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Direct ultrasonic visualization of fatigue cracks
Direct ultrasonic visualization of fatigue cracks Y. Bar-Cohen, S. Kovnovish, and E. Harnik The advantages of direct ultrasonic visualization over conventional pulse-echo methods in detecting and characterizing fatigue cracks is demonstrated. It is shown that visualization is especially advantageous when the plane of the crack is not perpendicular to the ultrasound beam. Angles of inclination of about 20° can be clearly discerned by visualization whereas the pulse-echo pattern becomes difficult to interpret. There is considerable interest in new non-destructive testing (ndt) methods for the detection of fatigue cracks in metals (see for example References 1 and 2) and the determination of their location, size and orientation. This information is necessary for the assessment of the serviceability of tested parts, the design of a proper corrective process, the followup of crack propagation process etc. Conventional pulseecho methods provide only limited information, while ultrasonic visualization instruments employing various scans are complex and expensive. In this paper the application of a recently developed direct ultrasonic visualization (duv) technique a to the detection and characterization of fatigue cracks in metals is described and discussed.
Experimental procedure and results A notched aluminium 7075 T6 specimen was treated by cyclic loading until a crack of about 5 mm in length was produced. One of the faces parallel to the plane of the crack was cut at an angle of •20 ° to the latter and lapped. The test piece thus obtained (Fig. 1) was tested via both the parallel and the inclined face by the conventional pulse-echo method using a Krautkramer USIP 11 pulser/receiver and a Krautkramer H5K wideband probe. In addition, photographs of duv images on a tv screen, viewed via the two faces, were taken.
Fig. 1
View of the specimen
The authors are with the Racah Institute of Physics, Hebrew University, Jerusalem, Israel.
NDT INTERNATIONAL. APRIL 1 9 8 0
Fig. 2 Visualization image (above) and echo pattern (below) with the ultrasonic beam perpendicular to the plane of the crack
0308-9126/80/020063-02 $02.00 © 1980 IPC BusinessPress
63
The results are shown in Figs. 2 and 3, in which the echo patterns obtained are compared with the corresponding duv images. When viewed via the parallel face F~ (Fig. 2), a sharp, well defined echo is obtained from the crack, implying a parallel plane defect. The corresponding duv photograph shows clearly the image of the crack (A) in continuation of the image of the notch (C)and parallel to the image o f thee F 1. The end face of the notch (B) is not seen because its angle of inclination is too large. The upper image is a permanent feature of the apparatus resulting from a repeated reflection from the ultrasonic transducer. With the viewing plane inclined (at %20 °) to the crack (Fig. 3), the echo looses its sharpness and definition ; it broadens and splits into a number of overlapping components. It becomes difficult to reconstruct its source. The duv picture, on the other hand, gives a clear description of the crack (A), its locatim~. and orientation with respect to the end face of ~he m~tch (B) and the viewing lace F 2.
Discussion
The results presented show clearly the advantages of the visualization method. When the crack is perpendicular to the ultrasound beam, duv offers more clarity and definition than the pulse-echo method. The advantages of duv are, however, obvious and outstanding when the crack is inclined to the beam. Little can be learned from the pulse-echo pattern on the defect, whereas, with visualization, the location, size and orientation of the crack can be readily determined from the image on the tv screen since the object-t~image magnification ratio is known,
References
Fig. 3
Visualization image (above) and echo pattern (below) with to the crack
the beam at an angle of %20 °
64
Hagemaier, D.J. and Bowles, D. 'Effect of stress on crack detection by radiography' Materials Evahtarion 37 No 9 pp 33-37 (1979) Joshi, N.R. 'Precrack damage and crack propagation study with ultrasonic attenuation' Materials Evaluation 37No 10 pp 57-61 (1979) Bar-Cohen, Y., Ben-Joseph, B. and Haxnik, E. "Compact sensitive instrument for direct ultrasonic visualization of defects' Rev Sci lnstrum 49 pp 1709-1711 (1978)
NDT INTERNATIONAL.
A P R I L 1980
Experimental procedure and results A notched aluminium 7075 T6 specimen was treated by cyclic loading until a crack of about 5 mm in length was produced. One of the faces parallel to the plane of the crack was cut at an angle of •20 ° to the latter and lapped. The test piece thus obtained (Fig. 1) was tested via both the parallel and the inclined face by the conventional pulse-echo method using a Krautkramer USIP 11 pulser/receiver and a Krautkramer H5K wideband probe. In addition, photographs of duv images on a tv screen, viewed via the two faces, were taken.
Fig. 1
View of the specimen
The authors are with the Racah Institute of Physics, Hebrew University, Jerusalem, Israel.
NDT INTERNATIONAL. APRIL 1 9 8 0
Fig. 2 Visualization image (above) and echo pattern (below) with the ultrasonic beam perpendicular to the plane of the crack
0308-9126/80/020063-02 $02.00 © 1980 IPC BusinessPress
63
The results are shown in Figs. 2 and 3, in which the echo patterns obtained are compared with the corresponding duv images. When viewed via the parallel face F~ (Fig. 2), a sharp, well defined echo is obtained from the crack, implying a parallel plane defect. The corresponding duv photograph shows clearly the image of the crack (A) in continuation of the image of the notch (C)and parallel to the image o f thee F 1. The end face of the notch (B) is not seen because its angle of inclination is too large. The upper image is a permanent feature of the apparatus resulting from a repeated reflection from the ultrasonic transducer. With the viewing plane inclined (at %20 °) to the crack (Fig. 3), the echo looses its sharpness and definition ; it broadens and splits into a number of overlapping components. It becomes difficult to reconstruct its source. The duv picture, on the other hand, gives a clear description of the crack (A), its locatim~. and orientation with respect to the end face of ~he m~tch (B) and the viewing lace F 2.
Discussion
The results presented show clearly the advantages of the visualization method. When the crack is perpendicular to the ultrasound beam, duv offers more clarity and definition than the pulse-echo method. The advantages of duv are, however, obvious and outstanding when the crack is inclined to the beam. Little can be learned from the pulse-echo pattern on the defect, whereas, with visualization, the location, size and orientation of the crack can be readily determined from the image on the tv screen since the object-t~image magnification ratio is known,
References
Fig. 3
Visualization image (above) and echo pattern (below) with to the crack
the beam at an angle of %20 °
64
Hagemaier, D.J. and Bowles, D. 'Effect of stress on crack detection by radiography' Materials Evahtarion 37 No 9 pp 33-37 (1979) Joshi, N.R. 'Precrack damage and crack propagation study with ultrasonic attenuation' Materials Evaluation 37No 10 pp 57-61 (1979) Bar-Cohen, Y., Ben-Joseph, B. and Haxnik, E. "Compact sensitive instrument for direct ultrasonic visualization of defects' Rev Sci lnstrum 49 pp 1709-1711 (1978)
NDT INTERNATIONAL.
A P R I L 1980