Direct tension indicators are sometimes used instead of torque wrenches to ensure that a bolt has a prescribed tension when used for connections. If a nut on the bolt is tightened so that the six 3 - mm high heads of the indicator are strained 0.1 mm/mm

Complete Question

The complete question is shown on the uploaded image

The tension on the shank is  

Explanation:

From the question we are told that

       The strain on the strain on the head is

         The contact area is    

Looking at the first diagram

           At  600 MPa of stress

               The strain is  

          At   450 MPa of stress

                 The strain is  

 To find the stress at   we use the interpolation method

Substituting values

Generally the force on each head is mathematically represented as

Substituting values

Now the tension on the bolt shank is as a result of the force on the 6 head which is mathematically evaluated as