PSI - Issue 37

Patrick Yadegari et al. / Procedia Structural Integrity 37 (2022) 500–507 P. Yadegari et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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behaviour, the endurance limit or mean-stress sensitivity as well as transfer functions for the transmission of these parameters to components. This knowledge is still unsatisfactory for ultra-high strength steels and was not yet defined in guidelines. This can lead to over dimensioning when using these materials and thus to a waste of resources or to a high dimensioning risk in lightweight construction. Furthermore, for ultra-high strength steels, usually time consuming experimental tests are necessary to characterize the cyclic material behaviour. The aim of a research project was to investigate different ultra-high strength steels and to extend existing estimation methods for the cyclic material behaviour. These input data are to be used for the fatigue life calculation on the basis of the local strain approach in order to evaluate components made of these materials. The “Guideline non - linear” of the Forschungskuratorium Maschinenbau (FKM) by Fiedler et al. (2019) , see also Fiedler and Vormwald (2016, 2018, 2021), provides a proof of structural durability of components based on the local strain approach, whereby elastic-plastic material behaviour is explicitly taken into account. Furthermore, the guideline offers methods for estimating the cyclic material behaviour on the basis of a minimum of experimental input data, without the need for time-consuming fatigue tests. To define the elastic-plastic material behaviour under cyclic loading, the cyclic stress-strain curve can be estimated based on the Ramberg-Osgood relationship. For the correlation of external load and the resulting fatigue life, two different damage parameter life curves can be estimated, which are used instead of S - N or - N curves. The estimation methods of the guideline are currently defined for three different material groups (steel, cast iron and wrought aluminium alloys). For steels they are only approved up to a tensile strength of 1250 MPa. For the application of ultra-high strength steels, these methods have to be adapted to match the results of the tests described in the following, which will extend the suitable range up to a maximum tensile strength of 2400 MPa. Nomenclature σ , ε , σ , ε Parameters for estimating the cyclic stress-strain curve M , M Parameters for estimating the mean-stress sensitivity P,Z , P,Z Parameters for estimating the fatigue strength of the damage parameter life curve P,D , P,D Parameters for estimating the endurance limit of the damage parameter life curve 1 , 2 Exponents (slope) of the damage parameter life curve a Strain amplitude grenz Parameter for estimating the cyclic stress-strain curve 2.5% Factor for decreasing the probability of failure to 2.5 % Young’s modulus Factor for consideration of the mean-stress sensitivity ′ Cyclic strength coefficient σ Mean-stress sensitivity Number of cycles to failure ′ Cyclic hardening exponent RAJ Damage parameter based on J of Vormwald RAM Damage parameters based on SWT of Smith, Watson and Topper and extended B of Bergmann m Tensile strength , ε Stress ratio, strain ratio a , m Stress amplitude, mean stress In the following studies, four high strength respectively ultra-high strength steels of different treatment types are examined: X3CrNiMoAl13-8-2 (through-hardened), 100Cr6 (through-hardened), as well as X40CrMoV5-1 (blind hardened or case-hardened). Round hourglass specimens with a minimum diameter of 5 mm and a polished surface ( < 1 µm) were manufactured for the testing of these materials. Initially, three displacement controlled tensile tests were done to determine the average ultimate tensile strength. Furthermore, total strain-controlled fatigue tests were performed under axial load with an alternating tensile-compressive strain ( ε = −1 ) as well as with a predefined tension or compressive mean strain ( ε = −0.5 or ε = −2 ). The results of these cyclic tests are used in the following