PSI - Issue 42

Matěj Mžourek et al. / Procedia Structural Integrity 42 (2022) 457 – 464 Matěj Mžourek / Structural Integrity Procedia 00 (2019) 000 – 000

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It is not uncommon for size effect to be studied along notch effect, as has been done by Sandberg and Olsson (2016) and many others. The interaction between the two effects (though it can be argued that they should be considered as one) has been shown by Buch (1984), where an increase in specimen size has had a larger impact on the fatigue strength of notched specimens than in the unnotched. This is reflected here through the presence of notched specimens in the experimental campaign, see Sec. 2. The values of V i and A i for each specimen and different values of n (and h crit ) were obtained via Abaqus FEM results post-processing in Matlab. Optimal values of the m , n and h crit parameters were found using an optimization routine in Matlab. The minimized objective function is given by the sum of squares of errors of logarithms of fatigue strengths. 2. Experimental campaign All six types of tested specimens were produced from hot rolled, quenched and tempered 42CrMo4+QT high strength steel bars of 35 mm diameter delivered from a single heat. Material parameters supplied by the manufacturer are available in Table 1. Table 1. Material parameters of the used 42CrMo4+QT high strength steel Ultimate tensile strength R m Yield strength R e Elongation A5 Cross section contraction Z Grain size Reduction ratio 1097 MPa 1001.5 MPa 16.5 % 62.5 % 8 25 The specimen designs are shown in Fig. 2. The four smooth (A01-A04) specimens feature two different diameters of the critical cross section, as well as different lengths of the central part, which lead to various values of critical volumes. Two V-type notched (A51-A52) specimen designs differ in the notch tip radius (0.5 and 2.0 mm, respectively), producing different stress gradients. All specimens have surface roughness of Ra = 0.8 µm, which has been achieved by turning ( Ra = 1.2 – 1.7 µm detected) followed by manual grinding. Ten samples of each design were available.

Fig. 2. Specimens A01-A04 (smooth) and specimens A51-A52 (notch tip radii are 0.5 mm and 2 mm, respectively)

The Amsler HFP422 resonator was used for all fatigue tests. Constant amplitude tests were run in the fully reversed push-pull mode. The life domain of interest was approximately between 5 000 to 2 000 000 cycles. Tests that ran for more than 10 000 000 cycles were considered as runouts. These specimens were considered undamaged and were used in a new test with a load level of at least 1.36 times the original value. The end condition of the tests was a 5-20 Hz drop in the loading frequency, which is caused by a stiffness loss due to crack propagation. The frequency tends to reduce quite rapidly once the crack is sufficiently large, thus, an arbitrary number of 5 (for smooth specimens) and 20