Shearography-Based Non-Destructive Evaluation of Composite Repairs Protected by Passive Fire Protection

Composite repairs used in industrial structures are often protected by passive fire protection (PFP) systems, which makes post-installation inspection more difficult. The presence of the protective layer reduces the surface response associated with internal defects and restricts the application of conventional excitation methods commonly used in shearographic testing. This paper presents an inspection approach based on shearography associated with electromagnetic induction loading, specifically aimed at evaluating composite repairs covered by PFP. A dedicated inspection system was used to generate controlled non-contact excitation and to record the corresponding shearographic response. The method was initially assessed in laboratory conditions using composite specimens containing artificial defects with predefined dimensions and positions. These tests allowed the detectability of defects beneath the protective layer to be evaluated in a controlled and repeatable way. The approach was then applied to field inspections of composite-repaired pipelines installed on offshore oil platforms. The results show that electromagnetic induction loading can produce sufficient deformation gradients for shearographic visualization of subsurface defects through PFP. The combined laboratory and field evidence indicates that the proposed method is suitable for detecting relevant defects in composite repairs under representative industrial inspection conditions.