Composite Materials Research Progress

Research Directions in the Fatigue Testing of Polymer Composites 217
2.3. Shear Dominated Fatigue
Fatigue testing in pure shear is very difficult. Lessard et al. [31] modified the static three-rail
shear test (ASTM D 4255/D 4255M – 01) to do fatigue testing on carbon/epoxy plates.
A much more common method are the tension-tension fatigue tests on a [+45°/-45°]ns
laminate, This test is based on the ASTM D3518/D3518M-94(2001) Standard Test Method
for ‘In-Plane Shear Response of Polymer Matrix Composite Materials by Tensile Test of a
±45° Laminate’. This standard explains how the shear stress-strain curve can be derived from
a static tensile test on a ±45° laminate, by measuring the longitudinal and transverse strain.
The test is also called a bias tension test, because the bias (or cross-grain) direction is the 45°
direction between warp and weft direction in case of fabric reinforced composites.
In both pure shear and shear dominated fatigue, the test frequency is a very important
parameter. The shear stresses can lead to significant autogeneous heating and once the
temperature exceeds the glass transition temperature, the deformations can be very large.
Figure 9 shows the localized yielding of a [(+45°,-45°)]2s carbon fabric/PPS composite in
tension-tension fatigue at 2 Hz. Temperature rises up to 90 °C were measured with a
thermocouple at the top surface.
Figure 9. Localized yielding of a [(+45°,-45°)] 4s carbon fabric/PPS composite in tension-tension fatigue
at 2 Hz.
Shear stress τ 12 [MPa]
60
50
40
30
20
10
Shear stress-strain curve for cyclic [+45°/-45°] 2s test IH2
IH2 cyclic test
IH4 static test
0
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07
Shear strain γ12 [-]
Figure 10. Shear stress-strain curve for the cyclic tensile test on a [+45°/-45°] 2s glass/epoxy specimen
and the envelope of the corresponding static test [32].