Effect of Water or Soap Solution on Fatigue Crack Growth in Welded Steel

It is common practice to conduct fatigue tests on full-scale girth welded pipes in resonance bending with a static axial mean stress induced by internal water pressure. Clean tap water is used, which gradually stagnates with time during a long endurance test, and therefore it is generally assumed that it has no significant effect on the fatigue lives of cracks propagating from the inside. However, important conclusions are drawn from such tests, especially when they relate to risers, and therefore there is a need to check this. In a different context there is a similar need to check the effect of soap solution on fatigue. This is often applied during fatigue tests on welded joints that fail from an accessible weld toe as an aid to crack detection. Furthermore, it may continue to be applied when a crack has been detected in order to produce beachmarks on the fatigue fracture surface by staining. The present paper presents the results of a series of fatigue crack growth rate (FCGR) tests that aimed to establish the effects of clean water or soap solution on the fatigue performance of welded structural steel. The tests were carried out on standard single edge-notched bend (SENB) specimens machined from girth welds in X65 grade steel pipe. Comparative tests were conducted at ambient temperature in air, tap water, de-ionized water and soap solution environments. Noting the general finding that corrosion-fatigue crack growth rate increases with decrease in load cycling frequency, the influence of frequency between 0.1 and 10Hz was investigated.