Testing A strain-controlled loading frame having a capacity of 100 KN was used for the load application with a strain rate of 0.08 mm/min. Disk specimens were diametrically loaded with different values of the crack inclination angle ( β) ranging from 0° to 75° with a 15° increment. The applied load and load-point displacement were acquired using a computerized data logger. Tests were made at ambient conditions, at high confining pressure σ3) of 28 MPa, and at high temperature of 116°. The details of the experimental program can be found elsewhere [9]. RESULTS AND DISCUSSIONS The values for mode I and mode II fracture toughness (KI) and (KII) were determined using equations 1 to 4. Table 1 summarizes the values of KIC and KIIC at different conditions for straight notch. For all conditions, KIC is smaller than KIIC, in contrary to the values provided by the famous failure theories. Figure 2-a shows the variation of KI and KII with β for D = 98 mm, at different conditions. It can be seen that the high confining pressure has a tremendous impact on the fracture toughness, while the effect of high temperature has a minor effect. The normalized fracture toughness values of (KI/KIC) and (KII/KIIC) were determine for various cases. The plot of (KII/KIIC) vs. (KI/KIC) is named the fracture toughness envelope, which is the fracture locus for the general mixed-mode I-II loading. Crack initiates when a point ((KI/KIC), (KII/KIIC)) falls on the envelope. Figure 2-b gives a comparison between fracture toughness envelopes at different conditions (ambient, σ3 = 28 MPa, and T = 116 °C). Figure 3-a shows fracture toughness envelopes for D = 98 and 84 mm, at both positive region (crack opening) and negative region (crack closing). Also, Figure 3-b shows similar results for straight and Chevron notches. A second-degree polynomial was used to fit the experimental data at various conditions, and at both positive and negative region, Figures 2 to 3. The general form of the fitting is: ( ) ( ) ( )2 I IC I IC II IIC K K A B K K C K K + = + (5) where, A, B, and C are the coefficient for the second order polynomial used for the regression. The values of A, B, and C for various experimental conditions are tabulated in Table 2. It can be seen from Figure 6 that the results for all three conditions fall into a relatively close bound in the positive zone; however, distinct regions of data exist in the negative side. Note that the results for the specimens tested at high temperature fall close to the data for ambient conditions in the negative zone revealing that the fracture toughness is not very much affected by the temperature used in this study. However, fracture toughness envelope at high confining pressure, in the negative region, is extremely lower than that at ambient condition. TABLE 1 COMPARISON BETWEEN KIC, KIIC, AND THEIR RATIO AT VARIOUS CONDITIONS Condition Diameter (mm) KIC (Mpa m1/2 ) KIIC (Mpa m1/2 ) KIIC/KIC 98 0.42 0.92 2.19 Ambient 84 0.35 0.75 2.14 84 1.19 1.49 1.25 σ3 = 28 MPa 98 1.57 2.18 1.39 T = 116°C 98 0.52 1.00 1.92
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