Composite Materials Research Progress

Research Directions in the Fatigue Testing of Polymer Composites 221
projection data, the reconstruction process is producing 2D horizontal cross-sections of the
scanned sample. These 2D images can then be rendered into 3D models, which enable to
virtually look into the object.
Figure 15 shows the micro-tomography images of a fatigue damaged 5-harness satin
weave carbon/PPS (left) and the embedded optical fibre sensors in a carbon thermoplastic
composite (right).
Figure 15. Micro-tomography images of a fatigue damaged carbon/PPS composite (left) and three
composite samples with embedded optical fibre sensor (right) [11].
4. Finite Element Simulation of Experimental Boundary
Conditions
In this paragraph it is shown that finite element simulations should support the experimental
work in order to be able to discriminate between intrinsic material behaviour and induced
effects by (insufficiently understood) experimental boundary conditions.
Four examples are given where the strong correlation between experimental
measurements and finite element simulations is proven.
4.1. Clamping Conditions in Tension-Tension Fatigue
As stated before, the composite coupons for tension-tension fatigue testing are parallel-sided
specimens, instrumented with aluminium or composite tabs. One of the main concerns in
tension-tension fatigue testing of composites is tab failure, i.e. the specimen fails just next to
or inside the tabbing area. Such failures are due to the inevitable stress concentration near the
clamped edges.
In Figure 16, two types of standard tensile machine fixtures are shown, with the
dimensions of the grips.
In order to optimize the shape and length of the tabs, it is important to simulate the stress
state near the clamped region. Therefore a simulation of part of the clamping mechanism has
been done in ABAQUS/Standard v6.6-2.