CF to Aluminum Bond Test (2nd Iteration)
Introduction
In the 1st iteration (Tensile test), we suffered from adhesive failure (skill issue) at low pullout force. In this second test, we will be generalizing the test process a little more. For the purpose of time, money, and ease of manufacturing – we will use a lap shear strength test of a carbon fiber and aluminum strip to get the basics down first.
We will also need to improve the design and manufacturing process of our aluminum inserts. In the compressive test, the flange of the insert sheared off.
Scope
The scope of this test is to find the best method for assembly by testing 4 interface scenarios. This is not to gauge the strength or characterize the material properties of the adhesive. Best is defined as least work for highest strength outcome.
Room Temp Cure - ‘Rough’ side to aluminum (Control)
Room Temp Cure - ‘Smooth’ side to aluminum
Oven Cured @ 150°F - ‘Rough’ side to aluminum
Silica Beads - ‘Rough’ side to aluminum
For a 1 x 1” area, maximum for lap shear is 21kN
ASTM Standards
*D1002 - Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal)
Important notes:
6.1 - The recommended length of overlap for most metals of 1.62 mm (0.064 in.) in thickness is 12.7 +/- 0.25 mm (0.5 +/- 0.01 in.).
9.2 - Apply the loading immediately to the specimen at the rate of 80 to 100 kg/cm2 (1200 to 1400 psi) of the shear area per min. Continue the load to failure. This rate of loading will be approximated by a free crosshead speed of 1.3 mm (0.05 in.)/min
D3165 - Strength Properties of Adhesives in Shear by Tension Loading of Single-Lap-Joint Laminated Assemblies
Important notes:
5.1 - The machine shall be capable of maintaining… approximately 1.27 mm/min (0.05 in./min) crosshead speed. It is recommended that the jaws of these grips shall engage the outer 25.4 mm (1 in.) of each end of the test specimen firmly
6.1 - The recommended thickness of most metal sheets is 1.62 +/- 0.125 mm (0.064 +/- 0.005 in.). The recommended length of overlap for most metals of 1.62 mm (0.064 in.) thickness is 12.7 +/- 0.3 mm (0.50 +/- 0.01 in.)
6.2 - To avoid yielding of the test coupon, maximum length of bonded area is given by equation (1)
Pretty Much same as D1002
*D2651 - Preparation of Metal Surfaces for Adhesive Bonding
Important notes:
Something about vapor degreasing
5.5.4 - Water break test. Distilled water is put on the coupon to test surface energy, no droplets should form if the surface is properly prepped.
7.2 - For aluminum: A typical pretreatment would normally involve the following steps: 7.2.1 Vapor degrease or non-etching alkaline cleaner, 7.2.2 Rinse, 7.2.3 Acid or alkaline, and 7.2.4 Rinse.
D4896 - Use of Adhesive-Bonded Single Lap-Joint Specimen Test Results
Important notes:
3.6 - Definitions: Creep, shear strength, strain, linear strain, shear strain, stresses (apparent, average, cleavage, peel, concentration, true), thick and thin adherend
Apparent shear strength is highly dependent on joint geometry, adherend stiffness, bondline thickness, and cure history, so results cannot be directly extrapolated to different joint designs
Single-lap tests are best used for comparative screening and process evaluation, not for determining allowable design stresses in structural joints
ISO Standards
ISO 4587 - Adhesives — Determination of tensile lap-shear strength of rigid-to-rigid bonded assemblies
Important Notes:
4.1 - tensile-testing machine, selected so that the rupture of the specimen falls between 10-80% of the full-scale capacity… The machine shall be provided with a suitable pair of self-aligning grips to hold the specimen.
5.1 - The length of overlap shall be 12.5 ± 0.25 mm.
7 - Locate the specimen symmetrically in the grips, with each grip 50 ± 1 mm from the nearest edge of the overlap. A shim may be used in the grips so that the applied force will be in the plane of the adhesive bond. Operate the machine at a constant test speed so that the average joint will be broken in a period of 65 ± 20 s. If a machine working at a constant rate of loading is used, apply the shear load at a rate of 8.3 MPa to 9.8 MPa per minute.
ISO 868 - Plastics and ebonite — Determination of indentation hardness by means of a durometer (Shore hardness)
Important Notes:
5.1 - Thickness of specimen at least 4mm
5.2 - Poke specimen at least 9mm away from any edges
8.1 - Apply the presser foot (4.1) to the test specimen as rapidly as possible, without shock, keeping the foot parallel to the surface of the test specimen. Recommended masses are for the type A durometer and for the type D durometer.
8.2 - Make five measurements of hardness at different positions on the test specimen at least apart and determine the mean value.
Materials and Tools
Carbon Fiber Coupon (1” x 3” x 0.25”) x 14
12” x 36” sheet
Aluminum Coupon (1” x 3” x 0.25”) x 28
5” x 24” x 0.25” sheet
Acetone
Isopropyl Alcohol (IPA)
Silica Beads
Instron Machine
Self Aligning Grips
Shore D Durometer | Hardness Tester
Methodology
Prepare Testing Materials
Layup CF
4” x 19” (only need 3” x 18”)
[0,0,0]T
Cut coupons
Cut Aluminum coupons
Create Template
Mix epoxy and apply to coupons in template
Pour mixed epoxy in a shallow cup (For cure validation)
After 24 hours, poke the epoxy in the cup using a Shore D durometer
If the minimum hardness is 76, the coupons have finished curing
Set up Instron
Method 1 - 5 samples (control)
Method 2 - 3 samples
Method 3 - 3 samples
Method 4 - 3 samples
Action Items
Future Possible Testing:
Chemical Etching:
Notes
Unidirectional Carbon Fiber will be used to reduce strain in the test and mimic the interior of CF pushrods and bulkheads
Template will be 3D printed for coupons bonded at room temp. For oven curing test, print using CF Nylon or machine out of aluminum as these materials won’t be significantly affected for our temperature regime. A fiberglass layup, will be used for this experiment using a resin-epoxy with high glass transition temperature to prevent creep.
For fixtures that must go in an oven, industry standards include:
Machine templates out G10 Fiberglass
Machine out of aluminum
Steel for tight tolerances (overkill)