Composite Materials Research Progress

Research Directions in the Fatigue Testing of Polymer Composites 215
• the bending moment is (piecewise) linear along the length of the specimen (3-point
bending, 4-point bending, cantilever beam bending). Hence stresses, strains and
damage distribution vary along the gauge length of the specimen. On the contrary,
with tension/compression fatigue experiments, the stresses, strains and damage are
assumed to be equal in each cross-section of the specimen,
• due to the continuous stress redistribution, the neutral fibre (as defined in the classic
beam theory) is moving in the cross-section because of changing damage
distributions. Once a small area inside the composite material has moved for example
from the compressive side to the tensile side, the damage behaviour of that area is
altered considerably,
• the finite element implementation of related damage models gives rise to several
complications, because each material point is loaded with a different stress, strain
and possibly stress ratio, so that damage growth can be different for each material
point. In tension/compression fatigue tests, the stress- or strain-amplitude is constant
during fatigue life and differential equations describing decrease of stiffness or
strength, can often be simply integrated over the considered number of loading
cycles,
• smaller forces and larger displacements in bending allow a more slender design of
the fatigue testing facility.
Basically, three types of bending fatigue tests can be distinguished: (i) three-point
bending [24,25], (ii) four-point bending [26], and (iii) cantilever bending [22,27-30]. The
success of these tests for fatigue of polymer composites is quite limited, because the
interpretation of the results is more difficult and in case of stiffness degradation, stress
redistribution across the specimen height comes into play.
Moreover, as long as the bending stiffness of the laminate is high enough (e.g. sandwich
composites), the deflections are small and linear beam theory still applies, but once that the
bending stiffness of the composite decreases (e.g. thin laminates), the deflections are large
and geometric nonlinearities and friction at the roller supports affect the fatigue results.
The authors designed a test set-up for cantilever bending fatigue tests as depicted in
Figure 7.
The power of the motor is transmitted by a V-belt to a second shaft. The second shaft
bears a mechanism with crank and connecting rod, which imposes an alternating
displacement on the hinge (point C in Figure 7) that connects the connecting rod with the
lower clamp of the composite specimen. At the upper end the specimen is clamped (point A
in Figure 7). Hence the sample is loaded as a composite cantilever beam.
A full Wheatstone bridge on the connecting rod is used to measure the force acting on the
composite specimen. Due to the (bending) stiffness degradation of the specimen during
fatigue life, the measured force will gradually decrease as the amplitude umax of the prescribed
displacement remains constant. In order to record the out-of-plane displacement profile, it
was necessary to develop a mechanism to hold the specimen fixed in this state, because
recording the profile while the test keeps running at a frequency of 2.2 Hz, gives rise to some
practical problems. A rotary digital encoder was attached to the second shaft. Its angular
position (relative to a certain reference angle) is directly related with the loading path of the