Integrated polar backscatter has been shown to have potential applications to composites, especially for the detection of matrix cracking, delaminations, fiber waviness, fiber fracture, inclusions and porosity [1–11]. The method was attractive because it avoided several measurement limitations inherent to conventional pulse echo techniques. Polar backscatter, however, has not been without its disadvantages. It has been reported that surface texture introduces unwanted artifacts in images made using the polar backscatter method [12]. One suggested method to overcome this limitation was the use of stripable coatings, which are paints that approximately match the impedance of the composite surface and have the effect of physically “smoothing” the surface impressions away [13]. After ultrasonic testing, these paints can be removed, but this method entails additional part handling and increases the cost of production

Bridal, S. Lori

Brush, Edwin F., III

Holland, Mark R.

Madaras, Eric I.

Miller, J. G.

E. I. Madaras

Edwin F. Brush

S. Lori Bridal

Mark R. Holland

J. G. Miller

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Measured Effects of Surface Cloth Impressions on Polar Backscatter and Comparison with a Reflection Grating Model

Integrated polar backscatter has been shown to have potential applications to composites, especially for the detection of matrix cracking, delaminations, fiber waviness, fiber fracture, inclusions and porosity [1–11]. The method was attractive because it avoided several measurement limitations inherent to conventional pulse echo techniques. Polar backscatter, however, has not been without its disadvantages. It has been reported that surface texture introduces unwanted artifacts in images made using the polar backscatter method [12]. One suggested method to overcome this limitation was the use of stripable coatings, which are paints that approximately match the impedance of the composite surface and have the effect of physically “smoothing” the surface impressions away [13]. After ultrasonic testing, these paints can be removed, but this method entails additional part handling and increases the cost of production.</p

Madaras, Eric

Brush, Edwin

Holland, Mark

Miller, J.

Digital Repository @ Iowa State University (ISU)

MEASURED EFFECTS OF SURFACE CLOTH IMPRESSIONS ON POLAR BACKSCATIER AND COMPARISON WITH A REFLECTION GRATING MODEL E.I. Madaras MS 231 NASA Langley Research Center Hampton, VA 23665 Edwin F. Brush, III Dept. of Physics Colorado College Colorado Springs, CO 80903 S. Lori Bridal, Mark R. Holland, and lG. Miller Dept of Physics Washington University St. Louis, MO 63130 INTRODUCTION Integrated polar backscatter has been shown to have potential applications to composites, especially for the detection of matrix cracking, delaminations, fiber waviness, fiber fracture, inclusions and porosity [1-11]. The method was attractive because it avoided several measurement limitations inherent to conventional pulse echo techniques. Polar backscatter, however, has not been without its disadvantages. It has been reported that surface texture introduces unwanted artifacts in images made using the polar backscatter method [12]. One suggested method to overcome this limitation was the use of stripable coatings, which are paints that approximately match the impedance of the composite surface and have the effect of physically "smoothing" the surface impressions away [13]. After ultrasonic testing, these paints can be removed, but this method entails additional part handling and increases the cost of production. The purpose of this paper is to address the nature of a typical composite surface and its effects on scattering. Once an understanding of the source of the signal artifacts is reached, more attractive methods may be developed to overcome the current limitations of the method. Using epoxy typical of that in composites and standard composite fabrication techniques we produced a sample with release cloth impressions on its surface. A simple model for the scattering from the surface impressions of this sample was then constructed and finally, polar backscatter measurements were made on the sample and compared with the predictions of the model. Review of Progress in Quantitative Nondestructive Evaluation, Vol. 12 Edited by D.O. Thompson and D.E. Chimenti. Plenum Press, New York, 1993 1799 TIIEORY In polar backscatter, the sound insonifies the surface at an angle e with respect to the surface normal. On a composite surface which has a fabric impression, the surface proflle will modulate the angle e so that specific locations are nearly perpendicular to the direction of the insonification and reflect the sound back onto the transducer. To model the surface, consider Figure 1 which shows a microscopic image of the surface of the panel used in this study. The surface impressions were formed by a teflon coated fabric (4 hamess weave, 60 threads per inch). Figure 2a shows an acoustic microscope image from the panel in which the flll fibers run horizontally and the warp fibers run vertically. Figure 2b shows the proflle of the surface which corresponds to the horizontal line located in the middle of Figure 2a. This pattern indicates a regularly repeated sequence which can be modeled as a series of planar reflectors as illustrated in Figure 3. This pattem represents a one dimensional reflection grating that will produce interference effects at the measuring transducer. In order to predict the interference effects, the phase relationship of the system of reflectors must be generated. Based on Figure 3, the following equation for the power is derived: In this equation, IE(f)12 is the reflected signal power, f is the frequency, and Eo(f) is the incident signal amplitude. X (R, A, a, b) is a function that depends of the reflection coefficient, R, the total area insonified, A, the reflector width, a, in the x direction and the reflector width, b, in the y direction. (For simplification, we have assumed that the two reflectors indicated in Figure 3 are of equal width, a 1 '" a2 = a.) M is the number of repeated patterns in the y direction within the beam. N is the number of repeated patterns in the x direction within the beam. (The number of fibers in the repeat pattern in both directions is 4 threads.) The phase terms ~, y, and 11 are defined by: and ~ = k P sin( e), y= k g sin(e), TI = k h sin(e), (2a) (2b) (2c) where k is the wave number of the ultrasound in the coupling media (water), p is the distance between reflectors denoted by a 1 in Figure 3, and g is the distance between the reflectors a 1 and a2 in Figure 3. The adjacent rows have a similar pattern, except that they are offset by a distance h from our reference point, x = O. It can be seen from Figure 2a that h = p/4. Eq. 1, in general, will produce narrow peaks in its spectrum that represent the effects of the surface impressions. Two additional improvements to this model can be made by allowing variation of reflector locations from the idealized patterns and accounting for the frequency dependence of the beam width. It should be expected that the locations of the individual threads in the fabric which led to the surface impressions will vary somewhat from the idealized pattern we have defined. By assuming a distribution for the location of the reflection points, we can include the effects of the variability of the thread locations within the beam area. For each reflector in the pattern, a Gaussian-like distribution centered about the ideal reflector location was assumed where the normal variance term, cr, was adjusted for each reflector to be K fX(). The symbol K is a constant variance which is scaled by the square root of the idealized distance of the reflector away from the origin, {XQ. (i.e., the farther from the x = 0 point, the larger the variance.) Other dependencies were investigated but this functional form provided the best fit with the measured data. 1800 Figure 1. A microphotograph of the surface of the sample. The image is 2 mm wide. The other improvement in this model is obtained by the inclusion of a frequency depen-dent beam width effect. Most calibration methods only calibrate the total energy incident on the part and ignore the beam width geometry. Of course, as the ultrasound frequency is increased, the beam width becomes smaller and the insonifying power per unit area will increase. This effect can be accounted for in our model by using a frequency dependent beam area, A, and frequency dependent numbers of reflection points, N and M. We can approximate the beam area as the area of the beam generated by a planar transducer out to the first diffraction minimum, or 1t rx ry' where rx is the beam radius in the x direction, and ry is the beam radius in the y direction. This approximation is used to generate estimates for N andM, N(t) = rx(f) / p (3a) and a) O. lmm I' b) Figure 2. a) An ultrasonic microscope image of the surface of the sample shown in fig. 1. The view shows a 6 mm by 6 mm area of the sample surface. b) The surface profIle along the marker shown in figure 2a. 1801 Reflector widths al Ultrasound Beam Direction Nonnal to e Figure 3. A simple model of the cross-section of the surface of a composite with cloth surface impressions corresponding to fig. lb. M(t) = ry(t) /4b. By including probability distributions for the reflector positions and the effects of a frequency dependent beam area, Eq. 1 can be amended to the following form: !B(fl2 = M2 XB (R, a, b) e-41C2(N~0+3T10+'Yo)x lEo ( £]2 ltrxry J sin2 ( (N + 1) ~o ) + sinh2 ( 2 (N + 1) 1C 2 ~o ) \ x (3b) ) sin2 ( ~o ) + sinh2 ( 2 1C 2 ~o ) J sin2 ( 4 Tlo ) + sinh2 ( 8 1C 2 Tlo )) I 2 ( ) . h2 ( 2 2 )} \ ( ICOS 'Yo + SIn 1C 'Yo . (4) sin2 ( Tlo ) + sinh2 2 1C 2 Tlo ) The function X(R, A, a, b) has been replaced by XB(R, a, b )/It r x r y which allows the frequency dependent beam area to be expressed explicitly. The terms ~, 'Yo, and Tlo, which are the average values of ~, 'Y, and TI, can be estimated by the dimensions of the weave and Eq. 2. Estimates of Nand M can be made by using Eq. 3 and theoretical values for the beam dimensions. The function XB(R, a, b) can be treated as a general constant in the fitting equation. The parameters in this equation can be determined by applying nonlinear fitting routines to the data. EXPERIMENTAL METIIODS Specimen The samples used in this study were fabricated at NASA Langley Research Center. They consisted of two plates that were 6" by 6" by 1/8" made from type 3501-6 epoxy which is commonly used in making composite materials. Both samples underwent handling and cure cycles identical to standard composite manufacturing practices. One was made with release cloth impressions on both sides while a control sample was made with smooth surfaces. Anisotropy Measurements Measurements of the anisotropy of integrated polar backscatter were performed using a broad band 5 MHz center frequency, 0.5" diam., 4" focus, immersible transducer in pulse echo mode (Figure 4). Data were obtained for polar angles (8) of 100, 200, and 300 with the focal region of the transducer placed at the top surface of the sample. The apparent anisotropy produced by the surface features of the epoxy was investigated by varying the azimuthal angle of insonification in one degree increments. (An azimuthal angle <I> = 00 corresponded to the warp direction of the cloth impressions.) At each angle, <1>, data were collected across a 3 by 3 array (1 mm between points) to obtain spatial averaging. The received backscattered rf signal was amplified (Metrotek MR 106), passed through a 51ls 1802 Computer Spectrum Analyzer or Digital Oscilloscope PulserlReceiver System Scan Controller Figure 4. Experimental equipment set up Metrotek MG 701 stepless gate (beginning slightly before the first reflection) and sent into a Hewlett Packard 8557 A analog spectrum analyzer. The power spectrum obtained at each site was normalized by the power spectrum corresponding to the specular echo from a polished stainless steel plate. Finally, the mean (spatially averaged) normalized spectrum at each azimuthal angle, cp, was obtained and frequency averaged across the useful bandwidth (3 to 7 MHz) yielding the integrated backscatter (Figure 5) Spectral Analysis Measurements Spectral analysis measurements were performed using the same geometric configuration as described above, but the azimuthal angle was fixed at cp = 9()0 for all scans. C-scans with the polar angle held constant at 100, 200, and 300 in tum, were taken while stepping through a 53 by 53 point (5.2 by 5.2 cm) grid. Two experimental arrangements were used to collect and process the data. In one configuration the rf signals were generated and amplified by a pulser/receiver (Metrotek MP215 and MR101), recorded using a digital oscilloscope (LeCroy 9400) sampling at 100 Msamples/sec, and stored in a computer for off-line analysis. The power spectrum corresponding to each digitized rf signal was determined and the spectra from all sites were averaged thus providing a mean spectral response ~ -20 ~ ... § -30 ~ () '" ~() ~ co -40 ... ~ -0 0... "0 -50 ~ 8 = 10° 8 = 20° 8 = 30° ... 01) 1:l -60_ 180 .5 -90 0 90 180 Azimuthal Angle Q> [degrees] Figure 5. Plot of the Integrated Polar Backscatter vs. azimuthal angle from an epoxy plate with surface impressions for e = 100,200, and 300. 1803 characterizing the panel. The mean data were normalized by the spectral response of the system using a planar stainless steel reflector placed in the focal zone of the transducer. Spectral response data were also collected using the amplifier/gate/analog spectrum analyzer arrangement as described in detail in the anisotropy measurements section. RESULTS The azimuthal scans illustrate of the severity of the effects of surface impressions. The data from the smooth (control surface) plate were near the noise level of the equipment Figure 5 shows the integrated polar backscatter (3 to 7 MHz) from the plate with surface impressions versus the azimuthal angle for polar angles of loo, 2oo and 3oo. The data exhibit azimuthal variations of 15 to 20 dB integrated backscatter values. Furthermore, in some azimuthal directions the backscatter from the surface would dominate even the high internal scattering from porosity in graphite/epoxy composite as measured in our prior work. In order to compare the theory with the data, we have plotted frequency dependent data generated from the plate with surface impressions with the most accurate Levenberg-Marquardt algorithm fits to Eq. 4 in Figure 6. The general nature of the data and the relatively accurate fit with our theory are apparent from the figures. Eq. 4 was specifically derived considering the cI> = 9oo direction shown in Figure 2; the resulting fit parameters are shown with their anticipated values in Table I. The fitted values for the parameter p are all in the range of 1.6 to 1.7 mm. corresponding to approximately 60 to 62 threads per inch. The fitted values of h are similar in magnitude (0.41 to 0.43 mm) and also correspond to 60 threads per inch. It is predicted from the weave pattern impressions that h would be 1/4 of p. The values of g range from 0.44 to 0.50. It should be noted that the variable g might deviate slightly from the value of h in the example used here because the exact points on Figure 2b where a reflection might occur could shift as the polar angle changes. Finally, the variable scaling parameter, K, has a range of about 0.13 to 0.22, which helps to give a measure of the spatial variability of the threads in the fabric. DISCUSSION It has previously been noted that the surface texture is a source of additional signal that compromises the quantitative results from integrated polar backscatter. The purpose of this work was to demonstrate the origins of those signals. By modeling the surface as a reflection grating, the general quantitative features of the data were produced. In this model, a simplified beam function was assumed and the variability of the thread impression pattern within the beam pattern was assumed. This model corresponds well with actual physical features present on the surface of the composite. Based on this work, several improvements can be readily suggested which will enhance the quantitative nature of integrated polar backscatter measurements. One of the simplest would be to lay up the release cloth in a direction where artifacts from the surface reflections would not dominate the desired polar backscatter measurement directions. This fact coupled with a judicious selection of polar angles could lead to adequate sensitivity in spite of the surface complications. The simple scans shown in Figures 5 and 6 would quickly identify the needed orientations. Another practical method would be to use the following equation (described graphically in Figure 7) to limit the frequency regions used for calculation of the integrated backscatter to those which are not dominated by surface reflections. Methods such as these would be relatively fast and efficient and avoid the need for special preparations of samples after manufacture. 1804 (5) 0.010 0.005 0.0002 '::i' i ~ 0.003 ~ c:: o .= 0.002 ~ ....... 0.001 e = 10° 3 0000000 456 7 8 Frequency [MHz] 456 Frequency [MHz] 9 10 11 7 8 0.000 t...o.c.a.~iZ-~~l!CIIlIC!!:~DIQ.c.c.c.~~~~ 2 3 456 7 8 Frequency [MHz] Figure 6. Plots of the polar backscatter power vs. the frequency. The data are shown as circles. The fit is shown as a solid line. a) e =100. b) e =200. c) e =300 ACKNOWLEDGEMENTS This work is supported by NASA's Office of Aeronautics and Space Technology, in part by grant NSG 1601. a) b) c) 1805 p, g, h, 1(, Table 1. List of Parameter Valnes Used to Fit the Data in Eq. 4. Parameters Anticillated Measured values from: Values 6..=.l.Qo 6=200 a..::..1Q0 the pattern repeat 1.65 rnm 1.6rnm l.7rnm 1.7rnm spacing the distance between > 0.42 rnm 0.50rnm O.44rnm 0.50rnm two adjacent reflectors the distance between 0.42 mm 0.42rnm 0.43 mm 0.41 rnm two warp threads the variability of 0.22,rom 0.16,rom 0.13 ,rom the thread locations I---------------=::::~I_ Contributions due f6 Frequency Figure 7. Useful spectral ranges for eq. 5. to surface effects Contributions due to "bulk" properues Only shaded regions would be used to calculate integTated polar backscauer REFERENCES 1. Y. Bar-Cohen and R. L. Crane, Materials Evaluation, 4Q, pp. 970-975, (1982). 2. Lewis J. Thomas, III, Eric I. Madaras, and J. G. Miller, Proc. IEEE Ultrasonics Symposium, 82 CH 1823-4. pp. 965-970, (1982). 3. D. E. Yuhas, C. L. Vorres, and Ronald A. Roberts, Review of Progress in Quantitative NDE, .5B" Edited by D. O. Thompson and D. E. Chimenti (Plenum Press, New York, 1986), pp. 1275-1284. 4. Earl D. Blodgett, Lewis J. Thomas, III, and J. G. Miller, Review of Progress in Quantitative NDE, 5B, 01'. cit., pp. 1267-1272, (1986). 5. Earl D. Blodgett, S. M. Freeman, and 1. G. Miller, Review of Progress in Quantitative NDE, 5B, 01'. cit., pp. 1227-1238, (1986). 6. S. M. Handley, M. S. Hughes, J. G. Miller, and Eric I. Madaras, Proc. IEEE Ultrasonics Symposium, 87 CH 2492-47, pp. 827-830, (1987). 7. Ronald A. Roberts, Review of Progress in Quantitative NDE, 6B, 01'. cit., pp. 1146-1156, (1987). 8. J. Qu, and J. D. Achenbach, Review of Progress in Quantitative NDE, @, Qll....£ib pp. 1137-1146, (1987). 9. Ronald A. Roberts, Review of Progress in Quantitative NDE, lB., ~,pp. 1053-1062, (1988). 10. J. Qu, and 1. D. Achenbach, Review of Progress in Quantitative NDE, N, Qll....£ib pp. 1029-1036, (1988). 11. T. Ohyoshi, and 1. D. Achenbach, Review of Progress in Quantitative NDE, lB., Ql2.. m" pp. 1045-1052, (1988). 12. S. M. Handley, J. G. Miller, and E. I. Madaras, Review of Progress in Quantitative Nondestructive Evaluation, .8H,,~, pp. 1581-1589, (1989). 13. Y. Bar-Cohen, McDonnell-Douglas Corp., Douglas Paper 7781,1987, (unpublished). 1806 

Measured Effects of Surface Cloth Impressions on Polar Backscatter and Comparison with a  Reflection Grating Model

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Measured Effects of Surface Cloth Impressions on Polar Backscatter and Comparison with a Reflection Grating Model

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