The United States Air Force has an operational need to reliably detect second-layer cracks around fastener holes in two-layer airframe structures with the fasteners in place. Because access to the second layer is usually not available, the inspection must be performed by placing a probe on the outer surface of the structure and detecting cracks through the first layer. Eddy current methods have been applied to this inspection problem [1–6], and have met with some success; however, much improvement is still needed to achieve the desired sensitivity to cracks and rejection of signals caused by the geometry of the structure under inspection

Beissner, R.

Burkhardt, G. L.

Creek, E. A.

Fisher, J. L.

English

The United States Air Force has an operational need to reliably detect second-layer cracks around fastener holes in two-layer airframe structures with the fasteners in place. Because access to the second layer is usually not available, the inspection must be performed by placing a probe on the outer surface of the structure and detecting cracks through the first layer. Eddy current methods have been applied to this inspection problem [1–6], and have met with some success; however, much improvement is still needed to achieve the desired sensitivity to cracks and rejection of signals caused by the geometry of the structure under inspection.</p

Burkhardt, G.

Creek, E.

Fisher, J.

Digital Repository @ Iowa State University (ISU)

EDDY CURRENT PROBE DESIGN FOR SECOND-LAYER CRACKS UNDER INSTALLED FASTENERS R. E. Beissner, G. L. Burkhardt, E. A. Creek, and J. L. Fisher Southwest Research Institute P.O. Drawer 28510 San Antonio, Texas 78228-0510 INTRODUCTION The United States Air Force has an operational need to reliably detect second-layer cracks around fastener holes in two-layer airframe structures with the fasteners in place. Because access to the second layer is usually not available, the inspection must be performed by placing a probe on the outer surface of the struc-ture and detecting cracks through the first layer. Eddy current methods have been applied to this inspection problem [1-6], and have met with some success; however, much improvement is still needed to achieve the desired sensitivity to cracks and rejection of signals caused by the geometry of the structure under inspection. BACKGROUND A typical two-layer airframe structure (taken from a T-38 lower wing) is shown in Fig. 1. Here, the structure consists of a second-layer rib or spar joined to the first-layer wing skin with a countersunk fastener. Difficulties with eddy current inspection of this type of structure are caused primarily by two factors: first, cracks in the second layer must be detected through the thickness of the first layer. Since the eddy current response decreases with increasing depth, the sensitivity to second-layer cracks is reduced. Second, the geometry of the structure (e.g., the presence of the edge of the second layer) causes signals that can be larger than the crack signal and that can mask the crack signal. In order to improve second-layer crack detection capability, a high-current density must be obtained in the second layer, which is necessary to obtain a suf-ficient flaw response. The region of high-current density also must be confined to an area immediately adjacent to the fastener and away from the second-layer edge to reduce the response from the second-layer geometry. Review of Progress in Quantitative Nondestructive Evaluation, Vol. 12 Edited by D.O. Thompson and D.E. Chimenti, Plenum Press, New York, 1993 1017 Fastener First Layer Noof-- Second Layer Fig. 1. Two-layer fastener configuration for a T-38 wing. MODEL CONFIGURATION A commercially available, three-dimensional, magnetodynamic, finite-element model was applied to design of an eddy current probe for second-layer inspection. The model calculated (1) the eddy current density in the two-layer configuration to determine the best excitation coil configuration and (2) the associated magnetic flux density above the surface of the first layer. Since the sensor output is propor-tional to the flux density, this latter calculation allowed the sensor output to be determined. The information was used to establish the optimum sensor location and orientation. The calculations for the sensor were performed with and without a crack, and with and without a second-layer edge so that the best ratio of flaw signal to edge signal could be determined. Cup and segment configurations of the excitation coil core were considered in the modeling calculations. The geometries of the coilj core combinations are shown in Fig. 2 (only half of the geometry is shown). Fig. 2(a) shows the cup core and a simple structure. The axisymmetric cup core covers an arc of 360 degrees. Fig. 2(b) shows the segment core, which has the same dimensions as the cup core except that the arc length is 45 degrees. The second-layer edge geometry, shown in Fig. 2(b), was used to calculate structure response, and the crack geometry was used to calculate flaw response; calculations were also performed with the same simple geometry shown in Fig. 2(a). Fig. 2(b) also shows sensor locations where the magnetic field density was calculated. In Fig. 2, the top- and bottom-layer thicknesses are 4.0 mm and 2.5 mm, respectively; the maximum crack radial length is 2.1 mm; and the maximum radial length of the cores is 13.6 mm. The structure material is aluminum, and the fastener material is titanium. MODEL RESULTS The current distributions (frequency of 500 Hz) in the structure for both the cup and segment core configurations are shown in Fig. 3. The contours are lines of 1018 (a) (b) Fig. 2. Model geometry for (a) cup core with simple geometric structure and (b) segment core with second-layer edge and crack geometries and sensor locations. 1019 • j (a) (b) Highest Current Density Highest Current Density ,. Fig. 3. Calculated current densities (at 500 Hz) for a (a) cup core and (b) segment core. 1020 constant-current density, and the scales are the same for both cores. The regions of highest and lowest density are labeled, and these are divided by regions of inter-mediate current density. The current density range from the highest to lowest is approximately an order of magnitude. The cup core produces an axisymmetric current distribution, while the current density from the segment core is concen-trated in the region of the outer leg of the core. Although the cup core gives a larger region of high-current density near the surface, the current density from the segment core falls off less with depth. For the cup core, the density fell to the fifth level at the edge of the fastener and bottom of the second layer, while the density for the segment core fell to only slightly past the third level. This suggests that the segment core will provide a better response from cracks in the second layer. To determine the response from a crack and a second-layer edge, the re-sponse from the simple geometry was subtracted from the response with a crack and with a second-layer edge, respectively. This was necessary because the changes caused by the crack and second-layer edge were very small with respect to the signal level from the simple geometry. The primary interest was in maximizing the crack signal with respect to the geometry signal; therefore, these data were used to compute a signal-to-noise ratio, which, in this case, was a ratio of the flaw signal to the second-layer edge signal. Some concern exists that the model may not be accu-rate enough to produce reliable results when working with the small differences obtained between the different responses; the results, therefore, need to be verified and should be considered preliminary. This was especially apparent for the circum-ferential component where the signals were even lower and meaningful results were not obtained. The ratios of crack to second-layer edge response are shown in Figs. 4(a) and 4(b) for both the segment-core and cup-core configurations, respectively. These figures show the ratio vs. radial position of the sensor for both the vertical and radial components of the magnetic field. For the segment core [Fig. 4(a)], the maximum ratio is obtained with the radial component for sensor positions near the center portion of the core (small sensor radial positions). The vertical component is relatively insensitive to sensor position, as the ratio is similar for all positions. For the cup core [Fig. 4(b)], the maximum ratio for the radial component is also obtained near the center portion of the core, but the ratios are somewhat smaller than for the segment core. This suggests that the segment core would provide somewhat better results than the cup core. CONCLUSIONS (1) The 45-degree segment core produces a higher current density in the expected crack location in the second layer than the 360-degree cup core. (2) The signal-to-noise ratio as determined by the ratio of the crack re-sponse to second-layer geometry response was higher for the segment core than for the cup core. (3) The signal-to-noise ratio was higher for the radial component of the magnetic field at the sensor location compared to the vertical com-ponent. For the radial component, the ratio was greater for sensor 1021 8 Q) 7 II) c: o Radial 0 a. 6 • Vertical II) Q) II: <l' 5 CJ) "'0 UJ 4 £ .::.: () 3 ~ () '0 2 a .~ II: 0 0 2 4 6 8 10 12 14 Sensor Radial Position (mm) (a) 5 Q) II) o Rad ial c: 4 0 a. • Vertica l II) Q) II: Q) 3 CJ) -0 UJ £ .::.: 2 () ('G '-() '0 0 iU II: 0 0 2 4 6 8 10 12 14 Sensor Radial Position (mm) (b) Fig. 4. Ratios of crack response to second-layer edge response vs. sensor radial position (0 is center of core) for a (a) segment core and (b) cup core. 1022 positions near the center portion of the core, while the ratio for the vertical component was relatively insensitive to sensor position. (4) The model needs to be verified because of potential inaccuracies from the small differences between the simple structure response and the crack and second-layer edge response. ACKNOWLEDGMENT Support was provided by Wright Laboratory, WLjMLLP, Contract No. F33615-91-C-5661. REFERENCES 1. G. L. Burkhardt, J. L. Fisher, and S. N. Rowland, Southwest Research Insti-tute (SwRI) Final Report 17-7958-850, Contract DLA900-84-C-0910, CUNOOOlBU (SwRI, San Antonio, Texas, 1990). 2. C. M. Teller and G. L. Burkhardt, SwRI Final Report 17-6193, Contract F41608-79-D-AOll-0033, Air Force Wright Aeronautical LaboratoryjMLLP ibid. (1981). 3. D. L. Bell and D. T. Mih, Systems Research Laboratories (SRL) Final Report AFWAL-TR-85-4095, Contract F33615-81-C-5100 (SRL, Dayton, Ohio, 1985). 4. M. Gibbs and J. Campbell, Mat. Eval.12. 51 (1991). 5. D. Hagemaier, B. Bates, and A. Steinberg, Mat. Eval. ~, 518 (1988). 6. D. J. Harrison, paper presented at Review of Progress in Quantitative NDE (1989), not published. 1023 

Eddy current probe design for second-layer cracks under installed fasteners

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1367&amp;context=qnde

Eddy current probe design for second-layer cracks under installed fasteners

Abstract

Similar works

Full text

Available Versions

Digital Repository @ Iowa State University (ISU)