PSI - Issue 68
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Mr. Parthasarathy Iyengar et al. / Procedia Structural Integrity 68 (2025) 446–452 P. Iyengar, J. Mardaras, S. Soe / Structural Integrity Procedia 00 (2025) 000–000
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While generically, Fracture toughness is a measure of its resistance offered to fracture (when loaded) in the presence of discontinuities; K 1C is a linear-elastic measure of the value of the applied crack tip loading, at which a crack grows to a pre-defined length under high constraint and without deformation in the thickness. The stress triaxiality is generally assessed under the title of “constraint” as it arises from some constraint on strain. It is a function of the geometry and the loading configuration. Increased thicknesses, deep cracks and loading in bending promote constraint. High constraint is targeted in order to achieve linear-elasticity. Explanatory images of a monotonic overload fracture are shown as Fig. 2 below reproducing (a) and (b) from Wanhill et al (2024).
Fig. 2. (a) monotonic overload fracture; (b) Deformation modes; (c) Image of ideal plane-strain coupon
Irrespective of material, ASTM E 399 constrains geometry of coupons (ex. W = 2* B ) with conditional allowance. Fracture toughness test is applicable both for structural design and production quality assurance. Although essential, conventional testing is premium on cost and resource.
Fig. 3. (a) Fracture on the surface on low thickness CT; (b) K c vs thickness from Wanhill (2024); (c) In-house observation on 202x-T3511
Fracture toughness of ductile materials tends to rise during the crack extension and is a function of a stable crack growth length against a certain load. While the principle is applicable to R-curves; K 1C is used by convention to assess material, semi-finished products or parts with a smaller plan/floor area. Hahn and Rosenfield (1965) have indicated that under plane strain conditions a ‘hinge’ type deformation is followed by flat fracture, whereas under plane stress - slant fracture occurs by shear after a hinge type initiation. A ‘size effect’ was observed with internally collated test results with CT specimen from solution treated, control stretched, naturally aged products such as 2024-T351, 2024 T3511, 2026-T3511. These products exhibit a high ductility which can lead to non-conforming numerical test results
when width of specimen is lower than the optimal for plane strain condition. 3. ‘Size insensitive’ Fracture toughness value as per ASTM E 399-23
The K 1si test procedure is based on a fixed amount of crack extension of 0.5 mm and as a result is less sensitive to specimen size than K 1C , whose force-displacement test record can be reinterpreted to calculate K 1si . Secant line S Q is first calculated and drawn akin to the 95% secant line, which is parallel to the force-CMOD register. “ P Qsi ” is then plotted akin to P Q which may be lower than P MAX , but if not, then this value is the P Qsi .
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