Strength Failure Mode-based Treatment of S-N curves and Haigh Diagrams for Brittle behaving Materials with emphasis put on Embedded UD Lamina Material

(a comprehensive collection of the topics in this field with special regard of compositeparts)

by Prof. Dr Ralf Cuntze

Experience shows that multi-directional laminates, composed of endless fiber-reinforced UD laminas, statically designed to a design strain limit of ε< 0.3%, do not fatigue. However, lightweight design requires a higher exertion of such materialsfor high-performance applications. The author presents a rigorous engineering-like method for fatigue life estimation which is equivalent stress-based according tohis World-Wide-Failure-Exercise (WWFE)-successful static ’Failure Mode Concept’ FMC. This FMC is the main brick of his ‘Mises-like’ UD strength failure criteria set which is thetool the damaging portions of this brittle behaving materialare calculated with. The proposed method consists of:

(1) Novel failure mode-based modeling of the varying operating loads depending stress states;

(2) Measurement and mapping of a minimum number of S-N curves which means one basic S-N curve for each activated failure modenamely 2 fiber FF and 3 inter-fiber or matrix fracture failure modes IFF;

(3) Novel determination of other necessary S-N curves within a one failure mode by employing Kawai’s ‘modified fatigue strength ratio’ together with the obtained basic S-N curve of thisfailure mode;

(4) generation of failure mode-linked FF and IFF Haigh diagrams which involve all S-N curves necessary for fatigue life estimation;

(5) Noveldesign-desired Constant-Fatigue-Life (CFL) curves, directly and automatically computed by the presented method;

(6) Application of Miner’s rule for the embedded lamina in order to accumulate the FMC-computed damaging portions.

The method will enable an effective and faster design development after the transfer from the embedded lamina to a general laminate will be validatedby further investigations.Tests for embedded laminas together with multi-layered multi-directional laminate test specimens are required to capture the occurring in-situ effect.The performed work hasbeen still applied in a research project to design energy storage rotors (wheels). These are composed by modular systems of different output classes, such as the Stornetic EnWheel: 188 kWh up to 188 kWh per day.