The presence of porosity in a wide range of materials, whether ceramics, steel or fiber reinforced composites, has a dramatic effect on the strength and mechanical properties of that material. Therefore, the presence of any porosity in a composite laminate during the manufacture of aerospace components is a basis for component rejection

CF Ying

JE Gubernatis

MD Fuller

Papadakis

PM Gammell

WD Kingery

M. D. Fuller

P. M. Gammell

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Ultrasonic Characterization of Porosity in Composite Materials by Time Delay Spectrometry

The presence of porosity in a wide range of materials, whether ceramics, steel or fiber reinforced composites, has a dramatic effect on the strength and mechanical properties of that material. Therefore, the presence of any porosity in a composite laminate during the manufacture of aerospace components is a basis for component rejection.</p

Fuller, M.

Gammell, P.

Digital Repository @ Iowa State University (ISU)

ULTRASONIC CHARACTERIZATION OF POROSITY IN COMPOSITE MATERIALS BY TIME DELAY SPECTROMETRY IND.ODUCTION M. D. Fuller Martin Marietta Aerospace Advanced Manufacturing Technology Baltimore, MD 21220 P. M. Gammell Naval Surface Weapons Systems Silver Spring, MD 20910 The presence of porosity in a wide range of materiala, whether ceramics, steel or fiber reinforced composites, has a dramatic effect on the strength and mechanical properties of that material. Therefore, the presence of any porosity in a composite !aminate during the manufacture of aerospace componenta is a basis for component rejection. This paper describes research being conducted to evaluate the effects of porosity on an ultrasonic field so that an accurate quantitative assessment of porosity can be made. Porous study specimens with controlled void volume percent, bubble size and distribution have been produced using careful moisture and thermal management techniques. Measurements of ultrasonic attenuation have been made at multiple frequency values utilizing Time Delay Spectrometry (TDS). Existing modela have been used to calculate apparent porosity volume percentages and pore sizes from the ultrasonic data. Suitability of the modela for NDT purposes is discussed. APPROACH To extract porosity values from absolute ultrasonic measurements, it is necessary to define the multiple and complex interaction of ultra-sound with voids, matrix material, and fibers. Since the pore size can easily be on the order of the wave length, scattering becomes the domi-nant mechanism. Exploiting this behavior, ultrasonic attenuation is the measurement of choice and is the focus of this experimental work. While it has been recognized that velocity measurements could also be useful for porosity characterization, this research was restricted in scope to measurements of attenuation. 1157 Attenuation has severa! sources: Reflection factor losses, diffrac-tion losses, hysteresis losses, mechanical damping losses and scattering losses. Clearly, a model was needed to accurately predict these inter-actions. As yet, no model exista to deal with the most complex cases. However, an excellent analytical formulations of attenuation have been suggested for scatterers placed in a homogeneous medium(l, 2, 3, 4). These works suggests that by extracting multiple measurements of attenuation at discrete wavelength values in the Rayleigh regime, a value of the characteristic porosity parameters can be deduced. Utilizing measurements in the Rayleigh region marks the uniqueness of this approach. Estimating the porosity by calculating the slope of the Rayleigh scattering behavior allows the discountance of extraneous sources of attenuation not within control of the experimenter. This includea, for example, reflection factor losses and nonoptimum coupling. The concept is illustrated in Figure 1, where poor coupling, for example, may lead to excessive estimates of porosity content when using the short wavelength approximator. In this paper experimenta are discussed in which neat resins were used as host media for porosity ranging from O to 22 percent in volume. Measurements of attenuation were made at discrete frequencies, and pore diameter and porosity volume calculated. SPECIMEN PREPAIATION Although porosity cannotes a very specific defect, there are, in fact a wide variety of porosity formations which may occur in a material, and which require variations in the approach to ultrasonic evaluation. A· total discussion of porosity classifications and its impact on materiala is contained in Reference 5. The defect of interest in this research is that encountered most commonly in production graphite epoxy laminates. This can be described as a continuous solid phase, isolated pore phase, structure with nearly spherical interlaminar pores, which are consider-ably larger than other system constituents (fibers). I-z w C3 ii: u.. w o CJ-z,g. o ~ :::1 z w .s LONGWAVE ~ LENGTH REGIM-E 1 SHORT WAVELENGTH LIMIT ----. IDEAL EXPERIMENT FREQUENCV f ~aDUETO INEFFICIENT COUPLING 1 Figure 1. Porosity Characterization by Measurements in the Rayleigh Region Eliminates Effects of 1 mperfect Experiments 1158 The porosity samples were manufactured using polyetherimide, a translusent thermoplastic resin supplied by General Electric (Ultem 1000 research grade). Although this work utilizes a thermoplastic resin sys-tem, the results are general and apply to all standard epoxy systems currently in use. It was also desirable to select a material tnat could dissolve and maintain lighter volatiles, and then, by virtue of its thermoplastic properties, allow controlled bubble formation at elevated temperatures. A translucent material was required so that porosity distribution and overall uniformity could be visually confirmed without requiring destructive sectioning. The details of specimen manufacture are interesting and are the subject of reference 6. They will not be discussed further here. DETERMINATION OF POROSITY CHARACTER The independent (nonultrasonic) characterization of porosity was an important consideration. The total porosity content was determined by comparison of the bulk density to the true density of the material. Density measurements were obtained by determining their weight in water in conjunction with the Archimedes method. The true density was determined for porosity free samples in the same manner as porous samples. The average measured value for pore free material was 1.286 g/cm3. Pore size was obtained optically with a calibrated gradicule. Forty measurements were obtained for each sample, averaged and the deviations obtained (accuracy 1/10,000 inch). Table 1 describes the samples and their porosity values and pore size. DATA ACQUISITION AND ANALYSIS The parameter of intrest in these studies is ultrasonic attenuation. The parameter (attenuation coefficient) must be obtained indirectly by the comparison of two ultrasonic amplitudes, each corresponding to a specific time of flight in a material. If a measurement of the difference in sig-nal amplitude A(f) and A0 (f) can be made, then a(f) can be calculated directly. Time Delay spectrometry was the technique used in this work. TDS is a multifrequency analog technique capable of accurate measurements in near real time. It is currently under development at the Naval Surface Weapons Center, Wnite Oak, Maryland.(7) With this technique, only a single pair of transducers is required, and an exceptionally l~rge hand width is obtainable (20 MHz). Time delay spectrometer primarily consists of a swept frequency source and a tracking filter. The tracking filter is comprised of a mixer and a filter which typically has a variable hand width and fixed frequency. Signals arrive at the receiver with a time delay that depends on the pathlength and the propagation velocity through a line of sight. Since the transmitter frequency is swept, the time delay is equivalent to a frequency offset. Now, the analytic signal and the energy spectrum can be obtained easily. The TDS system provides a direct measure of energy vs. frequency without numerical transform technique. The TDS has several advantages over traditional pulsed methods for this work. They are, for example, the very large dynamic range (120 dB), the great increases in noise tolerance, and the signal-to-noise ratio. A through transmission approach was coupled with this technique to calculate attenuation. Waterpath spectra and sample spectra were 1159 Table 1. Nonultrasonic Characterization of Porosity Samples Mean Samp1e Porosity Void Dia. T Fabrication* Designation (%) x (in.) (in.) Designation A 11.350 0.033 0.031 T4 B 0.311 o.ooo o.ooo DE7 c 5.365 0.021 0.005 r2 D 8.243 0.017 0.007 DE3 E 5.000 0.025 0.023 DES F 0.000 o.ooo o.ooo SD1 G 0.000 0.000 0.000 SD2 H 0.311 -- --- SD3 1 0.3!1 0.009 0.002 SD4 J o. 777 0.022 0.008 SD5 K 1.400 0.015 0.006 SD6 L 0.622 0.013 0.003 SD7 M 3.810 0.011 0.003 SD8 N 0.000 0.000 0.000 SD9 o 22.110 0.026 0.021 DE2 p 0.311 0.005 0.002 SD10 Q 3.344 0.015 0.013 DE6 R 18.420 0.017 0.007 DE9 s 8.130 0.012 0.005 UNKNOWN T 3.610 0.029 0.004 NTl u 6.610 0.016 0.004 NT2 V 3.110 0.017 -- T1 w 7.510 0.027 --- T3 *Fabrication Designation: DE - Extended dry series SD - Standard dry 168 hours series T - Time variation at nuc1eation temp. series NT - Nuc1eation in temperature variation series Width (cm) 1.30 1.08 1.30 1.30 1.24 0.30 0.49 1.29 1.21 1.30 1.30 1.20 1.35 1.28 1.25 1.28 1.31 1.44 ---------1.51 1.26 obtained, and the suitab1e deconvo1utions performed. Use of logar-ithmic amp1ifiers and displays simplified this calculation greatly. Data were acquired using 2.25 and 10 MHz transducers for each of 23 samp1es. First data were taken through a sample-free water path. The amplitude vs. frequency data were disp1ayed in a logarithmic fashion referenced to 1 mw terminated into 50 ohms. Next, samples of varying thickness and porosity level were placed between the transducers and data acquired. Subtraction of the sample spectra from the water path spectra yields insertion loss, which is total attenuation in addition to reflection losses at the surface. The attenuation coefficient for each porous sample was calculated by correcting for surface losses. In addition to sample attenuation, the attenuation attributed solely to the presence of porosity was calculated from these experimental data, and is the variable of interest. CALCULATION OF POROSITY PARAMETERS FROM DATA To extract actual values of porţ şize and volume fraction, the treatment of Gubernatis and Domany l8J was used. This treatment allows the enrichment of the original semiempirical model to accommodate 1160 pore size and the volume fraction of pores. The attenuation can be expressed in terms of scattering only as 3 4 a(f) • 31rc(~) 13 f (8.o8). (l) where B is a constant which depends strictl.y on the moduli of the material, C is the volume fraction and D is the pore diameter. 8.68 i s a constant conversion factor which allows a (f) to be expressed in dB instead of nepers. a(f)•7. 42 c D 4 as ţ3 f 4 (~) converges to a constant value. (2) It was possible to make several estimations of pore size and volume fraction using these enhanced expressions. An ultrasonic estimate of pore size was possible by noting the frequency of the characteristic bend in attenuation behavior. The frequency of this transition can be theoretically calculated for the general case of gas filled pores.(9) The transition frequency is located at the maximum reduced cross section or KD • 1.45 for this material where k is the wave number. Pore size cin be deduced from an individual experiment by noting the frequency of the transition point and solving the above equation for D. The volume fraction of porosity can now be calculated from either Equation 1 or Equation 2. Pore size has been calculated using this technique for all suitable test samples. In considering the result all ultrasonic estimates are well within the deviation of the optically determined diameter data. For the most part, pore sizes are overestimated as would be expected. Errors in size estimstes can be attributed to severa! sources, primarily the distri-bution of actual pore diameters. Large dispersion in actual pore size tends to make the transition point at ~ • 1.45 less distinct and there-2 fore quite difficult to nonarbitrarily assign a frequency value. Figures 2(A) and (B) illustrate the performance of the model in the IDEAL 20 ~ " ~ z i 12 .. ti o i a: ~ :> I'ERFORMANCE , SHORT WAVELENGTH \ / ....... .-APPROXIMATION ,ERFOAMANCE BASED /..~_. 0N VISUAL O /'' ,; •• • ,.,. .. -~-;-fi .... ·········· ··· 1 ...... ·' ~..:/ .... r·· S"ix ~4/ . , .. .. ". .,, • .- Sy1x • 2.81 ,:-/ y (A) V isual Determination of O 4 8 12 10 20 DENSITY DE'TEA.MINATION OF VOLUME% Figure 2. Performance of Short Wavelength Approximation 1 1 61 short wavelength regime, assuming that volume fractions obtained by density measurement are accurate. The ultrasonic estimates are generally higher with a deviation of 2.7 percent for optically based calculations and 3.2 fot ultrasonically based calculation. The performance in fact is probably much better. By restricting interest to volume fractions less than 6 percent, deviations less then 2 percent can be realized. In addition, much error can be associated with the inability to accurately determine the true volume fraction by density measurement. This also holda for long wavelength estimates. The source of the error is as follows: First some samples arc not uniformly porous; instead, a large portion of the sample is virgin resin surrounding a portion containing porosity from which data is acquired. Initial calculations indicate that some values may change by as much as 50 percent. Selective trimming will provide a better estimate of per-formance. Second, variations in the density of the virgin material itself restrict the accuracy of density derived volume fractions. Lastly, absorbed moisture has been shown to change overall weight of the sample thereby casting an additional uncertainty on the measured volume fraction. Alternate methods of volume fraction determination will help clear the performance picture. Figures 3(A) and (B) display the results of estimating the volume fraction of the samples in the Rayleigh regime. Since the attenuation varies greatly in this region (equation 1) it is possible to obtain a better estimate of the volume fraction than in the short wavelength limit. This has been done by obtaining the slope of the attenuation behavior in the Rayleigh region. An estimate of the volume fraction was obtained by using the following expression. c .. D(al az) 5.13(r1 -r2 5 (3) where r represents the reduced cross s ec t ion of a pore and is given by is in this frequency range. IDEAl~- _, ••• j'l' N - x~ PE.Af'ORMANCE OF LONG WAVELENGTH ... •:,.. :;~~~:~~~ : BA.SED ON VISUAL .... .-··; ~ • ~ ~ 16 .,.,..-···:// •• /········" ~~ .. .... --·...-· /.// k:·· ..····  .. · ~~ 12 •• •• ~~ ..... .-··· •••. ·•••· EST FIT u ....  -· ··-:.--·:: >< .. ······ '( ~ . / .. ··· " s.y,ll . 3.1:1 :li "" ••• •• .(..:'.~ . ~·~··'·/ (A) Visual Determination . of Void Diameter 12 " DENSITV DEH RMINATION OF VOID FRACTION N 20 (4) PfRFORMAHCE OF lONG WAVELIENOTH /.~· ' ~~~~=~~~~: BASED ON Ul TAASONIC ,ţ-·' • IDEAL~ ···f/' .. ··'/ .,.. .......... ··•· .. .... :.. : ·..-:~:\~::~(= . ,,.., .  ·• .-·;·•·' / : ••• ••••• BIEST FIT ,.········· "~-~-········· /~····' (8) Ultrasonic Determination ;}~/ of Void Diameter 12 18 OENSITY OEl"ERMINA TION OF VOI O FRACTION 20 Figure 3. Long Wavelength Approximation Performance Based on Visual and Ultrasonic Void Diameters 1162 This method is desirable over the short wavelength approximation as all physical conditiona at the time of the measurement may not be at the disposal of the analyst. As can be seen, the performance is enhanced when considering porosity values less than 6 percent. A deviation of 1.6 percent can be obtained which is well within the confidence limits estimated on the density determined volume fraction. The same sample errors exist, as already discussed. SUMMARY In summary, ultrasonic quantification of porosity by ultrasonic means has been shown possible. In some instances, the ultrasonic estimates are probably more accurate than those obtained by density determination. Restricting interest to less than 6 percent improves performance and is reasonable considering the application is to NDE of composites. Best results were obtained by estimating porosity from the Rayleigh scattering with an ultrasonically determined pore diameter. Large variations from the mean in pore diameter complicate the accurate sizing of pores and requires a more sophisticated treatment than used here. Too, the technique is quite sensitive to the void diameter, con-sidering the absolute size of the pore, factors of 2 in the attenuation can result from a few thousandths change in void diameter. REFERENCES 1. Papadakis, "Ultrasonic Attenuation Due to Grain Scattering in Polycrystalline Metals", Journal of the Acoustical Scoiety of America, 37, 711 (1965). 2. S. Hirsekern, "Scattering in Ultrasonic Waves in Single Phase Polycrystalline Metals Including Multiple Scattering, New Procedure in Nondestructive Testing", 265-285 (1982). 3. Thompson, et al., "NDE Technique for Detection and Characterization of Porosity", New Procedures in Nondestructive Testing, (1982) 287-303. 4. Richardson, Elsley, "Extraction of Low-frequency Properties from Scattering Measurements", Proceedings of the 1979 IEEE Ultrasonic Symposium, CH 1482, 336-341 (September 1979). 5. w. D. Kingery, H. K. Bowen, P. R. Uhlmann, "Introduction to Ceramics", (John Wiley and Sons, New York, 1976) 516-532. 6. M. D. Fuller, K. A. Barrett, R. S. Kiwak, "Production of Controlled Porosity Specimens for Ultrasonic Evaluation of Composite Struc-tures", Review of Progress in Quantitative Nondestructive Evaluation. Edited by D. O. Thompson and D. E. Chimenti, (P1enum Press, New York, 1985). 7. P. M. Gammell, "Coherent Processing of the Full Ana1ytic Signal Information of U1trasonic Wave Forms", International Advances in Nondestructive Testing (1984). 8. J. E. Gubernatis and E. Domany, Review of Progress in Quntitative Nondestructive Eva1uation, 2A. Edited by D. O. Thompson and D. E. Chimenti. (P1enum Press, New York, 1983) 833. 9. C. F. Ying and R. Truell, J. App1ied Physics li• (1956) 1086. 1163 

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Ultrasonic Characterization of Porosity in Composite Materials by Time Delay Spectrometry

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