Final Report - Strategic Environmental Research and Development ...

5.4.4 Results and Discussion.
5.4.4.1 Effect of coating thickness on stress at fracture of blank primer.
Blistering is unfortunately a common phenomenon in coated samples. Most of the primers used
in aerospace application are solvent-based epoxy primers, which are characterized by having low
elongation and high stiffness and therefore they are brittle. The purpose of primer application is
to create a pigmented slightly permeable barrier that adheres to the substrate. The pigments are
present for corrosion inhibition and therefore the primer needs to be slightly permeable for the
pigment to solubilize and inhibit corrosion of the substrate. However, blisters can be created due
to an internal pressure, which can be driven by osmotic blistering. A blister in a coated sample
can be assumed to be a spherical cap. If the pressure inside the blister increases, the stress can
reach the fracture stress and produce at least a microdefect, which will expose the substrate to the
corrosive environment. Therefore, the stress at fracture is an important mechanical property of
the primer that needs to be studied and no other test offers a better simulation of a blister than the
BT.
Samples were coated with blank primer for this specific study. The substrate was cleaned and
deoxidized 24 hours prior to primer application. Coated samples of different thicknesses were
prepared by applying several layers of primer. Figure 4.4 shows representative behavior of the
coated samples during the BT. The primer fractured for all of these samples. From the Griffith’s
failure criterion, a crack will propagate if the strain energy release rate is equal or larger than a
critical value. In this specific case, the crack is defined as the location where de-adhesion of the
organic coating from the substrate occurred. Therefore, it is expected that the pressure x radius
product, pxr, would reach a critical value that would be kept constant if the energy is continually
supplied to the system. When that critical value is reached and maintained, the crack propagates.
This critical value should be achieved at the highest pressure. However, the product Pxr
increased during the test without reaching a plateau and the radius started increasing after 10,700
seconds, before reaching the maximum pressure. This onset of blister growth before the
maximum pressure has been found by other authors. Gent [3], Kappes [4], and Wang [5] found it
in pressure-sensitive tapes adhered to PMMA and Teflon, carbon steel, and PMMA respectively.
This behavior has been related to the visco-elastic properties of the adherent layer. The criterion
that will be used to determine adhesion strength will be to take the maximum pxr value.
Nevertheless, there is not enough data to make these calculations due to the primer fracture. The
pressure and radius at rupture were used to calculate the stress at fracture, which is calculated
using the following equation [2],
pa
(Eq. 4.2)
2t
where σ is the stress in the blister, p is the pressure inside the blister, a is the radius of the sphere,
and t is the primer thickness. The sphere radius can be calculated from the radius of the blister
area on the substrate, r, and the blister height h:
r
2 2
h
a (Eq. 4.3)
2h
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