PSI - Issue 68

5

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

450

Fig. 4. (a) K 1C Plot types as per ASTM E 399; (b) K 1C Plot from 2024-T351 test showing P Q and P Qsi .

2024-T351 specimen of B =19mm to 21mm in L-T orientation taken from 25mm plate were tested at Arconic. • K 1si is generally seen to be in the vicinity of 3 MPa√m from the corresponding K Q . • Fracture toughness results lie within a range of 6 MPa√m as ligament varies from 15 mm to 32 mm. A similar observation was made with B ≤ 13 mm CT specimen in an internal effort using ductile 2026 extrusions. The incumbent K 1si test procedure requires a valid K 1C test specimen. It is therefore limited by thickness of specimen that is unlikely to fail for a given material and thus not optimized for ductile products of low thickness.

Fig. 5. (a) KQsi compared with variation in ligament length; (b) KQsi distributed against specimen thickness

While the thickness of most tested specimen is the same, ligament length is seen to have a negligible effect. 4. Cylindrical bar specimen used for indicative value comparable to K Q Another approach towards a value considered as indicative of K Q without the use of CT specimen in areas of constrained dimension is the proposal of the use of cylindrical specimen with a notch. In a planar specimen, crack front is constrained so it varies across thickness from plane stress on surfaces to plane strain in the middle (3D to 2D transition). Notched Round Bar (NRB) geometry has the advantage of constant circumferential constraint and can be modelled in 2D by using an axisymmetric form. While “withdrawn” standard ASTM E 602 previously provided a standard for NRBs in tensile testing, “apparent” K 1C was proposed by Wilson (1997). 8mm CCRBs were shown to conform statistically with 2024-T351 by Sarchamy and Burns (1996). However accurate concentric pre-cracking and geometric control remain challenges. The need for a high precision and sharp notch was therefore challenged. 10 mm and 8 mm dia. NRB specimen were lathe-turned from 25 mm plate sourced from the same mill as applied to K Qsi testing. The failure of an un-notched tensile coupon is markedly different from that of a notched specimen which