In the focus of large quantities and cost pressure, the design of lightweight structures made of carbon-fibre reinforced composite materials with fatigue as dimensioning criteria is always a challenging task in automotive industry. Beside extensive material characterization knowledge about fatigue damage is required as well to take the advantage of the full lightweight potential.
The triaxial braiding process may be suitable to overcome the challenge of composite manufacturing due to good deposition rates and material economy but this process inherently lead to yarn undulation which cannot be neglected. In the present investigation 24K carbon-fibres are used as roving for axial and braid yarns (braiding angle 45°) with a thermosetting epoxy matrix.
As base for cyclic testing quasi-static tests in tension and compression are carried out using flat specimens. The material damage is investigated with digital image correlation and acoustic emission measurements during the tests and microsections after fracture.
For constant amplitude cyclic loading in axial yarn direction two different R-ratios of 0.1 (tension-tension) and 10 (compression-compression) are chosen. The slopes of both S-N curves are quite comparable but damage modes and damage development as well as degradation levels are completely different. Tension-tension fatigue loading shows significant stiffness reduction (up to 35 % dependent on load level) due to inter-fibre fractures in the braid yarns and large-area delaminations. Additionally progressive stress-strain behaviour can be observed as a result of undulation straightening in the axial yarns. During compression-compression loading there is a lower stiffness reduction. As main damage mode kink band formation in axial yarns with subsequent delaminations can be observed.
Both R-ratios show high material property degradation during fatigue loading which is not allowed for lightweight structures with high stiffness and strength requirements during life. For dimensioning structures made of triaxial braided composite material it is suggested to use S-N curves based on stiffness degradation instead of load cycles to fracture.