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Ongoing concern regarding humanity's impact on the environment and declining fossil fuel reserves has inspired a global adoption and continual support of renewable power generation. Onshore wind is a major contributor to the world's renewable capacity. Ensuring or possibly extending the lifetime of current assets is key in gaining the most efficient power generation. Structural health monitoring (SHM) systems can be employed to identify the health state of a structure and provide information regarding lifetime. Many such systems are already incorporated into various infrastructure. Regarding onshore wind turbines, the supporting concrete foundations represent an integral structural feature, where failure could cause collapse of the entire turbine. Severe cracks on such foundations are, therefore, of great concern to turbine operators. SHM of such foundation cracks may provide a more detailed insight into the lifetime of the turbine as a whole. Sub-surface cracks are conventionally monitored using intermittent excavation and visual inspections that are carried out during turbine downtime. Other methods, such as fracture mechanics, consider the critical failure point of a cracked specimen and fail to provide an indication of increasing severity. Research presented in this thesis demonstrates the application of optical fibre Bragg grating (FBG) strain sensors for crack displacement monitoring on an operational wind turbine foundation. The primary original contribution to knowledge of this work is the development of a novel methodology in order to categorize and quantify crack deterioration measured by subsurface crack displacement sensors deployed on an operational asset with visually severe cracks. Results from this methodology should help decision making procedures in regards to acceptable crack displacements, repairs and the overall lifetime of the turbine. Accompanying investigation into the effectiveness of epoxy and metallic bonding for FBG attachment demonstrates that purely metallic bonding may provide an improved sensor design. Particularly, a humidity dependence was observed in industry standard epoxies in the form of swelling, with the metallic bonding technique immune to such effects but producing similar performance for strain measurement during direct static and fatigue experimentation. To validate industrial results and further test metallic bonding of FBGs, a unique low-cost small-scale fatigue testing machine is designed and demonstrated for cracked concrete beam fatigue tests. Transitioning from reactive to preventative methods for initial or additional damage is an important topic and could potentially constitute most future SHM work, including for wind turbine foundations. Prediction methodologies are explored using lab test-benches at this early stage, presenting promise for future application in SHM for preventing critical events. Further development of the sensors, deterioration methodology and prevention techniques as part of a limit alert system for cracks in an onshore wind turbine foundations is suggested.