Dynamic Force Distribution
testing of a coating is a destructive test. And because the coating
is tested to destruction a test cannot be repeated (unlike e.g.
measuring the dimensions of an object, which can be done repeatedly).
A pull-off test result cannot therefore be accurately checked, only compared
with a similar test done elsewhere. So
with no proper method of checking your test result you must instead
be able to trust your test equipment absolutely.
The most critical factor in pull-off testing is
trying to stress the material evenly during testing. Even
microscopic misalignment of the testing machine (which cannot be
controlled by mechanical, optical or other manual means) will cause
substantial unwanted stress concentrations in the tested material,
resulting in premature fracture of the sample.
choosing test equipment it would be a serious mistake to focus on a testing
machine's calibrated accuracy alone (which can be easily checked)
without taking into account the machine's ability to distribute the
pull stress evenly.
if the actual force applied has been
calibrated as accurately as you want, fracture may still
occur 10%, 20% or even 80% too soon. So to record a test
result of, say, 6.23 MPa with an accuracy within ± 0.001 MPa when
the result should have been eg.13 MPa, is not very
DFD® testing technology takes force, or stress distribution into account.
one occasion, five tests done with the DFD® technology on a
homogeneous material produced a repeatability (standard deviation)
of less than 0.5% of the average value. The only way of achieving
such repeatability is to measure at least 99.5% correctly every
time; random testing errors cannot be repeated with such
consistency. Most types of materials and coatings however are far
from homogeneous, so a higher deviation is normally expected.
The DFD® technology will always give as low deviation
as the tested material allows, not a higher figure due to misalignment or other
The Three Stages of the Evolution of Adhesion Testing
1) Manual alignment System
The first generation pull-off adhesion testers had
mechanical screws for adjusting the legs to achieve the
alignment. The fact was that however experienced the test
operator was the "alignment" was never sufficient. We now
know that even microscopic misalignments (A and B) (exaggerated for
illustration purposes) due to e.g. an uneven
surface under the test dolly (C) or uneven glue thickness
would have ruined the test. The pull
force was always concentrated on one side of the specimen causing
premature fracture at values far lower than the true strength.
2) Universal Link Type Mechanical Alignment
second generation adhesion testers attempted to deal with the
alignment problem by adding a universal link (D) between the test
element and the pulling device. The principle could either be
as illustrated on the right or other similar mechanical
devices. Even though the force indeed had been balanced
between the push legs (A) and (B) it had not solved the problem of
test dollies which were glued on an uneven surface (again, even
microscopic misalignments caused problems). The result was
that the force was concentrated on one side of the test dolly and
ripped off prematurely (C).
3) True Dynamic Self-Aligning System
the PAT tester was designed the question we asked was not
"How to achieve a perfectly perpendicular pull
direction". Perpendicular to what? The coating could be
on a pipe, a spherical surface, an uneven surface, even a flexible
surface. Only one thing was certain: It was never perfectly
flat! So instead the following question was asked: "How can the pull stress be distributed 100% evenly regardless
of substrate shape?"
The result was the patented DFD® testing principle
which is now being used world-wide.
The system continuously monitors the whole pressure
build-up period, self-correcting any imbalance thereby retaining
100% evenly distributed pull stress until fracture occurs at the correct
No mechanical system can detect the microscopic changes during
materials stress build-up and no other
system can take the corrective actions needed to retain the
||Animation of a PAT testing head in