Electric heating – Metal heating – By arc
Reexamination Certificate
2001-04-30
2003-03-04
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
By arc
C219S121600
Reexamination Certificate
active
06528763
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the detection of exfoliation corrosion in metal structures, and in particular, to a method for detecting exfoliation corrosion in the aluminum wing and fuselage sections of aircraft structures.
2. Description of the Related Art
Over the years, many different nondestructive evaluation methods or inspection techniques have been used such as electromagnetic, thermal, ultrasonic, radiographic and optical methods to inspect for hidden cracks or corrosion in the wing and fuselage sections of aircraft.
Exfoliation corrosion refers to the physical appearance of a specific type of intergranular corrosion which is layered or leafed in character and consists of alternating strata of corroding and non-corroding metal. It typically occurs in high-strength aluminum-alloy rolled sheet or plate which has a laminar-like microstructure consisting of grains flattened in the plane of the sheet or plate. The exfoliation or corrosion attack occurs along the grain boundaries in the rolling plane and predominantly in the rolling direction of the aluminum sheet or plate.
Exfoliation corrosion is a very common form of corrosion in precipitation-hardened 7XXX series aluminum alloy (AL—Sm—Mg) wrought products. Precipitation hardening, the process used to heat-treat aluminum alloys to the -T6 condition, tends to produce a somewhat continuous precipitate of Al—Zn, Al—Mg, and Mg—Zn intermetallic compounds in the grain boundaries of this series of alloys. These intermetallic compounds have an electrochemical potential anodic to the surrounding aluminum, hence causing them to preferentially corrode in certain corrosive environments. With the relative area of cathode (surrounding grains) to anode (small intermetallic particles) being quiet large, this attack can progress very rapidly with the grain boundary corrosion product wedging or separating the grains apart and causing the uncorroded material to lift or leaf. This effect produces the characteristic “blister” appearance of exfoliation. Current generation aluminum alloys, 7050 and 7150, exhibit high strengths in -T6-type tempers and when they are processed to the -T76, -T74 or -T73-type temper, to improve their resistance to stress corrosion cracking and/or exfoliation corrosion although this improvement is often achieved at some cost to strength vis-a-vis the -T6 condition. However, using these tempers does not resolve the exfoliation corrosion problem for aluminum alloys, such as, 7178-T6 still in use today that were produced prior to this current generation of alloys.
The typical aircraft application for these alloys is on the upper wing skins where failure originating from exfoliation corrosion is a significant concern to prevent failure of the wing structure. Exfoliation corrosion detection is required for early and accurate detection on all aircraft. The exfoliation corrosion is sometimes evident on the surface of the wing as it can cause the paint coating to have a blistered appearance in the corroded areas. When detected, these areas are sanded to remove the gross exfoliation corrosion until it is no longer visually observable. However, exfoliation corrosion can still be present in the sanded areas that is not visually observable even at higher magnifications. It may also be present in areas where exfoliation corrosion was not previously observed and is not detectable on the surface. This “hidden exfoliation” occurs when the amount of corrosion products between the grains is relatively small and has not caused the grains above the corroded boundaries to lift or separate from the subsurface grains.
Over the years, many different NDE methods or inspection techniques (electromagnetic, thermal, ultrasonic, radiographic and optical) have been developed and used to inspect for hidden cracks or corrosion in the wing and fuselage sections of aging aircraft. Each of these candidate techniques has its own inherent performance and detection limitations; therefore, new techniques or tools are being developed and validated for both production and field use to improve detection capability and reliability, and to reduce the cost of inspection.
Inspection for hidden exfoliation corrosion is typically conducted, for example, on the aircraft upper wing surface around fastener holes. The fastener holes are sometimes filled with steel rivets and a galvanic cell is created between the surface of the steel rivet and the surface of the hole in the aluminum wing skin, accelerating the rate of corrosion along the grain boundaries from the surface of the hole. There are several methods to locate the hidden exfoliation corrosion, but the current method utilizes a glass bead shot peening method and is called a search peening process. This search peening process is up to 95% efficient in detecting exfoliation corrosion. Once an area of corrosion has been detected, the area is sanded until the exfoliation corrosion is no longer visible in the sanded area. The area is then again glass bead search peened and inspected for additional exfoliation corrosion, revealed by the bead peening. This process is repeated until no more hidden exfoliation is detected after glass bead search peening.
Glass bead peening for inspection for exfoliation corrosion works by producing a compressive residual stress and cold work into the metal surface being inspected. These effects cause the surface material in areas having underlying grain boundary corrosion to exfoliate, i.e., blister or “leaf up,” and thereby expose only underlying corrosion. If there is no underlying corrosion, exfoliation does not occur.
Although generally effective, this method for exfoliation corrosion detection has several drawbacks. For example, it takes a significant amount of time to isolate the wing being search peened to contain the peening media and also to extricate the glass bead media from the surface being inspected. These requirements add three to six days to the Program Depot Maintenance (PDM) cycle for an aircraft. In addition to the added time, the additional workload increases the cost for maintenance of the aircraft.
Accordingly, the need exists for a cost-effective and time-efficient exfoliation corrosion detection apparatus and method which enables rapid inspection and evaluation of significant portions of the aircraft for exfoliation corrosion.
SUMMARY OF THE INVENTION
The present invention satisfies that need by providing an exfoliation corrosion detection method that enables rapid inspection for the detection and evaluation of exfoliation corrosion on aircraft with related cost savings.
The present invention utilizes laser shock peening to produce the compressive residual stresses and cold work into the metal surface, necessary to expose hidden exfoliation corrosion. Laser shock peening is currently being used in production to produce deep compressive residual stresses in titanium compressor blades and now this same technology can be adapted to search peening. The depth and magnitude of residual compressive stresses can be tailored by the application of specific overlays and controlling the intensity of the laser beam.
In the laser shock peening process, the surface of the part is first covered with two types of overlays, one transparent to the laser beam and the other opaque to the laser beam. The opaque overlay is applied directly to the surface of the part. The opaque overlay is typically paint or tape and has three functions. The first function is to protect the surface of the part from the intense heat of the plasma plume generated during the laser shock peening process. The second function is to enhance the strength of the shock wave from the plasma. The third and final function is to provide a consistent processing medium for the laser beam to couple to. The transparent overlay is typically water and is applied over the opaque overlay. The primary function of the transparent layer is to confine the plasma plume against the surface of the part in order to generate higher peak pressures during the laser shock peenin
Clauer Allan H.
Dulaney Jeff L.
Lahram David F.
Dunn Tom
Knuth Randall J.
LSP Technologies Inc.
Pittman Zidia
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