Numerical Analysis Of Material Surface Defect Depth Detection Using Rayleigh Wave

Surface defect is one of the most common damage in engineering material, which reduces the durability and safety of engineering structure. Traditional detection method is complex and the instrument is expensive. Rayleigh wave detection method has advantages of simple operation. Through general finite element software ANSYS, Rayleigh wave propagation was simulated when Rayleigh wavelength was longer than the surface defect depth and less than the surface defect depth, which concluded that the Rayleigh wave amplitude attenuation rule, propagation time delay rule and frequency blocking rule. Based on propagation law of Rayleigh wave in amplitude, propagation time, frequency, this paper studied material surface defect depth inspection method.From the aspect of Rayleigh wavelength longer than surface defect depth, how to use Rayleigh wave amplitude value attenuation method to detect surface defect depth was studied. The influence of material damping and geometric diffusion was eliminated through reference arrangement. Both reasonable distance of excitation source and the observation point and reasonable distance between observation point and surface defect were studied by quantitative calculation, which reduced clutter aliasing effect. The calculated transfer coefficient curve shows that when the surface defect depth and Rayleigh wavelength ratio(h/λ) is from 0 to 0.3, it meets a single mapping, and decline in steep, which is suitable for testing surface defect depth. The method can detect surface crack with a depth more than 0.5 m in the mass concrete structure.From the aspect of Rayleigh wavelength less than surface defect depth, how to use Rayleigh wave travel time method to detect surface defect depth was studied. Numerical simulation shows that when the excitation frequency is 8 MHz, and component height is more than 25 mm, the depth of the surface defects between 1 mm and 7 mm all have higher detection precision, and can realize mobile scanning detection and defect location.The maximum detectable depth is liminted by the the distance between excitation source and observation point and component height. This method has the advantage that the wave arrival time is easy to determine. A depth of 1 mm and 2 mm shallow surface defects can be well detected too. This method is suitable for detecting the steel surface defect depth which is hard for naked eye to find.Further, how to use Rayleigh wave frequency blocking method to detect surface defect depth was studied. By chopping diffraction body wave in front of the waveform curve, the corresponding critical frequency value for the depth between 4 mm and 10 mm was found out and also the effective excitation frequency range for different surface defect depth was determined. By data fitting, surface defect depth and critical frequency value is inversely proportional relationship. When surface defect is too deep, fitting curve trends gently and the critical frequency values of surface defect depth is too close. Restricted by the degree of differentiation of critical frequency value, the maximum detectable depth is around 10 mm. This method is suitable for testing fatigue crack depth on the surface of the steel.