A laser ultrasonics method is used to surface wave traveling on the surface of materials characterize the propagation properties of with sub-surface defect in 2D. Linear and nonlinear propagation properties of ultrasonics caused by the defects have been detected in experiment. A theoretical model is proposed and used to study the linear and nonlinear properties of ultrasonics caused by the defect. The numerical results indicate that the nonlinear ultrasonic wave will be excited when a finite amplitude ultrasonics propagates on the surface of materials with sub-surface defect. The theoretical analysis confirms that the nonlinear wave is caused by the ＂clapping＂ of the interface of defect instead of the mode conversions of ultrasonics.
