भंग के अधिकतम मुख्य प्रतिबल सिद्धांत के अनुसार, एक ही शाफ़्ट के लिए अनुज्ञेय मरोड़ी आघूर्ण है:

Concept:-

Maximum Principal Stress theory (M.P.S.T) -

According to M.P.S.T, the condition for failure is, Maximum principal stress ( σ₁) > failure stresses (S_yt or S_ut).

Condition for safe design, Maximum principal stress ( σ₁) ≤ Permissible stress (σ_per)

⇒ 

Where N = factor of safety

Maximum Shear Stress theory (M.S.S.T) - 

Condition for safe design, Maximum shear stress induced at a critical tensile point under triaxial combined stress ≤ Permissible shear stress (τ_per) 

Where Permissible shear stress = Yield strength in shear under tension test / Factor of safety

⇒ 

-- Torsion test is conducted under pure torsion, i.e., pure shear state of stress (σ_x = σ_y= 0; τ_xy = τ ).

Given:-

((T_max)_per)_MSST = T

((T_max)_per)_MPST =?

Calculation:-

Since it is the case of pure torsion, so there is only shear stress, and that is, 

Then according to MSST, τ_max =  = S_yt/2.....(Assuming factor of safety = 1)

From here, S_yt = ........Eqn(1)

Then according to MPST, for pure torsion case (σx = σy= 0; τxy = τ ),

Major principal stress = σ₁ = + τ 

Minor principal stress = σ₂ = - τ 

On applying the condition of safe design for MPST,

⇒ 

⇒ τ = S_yt ....(Taking factor of safety as 1)

Since τ = 16T_per/πd³ and S_yt can be written from the equation (1), 

⇒ T_per = 2T