Composite Materials Research Progress

218
W. Van Paepegem, I. De Baere, E. Lamkanfi et al.
A hardly studied phenomenon is the accumulation of permanent strain during shear
dominated fatigue loading. For composite materials with a thermoplastic matrix, creep effects
seem to be dominant, while in case of thermosetting materials, permanent strain is simply
neglected in most reported literature. Moreover, for both types of material, the phenomenon is
not understood quite well.
Van Paepegem et al. [32,33] studied the accumulation of permanent shear strain in
[+45°/-45°]2s glass/epoxy laminates under cyclic loading. They showed that the shear
modulus significantly degrades, but that the accumulation of permanent shear strain is even
more important. Figure 10 shows the accumulation of permanent shear strain in cyclic loading
of unidirectional glass fabric/epoxy composites.
3. Visualization of Fatigue Damage
3.1. Micrographs
The most easy inspection technique is visual inspection. Depending on the difference in
optical refractive index of the matrix and fibre materials, the transparency of the composite
laminate can be very high. Gagel et al. [34] reported an extraordinary high transparency of Eglass
multi-axial non-crimp fabric epoxy laminates. Matrix cracks, voids and inclusions could
be detected easily by transmitted light.
Optical or light microscopy provides a direct path from observations made with the naked
eye, to what is visible at magnifications up to about 1000 × [35]. Fracture surfaces are
embedded in resin and polished before observation. Figure 11 shows a microscopic image of
the damage in a plain weave glass/epoxy composite loaded in bending fatigue [36].
1 mm
Figure 11. Micrograph of the fatigue damage at the clamped end of a composite specimen loaded in
cantilever bending fatigue [36].
3.2. Ultrasonic Inspection
A very common inspection technique for fatigue damage in (textile) composites is
ultrasonics. Ultrasonics can be performed in various modes of operation, but the most
common for fatigue damage detection is the through-transmission (C-scan) technique.