Tech Toolbox Keeps Pace in Composite Repair

Long viewed as a black art, composite repair is adding more science to boost a mechanic’s confidence.

On any given day, thousands of aircraft repairs occur involving composite components, utilized in various applications from nose to tail within every military and commercial aircraft category. Use of these materials onboard aircraft go back 50 years, with wet layup repair mostly conducted hands-on, pegging the methodology as much "art" as science. The next 50 years will see increased demand for carbon fiber reinforced plastics in airframe, interior and system components, possibly tenfold by 2017. The commercial Boeing 787 and Airbus A380, as well as new military aircraft, are leading this growth spurt. Such an increased volume of composites in service will require the trained mechanics who can repair them.

Luckily, the aviation materials and technology toolbox has kept pace so that maintenance operations don’t get all hinky in the depot or the field when a composite repair is in order.

Electronic insight

The behavior of glass, carbon, boron or nanoparticle reinforcing fibers dispersed in a polymer matrix determines a composite part’s operating performance and so is analyzed extensively during design and manufacture. Obviously this data can be utilized to repair the component as well, which tells the story in a nutshell of the MRO application of VISTAGY’s FiberSIM composite design software.

By accessing a composite part’s original digitally-generated laminate specifications, FiberSIM can automatically guide every ply material type, location and orientation in sequence during a repair. This includes the creation of flat ply patterns in the exact shape needed for cutting prepreg or fabric, simulated drape of each ply and confirmation of minimal deformation under vacuum bagging or autoclave cure so that original strength and stiffness properties can be restored. Finally, the documented repair method and ply templates created with FiberSIM can be reused for similar repairs.

VISTAGY’s president and CEO, Steve Luby, says that the advent of composites into primary aircraft structure, such as several thousand new joint strike fighters (F-35s), "is bringing more science to the repair scenario by transferring manufacturing tools. This makes designing repair as important as designing the original 3D definition of the composite part." This synergy will be critical for repairing the 70 to 80-ply thick composite structures onboard the new 787.

Since military repair is often done out of the depot, VISTAGY and its customers have realized the importance of creating an audit trail of the accuracy of every repair. FiberSIM can be combined with the Automatic Ply Verification (APV) tool within the Laserguide ply location system from Assembly Guidance Systems to detect human factor errors in a repair and resolve them readily and cost effectively. Scott Blake, president of Assembly Guidance Systems, explains that Laserguide APV and the company’s handheld high accuracy manually positioned inspector (HAMPI) device captures and analyzes a digital image of the repair area so that an operator can validate material type, ply presence, ply orientation, and tolerance levels of ply edges.

Laserguide APV also generates a pass/fail analysis of the repair and formal report documentation for each part, including a digital archive. Blake reports that the APV 3D laser system "prevents scrap parts and also offers foreign object detection (FOD) should a full ply liner not be completely removed, which is a common human factor error that can significantly affect laminate performance."

Delta TechOps in Atlanta might be viewed as the poster company for FiberSIM’s value in composite repair. Todd Herrington, TechOps Support Engineer, tells AM that when Delta purchased version 1.4.2 of FiberSim in 2002, "we didn’t have plans for automated technology to assist in either the production of flat patterns or the positioning of patterns on the physical bond tool."

Using FiberSIM 5.0 today, he says Delta TechOps "has increased efficiency for ply design and importing flat pattern configuration to our Gerber cutter for quick repair or to manufacture quality, accurate plies. The time has easily been cut in half, if not better, by this upgrade. The interface is much simpler for the user, and the training needed is less intensive." Herrington estimates that the savings from operating the FiberSIM software suite for repair of the 452 aircraft in the Delta fleet main line is over $100,000 a year, and the investment is less than $10,000 a year.

Another application of FiberSIM is creating in-house repair tooling, as well as repair by re-engineering of parts. This capability has proven more cost effective than purchasing OEM replacement parts. Delta uses FiberSIM to generate repair and part manufacture protocols on 757-200, 767-200/300/400, MD 88 and C5 aircraft for fan and nose cowls, inboard aft flap trailing edge and spoiler bondments, inlet upper and lower skin assemblies, aileron skin and spar repair kits, fairing panels and main landing gear doors. In recent years, the group has extended services to the aircraft-on-ground (AOG) part manufacturing realm. In cases where an OEM no longer has original tooling for a part that needs repair or replacement, lead times can exceed 300 days, Herrington explains. By working with OEMs, Delta TechOps has developed response plans for delivering mission critical build information to the airlines so that CNC-machined molds can be created and AOG part manufacture accomplished. "This ability to engineer out-of-production parts has saved the airlines millions of dollars in out-of-service flight time."

Problematic leads times don’t just involve tooling. Mark Lloyd, lead engineer of composites, plastics and transparencies at American Airlines (AA) composite repair center in Tulsa, Okla., says that lead times to obtain composite materials are particularly frustrating — they have increased fourfold in the last five years, to 26 weeks for obtaining prepregs, film adhesives, and primers. AA conducts some 7,000 composite and metal bond repairs annually; at least half of these involve composites, with about 10 percent more glass fiber to carbon fiber reinforced materials. Up to 85 percent of all AA repairs originate from engine components such as ducts, liners, segments, inlets, and translating sleeves, as well as airframe flight controls, fixed panels, and interior items.

Military Coup

Even with the F-22 and F-35 coming into service, the U.S. Air Force continues to rely upon its F-16 and F-18 legacy jets. As of Oct. 2007, the USAF reported 1,280 F-16 C/D aircraft in operation, with a 20-year history compiled by this model. A unique hot bonder and vacuum bag repair addresses fatigue cracking in the fracture-critical 341 lower bulkhead.

This repair was originally designed by Southwest Research Institute and optimized for field installation by the Materials and Manufacturing Directorate of the Air Force Research Laboratory (AFRL) in 2006. Unlike most field repairs, this one involves thick airframe structure with complex geometry, and the bulkhead is expensive to replace (estimated 60 day in-depot repair time). During optimization, the AFRL determined that maintenance-induced damage to this component was occurring during main landing gear removal. Tension struts can impact the bulkhead and get a starter crack going in the flange radius that is attached to the boron/epoxy centerline keel beam.

The innovative bulkhead repair encompasses two aluminum doublers with a sacrificial and replaceable 0.648-cm-thick aluminum bumper plate over them, and a precured boron-epoxy keel beam doubler. All three doublers are adhesively bonded with two layers of epoxy film adhesive. Cure temperature was restricted to 260°F based on the aluminum’s thermal profile, and hot bonders controlled four separate zones. The location of the repair area is not easily accessible, and the boron/epoxy keel beam proved to be a large heat sink. The repair team customized a T-shaped heating blanket for the keel beam, added six layers of insulation around the repair area to optimize thermocouple locations in four zones, and applied hot air heating to the back of the bulkhead. This cut vacuum bagging time to about two hours.

For surface preparation of the composite keel beam, AFRL removed the peel ply on the doubler and grit blasted the beam prior to bonding. To keep grit out of engine and fuel areas, the team constructed a unique containment system from polycarbonate panels, bagging and breather material and tacky tape. By using this bonded repair, USAF estimates saving $16 million, primarily through the elimination of 63,400 maintenance hours and reduced downtime to one week.

Nondestructive inspection techniques were also developed to validate bond integrity of the patches, and impact testing proved that the repair can reduce stress at the flange radius cracking location by over 60 percent while carrying load, and stop fatigue crack initiation. The repair, impact testing and inspection protocol were successfully demonstrated last year and transitioned to F-16 engineering and maintenance personnel for fleetwide implementation.

As Good as Your Bond

The hot bonder may be one of the most indispensable items of repair equipment in the depot or field, and AM continuously covers the new features of various bonders ( AM August 2008, page 57). Wichitech Industries offers its HB-1 and HB-2 models with single or dual zone capability. Affordability and portability are key features of these bonders, and both have proven their merit over a decade of operating experience. At about the size of half a loaf of bread, the HB-1 is available in several different configurations, and priced from $4,200 to $10,000. The HB-2 has a larger footprint (21 inches long, 9 inches wide and 7 inches deep) and is priced in the $14,700 to $18,600 range.

Steve Caldwell, Wichitech’s president, describes this year’s explosion proofing as the lastest upgrade to the company’s bonders, "primarily for working on a fueled aircraft in an enclosed environment." While other bonders utilize inert gas purging to meet this requirement, Wichitech hermetically seals all elements of its bonders and includes an internal vacuum pump to speed up set-up time. "The units will work on voltages of 85 to 245v, frequencies of 50, 60 or 400 Hz, and can be run off a plane’s own auxiliary power. Eight TCs are standard, though 10 can be outfitted."

HEATCON Composite Systems provides hot bonders, heating blankets and other repair accessories, as well as composite training. Michael Landon, Regional Sales Manager for North and South America, highlights the company’s radome blankets, stretch blankets (with the abiility to deflect 35% of length and width in any direction), and customized form/fit blankets to ensure even heat distribution. Also, "our bonders have the ability to use up to 20 thermocouples for control and monitoring," he says. Two new models are now available: the HCS9200N-01, rated as common support equipment for use throughout the U.S. Navy and Marine Corps, offers electromagnetic interference shielding, and has been tested and certified to meet Mil-Std-810 and 461. The HCS9200M-03 unit, certified to meet Mil-Std-810, provides users with economical equipment for operating in hazardous enviroments.

Landon notes that HEATCON’s HCS9200FL bonder has been tested and certified to meet multiple standards and selected for F-22 and F-35 composite repair. He points out that the company’s new HCS3100 Repair Clave is designed to provide an affordable solution for shops that require heat and pressure to perform repairs, and can also accommodate complex tensioning procedures. For repair and remanufacture of aluminum/composite structures, the Repair Clave utilizes heat blankets and vacuum bagging along with positive pressure from shop air.

Times A’ Changin

The mix of materials in aircraft is changing, and the mechanic’s toolbox seems to be growing an electronic brain. Yet the artful tools of patience and finesse in a trained and experienced hand remain critical to the successful restoration of a composite part, or any aircraft part for that matter.