sentences:  An alternative definition of the plasticity index has been given by Mikic.Yield occurs when the pressure is greater than the uniaxial yield stress.Since the yield stress is proportional to the indentation hardness formula_133, Mikic defined the plasticity index for elastic-plastic contact to be
  
  In this definition formula_132 represents the micro-roughness in a state of complete plasticity and only one statistical quantity, the rms slope, is needed which can be calculated from surface measurements.