Specimen supply and preparation
The pipe of interest was 18in diameter lined pipe. PTL provided two 1m lengths each with a girth weld at mid-length. TWI then extracted and prepared for testing six waisted strips from these rings.
In service, the seawater would affect the outer surface of the pipe, and so the corrosion fatigue behaviour of the weld cap was of interest. TWI therefore removed the liner and ground flush the weld root to promote cracking from the cap side of specimens during testing. Figure 1 shows the specimen layout and a photograph of the root side of one strip specimen before the liner was removed.
Strain gauges were applied close to the girth weld on the cap side and ‘static tests’ carried out to determine the required loads for the tests. The strain gauges were then removed before the seawater cell was installed around the specimen.
Testing
The specimens were located inside a cell containing simulated seawater (to ASTM-D 1141 [2]). A separate reservoir was used to allow the seawater to be recirculated during the tests. Cathodic protection was set up to apply voltage of -1050mV Ag:AgCl to the specimen.
Tests were carried out in one of TWI’s 500kN servohydraulic fatigue test machines, as shown in Figure 2. Since the tests were being used to determine corrosion fatigue strength, the test frequency was an important parameter – it needed to be the same as occurs in service, to ensure that a representative amount of corrosion could occur in each cycle. The relevant loading frequency for this project was 0.1Hz.
Tests were continued until cracking occurred or a runout life was reached. Test durations ranged from seven to 60 days.