All the parts get a close visual inspection, and are inspected with D Sight and documented photographically. CAD drawings are created for many. IAR's NDI group has a robot and instead of taking spot measurements with eddy current (EC) or ultrasonics (UT) to check thickness loss or corrosion damage, it can provide maps of the entire joint. There is also in-house and collaborative research into multi-frequency and pulsed eddy current (PEC) which are able to better tolerate lift-off and quantify corrosion damage. Whenever a specimen is being screened for selection, inspections will be carried out with both traditional and experimental NDI techniques and these results are added to the database. Thermography has recently been added so that inspection data will be included, where appropriate, as well.
"Ultimately the researchers want to know how well their new NDI technique is performing or they want to accurately measure the damage. We will then take a section of the specimen, and very carefully disassemble it," Gould says. "We've developed techniques to remove the corrosion product chemically rather than mechanically so we clean it down to the point where only the remains of original alloy are left. Then we take a high resolution X-ray image along with a master calibration tool and digitize the X-ray film." The result is a highly accurate image representing the thickness loss. Each skin is processed separately and then the joint is put back together and the total thickness loss is determined. If a new NDI technology is being developed, specimens with quantified damage can be selected for use in determining the capability or sensitivity of the technology.
Fatigue and aging
In other studies aging degradation of material properties is also being analyzed. Through a reheating process named retrogression and re-aging, the fatigue and corrosion-resistant properties of various aluminum alloys can be re-established. According to Gould, this heat treatment process is about to be demonstrated on C-130 sloping longerons and the process should "save people a lot of money." One of the C130 Hercules specimens from the library is being used to qualify the technique.
The library includes specimens of both upper and lower wing skin material. A number of ongoing projects use the upper wing skin material to assess the structural significance of exfoliation corrosion damage, assess NDI techniques to detect and quantify the damage, and assess the effects of maintenance actions carried out to remove it.
IAR is comparing pristine, naturally corroded, and artificially corroded test coupons to understand how the structures will respond when they get old. Instrumented specimens are fatigue tested; these may be coupons of the material or built-up joint structures. IAR has designed built-up specimens that do not suffer from edge effects.
"When we first got involved in composites, we manufactured our own specimens from various materials and layups," Gould says, "and then we inflicted them with impact, delamination, and disbond damage. Over time the impact damage wasn't as spectacular as it used to appear in our enhanced visual inspections." This drove IAR to investigate further.
Indent depth and the number of damage sites in a given area are criteria when evaluating damage to composite structures. When an aircraft is taking off and a stone is thrown up by the wheels against a composite trailing edge flap, the impact damages that surface. The manufacturer's service manual stipulates how many or how deep an indent can be before repair is mandated.
"It's a concern as far as the structural integrity," Gould says, "and everybody bases that on being able to visually detect it and associate the internal damage to the measured indent depth. What we discovered was that even statically within a very short period of time or within one or two load cycles, the indent depth may be reduced by up to 40 percent. So any relationship you had between being able to observe, measure, and imply the significance of the damage is very much compromised by that relaxation. It's all driven by the fact that you have all these layers stuck together, you impact them, the adhesive lets go between the layers, and now they're free to move and the indent decreases."
Composite specimens are collected into the library. The latest include aluminum honeycomb core trailing edge flaps with moisture intrusion and corrosion plus an Airbus A320 vertical stabilizer recovered from a landing accident.
So how does this information affect mechanics? What should they be looking for?
One of the things predicted with finite element analysis of the different kinds of joint construction in the library was that they each respond differently when corrosion starts to build up. "Soon after it starts, the product buildup will push hard enough to force the material through the yield point and the skin becomes permanently deformed," Gould says.
The usual procedure is to look for cracks that appear at a fastener hole. "What IAR predicted and discovered was," Gould says, "rather than cracks running from rivet to rivet, we've detected nonsurface-breaking cracks that take off in all directions and they don't start at the rivet hole, they start away from the rivet hole and work both toward it and away from it. It is only much later that they come through the surface of the material and you can detect them from the outside."
Another thing IAR discovered is that when so much pressure is being exerted by this buildup of corrosion, the "guys in the field will notice when a rivet head pops off and they'll punch that one out and often replace it with a blind fastener because they can't get access to the inside," Gould says.
When taking specimens apart, rather than drilling out the rivets, IAR machines off the shop head and pushes them out, which allows for examination of the rivets. Adjacent to the replaced rivets are other rivets that are also suffering from this pressure and they're distressed and have cracks, according to Gould. In some cases, at total thickness, losses are between only 3 and 8 percent; and 40 to 75 percent of the adjacent rivets have been found to be damaged.
When dealing with corrosion removal in fuselage joints that may have permanently deformed skins, technicians are allowed to de-rivet a section, normally no more than 20 inches at a time, wedge the joint open, and reach in and mechanically remove the corrosion. Corrosion prevention compounds are added to try and stop the process and then it's buttoned back up. A lot of damage can occur to the remaining alloy when removing the corrosion. "And you only get to see that when you have the luxury of taking the joint completely apart.
"It's kind of a catch-22. You've been asked to do maintenance and do it quickly, and so people tend to use power tools," Gould says. "The SRM instructions don't say anything about power tools or rigid abrasive discs. But with the limited amount of space to work in, how else are you going to accomplish the task?" And the research continues.
Bombardier Aerospace offers 24/7 technical support for operators of Bombardier Learjet, Challenger, and Global aircraft. For Learjet technical support: (316) 946-6100 or firstname.lastname@example.org...
Corrosion Control Controlling corrosion damage through effective inspection and treatment By Joe Escobar September 2001 The average age of the aircraft in use today is getting...