Fatigue behaviour of composite tubes under multiaxial loading

12
Fifth International Conference on Fatigue of Composites
Fig. 2. Protocol of fatigue loading.
(1) Characterization step: In this step, quasi-static tests are performed with force-controlled ramps
(discrete pure tension/compression force and positive/negative torsion moments). These
characterization loads are chosen in a way that no further damage occurs and they are considerably
smaller than the applied cyclic loads in the fatigue damage steps. Nevertheless, all quasi-static
loads are adequate for calculating the Young‟s (tangent modulus of elasticity) and shear modulus.
(2) Fatigue damage step: After each characterization step, the fatigue damage step with cyclic loads
(sinus wave form) of constant amplitudes follows. Here, the different uniaxial and sequence fatigue
loads are performed. These steps introduce the fatigue damages like matrix cracking, delamination
and fibre failure. For recording the surface temperature of the tube specimen during the fatigue
damage steps, the thermographic camera (CEDIP Titanium) is applied. Furthermore, a photo
camera (CASIO Exilim Pro EX-F1) with integrated ring buffer and high-speed-mode is used for
observing the qualitative development of matrix cracks and for an optical fracture analysis
(recording the location and type of final failure). Consequently, an analysis of the location of high
surface temperatures (so-called hot-spots) and the location of the fracture is conducted.
(3) Discrete damage monitoring: Matrix cracking indicated by the crack density is the first occurring
type of fatigue damage and can be monitored with a light microscope (ZEISS Stemi 2000-C).
Based on the nearly similar refraction index of resin and glass fibre used for the tube specimens,
the specimen is transparent and matrix cracks are observable via transmitted light method.
Therefore, the fatigue test is stopped after several fatigue damage steps and the specimen is taken
out of the testing machine. Subsequently, photos of several areas (up to 15) of the specimen are
taken and matrix cracks are counted using the photographically documented cracking states.
Differentiating the cracks by their orientation angle leads to the numbers of cracks in each single
layer direction (0 o , 45 o , 90 o and -45 o -direction). Additionally, referencing of these crack numbers to
the fixed monitoring areas is conducted in order to calculate the specific crack densities, which are
average values of all observed areas. Beside the matrix crack monitoring close-up digital photos of
occurred damages such as delaminations are taken.
Concerning the different parts of the fatigue tests, the heating and cooling of the tube specimens