Research On Defect Quantification And Image Quality Improvement Of TOFD Detection For Carbon Steel Pipes

Time of Flight Diffraction(TOFD)is a non-destructive method for quantifying defectsusing the time difference of the diffraction angle of the defects.It has the advantages of high quantitative accuracy,high defect detection rate,and the detection result is not affected by defect orientation and echo amplitude.It is used in the detection of carbon steel pipelines in the petrochemical,power and natural gas sectors.The TOFD test results use the head wave signal as the quantitative basis for defects.The pipe surface causes the straight-through wave propagation path to not coincide with the surface of the workpiece,and there is a problem that the defect quantitative accuracy is low,the near-surface blind zone is large,and the image detection resolution is insufficient.Based on the diffraction longitudinal wave signal,considering the geometrical relationship between the radius of curvature of the pipe,the probe center spacing(PCS)and the upper and lower end points of the defect,the length and angle of the defect outside the blind zone of the pipeline are accurately quantified.It is larger than the acoustic propagation characteristics of the diffracted longitudinal wave,and is applied to the TOFD detection of the pipeline to realize the quantitative detection of cracks in the blind area near the pipeline.To improve the lateral resolution of the image on the basis of reducing the blind zone,Synthetic Aperture Focusing is adopted.Technology,SAFT)Reconstruct B-scan mode to convert wave images.TOFD testing is carried out for carbon steel pipes with an outer diameter of 148.0 mm and a wall thickness of 30.0 mm.The main research contents are as follows:1.Based on the TOFD detection of the longitudinal wave quantitative formula,considering the geometric relationship between the radius of curvature of the pipe,PCS and the depth of the upper and lower ends of the crack,the quantitative formula of the crack length and angle optimization in the blind zone is given.The simulation results show that the buried crack with a depth of 8.2 mm outside the blind zone,a length of 4.0 mm and an angle of 10°-50°,the optimized crack length error is reduced from 0.34 mm to 0.24 mm,and the angular quantitative error is reduced from 5.87° to 4.57°.The experimental results show that the artificial grooving with a depth of 8.2 mm outside the blind zone,a length of 4.0 mm and an angle of 30° is optimized.The quantitative error of the artificial grooving length is reduced from 0.30 mm to 0.27 mm,and the angular quantitative error is reduced from 2.74° to 0.28°.2.Based on the acoustic propagation characteristics of the mode-converted wave propagating sound larger than the diffractive longitudinal wave,combined with the relationship between the curvature radius of the pipe and the upper and lower end points of the crack,the formula of the crack length and angle in the blind zone of the pipe is given.The simulation results show that the depth and length of the crack in the blind zone are 3.0 mm,and the upper and lower end points of the crack are overlapped with the straight-through wave.The mode conversion wave is used to obtain the crack length quantitative error less than 0.27 mm.The error does not exceed 1.57°.The experimental results show that the artificial grooving with a depth and length of 3.0 mm and an angle of 30° in the blind zone is 0.14 mm and the angular quantitative error is 1.44°.3.The feature that minimizes the quantitative error at the top of the arc of the image using the mode-converted wave,using this point as the time reference for the A-scan signal.The SAFT technique is used to focus the mode-converted wave signal point by point,which weakens the xenon arc phenomenon.The lateral resolution of the image can be improved on the basis of reducing the near-surface dead zone.The simulation results show that the B-scan image of the open-surface groove with a depth of 3.0-6.0 mm in the blind zone is weakened by SAFT reconstruction,and the quantitative error of the end-opening groove depth is not more than 0.11 mm.The experimental results show that after SAFT treatment,the xenon arc phenomenon is suppressed.For the 3.0 mm bottom open groove in the blind zone,the depth quantitative result is 2.86 mm with an error of 0.14 mm.