A Revised Procedure for the Construction of a 3-Parameter J_R Resistance Curve and the Determination of Initiation Toughness J_(0.2)

摘要

Fracture toughness testing procedures often advocate the use of a 3-parameter offset power law fit to define a resistance curve, and subsequently an estimate of initiation toughness (e.g., J_(0.2)). J_r=A+α(Δa)β. Experience has indicated that the fit coefficients A, α, and β are highly variable. What is required is a consistent method of estimating A and thus a and β to reflect physical sense. In this paper it is proposed that the offset A is defined using a J-elastic term J_e given by A=J_e=W/2(σ_y~2/E')f(a_o/ W). Experimental data are provided in support of the use of J_e for a well characterized A533B-1 steel. Tests were made at ambient temperature using geometrically scaled plain-sided and side-grooved compact C(T) specimens of sizes 20 < W < 200 mm. The fixing of A through the use of J_e provides less variable a and β parameters. The exponent β is found to be independent of W, noting that β_(SG) < β_(plain), as is the α coefficient for side-grooved specimens (α_(SG)). The α coefficient for plain-sided specimens (α_(plain)) may be related to specimen width and thus thickness (B) for geometrically-scaled specimens, noting that α_∞ approximates to α_(SG)a. α_(plain) =α_∞ ( 1 +1 B~1/2). The results indicate that the J_R curves and thus J_(0.2) vary with specimen width W. For conventional laboratory-sized C(T) specimens (i.e., 25 < W< 100 mm) the variation in J_(0.2) with a size over this range is of the same order as the general level of scatter observed in the J_(0.2) measurement. However, for specimen sizes outside of this range size effects may be predicted.