-A PWASE VELOCITY AND ATTENUATION OF PLANE ELASTIC HAVES i/i IN A PARTICLE-REINF (U) COLORADO UNIV AT BOULDER DEPT OF M ECHANICAL ENGINEERING

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1 -A PWASE VELOCITY AND ATTENUATION OF PLANE ELASTIC HAVES i/i IN A PARTICLE-REINF (U) COLORADO UNIV AT BOULDER DEPT OF M ECHANICAL ENGINEERING S K DATTA ET AL AUG 87 7UCASFE UE-72NW76K88 / 14 M

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3 OTlO FILE Cut Contract N K WAVES PHASE VELOCITY AND ATTENUATION OF PLANE ELASTIC IN A PARTICLE-REINFORCED COMPOSITE MEDIUM S.K. Datta University of Colorado, CIRES * H.M. Ledbetter National Bureau of Standards - Y. Shindo Tohoku University A.H. Shah University of Manitoba CUMER 87-3 August, 1987 r DTIC Ip ELECTEII %- DISTRBUfTION SJ T'T.'EYrl Approved tot public xel el- -. ; p')

4 PHASE VELOCITY AND ATTENUATION OF PLANE ELASTIC WAVES IN A PARTICLE-REINFORCED COMPOSITE MEDIUM S.K. Datta Department of Mechanical Engineering and CIRES, University of Colorado, Boulder, Colorado 80309, U.S.A. H.M. Ledbetter Fracture and Deformation Division, Institute for Materials Science and Engineering, NBS, Boulder, Colorado 80303, U.S.A. Y. Shindo Department of Mechanical Engineering, Tohoku University, Sendai 980, Japan A.H. Shah Department of Civil Engineering, University of Manitoba, Winnipeg R3T2N2, Canada Received, Revised. 6ur study considere4 effective-plan -wave propagation, both longitudinal and shear, through a medium containing a random distribution of spherical inclusions. We assumed that t e particles and matrix are separated by a thin layer \of elastic sate ial with different properties. For some systems, we pre&i measurable ects for the thin layers. Especially, we considered /epoxy and ij'. 1. Introduction Wave propagation through a particle-reinforced composite medium has been studied by many authors [1-13]. Except 15], all these studies assume that the inclusions bond perfectly with the surrounding matrix material. In [5], for long wavelengths, the authors consider the effect of a thin viscous third layer. Recently, Sayers (11] examined the effect of this layer when the particles and the matrix possess the same properties. In the present study, we analyze the problem of wave propagation in a composite medium with a random distribution of spherical inclusions. The inclusions are separated from the matrix by thin layers of elastic material. The properties of the layers vary through the thickness such that there is a continuous transition from the inclusions to the matrix. rhe object of this study was to explore the practicality of using ultrasound to characterize properties of interface layers. Ultrasound is a practical tool for measuring properties of, and characterizing the state of, r ts. -. ", - -." -

5 2 a material with microstructure (or changes in microstructure). to such studies occur in i References 2. Scattering by a spherical inclusion with an interface layer Consider a spherical inclusion of radius a and elastic properties A,, pl, P, embedded in an elastic matrix of material properties A 2, A2, P2. Also, let the inclusion be separated from the matrix by a thin layer of uniform thickness h(<<a) with variable material properties A(r), p(r), and p(r). Here, A, p denote Lam& constants and p density. Let A(r), p(r) be expressed as A(r) + 2p(r) - (A, + 2p,)f(r), a < r < a + h, (1) p(r) - p,g(r), a < r < a + h. (2) Here, f(r) and g(r) denote general functions of r. A special case arises when f(a) - 1, g(a) - 1, f(a + h) - A2 + 2 p,, g(a + h) -, (3) A, + 2p 1 with the stipulation that f(r) and g~r) with their first derivatives are continuous in (a, a + h). Since h is assumed to be much smaller than a, it follows from (3) that f'(a) and g'(a) can be approximated by f'(a) - ( A, + 2A2) - (01 + 2,u,) h(al + 2p) (4) E El g'(a) - I - Jul Another special case arises when the interface material possesses 7.. constant properties. Then we have, ipo

6 3 f(r) - (Al + 2Ml)/(XI + 2 p,), g(r) - l/i'. (5) Here Al, pu are the Lam6 constants for the interface material. We also make the assumption that h is much smaller than the wavelength of the propagating wave. Then, to first order in h/a, A being the wavelength, t s i t S i rr rr rr' Tre re Ire t s i (6) Here rij is the stress tensor and superscripts t, s, and i denote the transmitted, scattered, and incident field quantities, respectively. Note that rij,,]j, and rij appearing above are calculated at r - a. The spherical polar coordinates r, 0, 9 are defined in Fig. 1. Boundary conditions (6) express the fact that, to first order in h/a, the traction components do not suffer any jump across the layer. However, the displacement components suffer jumps given by us + U i _ t hk, t (7) r r r A, + 2p, rr' s i t h t u + u O - ut - t (9) S0 Here, KI rf1 dx K f dx (0 I- f(a + hx)' K2 0 g(a + hx)" Using equations (3) and (4) in (10), we find that K 1 and K 2 are given approximately by -

7 4 A + 2 p, n 1 + A + 2p, - (At 2+ ) A 2 + 2A 2 - (A, + 2 p,) A, + 2j 1 K2 - Al ln [I + - ] (12) PA2 - JAI On the other hand, if eq. (5) is used, then K, - (A, + 2p,)/(Aj + 2pj), K 2 - A,/A!. (13) Mal and Bose [5] studied a problem similar to the one considered here. They assumed a thin viscous third layer between the sphere and the matrix and imposed the condition of radial-displacement continuity. We assumed the incident wave to be either a plane longitudinal wave propagating in the positive z-direction or a plane shear wave polarized in the x-direction and propagating in the positive z-direction. Thus, ui iklze + eik2ze. (14) -e -Z -x Here, k, - w/c, and k 2 - W/c 2. w denotes the circular frequency of the wave and c,, c 2 denote the longitudinal and shear wave speeds in the matrix. The factor e - iwt was suppressed. ul given above can be expanded in spherical vector wave functions as Suik 1 in(2n+1) L() - n-- on ' I 2n + 1 in LM(1)(6 + n(n +1) 6 2i n- lm- n(n + 1) mn ml m,-l (1)(6 - n(n + 1) 6 (15) k(1l)

8 5 Vector wave functions L M41 and N~ 1 appearing above are regular at r - 0 and are gi-en by L-(1n a~ 2- (k 1 r) Pm (cse + 0i kr) 1L P (case) + ne Jn (k 1 r) Pn (cose)] e Mm~~i - j (k 2 r) P tm (cose) - e in (k 2 r) 8L P 3 (cose)] 30o 1 rnr+) n(k 2 r) P tm (cose) + e - (rj k 2 r))x a P: (cose) + rin a- (rj (k 2 r)) P~es)]!iin* (16) The scattered and transmitted fields can be written!! f [: A (') 8 + B M()+ C N()(7 - rnil~ ano mn MoI mn -mn mn -mn '(7!! I L (A' Ll' 6 + B' M() + C' N i.(18 - n-0 rn-i mn Xmn mo mn -En mn-n Here, the prime denotes that k, and k 2 are to be replaced by kj (- W/cj) and k; (- w/cj), respectively. cl and c; are the wave speeds in the inclusion.. L - and N are obtained by replacing j nby n~ in (16). Note that in is the spherical Bessel function of the first kind, and h nis the spherical Hankel function of the first kind. The constants A, B, C, A', B', C' are found by using conditions (7) and (8)-(10). For this purpose, we define the following matrices: AA

9 6 [2: ::Sj. (20) Here, Fn(kia) - nhn(kia) - klahn~lck~a), Gn(k 2 a) - n(n + 1) hn(k 2 a), Hn(k~a) hnk-) In(k 2 a) - (n + 1) hn(k 2 a) - k 2 ahn~l(k 2 a), SFn(k~a) - Wn - n - % k~a 2 ) hn(k 1 a) + 2kjahn+l(k 1 a), SGn(k 2 a) - n(n+l) [(n - 1) hn(k 2 a) - k 2 abn+l(k 2 a)]. SHn(kja) - (n -1) hn(k 1 a) - klahn+l(k~a), SIn(k 2 a) - (n %kja 2 ) hn(k 2 a) + k 2 ahn~l(k 2 a). Equations to determine Amn and Bmn are found to be r2h A m x M+ Ma' L'-lL C lii 2 02 n {n t(inn) a - x + M' n (21) re(mn) Here 0 1] 2 K'

10 7 MA, LA are obtained from Mn, Ln, respectively, by replacing hn and hn+ 1 by in and in+l, respectively, in (19) and (20), and by replacing k, and k 2 by ki and k;, respectively. In writing (21) we express U i and 4 given by (15) as ui u i P m (cose)ei a r n-0 -i r(mn) n ui + mpm (22) e n0 ue (n) ae e ()( sine It then follows that {.rr(mn)} A -.T i I- -Lu L -M r(mn) O~ I' (23) re(mn) n n {:mn)} Ln and Mn are obtained from L n and Mn respectively, by replacing hn and hn+ 1 by Jn and in+4, respectively. The equation to find Bmn is [ h ((n- 1) hn(k 2 a) - k 2 ahn+l(k 2 a)) - h(k a) + (n - 1) hn(k 2 a) - k 2 ahn+l(k 2 a)( n2in ( n-i) j (k2a) - kajn(k2)] BJn2).: (n 1) n~ka ka n+1a j u21 -$ a h22 + (n-i 1,,(k;a)-1 21 e(mn) j - aj l(ka)j ma) rb(mn) Here, 2i p (n - 1) Jn(ka) - ka,,+l (kja) 2i re(mn) a Jn(k 2 a) Ue(mn) (25) S.-.,.... " " ; " " - "-". S - S ' - ", - -".. ". ".. " - -. ' ".. - '.'....'.' S'.

11 8 Once Amn, Cmn, and Bmn are determined by solving (21) and (24), the scattered field is then found from (17). Since the expressions for the field inside the inclusion will not be needed to derive the dispersion equation governing the effective wavenumber of plane-wave propagation through the composite medium, we omit these. 3. Distribution of inclusions In [5, 14] the scattered-field expressions were used to calculate effective wave speeds at long wavelengths in a medium with a distribution of spherical inclusions with interface layers. A 'quasicrystalline' approximation together with the assumption of no correlation was used to derive expressions for the effective wave speeds. As has been shown [12], particular forms of two-particle correlations can be included in the formalism. But this leads to complicated equations that require numerical solutions. In this study, we adopt the approach taken in [10, 11] to calculate approximate phase velocities and attenuation of plane-longitudinal and plane-shear waves. In this simple approximation, the effective wavenumber k is related to the forward scattered amplitude by the equation [15]: K 2 - k2 + 4*nf(K) (26) Here, k o is the wavenumber in the absence of scatterers, n the number * density of scatterers, and f the averaged forward-scattered amplitude. Equation (26) is an implicit equation for the determination of the (complex) wavenumber K. A further simplification occurs when the solution to (26) is taken as K 2 - k + 41nof(ko). (27) Equation (27) was derived by Foldy [16] and has been used by many authors to calculate the frequency dependence of phase velocity and attenuation of -4. 0i plane waves. Equations (26) and (27) are valid for low volume concentrations of inclusions.

12 9 Using (17) in (26), we find that for longitudinal waves the effective wave number is i ' - + 4k t - (-i)n Aon(K 1 K 2 ). (28) For shear waves we obtain + 4 n (_i)n hn(n+l) (C + k 2-1 k3 B n-i I-k-' B l1 (29) 2 in- n(n+l) n- 2 l( In the following section we present phase velocity and attenuation % calculated from the above two equations. Note that the real part of K/k gives the velocity ratio c/c, where c is the velocity in the matrix and C is the effective velocity in the composite. The attenuation of power is obtained from the equation -2 Im K kk (30) For dilute concentration this equation reduces to - no E. (31) Here, Z denotes scattering cross section. 4. Numerical results and discussion Computations were made for two particular composite materials: leadepoxy and SiC-Al. For both materials, we consider two interface thicknesses: zero and 0.1a, through which the properties vary linearly from the inclusion to the matrix. Numerical results for the lead-epoxy composite based on a simplified eq. V (27) were presented in [10]. Figures 2 and 3 show the attenuation and n

13 10 phase velocity of a longitudinal wave when the interface thickness equals zero. These results agree with those given in [10]. Also shown in these figures are the results for h/a It is seen that this thickness has little effect. At intermediate frequencies, the interface lowers the phase velocity and slightly increases the attenuation. Figures 4 and 5 show the results for the shear wave; the effect is slightly larger at moderate frequencies. We also computed the phase velocity and attenuation using eq. (26). Figure 6 shows that eq. (27) overestimates the attenuation. For phase velocity, however, as shown in Fig. 7, eq. (26) overestimates it at low frequencies, underestimates it at moderate frequencies, and gives nearly the same results at high frequencies. Finally, in Figures 8-11 we show results for the second example: * SiC-Al. These results are based on eq. (27) and show that the interface :N1 decreases both the attenuation and the phase velocity. 5. Conclusions We considered the effect of thin interface layers between the inclusions and the matrix in modifying the dynamic properties of composite materials. Dynamic effective properties were calculated by using Foldy's equations. We found that interface effects are larger in some composites. We also studied the predictions based on iterative solutions of modified Foldy's equations in which the scattering amplitudes were calculated assuming that the matrix had the properties of the composite. This iterative solution underestimates attenuation in general; at low frequencies it overestimates phase velocity. 6. Acknowledgment This study was supported in part by a grant from the Office of Naval Research (N K-0280; Program Manager: Dr. Y. Rajapakse). Partial support was also provided by National Science and Engineering Research Council of Canada (Grant *A-7988) and by the Office of Nondestructive Evaluation, NBS. This work was completed when S.K.D. held a Faculty Fellowship award from the University of Colorado and a Fulbright Research Award at the Technical University of Vienna. _% VA

14 11 References (1] P.C. Waterman and R. Truell, "Multiple scattering of elastic waves", L. Math. Phys. 2, (1961). [2] N. Yamakawa, "Scattering and attenuation of elastic waves", Geophys, Mag. (Tokyol 31, (1962). [3] J.G. Fikioris and P.C. Waterman, "Multiple Scattering of Waves, II.", J. Math. Phys. 1, (1964). [4] A.K. Mal and L. Knopoff, "Elastic wave velocities in two component systems", J. Inst. Math. Apl. 3, (1967). [5] A.K. Mal and S.K. Bose, "Dynamic elastic moduli of a suspension of imperfectly bonded spheres", Proc. Camb. Phil. Soc (1974). [6] G.T. Kuster and N. Toks6z, "Velocity and attenuation of seismic waves in two-phase media: Part I. Theoretical calculations", Geophys. 39, (1974). [7] S.K. Datta, "A self-consistent approach to multiple scattering of elastic waves", J. AxDl. Mech, 44, (1977). [8] J.G. Berryman, "Long-wavelength propagation in composite elastic media I. Spherical inclusions", J. Acoust. Soc. Am 68, (1980). [9] A.J. Devaney and H. Levine, "Effective elastic parameters of random composites", Apgl. Phys. Lett, 2Z, (1980). [10] C.M. Sayers and R.L. Smith, "Ultrasonic velocity and attenuation in an epoxy matrix containing lead inclusions", J. Phys. D: Appl. Phvs. 16, (1983). [11] C.M. Sayers, "Scattering of ultrasound by minority phases in * polycrystalline metals", Wave Motion 7, (1985). [12] V.K. Varadan, Y. Ma, and V.V. Varavan, "A multiple scattering theory for elastic wave propagation in discrete random medium", J. Acoust. Soc. Am. 77, (1985). [13] H.M. Ledbetter and S.K. Datta, "Effective wave speeds in an SiCparticle-reinforced Al composite", J. Acoust. Soc, Am. 29, (1986). [14] S.K. Datta and H.M. Ledbetter, "Effect of interface properties on wave propagation in a medium with inclusions", in: A.P.S. Selvadurai and G.Z. Voyiadjis, eds., Mechanics of Material Interfaces, Elsevier, Amsterdam (1986)

15 12 [15] J.E. Gubernatis and E. Domany, "Effects of microstructure on the speed % and attenuation of elastic waves in porous materials", Wave Motion 6, (1984). [16] L.L. Foldy, "Multiple scattering theory of waves", Phys. Rev. 67, (1945). 1', 4.." :'S 4 4.

16 13 Table 1. Properties of constituents Density (g/cm 3 ) E(GPa) 1A(GPa) Lead Epoxy sic Al

17 14 List of Figures Fig. 1. Fig. 2. Fig. 3. Fig. 4. Geometry of a spherical inclusion with an interface layer. Attenuation of longitudinal waves in a lead-epoxy composite with and without interface layer. Phase velocity of longitudinal waves in a lead-epoxy composite with and without interface layer. Attenuation of shear waves in a lead-epoxy composite with and without interface layer. Fig. 5. Phase velocity of shear waves in a lead-epoxy composite with and *, without interface layer. * Fig. 6. Comparison of attenuation coefficients for longitudinal waves in a lead-epoxy composite predicted by an iterative solution of eq. (26) and by eq. (27). Fig. 7. Fig. 8. Fig. 9. Fig. 10. Fig. 11. Comparison of phase velocities of longitudinal waves in a leadepoxy composite predicted by an iterative solution of eq. (26) and by eq. (27). Attenuation of longitudinal waves in an Sic-Al composite with and without interface layer. Phase velocity of longitudinal waves in an SiC-Al composite with and without interface layer. Attenuation of shear waves in an SiC/Al composite with and without interface layer. Phase velocity of shear waves in an SiC/Al composite with and without interface layer. 4 W A- - '

18 N. M.~p. 0. V V E S. '-

19 100 I Lead-Epoxy --- h/a = h/a w-0.1 'S k 2 a

20 Vft. Lead.Epoxy --- h/a =0.0 h/a=o.1 Z o k 2 a

21 100 II 10 Lead-Epoxy 6--3-h/a = h/a a k 2 a

22 1.0- CMJ S0.9 0 ~0.8 Lead-Epoxy IO -- h/a = o; h/a = L O ud2 k a NhtI, AJbAUA IcN

23 Lead-Epoxy 00 h/a = 0.0 _ c = k 2 a. N,, *S * *

24 "- i iterative J v, :. CC Lead-Epoxy h/a a 0.0 "0.8 c =0.15 Sayers, Smith 0.6 (1983) 0.5 I I I I I k 2 a.p. %" %

25 SIC-Alumifnum S1.04-ha k 2 a W. *90

26 SiC-Aluminum h/a = h/a = * k 2 a

27 1.15 I CJ SiC-Aluminum% --- h/a =0.0 hla= * ~1.001II k 2 a

28 I _ I I _ -.40 CM,ii SiC-Aluminum_ h/a m CF4 h/a = 0.1 * k 2 a., I" A. 4A.... "- ",;, " A, W.' A"E " ' - l ".

29 1*' K Aif L. * F9t :3 2 _ * * V *6 ft *~ 0-0 S S S S S 0 0 :5 0' -~. N%~~N~ ~ *-~. ~,.~- *.*~ -~..- *~*y**. ~ -~v-~: -~ 9~4