SAMPE Journal - January/February 2017 - 28

Feature Article
Figure 10. Crack length versus number of
cycles for reference samples, hybrid
bondline specimens and
pinning samples.

maximum tensile load of 9.28 kN is
used for all fatigue tests here. This
load level equals 35% of the static
tensile strength of the specimen (as
determined beforehand) and a strap
strain of 3000 µm/m (microstrain) in
the undisturbed area. This load level
considered to be most relevant since
3000 microstrain is often referred
as the maximum limit load strain
for aeronautical applications and
represents the relevant service load.
Furthermore, a stable crack growth
rate is determined for this load in27
that allows evaluation of the crack
stoppers.
A set of three specimens each is
tested for undisturbed reference
samples, hybrid bondline samples,
and pinning samples.
It is mandatory to measure the
developing crack in its length for
fatigue investigations. Therefore,
two microscopes (one for each side)
are used. To obtain a crack-opening
of high contrast, both edges are
painted in white by the use of paintspray. A mm-scale is manually
plotted on the surface for crack
length determination. Additionally,
one microscope is mounted to an
electric travel measurement system
allowing a crack length reading with
the accuracy of ±0.01 mm. Crack
growth readings are made every 28
000 cycles which equals a testing
time of 1 hour.
28

Test Results

Fatigue Testing of CLS Reference Tests
Comparison of the crack length
readings for both edges show only
minor deviations of the crack length
(over the specimen's width) of ±2
mm. Thus, it can be concluded
that the crack propagates almost
perpendicular to the direction of
loading. Averaged values of both
sides are used for evaluation of crack
propagation.
As shown in Figure 10, deviations
occur for both crack initiation
and crack growth rate for the
3 specimens tested. However,
deviations are considered to be
minor. Independence of the crack
growth from the actual crack length
is proven true by the predominant
linear course of the crack length
versus cycle number graphs. The
averaged crack growth rate da/dN
(amount of crack growth per load
cycle) is determined to 0.119 µm
per cycle with a standard deviation
of 4.3%. A thin layered cohesive
fracture pattern is observed for all
specimens without any delamination
as shown in Figure 11. Thus, surface
activation by use of atmospheric
plasma treatment is successful since
no adhesion failure is observed.

Fatigue Testing of the Hybrid
Bondline Concept
For hybrid bondline samples, crack
initiation and crack propagation
for the first 7 mm after the zig-zag
release film area is comparable
to reference samples (Figure 10).
Hence, no influencing effect of
the thermoplastic area is found.
However, directly ahead of the
weld (between 7 mm and 15 mm
crack length) crack growth is faster
than for the reference samples. The
average crack growth rate is here
determined to 0.174 µm per cycle.
Thin layered cohesive failure is again
observed as fracture pattern for
the epoxy adhesive. Nevertheless,
crack growth is fully stopped after
reaching the thermoplastic barrier at
a position that equals 15 mm of crack
length. No further crack growth is
found for the 106 load cycles tested
for all specimens tested.
Fatigue Testing of Pinned Samples
The pinning samples showed
a slightly different behavior. No
influence of the pins is found for 13
mm of crack growth, i.e. just ahead of
the pinning area. Fractured occurred
as thin layered cohesive failure type
as for the other samples. Crack arrest
is measured between 14 mm and 15
mm crack growth in between both
rows of pins as shown in Figure 10.
No further crack growth is found
for the 3 samples tested up to 106
load cycles. Thus, full crack stop is
achieved for the pinning specimens
as well.
Static Residual Strength Tests
Residual tensile strength tests are
conducted for all hybrid bondline
and pinning specimens after fatigue
tests. The results are summarized
in Table 3. Additionally, the results
are compared with the static tests of

SAMPE Journal, Volume 53, No. 1, January/February 2017



Table of Contents for the Digital Edition of SAMPE Journal - January/February 2017

Contents
SAMPE Journal - January/February 2017 - Cover1
SAMPE Journal - January/February 2017 - Cover2
SAMPE Journal - January/February 2017 - Contents
SAMPE Journal - January/February 2017 - 2
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