Progress reached? - Lifetime Prediction for UD-materials by using S-N curves, Kawai’s Modified Fatigue Strength Ratio and Novel Haigh Diagrams

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 suchmaterials. The author presents a rigorous engineering-like method for fatigue life estimation which is equivalent stress-basedon his WWFE-successful static ’Failure Mode Concept’, the basis of his ‘Mises-like’ UD strength failure criteria set and that tool the damaging portionsare calculated with. The proposed method consists of:

(1) Novel failure mode-basedmodeling of the varying operating stressstate;

(2) measurement and mapping of Master-S/N-curves for each activated failure mode (2 fiber and 3 matrix fracture failuremodes);

(3) determination of necessary other S/N curves within a single failure mode byemploying Kawai’s ‘modified fatigue strength ratio’together with the obtained master curveof the specific failure mode;

(4) generation of failure mode-linkedHaigh diagramswhichinvolve all S-N curves necessary for fatigue life estimation;

(5) application of Miner’s rule for the embedded lamina in order to accumulate the damaging portions. The method will enable an effective and faster design development after the transferfrom the embedded lamina to a general laminate will be validated.Special tests for embedded laminas together with multi-layered multi-directional laminates are requiredto capture the occurring in-situ effect.

by Prof. Dr. Ralf Cuntze