how to calculate bolt tightening torque

How to calculate bolt tightening torque.
By applying tightening torque, we are basically stretching the bolt. which in turns creates a tensile stress in the bolt.
The tensile load developed in the bolt due to tightening torque is known as preload. The tensile load in the bolt, creates a compressive force in the bolted joint. which is often known as clamping force.
When we apply tightening torque to a bolt, it seems that all applied torque converts into preload. But unfortunately only 10% of the applied torque convert into useful preload.
And about 50% of the applied torque is used to counter, frictional resistance between the nut and the washer or nut and joint.
And remaining 40% of the applied torque is used to counter frictional restraint between threads of nut and bolt.
The tightening torque also depends on the nut factor K.
The value of nut factor depends on Fastener Materials, type of Coatings and lubricant.
if we load the bolt up to its yield point. Than bolt will have permanent deformation and it cannot be used again. So in case of fasteners we define a term proof strength or proof load.
Proof strength can be defined as the maximum tensile stress that can be applied to a bolt, which will not result in plastic deformation. In other words, the material must remain in its elastic region when loaded up to its proof strength. Proof strength is typically between 85 to 95% of the yield strength.
In case of non-permanent bolted joint, where we want to reuse bolts. In such applications bolt should be loaded up to 75% of its proof strength, this is to avoid permanent deformation in bolt.
So for non-permanent bolted joint preload can be calculated as 0.75 times proof strength multiplied by bolt stress area.
And if the bolted joint is permanent, then we can load the bolt up to 90% of its proof strength. Therefore, in case of permanent bolted joint preload can be calculated as 0.90 times proof strength multiplied by bolt stress area.
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