The epoxy adhesive utilized in this research was D.E.R. 332 epoxy resin cured with Huntsman Jeffamine T-403. To reduce residual stress in the adhesive, the specimens were cured for 48 h at room temperature followed by a 2 h cure at 130 ºC. Borosilicate glass cut into the appropriate dimensions was used as the substrate.
SLBT specimens were (3.5 x 101.6 x 101.6) mm3 squares with an 8 mm diameter hole in the center. A pre-crack was fabricated by placing a 0.95 cm diameter piece of Kapton pressure-sensitive adhesive tape (PSAT) over the hole in the center of the quartz substrate. The Kapton PSAT consists of a 25 µm (1 mil) thick Kapton backing and a 37.5 µm (1.5 mil) thick acrylic pressure sensitive adhesive. The tape also provided additional mechanical reinforcement to the thin epoxy film at the highly stressed contact area between the coating and shaft-tip. To prepare the specimen, first the un-cured model epoxy was coated on to the quartz substrate. A 50 µm (2 mil) thick piece of Kapton-E film (no PSA) was then placed on top of the epoxy coating. The Kapton-E film also acts as a mechanical reinforcing layer for the epoxy coating. The resulting adhesive coating is therefore a composite of the model epoxy, Kapton PSAT (located solely in the center) and Kapton-E film. The average epoxy film thickness of each sample ranged between 100 µm and 150 µm ± 10 (based on one standard deviation and 4 thickness measurements).
To develop a sub-critical blister test specimen the sample must be self-loading, the integrity of the thin adhesive coating must be maintained, and the sample must be exposed to the environment. A schematic of the subcritical SLBT specimen is shown in Figure 1. To fabricate a simple self-loading constant load SLBT specimen, a hole is punched in the coating that can accommodate a 2.54 mm (or 4/40") stainless steel machine screw. The machine screw acts as the fastener from which to suspend the load (via a flexible copper wire). The punched hole containing the screw is sealed with a room temperature cure epoxy (Devcon 2-ton epoxy). The entire SLBT specimen was placed in a sealed vessel and conditioned at constant r.h. at room temperature using saturated salt solutions [19-21].
Figure 1. Schematic of the constant load sub-critical shaft-loaded blister test.
The strain energy release rate or crack driving energy, G, was calculated using the load-based equation (equation 1) [22]:
 |
Equation 1. |
where P is the load, a is the blister radius, E is the Young's tensile modulus of the composite film, and h is the thickness of the adhesive coating. The modulus of the coating (Ec) for the bi-layer film (Kapton and epoxy) can be estimated from a simple rule of mixtures (Ec = S vi Ei).
DCB samples were prepared from rectangular glass specimens (0.98 x 25 x 75) mm3. To prepare the specimens, a Teflon shim 0.127 mm thick was used to control the thickness of the adhesive layer and to confine the adhesive between two glass rectangular adherends. The shim was cut so that a strip of adhesive (12 x 56) mm2 was left in the center in the DCB once the shim was removed. The strain energy release rate was calculated using the following expression:
| Equation 2. |
where ∆ is the end opening, B and w are the width of the adherends and adhesive, respectively, t and h are the thickness of the adherends and adhesive, respectively, and E and Ea are the corresponding Young's moduli of the adherends and adhesive. A stainless steel razor (∆ = 0.25 mm) was used as the wedge. For both the SLBT and DCB sub-critical experiments, a micrometer was utilized to periodically measure the debond length. Like many adhesive tests, the uncertainty due to sample to sample variation is much greater than the uncertainty introduced by measuring the debond length. The average debond velocity, v (da/dt), was determined from the change in average debond length (a) over the elapsed time between measurements.