Abaris also teaches the use of laser shearography as a diagnostic tool. “Using a camera, an interferometric image of the part’s micro-surface is taken in an unloaded state,” says Dorworth. “The part is then exposed to loading with heat or weight or vacuum, and the image is compared. Information about the differences between the two photos is extracted, revealing surface strains associated with subsurface defects, anomalies and damage to the internal structure – as minute as one nanometer.”
Inspection of damage to wind turbines poses a unique challenge: height. “We have high-powered camera lenses to photograph the blades from the ground,” says Gary Kanaby, director of sales for MFG Energy Services. “We are just now seeing the use of remote drones with attached cameras for inspection – a much less expensive approach than raising platforms or using technicians on ropes.” Drone cameras can regularly track the progress of minor dings or cracks, enabling owners to make informed decisions on when to make a repair.
With detailed inspection information in hand, the technician or engineer drafts a repair plan. “This is a critical step,” says John Busel, vice president of ACMA’s Composites Growth Initiative. “The plan must take into account the loads and how this repair will provide continuity to the original structure. Understanding the materials, cure temperatures and rates are all factors in a successful repair.”
Step Two: Preparation
Removing the damaged material and debris from the compromised part may require the technician to cut out or grind out the various layers of laminate plies and inside core. During preparation, the technician also confirms the composite and core material, determines whether the repair is in a critical, highly loaded section and confirms the axis of the unidirectional, bidirectional or multiaxial fibers or fabrics.
Sophisticated preparation technology currently in use, particularly for aerospace composites repairs, includes computer-controlled milling for removal of damage and laser pretreatment to enhance the surface for bonding of the repair. The benefits of these more automated tools include improved consistency, removal of the least laminate necessary, accurate tapering to accept the new laminate plies or prepreg composite fiber materials and less opportunity for human error, as well as the opportunity to integrate the automated inspection, preparation and repair tools to form a manufacturing repair cell.
Step Three: Repair
Repairs should replicate the original laminate and core, matching the original strength, stiffness and weight. If the damage is extensive, reaching through the outer ply and into the structure, then the core and outer skin need to be addressed. In the most challenging scenario, damage to the inner skin plies, structure and outer skin plies require repair.