Fatigue behaviour of composite tubes under multiaxial loading

30
larger.
Axial stress x , MPa
Fifth International Conference on Fatigue of Composites
1000
800
600
400
200
Woven CFRP laminate
[(±45)/(0/90)] 3s
Experimental
1.0%/min
DVE
0
0 0.5 1 1.5 2
Axial strain x , %
RT
Tension
100¼C
150¼C
Fig. 2. Tensile stress-strain relationships at different temperatures.
No significant difference was found between the ultimate tensile failure modes at RT and 100 o C.
The tensile specimens tested at RT and 100 o C failed at a right angle to the loading direction, and they
accompanied neither remarkable pull-out of fibers nor delamination. In contrast, the tensile specimens
tested at 150 o C failed in a shear mode, and they accompanied local delamination that developed in the
45 o direction or in the ±45 o directions to the loading direction. These observations suggest that the
interlaminar and in-plane shear strengths of the woven CFRP laminate were significantly reduced by the
exposure to the high temperature of 150 o C. It is thus considered that the stress level of the knee point
observed in the stress-strain relationship at 150 o C corresponds to the onset of local deformation and
failure due to the reduction in strength.
Fig. 3 shows the compressive stress-strain relationships at RT, 100 and 150 o C. The axial compressive
strains shown in this figure are based on the longitudinal displacement of actuator that was measured
with LVDT on the testing machine. The reason is that the displacement measurement using DVE was
not successful for the short specimens used in the compression tests. In Fig. 3, it is seen that the tangents
of the compressive stress-strain relationships monotonically decrease with increasing strain, regardless
of temperature, and thus they exhibit small but clear nonlinearity over the range of deformation until
failure. In the compression test at 150 o C, the yielding behavior that appeared in the tension test at the
same temperature was not observed. The compressive elastic modulus decreased with increasing test
temperature, in line with the temperature dependence observed in the tension test results. However, the
fracture strain in compression decreased with increasing test temperature in contrast to the increasing
tendency in tension. The comparison of the results in Fig. 2 and Fig. 3 reveals that the reduction in
compressive strength with increasing temperature is more significant than the reduction in tensile
strength. No appreciable difference was observed between the fracture modes in compression at
different temperatures. All the specimens failed in an out-of-plane shear mode under compression,
regardless of test temperature.