154 International Journal of Applied Ceramic Technolog-Morscher and pujar Vol.6,No.2,2009 section; that is, is shown for comparison. In Fig. 1, the hysteresis loops f o=(Nply N )(epcm/10)(R)/t(1) were removed for clarity, while Fig. 2 shows represen- tative stress-strain curves with the initial loops and the where Noly is the known number of plies in the lay-up attendant residual stress for the different com- (eight for all the tested in this study); N posite specimens. From these figures and Tables I and the nominal number of fibers per tow: epcm/10 is the II, there are some general fiber-related observations that known tow ends per centimeter of the 2D weave(i.e can be made concerning the as-fabricated number of fiber tows per centimeter) converted to mil specimens. First, as expected from composite theory, limeter; R is the nominal fiber radius in millimeter; and increasing the fiber volume fraction increased the com- t is the measured specimen thickness in millimeter. Ta- posite secondary modulus as well as the ultimate ble l lists the calculated values for the total fiber volume. strength and strain. Second, for the higher modulus fi- bers(Er 380 GPa), increasing the fibe er volume fraction Table I are the nominal N and R values for each iber also increased the composite initial elastic modulus type, as well as the specimen t values and lulus for the Mi matrix in the loading direction is lower than that of the fiber. Third, composite specimen Room-Temperature Stress-Strain Bebavior witb AE with the higher modulus fibers showed that the matrix was under a mild compressive stress(ig. 2 and Table The average room-temperature mechanical proper- ID); in contrast, specimens with approximately the same ties from the stress-strain tests are listed in Table I l, and fraction of the lower modulus fiber showed the matrix some representative stress-strain curves are shown in essentially under zero to a very mild tensile residual stress Fig. 1 for individual specimens from each composite Fourth, for approximately the same fiber fraction, the system. In addition, the stress-strain behavior of an lower modulus fibers exhibited higher composite ultimat HNS-2 composite specimen from Morscher and pujar strain,with the HNS panels being an exception Table Il. Composite Room Temperature Mechanical Properties Average A A UTS (MPa) 8(%) on fibers(GPa) 0.005% AE onset Residual [#RT spec] [ specimens] [ specimens] [#RT spec] offset stress stress stress catter (scatter (s catter (scatter) (MPa) (MPa) (MPa) SYLiBN-1 247 3 0.35{3] 1997[2] 194[3 (+0.007/-0.006)(+36/-32)(+0.04/-0.06(+79-143)(+6/-9) SYLiBN-2 271 2 465[2 0.47[2] 18l[2 189[2]-60[2 (±12) ±0.03 +16 +10 SYLiBN-3 238 1 4[ 0.45[ 176[]155[1]-45[1] SA-1254[ 358[1] 0.33[1] 2000[ 152[ 145[1-20[1 SA-2 236[1] 372[1] 0.34[ 2047[ 178[]138[1 15[ SA-3 230[1 334[l 978[ 178[] 135[1 -30[1] HN 244[7 3l1[ 0.79[7 2272[7] 1266]114团6-46 43/-31)(+17/-10)(+0.12/-0.04)(+208/-141)(+4/-5)(+12/-8)(+7/-8) 213[4] 279[3 0.95[3] 973[4] 111(485[4]+12[4] (+5/-3) 9-6(+0.04/-0.03)(+66/-35)(+7/-6(+10/-15)(+5/-9) 0.83[4] 179[4] 12/-6)(+0.02/-0.03)(+49-53)(+5/-4)(+11/-14)(+8/-7) 1*262[ [ 0.63[1] 2278[1 154[1]150 412[1 147[1 135section; that is, fo ¼ ðNplyNfÞðepcm=10ÞðpR2 f Þ=t ð1Þ where Nply is the known number of plies in the lay-up (eight for all the composites tested in this study); Nf is the nominal number of fibers per tow; epcm/10 is the known tow ends per centimeter of the 2D weave (i.e., number of fiber tows per centimeter) converted to mil￾limeter; Rf is the nominal fiber radius in millimeter; and t is the measured specimen thickness in millimeter. Ta￾ble I lists the calculated values for the total fiber volume, f 5 2fo, for all specimens from each panel. Also listed in Table I are the nominal Nf and Rf values for each fiber type, as well as the specimen t values and specimen-to￾specimen scatter in these t values. Room-Temperature Stress–Strain Behavior with AE The average room-temperature mechanical proper￾ties from the stress–strain tests are listed in Table II, and some representative stress–strain curves are shown in Fig. 1 for individual specimens from each composite system. In addition, the stress–strain behavior of an HNS-2 composite specimen from Morscher and Pujar5 is shown for comparison. In Fig. 1, the hysteresis loops were removed for clarity, while Fig. 2 shows represen￾tative stress–strain curves with the initial loops and the attendant residual stress for the different com￾posite specimens. From these figures and Tables I and II, there are some general fiber-related observations that can be made concerning the as-fabricated composite specimens. First, as expected from composite theory,14 increasing the fiber volume fraction increased the com￾posite secondary modulus as well as the ultimate strength and strain. Second, for the higher modulus fi- bers (EfB380 GPa), increasing the fiber volume fraction also increased the composite initial elastic modulus. This is consistent with the hypothesis that the effective modulus for the MI matrix in the loading direction is lower than that of the fiber. Third, composite specimens with the higher modulus fibers showed that the matrix was under a mild compressive stress (Fig. 2 and Table II); in contrast, specimens with approximately the same fraction of the lower modulus fiber showed the matrix essentially under zero to a very mild tensile residual stress. Fourth, for approximately the same fiber fraction, the lower modulus fibers exhibited higher composite ultimate strain, with the HNS panels being an exception. Table II. Composite Room Temperature Mechanical Properties Panel Average E (GPa) [#RT spec] (scatter) Average UTS (MPa) [# specimens] (scatter) Average e (%) [# specimens] (scatter) Average stress on fibers (GPa) [#RT spec] (scatter) 0.005% offset stress (MPa) AE onset stress (MPa) Residual stress (MPa) SYLiBN-1 247 [3] 361 [3] 0.35 [3] 1997 [2] 194 [3] 192 [2] 60 [3] (10.007/0.006) (136/32) (10.04/0.06) (179/143) (16/ 9) 72 77 SYLiBN-2 271 [2] 465 [2] 0.47 [2] 2368 [2] 181 [2] 189 [2] 60 [2] (712) 737 70.03 775 74 716 710 SYLiBN-3 238 [1] 444 [1] 0.45 [1] 2210 [1] 176 [1] 155 [1] 45 [1] SA-1 254 [1] 358 [1] 0.33 [1] 2000 [1] 152 [1] 145 [1] 20 [1] SA-2 236 [1] 372 [1] 0.34 [1] 2047 [1] 178 [1] 138 [1] 15 [1] SA-3 230 [1] 334 [1] 0.30 [1] 1978 [1] 178 [1] 135 [1] 30 [1] HN 244 [7] 311 [7] 0.79 [7] 2272 [7] 126 [6] 114 [6] 4 [6] (143/31) (117/10) (10.12/0.04) (1208/141) (14/5) (112/8) (17/8) Z-1 213 [4] 279 [3] 0.95 [3] 1973 [4] 111 [4] 85 [4] 112 [4] (15/3) (19/ 6) (10.04/0.03) (166/35) (17/6) (110/15) (15/9) Z-2 202 [4] 261 [4] 0.83 [4] 1794 [4] 107 [4] 83 [4] 112 [4] (15/3) (112/6) (10.02/0.03) (149/53) (15/4) (111/14) (18/7) HNS-1 262 [1] 341 [1] 0.63 [1] 2278 [1] 154 [1] 150 20 HNS-2 232 [1] 412 [1] 0.60 [1] 2245 [1] 147 [1] 135 20 DiCarlo et al. 6 154 International Journal of Applied Ceramic Technology—Morscher and Pujar Vol. 6, No. 2, 2009