Short fatigue crack behaviour of a submarine hull steel in inert and aggressive environments

During the operational life of offshore structure numerous components experience the combination of seawater exposure and applied cyclic loading. As a consequence, corrosion fatigue, which consists of crack initiation and propagation has become one of the main causes of the numerous catastrophic accidents associated with offshore structures. This thesis reports on a study of the short crack growth behaviour of Q2N steel, designated for warship hulls, submarine hulls, deck railings and other offshore structural applications. Within this thesis three main aspects of fatigue have been conducted. They are as follows; 1. Evaluation of Short Fatigue Crack Growth in Air Three-point-bend fatigue crack growth tests were conducted at various stress levels using smooth polished specimens in laboratory air. In the initial stages of the tests, short cracks were observed and found to propagate at much faster rates than those of long cracks when correlated with the linear elastic fracture mechanics (LEFM) parameter ΔK. Furthermore a period of crack growth retardation was observed at crack lengths around 50 μm. The theory of the interaction between short cracks and grain boundaries fails to predict the occurrence of this deceleration at these crack lengths. In this respect a new short crack deceleration mechanism has been considered and studied. Observation of the characteristic behaviour of short cracks, using an optical microscope and an Scanning Electron Microscope (SEM) allowed a short crack growth model to be developed based on microstructural fracture mechanics analyses.2. Seawater Effect on Short Fatigue Crack Growth Behaviour Results obtained from a series of corrosion fatigue tests using smooth specimens in artificial seawater show that the first evidence of damage on a specimen surface is the formation of corrosion pits from which short cracks develop during the early stages of tests. Corrosion pitting is considered to play a major role during the early stages of corrosion fatigue damage. Models incorporating corrosion pitting and environment - assisted short crack growth regimes are proposed and discussed for this material - environment system. 3. Frequency Effect on the Behaviour of Short Crack Growth Frequency effects on short crack growth behaviour have been studied by conducting fatigue tests in both air and artificial seawater at cyclic load frequencies of 10, 1, 0.1 and 0.01 Hz. Experimental data obtained from tests conducted in air reveal that the fatigue lifetime, which includes both the initiation and propagation of short fatigue cracks, is little effected by changes in the applied test frequency. However fatigue strength and short crack growth behaviour within seawater appears to be heavily time depen dent with fatigue lifetime decreasing significantly as the cyclic test frequency is reduced. Detailed effects of the influence of frequency on the development and growth of short cracks have been studied. Quantification of frequency effects on short crack growth behaviour is made through the introduction of a time - dependent parameter. In addition to these effects the initiation of multiple cracks, crack-crack interactions and crack coalescence have been investigated and discussed.