The U-Concept: A simple add-on to the Local Strain Approach for the fracture fatigue life calculation of structures under constant and variable amplitude loading (#42)
M. Fiedler1, M. Vormwald1
1 TU Darmstadt, Material Mechanics Group, Darmstadt, Hesse, Germany
In February 2019 the new FKM guideline  was published. This guideline describes the Local Strain Approach (LSA), a method which considers the elastic-plastic material behavior when calculating the fatigue life for crack initiation. Inside the new guideline a method  is included, which is able to approximate damage parameter Woehlercurves and cyclic material parameters based on the ultimate tensile strength .
The crack propagation analysis is the most common method to calculate the fatigue life from crack initiation to fracture but requires 1.) an initial crack length, 2.) information about the crack propagation direction and 3.) crack propagation material parameters. None of this information are provided by the new FKM guideline.
This paper introduces the U-Concept , a simple method to approximate the fatigue life for failure criteria fracture based on a fatigue life calculation for crack initiation with the new FKM guideline for constant and variable amplitude loadings. Neither a specified initial crack length nor the crack propagation direction nor crack growth material parameters are required for using the U-Concept. This paper also compares the calculation results with experiments and shows the accuracy of the U-Concept.
 Fiedler, Wächter, Varfolomeev, Vormwald, Esderts; “Richtlinie nichtlinear”, VDMA-Verlag, Frankfurt, 2019
 Wächter; „Zur Ermittlung von zyklischen Werkstoffkennwerten und Schädigungsparameterwöhlerlinien“, Dissertation, TU Clausthal, 2016
 Fiedler; „Das Örtliche Konzept als Grundlage einer Bruchlebensdauerberechnung für Stahl*“, Dissertation, TU Darmstadt, unpublished (expected August 2019)
Keywords: crack initiation, fracture, variable amplitudes
Concepts for crack propagation under variable mechanical and thermal loadings based on the effective cyclic J-Integral (#48)
A. Bosch1, M. Vormwald1
1 Technische Universität Darmstadt, Materials Mechanics Group, Darmstadt, Hesse, Germany
Due to the increasing usage of renewable energies, conventional power plants operate more and more flexible. The original stationary operation, just interrupted by shut downs and start ups for regular inspections, turned to a demand-oriented operation. By this kind of operation, the additional loading expresses in a high number of smaller load cycles. The load cycles itself are mainly induced by changes in temperature. The guidelines for the design and the assessment of such components (e.g. ) just considering the strain ranges on the load side, mean stresses and load history are considered blanket on the resistance side. Based on an experimental programme including several types of specimens (smoot, notched, welded and Gleeble-specimens) and several types of loadings (constant, two step, eight step and operational loading tests) exemplary for the austenitic stainless 1.4550 (X6CrNiNb18-10 / AISI 347), concepts were developed on fracture mechanical principles. As crack driving force, the effective cyclic J-Integral is used. The developed concepts, considering mean stresses and load history as well as the temperature dependency individually on the load side, base on the PJ-concept by Vormwald  (damage parameter PJ: effective cyclic J-Integral normalized to crack length). Improvements are made by the substitution of the crack propagation law and the execution of a crack propagation calculation instead of a damage calculation. Further improvements are made for the modelling of the transition of the threshold for small cracks to long cracks and the consideration of transient crack closure. The concepts variants differ in the crack propagation law and the consideration of transient crack closure in style of Anthes , but all of them can be applied even under variable temperatures. All concept variants share the relatively easy identification of the required parameters, which are to be determined mainly on constant amplitude loading and ultimate tensile tests. The advantages of the improvements and the advantages of fracture mechanical approaches are shown in contrast to the results of conventional approaches. By the developed concepts, it is possible to assess the lifetime with much higher accuracy and less scattering. Regarding to the application in the context of power plants, the economic efficiency can be improved by an appropriate (depending on the load history) planning of inspection intervals.
 KTA 3201.2: Komponenten des Primärkreises von Leichtwasserreaktoren: Teil 2: Auslegung, Konstruktion und Berechnung: Kerntechnischer Ausschuss, 2013
 Vormwald, M.: Anrißlebensdauervorhersage auf der Basis der Schwingbruchmechanik für kurze Risse, PhD Thesis, 1989. –TH Darmstadt ; 47
 Anthes, R. J.: Ein neuartiges Kurzrißfortschrittsmodell zur Anrißlebensdauervorhersage bei wiederholter Beanspruchung, PhD Thesis, 1997. – TH Darmstadt; 57
Keywords: crack propagation, variable mechanical loading, variable thermal loading, load sequence effects, short crack growth
Fatigue crack growth in the structural elements of the RRJ-95 aircraft under bench testing and service conditions (#52)
A. A. Shanyavskiy1, A. Soldatenkov1, A. Toushentsov1
1 Aviation Register for Russian Federation, Moscow region, Russian Federation
The life tests of the RRJ-95 aircraft elements were carried out according to the programs simulating the operating conditions. In the course of these tests, damage of the lower power panel of the left detachable part of the wing and the right lower console of the wing occurred. During operation, the attachment bracket lugs of the aircraft rudder actuator were fractured.
Fractographic analysis showed that fatigue cracks originated from holes in the investigated structural elements. The zones in which the advanced development of fatigue cracks occurs were revealed. The regularities and mechanisms of crack development in the investigated structural elements were shown.
The regularities of crack growth were compared with the predicted durability under variable block loading conditions during life tests. Recommendations on the modernization of structures were provided. The in-service inspection rates of the zones with the highest stress intensity were substantiated.
Keywords: fatigue, fractography, aircraft elements, number of flights, number of cycles