Entwicklung eines Lebensdauervorhersagekonzepts im VHCF-Bereich auf Basis kovariater mikrostruktureller Merkmalsgrößen

The prediction of fatigue life of metallic materials in the very high cycle fatigue (VHCF) regime is still a challenge for material science. Recent studies show that a true durability limit does not exist for many metallic materials and fatigue failures can occur even after 107 loading cycles. Moreover, the huge scatter of experimental results at low stress amplitudes relates to a strong influence of microstructural heterogeneities such as varying grain boundary character, distribution of grain size, inclusion size etc., which together with the applied stress amplitude determine fatigue life. The influence of the microstructure has a probabilistic character and increases the scatter band width in the area of VHCF up to three decades for the SN-curve, thus impeding the application of a reliable fatigue life prediction concept by means of traditional statistical approaches and requiring a detailed investigation of the relevant fatigue damage mechanisms.
In the present work the dependence between the size and space distribution of defects relating to the material quality and the size and location of failure initiation defects in fatigue specimens correlating with their fatigue lives in the VHCF-range was investigated. For this purpose fatigue tests with clarification of typical crack initiation sites and damage mechanisms were carried out. The investigations were made for three reference materials that belong to different material types introduced by H. Mughrabi (2006). A nickel-based superalloy Nimonic 80A (material type I), a metastable austenitic stainless steel 1.4301 with a high deformation-induced martensite volume fraction as well as a welding joint of aluminium sheets from EN AW-6082 T651 (material type II) were investigated. The effect of typical damage-relevant defects for the investigated materials was modeled by corresponding failure-relevant parameters. The stress concentration at crack initiation twin boundaries as well as regular grain boundaries in Nimonic 80A was quantified using a misorientation factor by Blochwitz et al. (1997) and a developed crack initiation parameter. The effect of size and location of extrinsic defects in the type-II-materials was estimated by means of a stress intensity factor with consideration of the local stress at defects. The investigation of distribution of failure-relevant parameters in the single specimens showed that crack initiation predominately takes place at defects with the maximum values of the defined parameter. Using the fatigue test results the observed dependence between the failure-relevant parameters and corresponding numbers of cycle until failure or crack initiation was modeled.
The analysis and statistical modeling of the defined damage-relevant defects was carried out on the basis of metallographic investigation of the reference materials in as-received condition. Using the extreme value statistics the size and (if necessary) space distributions of the larger values of defined damage-relevant defects was modeled in metallographic samples. These models were used in order to evaluate the value of failure-relevant parameters in fatigue specimens. The fatigue life of different specimens was predicted on the basis of evaluated failure-relevant parameters and compared with own as well as adopted fatigue test results. The agreement of experimental and modeling results as well as application of the used method on other alloys were discussed in the conclusions.