R-CURVE GENERATION AND CTOD EVALUATION CONSIDERING MAXIMUM CRACK-GROWTH SIZE AND PARABOLIC CRACK FRONT

The standard methods of estimating the average size of the slow crack growth region as described in BS 5762 (Crack Opening Displacement (COD )) and ASTM E 1290 (Test Method for Crack-Tip Opening Displacement (CT OD) Fracture Toughness Measurement) are cumbersome, painstaking, time consuming, and dependent on several factors; e.g., section thickness, the nature of crack front profile, and the number of measurements. Oft en errors are introduced in determining the average crack size which i n turn affect the determination of critical CTOD value. However, sever al merits are associated with the use of maximum crack size. A theoret ical relationship is established between the two physical quantities b y assuming a parabolic nature of the slow crack growth front profile, which is supported by experimental data points. In situations of large crack growth, the crack profile tends to change from a parabolic to a semicircular crack front. A new approach is proposed to make use of t he maximum crack size to generate R curves that can be extended to yie ld critical CTOD values. The nonlinear regression analysis of the CTOD -maximum crack size R curve yields critical CTOD values in agreement w ith the values that may be obtained by BS 5762. However, the linearly regressed and extrapolated critical CTOD values are found to be highly conservative. Alternatively, the average crack size may be obtained t heoretically by using maximum crack size data to determine CTOD values following BS 5762.