Study On Photoacoustic Detection Technology For Quantitative Measurement Of Microdefects

The appearance of micro-defects on the surface of a workpiece is often the beginning of a large crack.If not detected in time,it can expand further and even lead to the fracture of the workpiece,posing a great safety risk to production and life.In order to effectively monitor the mechanical state of the workpiece surface,this paper selects the laser ultrasound method to achieve quantitative monitoring and measurement of micro-defects in materials.This paper uses a combination of numerical simulations and experiments to measure defects in aluminum and glass plates of different thicknesses.Surface waves and guided waves are used to measure the defects respectively.The details of the study are as follows:(1)In this paper,laser ultrasound theory is analyzed and a finite element simulation model is established to simulate the process of laser-excited ultrasound,and the directivity of surface,transverse and longitudinal waves are analyzed.Through numerical simulations,the acoustic field diagrams of the interaction between acoustic waves and defects at different angles are observed and the simulated time domain signals are analyzed,and a method for measuring the angle of defects based on the energy of laser ultrasound diffracted waves is proposed.A measurement experimental scheme is designed and a laser ultrasonic inspection experimental platform is built to locate surface defects and quantify the angles of surface defects by B-Scan diagram.Combining simulation and experiment,the feasibility of measuring surface defects by diffracted waves is verified.(2)To realize the measurement of defects in thin plates,the principle of guided wave generation and the dispersion characteristics are analyzed in this paper,and the dispersion curves of the guided waves are plotted.In S1 mode,there is a group of points with zero velocity and non-zero phase velocity,and this position is the Zero Group Velocity(ZGV)point.The ZGV guided wave is generated by the coupling of the forward and backward waves.In this paper,an elastic oscillator dot matrix model is established to simulate the guided wave propagation process in a thin plate.When the pulsed laser emission and reception points are close to the defect,the ZGV guided wave resonance frequency is shifted,and quantitative measurement of the defect can be achieved through a two-dimensional scan map based on this property.The combination of simulation and experiment validates the conclusion that laser ultrasound ZGV guide technology can achieve the measurement of surface defects in thin plates.To achieve quantitative measurement of surface defects in workpieces,this paper proposes the diffracted wave energy detection method and the ZGV guided wave detection method and conducts numerical simulations and experimental studies,the results can provide simulation and experimental reference for laser ultrasound surface wave and guided wave detection of surface defects.