G N. Morscher Composites Science and Technology 64(2004)1311-1319 0.12 01· onset stra typically had lower elastic modulus values, M220+ we 0.1 Onset stress 口·g044+200 20 MPa, compared to the 250+ 30 MPa measured for ID composites. Note that the two composites with the higher stress-distributions for matrix crack ing were OD(044)and MD(Ol1) specimens (2)The matrix crack density varied from 5 to 12 cracks/ mm and did not appear to show any correspondence with the fiber volume fraction or interfacial shear strength(Table 1) 0.120.140.16 (3)Many of the composites did not fully saturate with matrix cracks. Specifically, those composites with Fig. 6. Eonset and onset versus fraction of fibers in the loading direction lower fiber volume fractions where the rate of ae activity remained high until failure(e. g, 007, 012, activity to the abscissa for strain and stress, respectively 016, and 018 in Fig. 5). Matrix crack saturation oc- The initial low energy ae prior to the AE energy in curs when the rate of cumulative Ae energy dimin crease corresponds to the formation of microcracks or ishes to near zero(slope of Fig. 2(b) plateaus at tunnel cracks that form in the 90 bundles but do not higher stress). This is evident in Fig. 5 for 00 penetrate, at least not very much, into the load-bearing 011, 068, 044, and probably 017 fibers. Eonset and onset was determined for all the speci- men data in Fig. 5. In general, Eonset and onset increase 3. 2. Normalizing matrix cracking behavior for standard with fiber volume fraction in the loading direction single-tow woren composites (Fig. 6). However, composites with the same fo but higher E have lower Eonset. Two examples are shown in It is evident that there is some relationship between Fig. 6 for specimens with a fo=0. 2(068 and 044)and fiber volume fraction and stress-distribution for matrix specimens with a fo=0.14(012 and 018). For both cracking and it would be useful to characterize matrix examples, onset was very similar but Eonset was less for cracking based on the constituent properties of the the higher E material. The stress-dependent crack den- composites and the matrix in particular. Matrix cracks sity distribution in general follows the same trend as originate within the 90 tow-minicomposites or large, Conset and occurs over a higher stress range for higher unreinforced matrix regions and then propagate fiber volume fraction through the load-bearing minicomposites [7, 8, 20). In Even though the composite panels were fabricated by other words, matrix cracks do not originate for these the same vendor over a relatively short period of time, l materials in the load-bearing fiber, interphase, CVI SIC year, there were several anomalies observed for the tow-minicomposite. If the volume fraction and elastic composites tested in this study modulus of an average""minicomposite"can be deter (1) Two types of interfacial debonding and sliding be- mined from the processing data, then the average stress haviors were present in the data set for this study in the matrix region excluding the bn and CVI SiC in and noted in Table 1. Most of the specimens exhib- the load-bearing minicomposite could be backed out ited debonding between the fiber and the BN-inter- from a rule-of-mixtures approach and a relationshi phase, this was referred to as""inside debonding between matrix cracking and matrix stress can be (ID). Two specimens exhibited debonding between established the BN-interphase and the Cvi SiC portion of the The fraction of load-bearing minicomposites, mini matrix, this was referred to"outside debonding was estimated from half of the combined fraction of fi (OD). Also, two specimens exhibited a mixture of in- ber, BN, and CVI SiC determined from the processing side and outside debonding, i. e, "mixed debonding" data sheet supplied by the composite fabricator. The (MD). This was pointed out in[14] and has been de- elastic modulus of the minicomposites, Emini, was esti scribed in greater detail in a recent paper [19]. od mated via the rule-of-mixtures from the elastic moduli of composites have much lower interfacial shear stres- each constituent of the minicomposite (Er=380 GPa, ses, 10 MPa, compared to the 70+10 MPa mea- EBN=60 GPa, and ECVLSiC 425 GPa)and the frac- sured for ID composites. Also, OD composites tion of each constituent in the loading direction. Again, a rule-of-mixtures approach can be used to"back-out the stress in the"minimatrix'"surrounding the load-bearing The interfacial shear stress. t [12, 19]:(1)fiber"push-in, a direct measure of t, and (2)best fitting the stress-strain curve for t using Eqs. (3H5)(below) and assuming the matrix crack distribution from AE energy and final matrix crack The raw data from the manufacturer is base density, an indirect measure. Both methods produced consistent each processing step. The constituent fractions determined from results processing data are tabulated in Table I for each panelactivity to the abscissa for strain and stress, respectively. The initial low energy AE prior to the AE energy in￾crease corresponds to the formation of microcracks or tunnel cracks that form in the 90
bundles but do not penetrate, at least not very much, into the load-bearing fibers. eonset and ronset was determined for all the speci￾men data in Fig. 5. In general, eonset and ronset increase with fiber volume fraction in the loading direction (Fig. 6). However, composites with the same f0 but higher E have lower eonset. Two examples are shown in Fig. 6 for specimens with a f0 ¼ 0:2 (068 and 044) and specimens with a f0 ¼ 0:14 (012 and 018). For both examples, ronset was very similar but eonset was less for the higher E material. The stress-dependent crack den￾sity distribution in general follows the same trend as ronset and occurs over a higher stress range for higher fiber volume fraction composites. Even though the composite panels were fabricated by the same vendor over a relatively short period of time,
1 year, there were several anomalies observed for the composites tested in this study: (1) Two types of interfacial debonding and sliding be￾haviors were present in the data set for this study and noted in Table 1. Most of the specimens exhib￾ited debonding between the fiber and the BN-inter￾phase, this was referred to as ‘‘inside debonding’’ (ID). Two specimens exhibited debonding between the BN-interphase and the CVI SiC portion of the matrix, this was referred to ‘‘outside debonding’’ (OD). Also, two specimens exhibited a mixture of in￾side and outside debonding, i.e., ‘‘mixed debonding’’ (MD). This was pointed out in [14] and has been de￾scribed in greater detail in a recent paper [19]. OD composites have much lower interfacial shear stres￾ses,
10 MPa, compared to the 70 10 MPa mea￾sured for ID composites. 3 Also, OD composites typically had lower elastic modulus values,
220 20 MPa, compared to the 250 30 MPa measured for ID composites. Note that the two composites with the higher stress-distributions for matrix crack￾ing were OD (044) and MD (011) specimens. (2) The matrix crack density varied from 5 to 12 cracks/ mm and did not appear to show any correspondence with the fiber volume fraction or interfacial shear strength (Table 1). (3) Many of the composites did not fully saturate with matrix cracks. Specifically, those composites with lower fiber volume fractions where the rate of AE activity remained high until failure (e.g., 007, 012, 016, and 018 in Fig. 5). Matrix crack saturation oc￾curs when the rate of cumulative AE energy dimin￾ishes to near zero (slope of Fig. 2(b) plateaus at higher stress). This is evident in Fig. 5 for 009, 011, 068, 044, and probably 017. 3.2. Normalizing matrix cracking behavior for standard single-tow woven composites It is evident that there is some relationship between fiber volume fraction and stress-distribution for matrix cracking and it would be useful to characterize matrix cracking based on the constituent properties of the composites and the matrix in particular. Matrix cracks originate within the 90
tow-minicomposites or large, unreinforced matrix regions and then propagate through the load-bearing minicomposites [7,8,20]. In other words, matrix cracks do not originate for these materials in the load-bearing fiber, interphase, CVI SiC ‘‘tow-minicomposite’’. If the volume fraction and elastic modulus of an average ‘‘minicomposite’’ can be deter￾mined from the processing data, then the average stress in the matrix region excluding the BN and CVI SiC in the load-bearing minicomposite could be backed out from a rule-of-mixtures approach and a relationship between matrix cracking and matrix stress can be established. The fraction of load-bearing minicomposites, fmini, was estimated from half of the combined fraction of fi- ber, BN, and CVI SiC determined from the processing data sheet supplied by the composite fabricator. 4 The elastic modulus of the minicomposites, Emini, was esti￾mated via the rule-of-mixtures from the elastic moduli of each constituent of the minicomposite (Ef ¼ 380 GPa, EBN ¼ 60 GPa, and ECVI–SiC ¼ 425 GPa) and the frac￾tion of each constituent in the loading direction. Again, a rule-of-mixtures approach can be used to ‘‘back-out’’ the stress in the ‘‘minimatrix’’ surrounding the load-bearing Fig. 6. eonset and ronset versus fraction of fibers in the loading direction. 3 The interfacial shear stress, s, was determined by two methods in [12,19]: (1) fiber ‘‘push-in’’, a direct measure of s, and (2) best fitting the stress–strain curve for s using Eqs. (3)–(5) (below) and assuming the matrix crack distribution from AE energy and final matrix crack density, an indirect measure. Both methods produced consistent results. 4 The raw data from the manufacturer is based on weight gains after each processing step. The constituent fractions determined from the processing data are tabulated in Table 1 for each panel. G.N. Morscher / Composites Science and Technology 64 (2004) 1311–1319 1315