Study On Ultrasonic NDT Method Of The Thickness Of Coated Layer In Thermal Spraying

Ultrasonic measuring on the thickness of thermal spraying coated layer is one of the hotspot in the research on quality evaluation, and is a difficulty in ultrasonic NDT. So the research on ultrasonic NDT method has an important effect on the thickness of thermal spraying coated layer. In the present work, the advanced digital signal processing (DSP) technology which is the wavelet transform modulus maxima has been applied to the transform and processing of time-domain signals which acquired from ultrasonic non-destructive testing, the purpose is to find useful information from the ultrasonic signals which can show the thickness of coated layer. Thus, the applicability and accuracy of ultrasonic non-destructive testing on the thickness of coated layer can be valued. In the project, surface coated layers of thermal spraying sample have been studied.This paper firstly established ultrasonic echo module, and based on this module characteristic information is analyzed which is connected the thickness of coated layer, and this information is the time which reflection echo is incepted by detector from the interface of coated layer and basal body. Based on good performance of wavelet transform in time-frequency domain and signal singularity detecting, wavelet transform maxima method was accessed to process the signals of coated layer, then excavated the useful information on the thickness of coated layer in the figure of wavelet transform modulus maxima, and then calculated the thickness of coated layer. The result on the thickness of epoxy layer with ultrasonic NDT method shows that the principle of wavelet transform modulus maxima method is applicable in the singularity detection of the coated layer signals, and the singularity corresponded to the time ultrasonic arrived on the interface of coated layer and basal body. And ultrasonic non-destructive testing method is applicable on the thickness of coated layer.In the research of ultrasonic NDT method on the thickness of coated layer, the sample of Al coated layer, SAM and HA coated layer were detected. A-signal was pick up by directed contacting , and characteristic information was pick up on wavelet transform modulus maxima method by Mexh wavelet. The results on the thickness of Al , SAM and HA coated layer with ultrasonic NDT method showed that the value of ultrasonic measuring has an agreement with that on metallographical method. And the relative error is in the range of 1%-16%.Against the accuracy of ultrasonic non-destructive testing on the thickness of coated layer, experimental investigation was proceeded in four major influencing factors. The results of analysis show that varied thickness, varied sampling frequency, varied extraction method and varied wavelet greatly impacted on accuracy of ultrasonic measuring on the thickness of coated layer.