Effect of Crack Front Curvature and Side Grooving on CTOD δ5 and <i>J</i>-Integral in CT and 3PB Specimens

Abstract

Abstract Fracture testing on compact tension (CT) and threepoint bend (3PB) specimens is preceded by fatigue precracking. Fatigue cracks have curved crack fronts. Nevertheless, crack front curvature is not taken into account in numerical modeling of fracture toughness testing procedures. In this paper we analyze actual fatigue cracks in CT specimens and show that the difference between crack depth at the center plane and at the surface of specimens is typically 0.01 to 0.03 of the specimen width. Three-dimensional elastic-plastic finite element analyses were performed for CT and 3PB specimens with crack fronts of different curvatures, and the CTOD δ5 and the J-integral were determined. It is found that the ratio δ5C∕δ5 is sensitive to the crack front curvature and strongly affected by side grooves. It is also shown that crack front curvature variations that are within the limits given by the test standards affect to a certain extent the η factor employed for the determination of the experimental value of the J-integral using the area under the force versus load-point displacement record. These facts should be kept in mind when toughness is measured.