Axial Load
        Consider a bar with a slowly changing cross-section that is loaded centroidally.
          The internal load at x from one end is N , and the normal stress is N / A . using Eq. (6-
           4) the strain energy is:
                                            2          N2
                                  U i   x dV       2
                                                             dV      
                                          V 2E       V 2EA
          The volume dV can be expressed as Adx and:
                                                   L
                                                          N2
                                              Ui       2AE
                                                             dx                          (6-10)
                                                 0
          If the cross-sectional area is constant:
                                                   N 2L
                                              Ui                                        (6-11)
                                                     2AE
          Note:
              o    L , Ui    
              o    A, Ui 
           o   E , Ui 
           o i.e. something that is easy to distort will store more strain energy.