Composite Materials Research Progress

Research Directions in the Fatigue Testing of Polymer Composites 211
gauge length, because the inevitable slip in the clamps can lead to serious errors in the strain
calculation. Depending on the fibre and matrix type and the stacking sequence, the stiffness
degradation can range from a few percent to several tens of percent [2-7].
If the transverse strain is measured additionally, the Poisson’s ratio νxy can be followed as
well. It has been recently showed by Van Paepegem et al. [8] that the evolution of the
Poisson’s ratio is a very sensitive parameter for fatigue damage. Figure 1 shows the evolution
of the Poisson’s ratio for a [0°/90°]2s unidirectional glass fabric/epoxy composite in tensiontension
fatigue. The νxy – εxx curves in strain-controlled fatigue between 0.0006 and 0.006
show a highly nonlinear behaviour and are upper-bounded by the static degradation of the
Poisson’s ratio.
ν xy [-]
0.20
0.15
0.10
0.05
-0.00
0.000 0.005 0.010 0.015 0.020
-0.05
-0.10
-0.15
-0.20
ν xy versus ε xx for [0°/90°] 2s fatigue test W_090_8
ε xx [-]
static [0°/90°] 2s test IF4
static [0°/90°] 2s test IF6
[0°/90°] 2s fatigue test W_090_8: cycle 600 + 5
[0°/90°] 2s fatigue test W_090_8: cycle 3600 + 5
[0°/90°] 2s fatigue test W_090_8: cycle 37200 + 5
Figure 1. Evolution of the Poisson’s ratio ν xy in function of the longitudinal strain ε xx for a glass/epoxy
[0°/90°] 2s specimen at three chosen intervals in the fatigue test [8].
Another online technique is the use of embedded optical fibre sensors with a Bragg
grating. The Bragg grating is a periodical variation of the optical refractive index that is
written in the core of the glass fibre and is typically a few millimetres in length (Figure 2).
When broadband light is transmitted into the optical fibre, the Bragg grating acts as a
wavelength selective mirror. For each grating only one wavelength, the Bragg wavelength, λB
is reflected with a Full Width at Half Maximum of typically 100 pm, while all other
wavelengths are transmitted. The Bragg wavelength is directly proportional with the period of
the Bragg grating. If the optical fibre sensor is embedded in a composite laminate, the strain
in the loaded laminate will cause the period of the Bragg grating to change, and hence the
value of the reflected Bragg wavelength.