SAE 4340 Fracture Toughness Evaluation Following Wang’s Formulation for Spiral Notch Torsion Test Specimens

Abstract

The fracture toughness parameter (fracture toughness in plane strain-state [KIC], fracture toughness near the onset of crack growth, or crack tip open displacement) is essential information for understanding fracture behavior in mechanical and structural components. To determine this property, several methods have been developed; these require specimens with sufficient thickness to ensure a plane-strain condition at the crack front, which depending on materials mechanical properties, may lead to the need of large amounts of material. To overcome this problem, the spiral notch torsion test (SNTT) was proposed by Wang in 2000, and over these years, methodologies for testing many different materials have been presented, as well as the elastic compliance equations for assessing crack size (essential for introducing the fatigue precrack), also presented in 2012. Although the SNTT presented advantages and potentialities, its application is still quite restricted, and information on its use is limited. Implementation of the SNTT is a challenge, mainly because of the inexistence of a specific standard, thus producing ambiguities when dealing with the application of the test, methodology, and limitations because the SNTT implementation is principally based on previously published theoretical and experimental studies. The study presented herein aims at organizing already existing information based on work conducted by Wang while presenting in detail a test chart and the necessary techniques for obtaining a fracture toughness parameter, in mode I loading and in the domain of a plane strain-state. Determining KIC using the SNTT was performed on a quenched and tempered AISI 4340 steel (45 HRC) in air at room temperature. The obtained results proved the great potential of SNTT methodology for KIC determination. The KIC values obtained by the SNTT methodology exhibited a maximum error of 3.5 % and were statistically equal to those provided by the conventional method compact tension (C(T)) that follows ASTM E399-20, Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness KIC of Metallic Materials. The fracture surfaces observed in SNTT and C(T) specimens showed similar fractographies and mechanisms.