Fracture Studies on Through-Wall Cracked Straight Pipes and Elbows under Internal Pressure and Bending

In service, the piping components (pipes/elbows) are subjected to different operational loadings such as thermal stress, internal pressure, self-weight, external loading from soil movement, flow induced vibration, vortex induced vibrations and third party damage. Structural integrity assessment of these piping components under postulated accidental conditions is essential for ensuring smooth operation of power plant piping system. In order to understand crack growth behaviour of piping components under operating conditions, fracture studies were carried out on two straight pipes and three elbows of 219 mm OD having circumferential through-wall notch with internal pressure. Fracture tests on pipes were carried out under four-point bending and elbows were tested under in-plane bending. A sealing arrangement consisting of stainless steel plate as backing material and silicon rubber sheet with two-component super strength epoxy based adhesive was used to seal the through-wall notch in the pipes and elbows. The pipe and elbows specimens were filled with water through threadolets and the desired internal pressure was achieved using an automatic high-pressure hydraulic pump. The internal pressure was varied from 10 to 25 MPa. The load carrying capacity of pipes decreased by a factor 1.95 with the increase of initial notch angle from 90.5° to 152.8° under an internal pressure of 10 MPa. In the case of elbows, with an increase of internal pressure from 10 to 25 MPa, load-line displacement corresponding to maximum load increased by 41.1%. Ovalization (intrados-extrados) in the elbows was found to be varying from 2.74% to 6.85% at the end of fracture tests. The pipe specimens have undergone initial stable crack growth followed by crack instability and crack growth deviated from the initial plane of the notch. The collapse load obtained for through-wall cracked elbows under internal pressure was found to be conservative compared to the values obtained using twice elastic slope method.