The nonlinear ultrasonic theory is that ultrasonic waves interact with micro defects and the higher harmonic waves appear when the ultrasonic waves propagate in materials. The acoustic nonlinear parameters are sensitive to micro defects in the nonlinear materials. They are used to characterize the nonlinear characteristics of materials. The nonlinear ultrasonic theories and the characterization of stress in metal by the nonlinear ultrasonic are studied. The nonlinear ultrasonic testing system is built to detect the received signals which propagate in materials with stress. The variation trends of the nonlinear parameters is studied in tensile testing. This paper studies the relationship between nonlinear parameter and stress in different coupling states including contact coupling, air coupling, water coupling. Use manipulator to measure residual stress based on nonlinear ultrasonic and achieve a rapid, non-contact and non-destructive testing
