TASK 70-71-06-700-007
1 . General.
This procedure describes the method and test specimens required for a climbing drum peel test to determine the strength of a face sheet-to-honeycomb bondment. Use this test procedure when specified in Standard Practice film adhesive bonding procedure or Engine/Shop Manual repair.
2 . Equipment.
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A.Tooling to prepare specimens.
B.Specimens per Figure 3 and Figure 2.
C.Peel test apparatus per Figure 1.
Figure 1   Climbing Drum Sandwich Peel Test
Figure 2   Sandwich Peel Specimen
Figure 3   Test Specimen Bondment
3 . Materials.
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D.Material to be tested is specified in each Engine/Shop Manual repair.
4 . Preparation of Specimen.
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E.Panel assemblies are fabricated sufficiently large to cut into 4 each 3 x 12 inch (76 x 305 mm) specimens. When repair also calls for flatwise tensile tests ( TASK 70-71-07-700-008, Adhesive Bond Strength Flatwise Tensile Testing), the panel should be constructed large enough to contain specimens for both tests. See Figure 3.
(1)The panel assembly consists of 2 face sheets and a honeycomb core. These pieces are to be cleaned, prepared and primed in an identical manner and concurrently with the detail parts of the repair being performed.
NOTE:
All edges of the metal panels and specimens which will be within (or which will bound) the joints shall be machined true(without burrs or bevels and at right angles to faces) and smooth(16 AA maximum) before the panels are surface treated and bonded.
(2)Unless otherwise specified by Engine/Shop Manual repair, both face sheets are made 10 x 28 x 0.020 inches (254 x711 x 0.51 mm) 2024-T3 aluminum alclad. The face sheets are to be etched and primed identically to the detail parts in the repair process.
(3)Unless otherwise specified by the Engine/Shop Manual repair, the honeycomb core is made 10 x 28 x 0.50 inches(254 x 711 x 12.7 mm) high, 7.8-1/4-40 (5052) NP aluminum honeycomb core. The ribbon direction is parallel to the 28 inch(711 mm) dimension. The core is degreased, dried, primed and baked identically to the detail parts in the repair process.
(4)The panel assembly is made with one sheet bonded to core with 0.095 lb/sq. ft. supported adhesive and the other with 0.075 lb/sq. ft. unsupported adhesive. On the sheet being bonded with the supported adhesive, mark the word "up". Bond this sample specimen panel to a procedure identical to that used in the Engine/Shop Manual repair.
NOTE:
Prior to bonding unsupported adhesive, a hole should be pricked in each cell to aid reticulation (beading). This may be accomplished by perforating the adhesive on a bed of phonograph needles, spaced to conform to honeycomb cell width. Back up adhesive with plywood covered with a sheet of rubber.
(5)Upon completion of the specimen panel bonding, strip the skin from the supported adhesive side, which was marked "up". This produces a specimen which should be similar to and is to be processed with the parts being repaired.
(6)Prepare the surface of the honeycomb for a second bond. Clean off excessive adhesive, degrease, dry, prime and bake identically to the detail parts in the repair process. Prepare the face sheet for a second bond, etch and prime as for detail parts of the repair.
(7)Bead unsupported adhesive to the prepared honeycomb surface. A heat gun may be used. Prick each cell to reticulate adhesive to cell wall. Apply supported adhesive to the prepared surface of the face sheets. Be sure that the face sheet is marked "up".
(8)Bond the specimen panel assembly identically to and con-currently with the detail parts of the repair being performed.
NOTE:
Process control specimens may be bagged on a separate tool provided that the vent line from this tool is interconnected with at least one of the vent lines from the assembly being bonded. Interconnection of the vent lines shall be accomplished in such a manner that any pressure variation on one assembly will be accurately reflected on the other.
(9)Cut 4 climbing drum peel specimens from the panel assembly (see Figure 3 and Figure 2). Identify the "up" side on each specimen, as in paragraph 4.A.(4). Panels should not be cut into specimens until at least 24 hours after bonding. Panels may be cut with a bandsaw with the setting and spacing of the teeth and operational speed adjusted to hold frictional heating of the bond to a minimum.
NOTE:
Ribbon direction of the core is to be parallel to the long side of the specimen.
(10)Prepare specimens by cutting back one face sheet and the core at both ends. See Figure 2.
5 . Testing.
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F.A suggested peel test apparatus consists of a drum with flanges, flexible loading straps around the flanges and clamps for holding the test specimen. See Figure 1. The drum should have a counterweight, diametrically opposite the clamp and of such a mass as to balance the drum about its axis. The drum with flanges and clamp and the counter-weight should weigh 8 lbs. (3.6 kg) maximum. The drum radius is 1.995-2.005 inches (50.67-50.93 mm). The drum clamp fixes the specimen face sheet such that its center-line lies on the drum radius. The flange radius is to the centerline of the strap and is 0.495-0.505 inch (12.57-12.83 mm) greater than the drum radius.
G.The test setup is made by clamping the sandwich securely in place at the top (see Figure 1) and loading the apparatus in tension at a head speed of 1 ± 0.01 inch (25 ± ??? 0.3 mm) per minute. A head speed of 2 inch (51 mm) per minute peels the sandwich facing at a rate of 4 inches (102 mm) per minute. The test machine should be capable of measuring load accurately within 1 percent. Prior to the start of the peeling test, the specimen and apparatus is suspended from the top head of the testing machine and the weighing apparatus balanced to zero. The loading bar is then pinned to the lower fixed fitting and an initial load (FO) is applied that is equal to that required to overcome the resisting torque exerted by the weight of the drum and that required to bend the facing.
NOTE:
This load (FO) can be determined by inserting a strip of metal of the same width, thickness, and material used in the facing of the sandwich panel to be tested in place of the facing of the sandwich specimen and applying a load sufficient to roll the drum upward around the metal sheet.
H.During the peel test, an autographic recording is made on the load (Fp) versus head movement or of the load versus distance peeled. The average load required for peeling shall be taken from the autographic curve for the 5 inches (127 mm) of peeling of the sandwich specimen and between 1 and 6 inches (25 and 152 mm) corresponding to 1.25 inches (31.8 mm) of head travel after the first 0.25 inch (6.4 mm). The average load is determined by scaling the curve by means of a planimeter.
NOTE:
For an alternate method, if autographic equipment is not available for acceptance testing, the approximate average may be obtained by averaging recorded loads at fixed increments of time after the start of the test. The load should be recorded 15 seconds after the start of the test and at each 5 second interval thereafter until 6 inches (152 mm) of the specimen are peeled.
NOTE:
During testing, specimens must be at a stabilized temperature of 70°-80°F (21°-27°C).
I.To peel specimen, load Fp is applied to the loading bar (see Figure 1) to peel as in paragraph 5.C. The torque required to peel the adhesive bond is calculated from the formula:
Torque = (Fp-Fo) (R2-R1) divided by W
where R1 and R2 are the 2 drum radii (see Figure 1) and W is the width of the specimen to the nearest 0.010 inch (0.25 mm). See Figure 2.
J.The required torque must meet or exceed the minimum values given in Table 1 (Process Control Specimen Strength Requirements) in TASK 70-70-01-700-001, Testing and Quality Analysis for Metal-to-Metal Bonding.