Acoustic Emission Method for Determining Damage-Initiating Stresses in Repaired Glass-Fibre-Reinforced Polymer (GFRP) Structural Composites

Abstract Composite structures are widely used for marine applications such as yachts and wind turbine blades, due to their high mechanical strength, corrosion resistance, and favourable strength-to-weight ratio. In marine environments, however, they are exposed to numerous types of damage, resulting from mechanical, environmental, and fatigue-related factors. Effective repair methods for these structures, such as patch bonding, resin injection, and ply replacement, allow for partial restoration of their functionality without the need for costly replacement. A key aspect is the proper selection of the repair technology, which should take into account the type of damage, operating conditions, and material properties. The application of advanced diagnostic techniques, such as acoustic emission, enables early detection of defects and effective quality control of repairs; moreover, acoustic emission provides insights into the service parameters of such repairs. In this study, a damage scenario involving a composite structure was considered, and a stepped repair was designed and executed. Samples of composite material, both before and after repair, were prepared for static tensile testing to determine their strength parameters. Acoustic emission was also employed during the tensile tests to identify the values of stress at which damage initiation occurred, as well as to obtain the serviceability parameters. The results showed that the developed stepped repair scheme yielded strength parameters that were only slightly lower than those of the undamaged base composite. However, acoustic emission data revealed that the onset of damage in the repaired specimens occurred at significantly lower stress levels, indicating a considerable weakening of the composite structure as a result of the repair process.