The pipe studied was seamless to API 5L PSL2 Grade X65. Tensile testing was carried out to determine the stress-strain curves of parent and weld metals.
Fracture toughness was determined using 2BxB SENT specimens using a multiple specimen method to derive CTOD resistance curves (R-curves) at room temperature. The tests were compliant with BS 8571.
For the purposes of testing, the pipes were cut to a length of 2000mm. The girth weld was located in the middle of the pipe length and the weld cap at the designated notch location was ground flush with the original pipe surface and etched to reveal the weld. This procedure enabled the notch to be located along the weld centreline and also facilitated fitting of the knife edges close to the notch mouth, for subsequent instrumentation with clip gauges, as illustrated in Figure 1.
A single electrical discharge machining (EDM) notch was inserted at 4 o’clock in the pipe, which was subjected to tension only. Figure 2 shows the installation of the pipe test in the tension machine. Before this, the pipe was instrumented with strain gauges and linear variable differential transformers (LVDTs), as shown in Figure 3.
The existing strain-based methods were divided into five groups:
- Group 1: Modification to existing stress-based approaches
- Group 2: Extension to the EPRI scheme for fully plastic J estimate
- Group 3: Reference strain method
- Group 4: Strain capacity method
- Group 5: Others
The strain capacity predicted using the methods in each group was compared with the large-scale test results. Figure 4 shows that most of the existing methods are conservative compared with the test results.