JEC COMPOSITES MAGAZINE - Issue #105 - June 2016 - 53

be limited to prevent the specimen from
overheating. Fatigue loading cycles, which
include compression loading, are not yet
common due to the difficulty of preventing
specimen buckling, but there is increasing
demand to test and analyse under such
loading conditions.
High-rate testing of composite materials
is required to predict their behaviour in
the event of a crash. Common examples
of high-rate testing of composite materials include impact testing and high-rate
tension/compression testing. In a high-rate
impact test, a composite panel or part is
subject to an impact from a drop weight or a
hydraulically-driven indenter. After impact,
damage is evaluated by either visual or
ultrasonic inspection and/or the determination of residual strength in a compression
after impact (CAI) test. In a CAI test, an
impact-damaged composite panel is mounted in a support to prevent buckling and
loaded in compression until it fails, giving a
measurement of the strength reduction due
to the impact damage. High-rate tension
or compression testing can be performed
using either a drop tower (figure 3) or with

Fig. 5: DIC axial and shear strain maps for an
open-hole composite specimen

Fig. 6: General principle of a modern industrial CT scanner

a high-rate servohydraulic testing machine
(figure 4). The servohydraulic machine provides a more flexible platform for this type
of work. For example, it can test over a wider
speed range and can maintain a constant
speed during the test.

Full-field strain measurement
for model validation
Mechanical testing of prototypes is a long
established practice, but recent developments in full-field strain measurement have
revolutionised this process.
The most widely used approach to fullfield strain measurement is digital image
correlation (DIC). This technique works by
applying a random pattern to the surface of
the component under test, capturing a series
of images of a specimen during a test, and
then analysing the images with an algorithm
that determines first the displacement field
and then the strain field for each image. The
first image - also known as the reference
image - is captured when there is no load on
the sample. The image is then split into small
subsets, the patterns within each subset of
subsequent images are compared to the reference image and displacements are calculated. From these displacements, a strain map
is calculated. The strain maps of all the strain
components (axial, transverse and shear
strain), along with maximum and minimum
normal strains, can be determined. Example
strain maps for an open-hole composite
tensile test coupon are shown in figure 5.
The magnitude of the strain is represented

by different colours. The first image shows
the axial strain distribution in the vertical
direction and the second image shows the
shear strain distribution.
Even the most efficient DIC algorithms
require a large amount of computing power
and it is generally not possible to perform
the analysis in real time, i.e. during the test.
The usual approach is to acquire and store
the images during the test and then perform
the analysis afterwards. One challenge when
using DIC systems with testing machines
that incorporate other measurement systems
(force, displacement and temperature) is
how to synchronize and record all of the
test data. A recent, integrated DIC system
for use with material testing machines has
solved this problem by synchronizing the
recording of the images and the other test
data digitally.
Compared to traditional methods measuring local strains (e.g. strain gauges), full-field
strain measurement yields an enormous
amount of additional information that
can help engineers and scientists better
understand material behaviour and validate
complex models.

Computerised tomography
for the non-destructive inspection
of composites
Industrial CT (figure 6) uses a series of
two-dimensional images taken at specific
intervals around the entire sample. An industrial CT system uses three principal components: an X-ray tube, an X-ray detector and a

No105 June 2016 / jec

composites magazine 53

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #105 - June 2016