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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to develop and validate the methods for estimating the fatigue strength of ultra-high strength steels. Since the only experimental input value needed for the estimation methods is the ultimate tensile strength, the results of tensile tests are shown in Figure 1. As can be seen in the stress-strain curves, the four materials differ greatly in terms of strength and ductility. The high-strength steel X3CrNiMoAl13-8-2 achieves a tensile strength of 1600 MPa with a total elongation at break of more than 15 %. In contrast, the three remaining materials reach a tensile strength of 2100 to 2400 MPa and are thus in the range of ultra-high strength steels. Remarkable here is the exceptionally high ductility of the blind-hardened X40CrMoV5-1 with a total elongation at break of 25 %. The aim is now to find a single method for describing these four steels, which obviously show a very different static material behaviour.

Fig. 1. Stress-strain curves of the tensile tests for all four materials.

2. Estimation methods 2.1. Cyclic stress-strain curve

The cyclic stress-strain curve is defined by the model of Ramberg and Osgood (1943) and estimated by determining the cyclic strength coefficient K´ on the basis of an experimentally measured ultimate tensile strength and material group-dependent calculation values, according to the procedure of the “Guideline non - linear” (Fiedler et al. (2019)), Equation 1. = + ( ′ ) 1/ ′ with ′ = σ ∙ ( m MPa ) σ (min ( grenz ; ε ∙ ( m MPa ) ε )) ′ (1) The calculation values required for the estimation are given in Table 1 for steel, as already defined in the guideline (Fiedler et al. (2019)), and for the group of ultra-high strength steels. These were determined empirically on the basis of the test results.

σ in MPa ε σ ε grenz ′ in MPa 3.1148 1033 0.897 -1.235 0.338 0.187 206000

Table 1. Calculation values for estimation of the cyclic stress-strain curve Material group

Steel

Ultra-high strength steel

2.66

1400

0.895

-1.235

0.099

0.085

206000