A through-wall, 1.7-m-long crack grew suddenly from an outer diameter (OD) notch in a 285-mm-OD A723 steel overstrained tube that was undergoing plating operations with no externally applied loads. A description is given of the fracture mechanics tests and analyses and the fractography that were performed to characterize the cracking. Key material, residual stress, and environment information are: 1200 MPa yield strength; 150 MPa √m fracture toughness; composition typical of air-melt A723 steel; tensile residual stress at the OD of about 600 MPa; and electro-polishing bath of sulfuric and phosphoric acids at 54°C.
The bolt-loaded test for threshold stress intensity factor for environmentally controlled cracking described by Wei and Novak was used here with two significant modifications. Some tests included only a notch with radius matching that of the tube, and a new expression for K in terms of crack-mouth displacement was developed and used. Scanning electron microscopy (SEM) fractography and energy dispersive X-ray spectra were used to identify cracking mechanisms.
Results of the investigation include: (a) a measured threshold of hydrogen stress cracking for the material/environment below 20 MPa √m, (b) da/dt versus K behavior typical of classic environmental control, (c) an improved K/v expression for the bolt-loaded specimen and associated criteria for determining plane strain test conditions in relation to the Irwin plastic zone.