PSI - Issue 68
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
447
2
1. Introduction and limitation The Compact Tension (CT) specimen as per the ASTM E 399 is the most widely accepted type of K 1C specimen in the commercial aircraft industry and is a key point of focus in this presentation. MMPDS-14 by MMPDS (2019) is used to refer to baseline material properties. A discussion on the comparison of two alternatives to K 1C based upon Compact Tension (CT) specimen is based upon observations from certain 2xxx and 7xxx Aluminium alloys and their relationship with this established material indicator is explored. Reference to ASTM E 399 by ASTM (2023) is also used to cover ASTM B 645. Aluminium-Lithium alloys however are not covered. Alternatives considered are K 1si discussed by Wallin et al (2023) and cylindrical bar specimen inspired by Sarchamy et al (1996) and Wilson (1997), given the challenges of testing thicknesses which are less than or equal to 14 mm and obtaining valid results with conforming plane-strain fracture toughness values. K 1si is based upon CMOD estimation by assuming a fixed amount of crack extension of 0.5 mm irrespective of tested specimen size and round specimen have been tested for applicability when dimensionally constrained. Centered upon aircraft aluminium alloys, 2024-T351 was chosen for conceptual testing for convenience of discussion given open-source data availability. The goal is to present a discussion regarding practical applicability of alternative approaches to K 1C in the aircraft industry, but not to challenge established theoretical understanding. Incumbent ASTM E 399 validity criteria are ignored within the present discussion. This discussion is therefore focused upon the value of K Q , which is an indicative value of Fracture Toughness Plane-strain fracture toughness as defined by ASTM E 399 K Q \ K Qsi Calculated value of Plane-strain fracture toughness without considering specimen validity criteria K 1si Size-insensitive fracture toughness CMOD Crack Mouth Opening Displacement CT Compact Tension specimen UTS Ultimate Tensile Strength TYS Tensile Yield Strength (0.2% is assumed) v Poisson’s ratio E Young’s modulus of elasticity G Rigidity modulus of elasticity i.e., shear modulus ρ Density σ Stress ε Strain e % % Elongation i.e., indicating strain at failure ε % Strain at UTS (observed from indicative Stress-Strain curves) D External diameter d Internal diameter Nomenclature K 1C
2. Overview of relevant understanding of static fracture 2.1. A high-level comparison of mechanical properties of 2024-T351 with other alloys
A comparison of select mechanical properties of aircraft materials is presented in the Table 1 below. It is noted that aluminium alloy 2024-T351 has a greater strain at failure ( e %) than Ti-6Al-4v but significantly lower elastic modulii and tensile strength alongside a comparable Poisson’s ratio. However, the strain at UTS for AISI 316 lower than 2024 T351, despite it having a significantly greater strain at failure (i.e., 40%). Strain occurring after the UTS therefore represents a rapidly reducing resistance to an applied force value, until failure occurs. Assumed properties are of the thinnest section with the exception of Ti-6Al-4v.
Made with FlippingBook - Online Brochure Maker