Minimum Test Effort-Based Derivation of Constant-Fatigue-Life Curves - Displayed for the Brittle UD Composite Materials

Series production of safety-relevant structural parts requires a Design Verification (DV) guaranteeing Structural Integrity. This means, it is to demonstrate that No relevant limit failure state is met considering all Dimensioning Load Cases (DLCs). These DLCs involve static, dynamic, and cyclic loading focusing lifetime. However, lifetime prediction is a pain point for a better use especially of laminated composites in lightweight design. A generally practical tool is not available. Hence, each novel high-performance UD-lamina (ply)-composed laminate requires a new effortful test campaign. Therefore, the idea of the author-founded Germany-wide group BeNa (in 2010) was to base fatigue life prediction embedded and laminawise in order to become more general in future. This idea also fits to Cuntze’s “modal” Failure-Mode-Concept (FMC), which is based on material symmetry facts dedicating a ‘generic’ number to ideally homogeneous materials, namely 2 for the isotropic material and 5 for the transversely-isotropic UD lamina material. Fracture morphology gives evidence: Each strength property corresponds to a distinct strength failure mode or to a strength failure type Normal Fracture (NF) or Shear Fracture (SF). In the case of UD materials 2 FiberFailure (FF) and 3 InterFiberFailure (IFF) modes are faced. In lifetime prediction strain-life and stress-life models are used. For ductile materials one single plastic strain-linked yield mechanism dominates and strain-life models are applied. However for brittle materials the elastic strain becomes dominant and stress-life models are used. Micro-damage mechanisms drive fatigue failure and several fracture mechanisms come to act. This asks for a so-called modal approach that captures all fracture failure modes. The automatic establishment of the not piecewise straight Constant Fatigue Life (CFL) curves is the challenging task. All SN-curves’ information and all the CFL curves (N = const) are captured by the Haigh Diagram σa (σm), with σa the stress amplitude and σm the mean stress. The author’s idea for the generation of such an automatically deducible SFC-curve includes to provide: 1. At minimum one single SN-curve as Master curve of each mode (by measurement). 2. A strength failure criterion (SFC) that can quantify the micro-damage portions under cyclic loading (due to experience, in the brittle case given by a static one). 3. A model that can predict other SN-curves on the basis of a mode Master Curve. (by Kawai’s Model ‘Modified Fatigue Strength Ratio Ψ’). 4. A physical model to map the test data in the transition domain as most problematic region in the Haigh diagram, where the modes interact and the CFL curve heavily decays (a decay function was found). A first model validation of this private investigation, using test data from Dr. C. Hahne, AUDI, looks very promising and asks for funding.