Composite repair is an art, relying on high-tech tools and processes – in the hands of skilled technicians – to extend product life.
While composites are strong and durable, they are not immune to damage. Composite parts on everything from boat hulls to wind turbines can be impaired through collisions, lightning strikes, environmental exposure and other causes. In addition, as parts age fatigue may also be a factor. With applications for advanced composites ever increasing, so too is the demand for repair services and experts.
While the basic principles of repair have not changed much recently, the size and complexity of the repairs have. For example, last December a team of Boeing engineers and technicians repaired an Ethiopian Airlines 787 Dreamliner that was damaged extensively by fire. It was the first major repair to an airliner made largely from carbon-fiber reinforced plastic. In the wind market, technicians are challenged to perform maintenance on increasingly larger wind blades, some as long as 250 feet.
The repair of advanced composite parts follows a standard series of steps: damage assessment/ inspection, preparation, repair, curing, final inspection and finishing. But in the field, each damaged part poses unique requirements based on the type of composite material and the manner and extent of the damage. Each repair requires its own solution that must be uniquely engineered, particularly if the structural integrity of the part has been compromised.
A student at Abaris Training Resources removes damage and prepares the taper scarf for repair.
Step One: Damage Assessment/Inspection
Since the repair’s design is driven primarily by the type of damage incurred, proper assessment and inspection is critical. Beyond a preliminary visual inspection, there are a range of nondestructive inspection techniques available in the repair technician’s tool kit.
Manual audio sonic testing, or “tap testing,” is the most straight-forward method to detect voids, degradation and delaminations in a composite structure. Tap testing used to be as simple as taking out a coin or tap hammer and listening for a change in tone where the laminate is damaged or has delaminated from the core material. Now repair technicians use digital tap hammers to more accurately identify and measure the damaged area. Along with ultrasonic A-scan methods to determine the depth and size of damage, these are the workhorses of inspection.
Louis Dorworth, division manager of direct services for Abaris Training Resources Inc., a provider of composites repair training, notes that repair technicians are increasingly using higher-tech methods for inspection. “Thermal imaging (thermography) is performed with infrared cameras to measure different levels of heat transfer, indicating where defects within the composite part are located,” he says. “More sophisticated, yes, but environmental factors can affect heat transfer, making thermography just one more tool in the repair technician’s toolbox.”