Studies On Instrument Calibration Optimization And Defect Quantitative Evaluation Methods In Ultrasonic Testing

Pressure equipments which widely used in nuclear power,chemical,petrochemical and other industries usually difficult to avoid some manufacturing defects as weld cracks,porosities,slags and in service defects as local thinning,corrosion cracks,fatigue cracks.These defects seriously threaten the structural integrity of the equipments and their safe operations.It is necessary to detect these defects as soon as possible and evaluate their fitness for service to avoid catastrophic accidents.Ultrasonic testing is one of the most commonly used quantitative testing techniques in nondestructive testing for pressure equipments,which can characterize the sizes and locations of defects.As a nondestructive testing technology,the reliability of ultrasonic testing technology directly affects the structural integrity assessment results of pressure equipment and causes potential risks to the safe operation of equipments.This study focus on the reliability of the ultrasonic testing technology.A Probability Of Detection(POD)model was established.An instrument calibration strategy was proposed,and according to the refraction angle calibration error to corrected the defect height distribution model.Then the defect quantitative models based on reliability were formed.The main research contents are as follows:Ultrasonic wave is continuously attenuated in the process of propagation and the ultrasonic attenuation will affect the probability of detection(POD).Therefor,in this paper,the experiment of sound pressures measurement is designed to explore attenuation law of ultrasonic sound field.The curve of standard deviation of sound pressures and sound path distances was obtained in this experiment.The response amplitude model was improved for sound path distance parameter and the estimation flow of model parameters were introduced.The results show that the POD curve in ultrasonic testing tends to be flat with the increase of sonic path distance in the far-field area.The POD model which introduced can fuse the defect information at different depths.It overcomes the deficiency of original POD curve and reduces the workload of manufacturing POD curve and the number of test plates greatly.The instrument muse be calibrated by inspector before ultrasonic shear wave testing.It is inevitable that there have some errors in the calibration procedure.Especially,the error of the refraction angle and time base will affect the final measurement results in ultrasonic shear wave testing.The mathematical relationships between the depths and sonic path distances of side drill holes were often ignored in the current DAC curve manufacture procedure.Therefor,in this paper,the mathematical relationships were converted into refraction angles and time bases,and used them to fusion values in calibration.The fusion methods were arithmetical measurement fusion and weighted measurement fusion.For correlation of each information,a method of calculating the optimal weights was proposed.The accuracy of the calibration of the refraction angle and time base was improved by calibration values fused.In addition,it has found that the calibration accuracy of the refraction angle decreases with the increase of the refraction angle.Base on studing the data fusions of refraction angle information and time base information in instrument calibration,the applicability and the fusion effect of arithmetical measurement fusion and weighted measurement fusion were analyzed by the actual defectss.The results show that the arithmetical measurement fusion of refraction angles is helpful for all defectss measurement,but the weighted measurement fusion of refraction angles is only helpful for bottom-surface connected defects and embedded defects measurement.Arithmetical measurement fusion of time bases can't improve the accuracy of measurement,but the weighted measurement fusion of time bases can improve the accuracy of top-surface connected defects measureme.By comparing the fusion results of defects of different sizes,it is found that the effect of refraction angle fusion to improve the precision of defects increases with the increase of defect size.For the probe with different refraction angles,there are few difference between the optimal weight of refraction angles fusion,and it could be ignored.But there have some difference between the optimal weight of time bases.This paper verified the normal distribution model and lognormal distribution model in structural integrity assessment by experimental data.The probability distribution parameters of refraction angle were introduced by combination with the study results.The normal distribution models and lognormal distribution models of bottom-surface defects and buried defects are updated,and the estimation formulas of model parameters were given.According to the modified model,a defect height estimation method which based on Bayes' theorem was proposed.This method takes the unfused measurement information as the prior values and the fused measurement information as the likelihood values to estimate the posterior distribution of defect height.After bayesian analysis,the variance of defect heights decreased gratelly.