January 1999 Fiber Efects on Minicomposite Mechanical Properties for Several SiC/Cvl-SiC Matrix System associated with the relief of residual stres fiber did seem to show some pullout(Figs. 6(b)and(c)). The mposite with matrix cracking. The fiber had a thermal ex- Sylramic fibers that did pull out a longer distance(Fig. 6(b)) pansion coefficient of-3. 1 x 10-b/C, according to the product were always the outer-tow fibers, which had a thicker BN literature from the manufacturer for CG on. Assumin ayer. that the matrix has a thermal expansion coefficient of -4 Another observation is the presence of matrix cracks near the 10/C and the minicomposite was processed at-1025C, the fracture surface. For a HN-PBN minicomposite, the nearest anent deformation due to full decoupling of the fiber from matrix cracks to the fracture surface were -300 and 1200 um matrix would result in a permanent strain of-0.1% away from the fracture surface(Figs. 5(a)and(b). For a Syl value, of course, is an overestimate, because complete PBN minicomposite, the matrix-crack spacing is very smal rever. it is evident that more th (<100 um) near the fracture surface( Fig. 6(a)) of the permanent deformation measured for the Nic minicomposite is not from the relief of residual stress in the (3) Determination of the Matrix-Crack Spacing gauge section of the minicomposite. Instead, at least half of the Polished minicomposite longitudinal sections were used anent deformation is associated with excessive damage determine the average crack spacing after failure. Figure 7 and alignment that occurs at the tabs because of the gripping shows examples, at different of the hN-pbn rrangement. This result is also probably the cause of the lar and Syl-PBN minicomposites. The crack spacings near the d decreases that occur in the Nic-3MBN stress-strain curve fracture surface are indicative of those measured at higher stresses(strains)in Fig. 1. Less permanent deforma- length of the minicomposites. The average crack spac tion was observed for the other two systems(Fig. 2) determined from the number of cracks and the length of mini Figure 3 shows typical aE data for the three minicomposite omposite examined, as listed in Table II types, as a function of minicomposite stress and strain. The AE The Nic-3MBN and HN-PBN minicomposites had fairly energy can be attributed to matrix cracking. The first matrix ell-distributed cracks; i.e., the spacing between two indi- cracking stress and strain for the different minicomposites was vidual cracks ranged from 0. 1 mm to -3 mm along the entire determined from the onset of AE activity(Fig. 3)and is tabu- minicomposite length observed. The Syl-PBN minicomposite was entirely different. Most individual crack spacings were are approximately the same for the three minicomposite sys- very small (Fig. 7(b))or very large along the length of the tems. At -0.04% strain, there is a significant deviation fro con osite. There were several regions of the minicomposite linearity on the stress-strain curve for the Nic-3MBN(Fig. 1) where dozens of cracks were spaced <0. 1 mm apart. There and HN-PBN(not shown) minicomposites; however, for the were other regions where cracks were separated by large dis Syl-PBN minicomposites, there is no significant deviation tances, the largest separation being 9 mm. In fact, for the Syl- from linearity until a strain of-0 1% is reached. The aE data PBN minicomposite, -92% of the cracks occurred in regions so implies that the rate of cracking with stress is much greater for Nic-3MBN minicomposites, at least at the onset of crack observed. In other words, there was very little cracking in ing. The HN-PBN and Syl-PBN minicomposites have similar >70%of the sample, which indicates that this system is far AE behavior(cracking trends) with stress, however, the Syl- from matrix-crack saturation the different crack. ng dis- PBN minicomposite has a rapid crack-growth regime for tributions are shown in Fig. 8 for the HN-PBN and Syl-PBN pared to that of the HN-PBN minicompos- minicomposit e. The rate of AE energy for the Nic-3MBN minicomposite decreases at -0 32% strain(-200 MPa stress). The HN-PBN IV. Analysis and Syl-PBN minicomposites do not show a decreasing rate o AE activity. All three minicomposit aturate in matrix cracks prior to the failure load a (1) Determination of the Interfacial Shear Stress fro Hysteresis Loops Scanning electron microscopy(SEM) micrographs of typical Lamon hysteresis loops were analyzed using the approach of (2) Examination of the Fracture Surface aly and v PBN, respectively ) Significant pullout lengths were observed for the Nic-3MBN and HN-3PBN minicomposites, whereas the da Syl-PBN minicomposites only had very short pullout lengths The Nic-3MBN fracture surfaces were always more frag- mented and jagged, in comparison to the other two minicom where de is the hysteresis loop width, o the peak stress of the posite types. It seems that the Sic did not infiltrate as well into hysteresis loop, o the stress where de is measured, and o min the the tow, compared to the other two minicomposite types, which minimum stress of the hysteresis loop. The inelastic strain in- resulted in the appearance of a thicker SiC"sheath"that sur dex, is determined from the relationship rounded the infiltrated tow. This phenomenon made it difficult to determine the pullout length for the Nic-3MBN minicom -a0(8) posite. The pullout-length distributions were determined for the HN-PBN and Syl-PBN minicomposite fracture surfaces. The d=b 4T-TE mean pullout lengths were 240 and 3 um for the HN-PBN and Syl-PBN minicomposites, respectively. Even though the Syl- where R is the average fiber radius, d the average matrix-crack PBN minicomposite fiber pullout is very small, almost ever spacing(number of cracks divided by minicomposite length), f Table Il. Mechanical Properties of the minicom crack Estimated interfacial Ultimate stress shear stress(MPa) Nic/3MBN Hi-Nic/PBN 15±10 -14 0.035 440±20 65-175 0.035 450±20be associated with the relief of residual stresses in the mini￾composite with matrix cracking. The fiber had a thermal ex￾pansion coefficient of ∼3.1 × 10−6/°C, according to the product literature from the manufacturer for CG Nicalon. Assuming that the matrix has a thermal expansion coefficient of ∼4 × 10−6/°C and the minicomposite was processed at ∼1025°C, the permanent deformation due to full decoupling of the fiber from the matrix would result in a permanent strain of ∼0.1%. This value, of course, is an overestimate, because complete decou￾pling does not occur; however, it is evident that more than half of the permanent deformation measured for the Nic-3MBN minicomposite is not from the relief of residual stress in the gauge section of the minicomposite. Instead, at least half of the permanent deformation is associated with excessive damage and alignment that occurs at the tabs because of the gripping arrangement. This result is also probably the cause of the large load decreases that occur in the Nic-3MBN stress–strain curve at higher stresses (strains) in Fig. 1. Less permanent deforma￾tion was observed for the other two systems (Fig. 2). Figure 3 shows typical AE data for the three minicomposite types, as a function of minicomposite stress and strain. The AE energy can be attributed to matrix cracking.11 The first matrix￾cracking stress and strain for the different minicomposites was determined from the onset of AE activity (Fig. 3) and is tabu￾lated in Table II. The first matrix-cracking stresses and strains are approximately the same for the three minicomposite sys￾tems. At ∼0.04% strain, there is a significant deviation from linearity on the stress–strain curve for the Nic-3MBN (Fig. 1) and HN-PBN (not shown) minicomposites; however, for the Syl-PBN minicomposites, there is no significant deviation from linearity until a strain of ∼0.1% is reached. The AE data also implies that the rate of cracking with stress is much greater for Nic-3MBN minicomposites, at least at the onset of crack￾ing. The HN-PBN and Syl-PBN minicomposites have similar AE behavior (cracking trends) with stress; however, the Syl￾PBN minicomposite has a rapid crack-growth regime for strains >0.1%, compared to that of the HN-PBN minicompos￾ite. The rate of AE energy for the Nic-3MBN minicomposite decreases at ∼0.32% strain (∼200 MPa stress). The HN-PBN and Syl-PBN minicomposites do not show a decreasing rate of AE activity. All three minicomposite systems probably do not saturate in matrix cracks prior to the failure load. (2) Examination of the Fracture Surface Scanning electron microscopy (SEM) micrographs of typical minicomposite fracture surfaces are shown in Figs. 4–6 for the three minicomposite types (Nic-3MBN, HN-PBN, and Syl￾PBN, respectively). Significant pullout lengths were observed for the Nic-3MBN and HN-3PBN minicomposites, whereas the Syl-PBN minicomposites only had very short pullout lengths. The Nic-3MBN fracture surfaces were always more frag￾mented and jagged, in comparison to the other two minicom￾posite types. It seems that the SiC did not infiltrate as well into the tow, compared to the other two minicomposite types, which resulted in the appearance of a thicker SiC ‘‘sheath’’ that sur￾rounded the infiltrated tow. This phenomenon made it difficult to determine the pullout length for the Nic-3MBN minicom￾posite. The pullout-length distributions were determined for the HN-PBN and Syl-PBN minicomposite fracture surfaces. The mean pullout lengths were 240 and 3 mm for the HN-PBN and Syl-PBN minicomposites, respectively. Even though the Syl￾PBN minicomposite fiber pullout is very small, almost every fiber did seem to show some pullout (Figs. 6(b) and (c)). The Sylramic fibers that did pull out a longer distance (Fig. 6(b)) were always the outer-tow fibers, which had a thicker BN layer. Another observation is the presence of matrix cracks near the fracture surface. For a HN-PBN minicomposite, the nearest matrix cracks to the fracture surface were ∼300 and 1200 mm away from the fracture surface (Figs. 5(a) and (b)). For a Syl￾PBN minicomposite, the matrix-crack spacing is very small (<100 mm) near the fracture surface (Fig. 6(a)). (3) Determination of the Matrix-Crack Spacing Polished minicomposite longitudinal sections were used to determine the average crack spacing after failure. Figure 7 shows examples, at different magnifications, of the HN-PBN and Syl-PBN minicomposites. The crack spacings near the fracture surface are indicative of those measured along the length of the minicomposites. The average crack spacing was determined from the number of cracks and the length of mini￾composite examined, as listed in Table II. The Nic-3MBN and HN-PBN minicomposites had fairly well-distributed cracks; i.e., the spacing between two indi￾vidual cracks ranged from 0.1 mm to ∼3 mm along the entire minicomposite length observed. The Syl-PBN minicomposite was entirely different. Most individual crack spacings were very small (Fig. 7(b)) or very large along the length of the composite. There were several regions of the minicomposite where dozens of cracks were spaced <0.1 mm apart. There were other regions where cracks were separated by large dis￾tances, the largest separation being 9 mm. In fact, for the Syl￾PBN minicomposite, ∼92% of the cracks occurred in regions where the total length was ∼30% of the minicomposite length observed. In other words, there was very little cracking in >70% of the sample, which indicates that this system is far from matrix-crack saturation. The different crack-spacing dis￾tributions are shown in Fig. 8 for the HN-PBN and Syl-PBN minicomposites. IV. Analysis (1) Determination of the Interfacial Shear Stress from Hysteresis Loops The hysteresis loops were analyzed using the approach of Lamon et al.9 and Vagaggini et al.19 The hysteresis loop width was related to stress for various peak-stress hysteresis loops as follows: d« sp 2 = 2+S s sp − smin sp DS1 − s sp D (1) where d« is the hysteresis loop width, sp the peak stress of the hysteresis loop, s the stress where d« is measured, and smin the minimum stress of the hysteresis loop. The inelastic strain in￾dex, +, is determined from the relationship + = b2 ~1 − a1 f! 2 S Rf d D 4f 2 tEm (2) where Rf is the average fiber radius, d the average matrix-crack spacing (number of cracks divided by minicomposite length), f Table II. Mechanical Properties of the Minicomposites Minicomposite Average crack spacing† (mm) Estimated interfacial shear stress (MPa) First cracking Ultimate stress (MPa) Stress (MPa) Strain (%) Nic/3MBN 0.45 25 ± 10 ∼140 0.035 310 ± 20 Hi-Nic/PBN 0.58 15 ± 10 ∼140 0.035 440 ± 20 Syl/PBN 0.34 65–175 ∼140 0.035 450 ± 20 † At failure. January 1999 Fiber Effects on Minicomposite Mechanical Properties for Several SiC/CVI-SiC Matrix Systems 149