Ultrasonic properties of composites (polymer-fibre glass)

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Bull. Mater. Sci., Vl. 2, N. 7, Octber 1997, pp. 981-99. Printed in India.

1 Bull. Mater. Sci., Vl. 2, N. 7, Octber 1997, pp Printed in India. Ultrasnic prperties f cmpsites (plymer-fibre glass) N A ABD EL-MALAK Natinal Institute fr Standards, Egypt MS received 15 Octber 1996; revised 9 June 1997 Abstract. Majrity f cmpsite materials used in industry cnsist f plyester as a matrix and fibre glass as a reinfrcement material. Five samples with different fibre glass cntents were studied ultrasnically, using pulse ech technique t determine their ultrasnic prperties. Results shwed that the ultrasnic velcity, elastic mduli and activatin energies assciated with their relaxatin prcess increase with the increase f the fibre glass cntent. The study shwed the utility f using these inexpensive materials as sund absrbing devices and as backing materials in the manufacture f ultrasnic transducers. Keywrds. Fibre glass; cmpsite. 1. Intrductin Cmpsites are made, in general, frm tw r mre materials cmbined n a micrscpic scale t frm a material f required prperties. The first material frming the cmpsite is the matrix, which gives the bulk frm f the cmpsite. It enclses the ther cnstituents and essentially prtects them chemically and thermally. The secnd material is the reinfrcement, which plays an imprtant rle in the mechanical prperties and behaviur f the cmpsite. Matrix and reinfrcement are bnded tgether and create a phase called the interphase. This interphase has an imprtant effect n the mechanical prperties f the cmpsite, especially in highly reinfrced cmpsites with large interphase area. The main advantage (Yunis 1995) f cmpsites is that they usually exhibit the best qualities f their cnstituents cmpared with the sum f the prperties f each cnstituent taken separately. There has been a rapid grwth in the use f fibre-reinfrced-materials, which has been achieved mainly by the replacement f traditinal materials, primarily metals. On the basis f strength and stiffness alne, fibre-reinfrced cmpsite materials d nt have a clear advantage, particularly when it is nted that their elngatin t fracture is much lwer than that f a metal with cmparable strength. Hull (1981) shwed that the large nnlinear defrmatin evident in the stress/strain curves f shrt fibre-reinfrced thermplastics is due t defrmatin and creep f the matrix as thermplastics can underg large amunts f plastic flw befre fracture. Sat et al (1988) carried ut electrn micrscpic bservatins n the tensile side f a sample in a bending test. The fibre ends were fund t separate frm the matrix and this was fllwed by the appearance f interfacial cracks. The debnded regins f the neighburing fibres were then jined by bands f micrcracks within the matrix. It was als bserved that plastic defrmatin ccurred arund the fibre ends as a result f shear stress cncentratin. Gupta et al (199) made an attempt t identify the energy absrbing mechanism in a stress/strain test by first btaining the tensile lad-elngatin curves at a lw-strain rate n samples cut in different directins frm an extruded shrt-fibre-reinfrced 981

$The acustic emissin measurements during the tensile test and scanning electrn micrscpy f fracture surfaces were als undertaken t btain an understanding f the energy-absrbing mechanisms.$

2 982 N A Abd El-Malak plyprpylene sheet with gd fibre alignment. The acustic emissin measurements during the tensile test and scanning electrn micrscpy f fracture surfaces were als undertaken t btain an understanding f the energy-absrbing mechanisms. The usefulness f ultrasnics fr detectin and characterizatin f flaws in a material is well knwn. Cnsiderable prgress has been made twards a theretical descriptin f the scattering prcess in plycrystalline materials. Many authrs (Klinman et al 198; Vary 198; Smith and Reynlds 1982), shwed that ultrasnics has prved useful fr the nndestructive evaluatin f materials prperties such as stress, tughness, hardness etc. Hwever, the theretical understanding f ultrasnic prpagatin in such materials is usually based n the scattering prperties f a single inclusin r a vid. It is nt knwn t what cncentratin the single-scatterer apprximatin remains useful and f what frm are the multiple-scattering crrectins required. Kinra et al (1982) investigated the frequency dependence f the ultrasnic velcity in samples with lead inclusins in an epxy matrix. Sayers and Smith (1982) cmpared their experiments with spherical lead inclusins with the predictins f the singlescattering thery f Ying and Truell (1956) and Truell et al (1969). They fund that samples with 5 wt.% cncentratin f inclusin are in reasnable agreement with single-scatterer thery. Hwever, measurements n samples with t 5 wt.% lead shwed nticeable deviatins, which were interpreted as due t multiple-scattering effects. Madhava et al (1983) reprted the results f ultrasnic velcity and attenuatin measurements in randmly riented chpped cir-plyester cmpsites, cntaining up t 3v1.% as received r alkali-treated cir fibres. These measurements were perfrmed with a view t examine, nn-destructively, the bnding between the fibre and the matrix, and als t assess the sund absrptive ptential f these cmpsites. They fund that the lngitudinal ultrasnic velcity decreases with the increase in fibre cntent fr bth as-received and treated fibre cmpsites, but this decrease was clear in cmpsites cntaining as-received fibres than in thse cntaining alkali-treated cir. This behaviur was understd within the framewrk f theries f effective elastic mduli f randmly riented chpped fibre cmpsites. In the case f cir-plyester cmpsites where the elastic mduli f cir are smewhat higher than thse f the plyester matrix, the effective mduli theries indicate an increase in lngitudinal velcity with increase in the fibre cntent. These theries assume ideal cnditins: an unifrm distributin f the randmly-riented fibres in the matrix, absence f vids and a perfect interfacial bnding between the fibres and the matrix. It was fund that the fibre matrix bnding is better in case f cmpsites cntaining alkali-treated cir fibres than in thse cntaining as-received fibres. Fr bth treated and untreated fibre cmpsites, ultrasnic attenuatin increases als with the increasing fibre cntent. Reger et al (1992) determined Yung's and shear mduli f an unidirectinal glass fibre reinfrced epxy cmpsite, using ultrasnics and mechanical tests. A cmparisn between the tw methds shwed n significant difference in Yung's mduli, but a small difference in shear mduli, that can be related t a larger scatter encuntered with the three-pint bend test. The authr als illustrated that the ultrasnic measurements technique is a nn-destructive test that is well adapted fr the determinatin f all the stiffness cefficients f anistrpic materials. The present wrk deals with the characterizatin f ne f the mst versatile plymeric material lcally prduced and used in many glass fibre-reinfrced plyester industries.

3 2. Experimental 2.1 Materials Ultrasnic prperties f cmpsites (plymer-fibre 9lass) 983 Specimens used in this wrk were machined frm plyester-fibre glass sheets, which were prduced and prvided by Egypt Fibre Cmpany. The sheets were fabricated by the manufacturers using hand lay-up methd. Fibre weight fractin was determined prir t the lay-up. A certain amunt f the matrix material was then weighed and the number f fibre glass layers required t prduce the cmpsite material f specific thickness were als weighed; hence the weight fractin culd be calculated. T ensure that the calculated fibre weight fractin was equal t that f the resulting cmpsite, within acceptable limits, a suitable specimen frm the fabricated cmpsite was weighed prir t placing it in a furnace at a temperature 923 K fr abut 3 min. As a result, the plyester was burnt and the fibre glass culd be weighed t determine the fibre weight fractin. Cmpsitin f specimens used in this study were (i) 1wt.% pure plyester, (ii) 72 wt.% plyester, 28 wt.% fibre glass, (iii) 7 wt.% plyester, 3 wt.% fibre glass, (iv) 6wt.% plyester, 4wt.% fibre glass and (v) 4wt.% plyester, 6wt.% fibre glass. 2.2 Density measurements Densities f the cmpsites were measured by the displacement methd using tluene. 2.3 Ultrasnic measurements Attenuatin f lngitudinal ultrasnic waves in the investigated specimens was measured using a cnventinal pulse ech technique. An ultrasnic flaw detectr USM3 prduced by Krautkrgmer was used. This apparatus perates in the frequency range.5-12 MHz and usually ne transducer is used as a transmitter and receiver at the same time. Attenuatin cefficient f lngitudinal ultrasnic waves (~) is calculated frm (Mngy et ai 1982): 2 lg(l 1/l 2 ) 2x where 11 and 12 are the amplitudes displayed n CRO f tw successive reflected eches and x the sample thickness. The attenuatin measurements were carried ut at fur different frequencies: 2, 4, 6, and 8 MHz, in the temperature range K. A special sample hlder fr munting the transducer and the sample was cnstructed at the Natinal Institute f Standards wrkshp and was used fr the purpse. Attenuatin cefficient (~) measurements were taken at intervals f every three degrees. The ultrasnic lngitudinal velcity and shear velcity were measured using delay time between tw successive eches.

4 984 N A Abd EI-Malak The cmplex lngitudinal mdules L* f the material can be derived by the fllwing relatins (Nguyen et al 1995) L* = L' + L", L" tan 6 = -- Z" L'= V~p, L" = ~ 3 -~--f V L P, where L', L" and tan 6 are the real and imaginary parts f the lngitudinal mdulus and the lss tangent VL, a, f and p are the lngitudinal velcity, attenuatin, frequency and density. Cnsidering the effective mdulus f a lngitudinal wave, the elastic mduli can be calculated frm the equatins: V 2 L=p L, G= pv2~, K = L - 4/36, E=(1 +a) x 26, n = (1-2a)E/6(1 + a), where L, G, K and E are lngitudinal, shear, bulk and Yung's mduli, H the micrhardness, Vs the shear velcity and - the Pissn's rati. 3. Results and discussin The results f the relatin between ultrasnic velcities and fibre glass fractin f the cmpsites under investigatin are shwn in figure 1. It is evident frm the figure that the velcity did nt suffer frm dispersin f fibre glass cntent (reinfrcement). Bth V I and V s increase with the increase f fibre glass cntent. Hwever, this increase is mre prnunced in cmpsites cntaining mre than 3 wt.% fibre glass. This behaviur can be understd within the framewrk f theries f effective elastic mduli f randmly-riented cmpsites (Chang and Weng 1979). The effective mduli theries indicate an increase in lngitudinal velcity with increasing fibre cntent. These theries, hwever, assume ideal cnditins such as absence f vids and perfect interfacial bnd between the fibre and the matrix. The bserved V~ and V s variatins indicate that the fibre glass is unifrmly distributed in the matrix. Figure 2 shws the relatin between ultrasnic attenuatin and fibre glass cntent at 2 and 4 MHz frequency. The results f ultrasnic attenuatin shw these samples t have practical value as the results (Sayers and Smith 1982) suggest that the ultrasnic attenuatin is rather less sensitive t multiple scattering effect. The curves f figure 2 shw that the ultrasnic attenuatin (a) increases with the increase f fibre glass cntent and als a-values are smewhat higher fr samples with cmpsitins cntaining mre than 3wt.% fibre glass. This increase in the attenuatin values prves the sund

5 Ultrasnic prperties f cmpsites (plymer-fibre glass) lng.v 8hear V E >,, _.2 2 t-. U t..,e.,, Figure 1. L I I I I I f.g fractin (wt.%) Relatin between U.S. velcity and F.G. fractin f._..-+ C _ ~ Figure 2. I I I I I I 1 2 a f.g fractin (wt.%) Relatin between F.G. fractin and absrptin cefficient. 7 absrptive ptential f these inexpensive materials. These cmpsites can thus be used in the manufacture f ultrasnic transducers where such cmpsites can be used as backing materials.

6 986 N A Abd EI-Malak /* S" ar I... El 8 E 8 D [ ]_ k" i I I I I f.g fractin (wt.%) 7 Figure 3. Relatin between elastic mduli and F.G. fractin..7,6 "'7" ,2.1 L I I I I I f.g fractin (wt.%) Figure 4. Relatin between micrhardness and F.G. fractin. The variatin f the elastic mduli values with the fibre glass cntent is shwn in figure 3. The elastic mduli increases with the increase f fibre glass cntent and the increase in the elastic mduli values is mre prnunced in cmpsitins cntaining mre than 3 wt.% fibre glass. The values f lngitudinal elastic mduli determined ultrasnically were cmpared with thse measured mechanically (Yunis 1995)

7 Ultrasnic prperties f cmpsites (plymer-fibre glass) 987 and gd agreement was fund. Fr example, the values fr specimen having 3 wt.% fibre glass, as determined ultrasnically and mechanically, are 7.17 and 6"63GPa respectively. Fr specimen cntaining 4wt.% fibre glass the values are 1.5 and 9"5 GPa. This indicates that experimental measurements carried ut n the cmpsite samples are very reliable. Figure 4 als shws that the micr hardness increases with the increase f fibre glass cntent, which gives the same trend as abve. Ultrasnic attenuatin fr plyester and different cmpsitins (28, 3, 4 and 6wt.% f.g) was measured at 2, 4, 6 and 8 MHz frequency. Over the whle temperature range, there is nly ne defined peak which shifts its psitin twards higher temperature with increasing frequency (Mngy et a11982). It is bserved that as the frequency increases, bth the attenuatin and the transitin temperature increase. Figure 5 shws the relatin between the ultrasnic attenuatin and temperature fr plyester at 2, 4, 6 and 8 MHz frequency. Figure 6 shws the relatin between ultrasnic attenuatin and temperature fr different cmpsites (28, 3, 4 and 6 wt.% f.g) at 2 MHz frequency, The attenuatin and transitin temperature increases with increasing fibre glass fractin. Figure 7 depicts the variatin f the reciprcal f transitin temperature and lgarithm f the applied frequency fr the samples under study. Frm the slpe f the straight lines representing this relatin, the activatin 1 U [] 2MHz 4MHz 6MHz 8MHz v + D t) 2 25 Figure 5. I I I I T (K) Absrptin cefficient f plyester. I I I

8 988 N A Abd El-Malak 1 " 8- j=a! /!,Yi + 28 wt% f.g * 3 wt% f.g 4 wt% f.g ~* 6 wt% f.g v e e~ < N,- Ot O 6.6 "J ' I ' I ' T ' I ' T(K) Figu~ ~ Absrptin ce~cient ~r cmpsites (with 28, 3~ 4 and 6 wt.% F.G. fractin) ~ frequency 2 MHz, ~ ~ ~ ~ 1% Plyester 28% F.G (- -F ~ 3% F,G C) 4% F,G X 6% F.G 6 I t I I /T x(lo) Figure 7. Relatin between lgfand 1/T(max).

9 Ultrasnic prperties f cmpsites (plymer-fibre glass) 989 Table 1. Sample parameters assciated with transitin and activatin energy. Density Peak Relaxatin Activatin Sample (kg/m 3) temp. (K) frequency (MHz) energy (Jule) Plyester x l ~ x 1 2z 28% f.g x 11~ x 1 2z 3% f.g x x % f.g x x % f.g x x 1 22 energy, Wand the relaxatin frequency f are determined. Table 1 gives the parameters assciated with the transitin in these samples determined frm Arrhenius equatin given as, f = f e- w/k~ where f is the applied frequency, f the relaxatin frequency, W the activatin energy assciated with the relaxatin prcess, K the Bltzmann cnstant and T m the transitin temperature. The results f the experimental investigatin carried ut at high ultrasnic frequencies shw that the attenuatin f ultrasnic waves in cmpsites depends n the temperature and that nly at thse pints where the mde f mlecular mtin is changed des the temperature cefficient f ultrasnic attenuatin change discntinuusly. Thus, the change f the mlecular mtin f the cmpsite can be judged frm the change in the temperature dependence n attenuatin ~ = f(t). This crrespnds t the cnditin under which a given type f mlecular mtin begins t start the relaxatin prcess. The dependence f the transitin temperature n the frequency f the applied ultrasnic field is assciated with the phenmenn f glass transitin which is due t the relaxatin nature f the transitin frm glassy state t rubbery state. Mrever, this phase transitin is f the secnd rder where the internal energy and specific vlume change smthly while the heat capacity (Ov/c~T)v and cmpressibility (Ov/~p) 7, change jump wise (Andrews and Hammack 1965). It is, therefre, suggested that this transitin r relaxatin, near these temperatures, is assciated with thermally activated r rerientatin f a given type f segmental r mlecular mtin. The results shw that the transitin bserved at ultrasnic frequencies invlves the same relaxatin prcesses as the glass transitin. The glass temperatures f plyester was fund t be 268C'K, whereas it increases with the increase f fibre glass t reach 353 K fr 6 wt.% fibre glass. The reprted increase in activatin energy values with increasing fibre glass cntent is in agreement with the results reprted fr elastic mduli. 4. Cnclusin Results shw that increase in activatin energy values with increasing fibre glass fractin is in agreement with results reprted fr elastic mduli. Gd agreement was fund between the lngitudinal elastic mduli determined ultrasnically and thse measured mechanically.

10 99 N A Abd El-Malak Acknwledgement The authr thanks Miss E H H Yunis, Testing f Material Department, Natinal Institute fr Standards, fr the help she ffered thrughut this study. References Andrews R D and Hammack T 1965 J. Plym. Sci. B3 659 Chang D C and Weng G J 1979 J. Mater. Sci Gupta V B, Mittal R K and Malti Gel 199 Cmpsites Sci. & Technl Hull D 1981 An intrductin t cmpsite materials (Lndn: Cambridge University Press) Kinra V K, Ker E and Datta S K 1982 Mech. Res. Cmmun Klinman R, Webster G R, March F J and Stephensn E T 198 Materials Evaluatin p. 26 Madhava M R, Raman S, Pavitran C, Prasad S V and Rhatgi P K 1983 J. Pure Appl. Ultrasn Mngy M, Abd E1-Malak N A and EI-Sabe~ M 1982 Acustica Nguyen N T, Lethiecq M and Gerard J F 1995 Ultrasnics Reger M, Seeleuthner P and Baptiste D 1992 Eurpean cnf. n cmpsite testin 9 and standardizatin (Amsterdam: ECCM-CTS) p. 297 Sat N, Kurauchi T, Sat S and Kamigait O 1988 J. Cmp. Mater Sayers C M and Smith R L 1982 Ultrasnics 2 21 Smith R L and Reynlds W N 1982 J. Mater. Sci Truell R, Elbaum C and Chick B B 1969 Ultrasnic methds in slid state physics (New Yrk: Academic Press) Vary A 198 Research techniques in nndestructive testin 9 (ed.) R S Sarpe (Lndn: Academic Press) Vl. IV Ying C F and Truell R 1956 J. Appl. Phys Yunis E H 1995 Studies n the envirnmental fatiue behaviur and the characterizatin f reinfrced plyester cmpsites, M Sc Thesis, Cair University, Cair

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