Carbon fiber-reinforced-plastics (CFRP) are already used under high-cycle- (HCF) and very-high-cycle-fatigue (VHCF) at relatively low loads e.g. in aerospace industry. While the static strength of CFRP is satisfyingly predictable by means of layerwise-strenght-analysis and material data from single layers, it is uncertain to predict residual strength or lifetime as the design lifetime increases. Previous studies showed that local minor cracks and inter-fiber-fracture (IFF) originate long before total failure of single layers. Reliable fatigue estimation for fiber-reinforced-plastics is not available at present. Subsequently safety-related primary structures made out of FRP are over dimensioned.
It is possible to detect the change of inner surface in a specimen by means of X-Ray-Refractography. This includes voids, intact fiber-matrix-interfaces as well as defective ones, cracks and all interfaces in plane with the X-Ray-beam. As the crack-density increases, the inner surface of the material correspondingly increases.
The borderline where even at high cycles no early cracks were detected was identified for selected laminates out of woven and non-crimped glass and carbon fabric with epoxy-matrix. When no micro-cracks were detected up to 106 cycles, the specimens also lasted in the VHCF-range. Furthermore for the epoxy reinforced with carbon fiber it was distinguished that at a load minor to 50% IFF-load level, no micro-cracks were detected and consequently the VHCF-endurance reached.
The future integration of the set-up small-sized testing machine into the X-Ray buildup in order to achieve non-destructive detection of cracks while loading the specimens at the same time will increase the performance of the studies.
In earlier investigations at BAM it was shown, that the matrix has a strong influence on micro-crack formation and total lifetime. In order to determine the influence of the matrix properties on the boarderline to infinite life, research will be done on laminates while varying matrices and their fracture mechanical properties.