Fatigue behaviour of composite tubes under multiaxial loading

2
Fifth International Conference on Fatigue of Composites
the same thickness (1.5 mm) were produced. Samples with a different lay-up were also manufactured,
by replacing the inner layer of UD tape with a balanced fabric layer. In this way, the initial [90]4 lay-up
became [0T/90UD,3]. The [90]4 lay-up allowed us to quantify the influence of the shear stress component
on the transverse fatigue strength, in the absence of the longitudinal (fiber-direction) stress. The
[0T/90UD,3] facilitated instead a stable and measurable growth of the fatigue damage. The following
prepregs were considered: UD tape UE400-REM produced by SEAL-Italy, area weight = 645 g/m 2 ,
Balanced fabric VV345T-DT107A produced by Deltatech-Italy, area weight 345 g/m 2 ).
3. Experimental testing and damage investigation
The tensile properties of the prepregs used for manufacturing the tubes were first measured by testing
flat coupons under tension loading. The average values of elastic and strength properties measured for
the UD and fabric tape are summarised in table 1.
The multiaxial fatigue behaviour of the tubes was investigated by means of pulsating tension- torsion
fatigue loading. Fatigue tests were carried out on a MTS 809 axial-torsional machine, under
load/torque control at a frequency of 10 Hz. S-N fatigue curves for [90]4 tubes are shown in figure 1,
for pure tensile loading and for biaxiality ratio 12 (6/2) equal to 1.0 and 2.0.
Table 1. Average in-plane properties of the materials used for tubes manufacturing
EL (MPa) ET (MPa) GLT (MPa) LT L (MPa) T (MPa) LT (MPa)
UD400-REM 34860 9419 3193 0.326 973 50 98
VV345T-DT107A 21700 20800 3351 0.159 448 431 85
Fig. 1. Fatigue results for [90]4 glass/epoxy tubes under tension (12=0) and tension-torsion loading (12= 1 and 2) (L denotes tube of 38 mm
external diameter, S for 22 mm external diameter; thickness= 1.5 mm).
The presence of a shear stress component turned out to have indeed a significant influence, with a
40% drop in the fatigue strength at 2 million cycles for a biaxiality ratio equal to 2 .
As shown again in figure 1, tubes with 38 or 22 mm diameter turned out to behave similarly and
therefore it was decided to continue testing with the smaller diameter tubes which can be clamped