PSI - Issue 54

Naveen Kumar Kanna et al. / Procedia Structural Integrity 54 (2024) 196–203 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1.1. The Kitagawa-Takahashi diagram The Kitagawa-Takahashi (KT) diagram, (Kitagawa and Takahashi (1976)), is a concept for determining the fatigue limit in the presence of a defect or a crack. The KT diagram is a double-logarithmic plot with the fatigue limit Δσ 0 on the y-axis and the crack depth a on the x-axis (Fig.1). The KT diagram is controlled by the threshold against the fatigue c rack propagation, ΔK th . The KT diagram can be described with Eqn. (2), with Y being the boundary correction factor. ∆ 0 = ℎ √ ∙ ∙ (2)

Fig. 1. Kitagawa-Takahashi diagram

The mathematical description of the KT diagram was improved by El Haddad (1979) by adding the effective crack length ‘l 0 ’ to the equation and employing a linear elastic solution. The resulting curve is smooth and differs especially in the transition region as shown in Fig. 1. This model was again modified by Chapetti (2003) by considering the crack closure phenomenon which differs for both short and long cracks and the curve again differs in the transition region. All the models differ in the transition region which is also the technical interesting region. The determination of these models is coupled with complicated and time-consuming experimental effort. An alternative experimental method to detect the crack initiation and crack arrest or propagation points on the KT diagram is the DCPD measurement technique. 2. Experimental procedure The experiments were carried out on a flat specimen (Fig. 2) of an unalloyed C45E steel. The material was supplied by ThyssenKrupp Schulte GmbH as rolled sheets with a thickness of 4 mm. The material is a quenched and tempered steel with a ferritic microstructure.