JEC COMPOSITES MAGAZINE - Issue #105 - June 2016 - 52

testing

Fig. 2: Instron 5985 composites testing system

In-plane tension testing of composite laminates is similar, in principle, to a traditional
metals or plastics tension test. The test
specimen is usually a rectangular coupon cut
from a laminate panel in a specific direction relative to the fibre directions. Glass
fibre composite tabs are usually bonded to
the gripped ends of the specimens to help
prevent the grip jaw faces from damaging
the laminate and causing premature failure.
When required, through-thickness properties of thick laminates can be obtained from
direct tension tests on cylindrical specimens
cut from the laminate. Through-thickness
tension testing on thin laminates can also be
performed, indirectly, by subjecting a curved
laminate beam to a four-point bending test
to generate a through-thickness tensile stress
in the curved section.
In-plane composite compression test
methods provide a means of introducing a
compressive load into the specimen while
preventing buckling of the specimen. In
practice, there are several different compression test methods and associated fixtures
in widespread use employing different
methods of load introduction and anti-buckling. All composite compression fixtures are
required to have excellent alignment and
high lateral stiffness in order to prevent specimen bending and premature failure. Most
compression test methods require specimen
bending to be monitored by independently
measuring the strain on both sides of the
specimen using bonded strain gauges and set

Fig. 3: Instron drop tower for impact testing

a limit for the maximum allowable bending
strain during a test. Through-thickness compression testing is also performed.
Variations on in-plane tensile and compression tests for laminates include tests on
specimens with both open and filled holes.
These tests provide information on the
strength reductions associated with such
common features.
Shear properties for design databases are
generally determined using a V-notch
specimen subjected to shear loading using
either ASTM D5379 (Iosipescu) or ASTM
D7078 test fixtures. The shear strain is
measured using strain gauges bonded to the
specimen. Specimen preparation and testing
for V-notch shear tests are complex. Simpler
shear tests that are suitable for comparative
testing, screening, and quality control (QC)
are available. The in-plane shear (IPS) test is
a tension test on a specimen cut from a 0/90
laminate panel so that the fibre directions
are +/-45° to the specimen axis. This test
enables shear modulus and shear strength
to be determined. The inter-laminar shear
strength (ILSS) test is in widespread use
for QC testing and screening. It uses a small
specimen subject to three-point bending.
This configuration results in large shear
stresses along the mid-plane of the specimen, resulting in a shear failure.
Flexure testing can also be used to determine a number of composite material
properties. Compared to other test methods,
flex testing has the advantage of requiring

52 jec composites magazine / No105 June 2016

Fig. 4: Instron high-rate servohydraulic testing
system

simple rectangular specimens without tabs
or complex machining.
In addition to tests designed to determine
the bulk properties of composites, a number
of tests have been developed to evaluate the
fracture toughness properties of composite
laminates. Examples of such tests include
Mode I fracture testing using a double cantilever beam (DCB) specimen and Mode II
fracture testing using an end notched flexure
(ENF).
Figure 2 shows a typical electromechanical
testing system capable of performing most
of the mechanical tests on composite materials over a range of temperatures.

Determining material properties
for modelling and simulation -
fatigue and high-rate properties
In addition to static properties, designers
need to be able to assess the service life
of structures subject to the time-varying
loads and environmental factors that will be
encountered in use, and possibly to assess
the behaviour of a structure under extreme
conditions such as an impact or crash.
Fatigue testing of composite materials
is most commonly performed using
tension-tension cyclic loading of rectangular specimens. Typically, a number of
specimens are subjected to cyclic loading
at various stress amplitudes in order to produce an S-N curve that relates the varying
stress conditions to the number of cycles to
failure. Fatigue testing of composites is time
consuming because the test frequency must

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