N. Eswara Prasad et al. Engineering Fracture Mechanics 71(2004)2589-2605 Table l Plane strain fracture toughness (KIc, MPa ym) data of the 2D silica-silica CFCC material f(a/w) Po and p (a) Crack divider orientation 123 9.76 7.23 3.12and0.32 234and246 2.15and2.26 993 4.39and0.44 205and206 2.43aand24 994 5.60and0.56 .25 89 and 101 1.63a and 1.84 7.26 6.35and0.65 4.63 52 and 58 1.36andl52 (b) Crack arrester orientation 7.74 7.53 58and66.5 0.78and0.89 23456 7.59 8.95 06and0.403 2.15 58 and 60 0.84and0.87 9.1 53 and 60 0.98aand1.10 976 4.84and0. 3.5 33 and 0.72and0.86 4.9and0.64 37 and 49 theo Decimen width, in mm; B-specimen thickness, in mm; L-specimen span length(fixed value of 40 mm); Pg--conditional load for nset of fracture, in N; Pmar-maximum load in the load-displacement curve, in N; Ko-conditional fracture toughness, in MPa vm; Kmar-maximum stress intensity factor, in MPa vm. Valid Klc. crack lengths, as discussed in the previous section, the fracture mode gradually changes to predominant shear, involving mode II (in-plane shear or sliding) fracture components. This is true for both test orien tations 3.3. Elastic-plastic fracture toughness (JIc) The procedure suggested by Landes and Begley [23] and the ASTM standard E-813 [24] provide details of the latest standard practice for the determination of elastic-plastic fracture toughness, Jic. Alternatively, another widely accepted methodology, again suggested by Landes and Begley [22], can also be employed for Jie determination. Both these methodologies are based on Rice proposed J-integral [21]. The later proce- dure principally involves the determination of fracture energy o) from the energy absorbed in the fracture process(Eini, area under the load-displacement, usually the displacement considered here corresponds to the load line) by specimens with different crack lengths, up to either a constant displacement or a constant load. In the present case, the load-displacement data given in Figs. 2 and 3 are used to calculate the energy for the crack initiation(Eini), which event is assumed to occur at the displacements corresponding to the peak load. Eini values thus determined are used to calculate the fracture energy, Jo as [21, 22 o=△Emn/B(△a), where(AEini)is the difference in the fracture energies(corresponding to peak loads in the load displacement curves)of two specimens with different initial crack lengths(their difference is Aa). The values of Jo, determined from the load-displacement curves in crack divider and crack arrester orientations, are shown as a function of crack length in Fig. 5. As to be expected, the material shows constant values of o in the crack divider(1.36 kJ/m")and crack arrester(0.66 kJ/m")orientations. The scatter in o values is higher for crack arrester orientation as compared to the crack divider orientation. However, all these values were found to satisfy the validity conditions and hence, can be termed as elastic-plastic fracture toughness, JIc of the CFCc As stated above, the lc corresponds to the peak load (assumed to correspond to the crack initiation) and ence would encompass only those fracture events that occur in the CFCC material before or up to thecrack lengths, as discussed in the previous section, the fracture mode gradually changes to predominant shear, involving mode II (in-plane shear or sliding) fracture components.This is true for both test orien￾tations. 3.3. Elastic–plastic fracture toughness (JIc) The procedure suggested by Landes and Begley [23] and the ASTM standard E-813 [24] provide details of the latest standard practice for the determination of elastic–plastic fracture toughness, JIc.Alternatively, another widely accepted methodology, again suggested by Landes and Begley [22], can also be employed for JIc determination.Both these methodologies are based on Rice proposed J-integral [21].The later proce￾dure principally involves the determination of fracture energy (JQ) from the energy absorbed in the fracture process (Eini, area under the load–displacement, usually the displacement considered here corresponds to the load line) by specimens with different crack lengths, up to either a constant displacement or a constant load.In the present case, the load–displacement data given in Figs.2 and 3 are used to calculate the energy for the crack initiation (Eini), which event is assumed to occur at the displacements corresponding to the peak load. Eini values thus determined are used to calculate the fracture energy, JQ as [21,22]: JQ ¼ DEini=BðDaÞ; ð1Þ where (DEini) is the difference in the fracture energies (corresponding to peak loads in the load displacement curves) of two specimens with different initial crack lengths (their difference is Da).The values of JQ, determined from the load–displacement curves in crack divider and crack arrester orientations, are shown as a function of crack length in Fig.5.As to be expected, the material shows constant values of JQ in the crack divider (1.36 kJ/m2) and crack arrester (0.66 kJ/m2) orientations.The scatter in JQ values is higher for crack arrester orientation as compared to the crack divider orientation.However, all these values were found to satisfy the validity conditions and hence, can be termed as elastic–plastic fracture toughness, JIc of the CFCC. As stated above, the JIc corresponds to the peak load (assumed to correspond to the crack initiation) and hence would encompass only those fracture events that occur in the CFCC material before or up to the Table 1 Plane strain fracture toughness (KIc, MPa pm) data of the 2D silica–silica CFCC material Specimen no. WB a and a=W f ða=W Þ PQ and Pmax KQ and Kmax (a) Crack divider orientation 1 9.76 7.23 3.12 and 0.32 1.6 234 and 246 2.15 and 2.26 2 9.93 7.54 4.39 and 0.44 2.22 205 and 206 2.43a and 2.43 3 9.94 7.17 5.60 and 0.56 3.25 89 and 101 1.63a and 1.84 4 9.76 7.26 6.35 and 0.65 4.63 52 and 58 1.36 and 1.52 (b) Crack arrester orientation 1 7.76 7.60 2.52 and 0.325 1.62 70 and 74 0.86 and 0.90 2 7.74 7.53 2.7 and 0.35 1.73 58 and 66.5 0.78 and 0.89 3 7.59 8.95 3.06 and 0.403 2.15 58 and 60 0.84 and 0.87 4 7.68 9.13 4.0 and 0.52 2.84 53 and 60 0.98a and 1.10 5 7.61 9.76 4.84 and 0.58 3.5 33 and 40 0.72 and 0.86 6 7.67 9.89 4.9 and 0.64 4.43 37 and 49 0.99 and 1.3 W ––specimen width, in mm; B––specimen thickness, in mm; L––specimen span length (fixed value of 40 mm); PQ––conditional load for the onset of fracture, in N; Pmax––maximum load in the load–displacement curve, in N; KQ––conditional fracture toughness, in MPa pm; Kmax––maximum stress intensity factor, in MPa pm. a Valid KIc. N. 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