JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition - 40

testing

measure and analyze the 3D displacement and strain of the entire field for at
least one representative sample of each
specimen type (Aramis, v6.2). For fullfield coupon measurement, a consistent
measuring volume of approximately
175 mm by 150 mm was used with a
relatively consistent depth since out-ofplane displacement was significantly less
than in-plane displacement during these
tests. As such, a set of 17 mm lenses were
used and the resulting pixel size was
approximately 0.005 mm2. To improve
accuracy and resolution during the open
hole tension (OHT) testing, a different
lens and smaller measuring volume
were used, 50 mm and approximately
50 mm by 42 mm, respectively with a
pixel size of 0.0004 mm2. Load data was
input automatically from the load cell
and tabulated by the system allowing for
immediate analysis of stress and strain.
Image capture rates were varied between
0.5 and 3 Hz throughout the testing.
For each case, strain analysis and stress/
strain curves were compared for localized
areas, using full-field and far-field where
appropriate.
Laminates were manufactured with CPly material in a [0,±30,0,±30,90,±60]
S configuration and cut into coupons
approximately 25.4 mm wide with a thickness of approximately 2.34 mm. A wider
coupon, approximately 38.1 mm with a
6.4 mm centre hole, was used for open
hole testing. A quasi-static, monotonic
tension test was performed on each coupon and DIC was used to capture strains
during loading. In addition, strain gauges
were used on coupons without holes for
direct comparison to DIC data.
It is clear from Fig. 2, left, that the
full-field strain allows visualization of
not only the average strain, but also
localized strain concentrations. It is
interesting to note that initial failures
occur near the edges and then delaminate at the approximately 30° oriented
plies. Regardless, the initial average fullfield strain was found to be consistent
between the DIC (circles, squares, and
triangles) and strain gauges (lines) both
longitudinally and transversely, as noted

Fig. 2: Full-field strain of a laminate with initial fibre
failures just prior to ultimate failure (left), and outof-plane displacement associated with unbalanced
laminate (right)

in Fig. 3. This resulted in consistent
modulus values in both directions of
approximately 10.2 GPa. Comparing
the longitudinal DIC and strain gauge
data, it is apparent that beyond 0.3%,
strain varied between the two methods
of strain acquisition. This variation may
be attributed to the larger area, yet higher
point resolution of the DIC. In short, the
strain gauge offers one value that is the
local average of the area the gauge covers.
However, DIC is able to measure strain
related to the pixel size, which is several
orders of magnitude less than the area of
the strain gauge, allowing for a comprehensive visualization of the strain in
the coupon during testing. Further, it is
interesting to note that the out-of-plane
effects may also be captured as noted
in Fig. 2, right, where a variation of Z
displacement is noted across the sample
due to the non-zero terms in [B]. This
variation was noted with strain gauges
as well by variations in the calculated
Poisson's ratio depending on which side
of the coupon the strain gauges were
placed. Again, the calculated values were
consistent between the DIC and strain
gauges.

Both methods are
relatively consistent
Since both methods are relatively
consistent in determining initial modulus
and Poisson's values for a laminate, it was

40 jec composites magazine / No88 April 2014

Fig. 3: Longitudinal (up) and transverse (below)
stress-strain curves comparing strain gauge (lines)
and DIC (circles, squares, triangles) data collection
methods

suggested that DIC may be useful for
determining these properties while also
capturing damage around a hole during
an OHT test. It is clear from Fig. 4 that
the damage progression and areas of
increased strain due to the hole are visualized. Two measuring volumes were used
for OHT testing as noted in Fig. 4, where
the full coupon was measured (left) and
a higher resolution volume was used to
determine the effects surrounding the
hole (right). First, for the full coupon,
average strain was compared between
the full field and the far field which
was calculated from approximately one
eighth of the entire area at the extreme
ends of the coupon to ensure the effects
of the hole were minimized. As noted in
Fig. 3, little variation between these two
measurement methods was noted and
no difference was found in calculated
laminate properties. This indicates that
the average of the increased and zerostrain areas around the hole are similar
to the overall average strain. Further
support of this observation was noted
once the measuring volume was reduced
(Fig. 4, right), where the localized area
more closely matched the results seen

JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition

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