Glass fibre reinforced polymer composites are frequently used in marine applications where the combined effects of cyclic loads and the seawater environment limit their fatigue life. This paper aims to demonstrate the degradation that seawater causes to the stiffness of the composites. Three-point bending fatigue properties of cross-ply woven glass fibre composites commonly used to manufacture tidal turbine blades are reported for both wet and dry conditions. Failure analysis based on the Digital Image Correlation method was performed to identify damaged zones on the test coupon surface and to follow failure progression during the fatigue tests. To characterize the damage in the composite, stiffness degradation has been monitored during the entire fatigue history. Scanning electron microscopy was used to identify multiple failure mechanisms on the specimen fracture surface. In addition, for further verification of microscopy results, X-Ray Micro-computed tomography, was used to characterize the internal damage such as delamination. From the full-field strain measurement technique and microscopic examination of failed samples, it was found that distributed localized strains are evidence of the number of resin cracks and de-bonded areas. SEM examination shows a degraded fibre/matrix interface region due to the action of seawater.

Metallographic examination performed on a number of micropitted gears has revealed microstructural features similar to those reported in the literature in fatigued bearings, namely, dark etching regions and white etching bands. In addition, a new type of fatigue-induced microstructural constituent was observed close to the surface below asperities referred to as the plastic deformation region. Finally, it is shown that the initiation and propagation of cracks leading to the formation of micropits are related to the phase transformations mentioned previously.

The elastic properties of \(\eta\hbox{-Fe}_2\hbox{C}\) (eta carbide) have been determined from ab initio density functional theory calculations using the generalized gradient approximation. The isotropic polycrystalline elastic modulus of \(\eta\hbox{-Fe}_2\hbox{C}\) has been calculated as the average of anisotropic single-crystal elastic constants determined from the ab initio simulations. The calculated polycrystalline elastic modulus was used to compute the elastic modulus of a case carburised gear steel subjected to shallow cryogenic treatment (SCT) and deep cryogenic treatment (DCT). This value was then compared with experimental values obtained from nanoindentation. The results confirmed that the changes in elastic modulus observed in the DCT steel can be attributed to the precipitation of \(\eta\hbox{-Fe}_2\hbox{C}\). No changes in hardness have been observed between the SCT steel and the DCT steel. These data were then used to assess the surface contact fatigue behaviour of the SCT and DCT steels tested under elastohydrodynamic lubrication conditions.