Composite components are known for their complexity. These materials—layers of carbon fibers encased with a matrix of epoxy resin—often occupy engineers with the dilemma of how to best build them back up to their original state after suffering on-wing damage.
John Cornforth, GKN Aerospace's VP technology, realized that technicians spent hours preparing these materials before they can start repairing them. Because of this, he set off to research ways to turn this manual process into a semi-automated one. The result is laser ablation.
With the help of a laser specialist, special software and a team of composites researchers in Munich and Cowes, U.K., the project that started in 2007 has been proven on simple part geometries and now advanced into its second phase to test the technology on more complex components.
Developing a semi-automated preparation process is important to GKN because it is manufacturing more types of thick composite products, such as the all-composite wing spar for theand the composite rear wing spar on the . These structures have more layers than parts in the past—about 100 plies—which elongates the preparation process because there are more layers to grind away.
Preparing composite materials is a time-consuming business generally done by hand, with air powered tools. A technician grinds or sands ever-increasing layers of the damaged composite material in circles or other defined shapes, to gradually remove the resin. This method of preparation sets up the part for a “stepped scarf” repair, which is designed to remove damage on composite parts one layer at a time. This “stepping” allows a precise repair without greatly impacting the structure.
When the technician thinks that he or she is done preparing the surface, the part goes to a non-destructive testing lab to determine that the damage has all been sanded away. If not, the part goes back for deeper grinding. According to Cornforth, the preparation phase on a part like a wing trailing edge could take more than 20 hours.
The idea of the laser ablation preparation, says Cornforth, is not to replace a technician, but to make his or her job faster and easier with laser technology. By eliminating the need to manually grind each composite part by hand, that engineer can spend more time working on other projects while the machine is doing its job. GKN estimates that the system saves about 60% of preparation time for each part, and the operating cost for the laser is about half as expensive as what it costs for a human to manually prepare the part.
GKN's composite team launched the first phase of testing in 2007, working with SLCR Lasertechnik, a small German company that specializes in removing coatings with laser technology. The objective of this phase was to prove that a laser could perform non-contact ablation on flat parts with simple geometries. To do this, SLCR produced a rotating head that connects to a laser source and delivery system for its beam, which is housed at GKN's composite research center on the Isle of Wight in the U.K. The damaged part sits under this head, and the laser moves in a circular motion over the part to ablate the resin coating.
The laser ablates the composite's resin matrix by vaporizing it. Unlike a regular grinding process, this semi-automated process does not induce any vibration of the part, which ensures that the composite fibers are not damaged even more during preparation. GKN has tested the new technique with built-in thermocouples to ensure that the epoxy does not become overheated as the laser passes over it. This motion is programmed by a specialized software code tailored to each repair.
Because the ablation technique with lasers is a non-contact process, it does not require tuning the equipment to be as precise as a traditional grinding method. The laser has a focal lens of 5 mm, so it will do the same work on the part anywhere within that length. Plus, after each layer has been penetrated by the laser, the ground layer leaves fluffy, brittle carbon fibers behind. Until they are brushed away, these fibers protect the next layer until it is ready to be ablated.
“It's a very gentle process,” says Cornforth. “In other words, you could go back over to the same area and you wouldn't do any damage.”
As the laser moves over the composite structure, the laser beam takes off 1/10 of each layer, which are each approximately 0.13 mm thick. The laser head “steps” the circles it makes on each composite layer, which means that it ablates the resin from the laminate material surrounding the damage in gradual increments to form a downward slope. Therefore, the deepest circle is at the damage. Cornforth says that the ablation equipment to be assembled in the second phase could easily prepare an area about 2 sq. ft. for repair, with the option of increasing the size if needed.
After Cornforth's team proved that the laser could execute the laser ablation technique on a flat laminate, the project advanced to the current second phase in late 2011. The purpose of this phase is to tailor the laser ablation method for real world applications, which will require the laser technology to remove the epoxy resin on complex geometries, such as curved surfaces. To do this, GKN is working with SLCR to perfect a more advanced equipment assembly, with the goal of obtaining OEM approval for the preparation in the future.
“Now we're trying to create a piece of equipment that will cope with more complex shapes and more complex surface geometries,” says Cornforth.
Cornforth says that the idea of adapting the laser ablation method for on-wing surface preparation is not out of the question in the future. In fact, he sees potential high demand for this type of advancement. Developing an on-wing version of the equipment could be used to restore newly manufactured composite structures that had a flaw when they were constructed, or it could be used to prepare a part for repair that has been taken out of service because it suffered damage.
For now, GKN is focused on making this semi-automated technique a usable tool. However, the possibilities for laser ablation can be taken even further, possibly becoming more integrated with the actual composite repair process that the materials will undergo after the initial preparation phase. For example, Cornforth indicated that his team could create a repair cell that includes not only the surface preparation equipment, but a robotic arm to do non-destructive testing after the laser completes the ablation process. Even further down the road, a fiber placement machine could be added to the mix, creating a patch to actually carry out the repair.