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This study examines the limits of validity of the Q ≈ 0 assumption under monotonic loading along the crack front in the “high constraint” 2-in.- and 4-in.-thick 4T-planform CT specimen geometries. Preliminary results indicate that progressive deviation of the in-plane crack-tip fields from Q ≈ 0 occurs with increasing load, along the crack front up to the symmetry plane of the geometries. The nature of the in-plane deviation can be described using J-Q theory. Significantly, no indication of possible 3-D transverse loss-of-constraint effects on the crack-tip fields is manifested in a 2-D plane strain analysis of the 4T-planform geometry. Uniformity of J or K values along the crack front does not imply either the existence of single-parameter-dominant conditions or the absence of substantial 3-D variations in the in-plane crack-tip fields. Preliminary results also suggest that current KIc-related size requirements in ASTM-E399 might be sufficient to preclude the onset of significant 3-D transverse Q-stress effects for the 4-in.-thick geometry. However, significant deviation from J-dominant conditions could occur within the existing JIc- and JR-related size requirements in ASTM-E813 and ASTM-E1152 for both geometries.
J-Q theory, crack-tip constraint, fracture toughness, crack-tip fields, small-scale yielding, CT, warm prestress
Research Associate, Saint-Gobain/Norton Industrial Ceramics Corporation, Northboro, MA