Experimental investigation and numerical description of the damage evolution in a duplex stainless steel subjected to VHCF-loading

Abstract

The present study documents how the irreversible fraction of cyclic plastic strain, induced by loading amplitudes close to the durability limit, causes fatigue damage such as (i) slip band development, (ii) fatigue crack initiation and (iii) short fatigue crack propagation. The damage evolution of the austenitic-ferritic duplex stainless steel X2CrNiMoN22-5-3 (318 LN) was investigated up to one billion load cycles by means of high resolution electron microscopy (HR-SEM, TEM), focused ion beam (FIB) cutting, confocal laser scanning microscopy (CLSM), in-situ far field microscopy and high-energy (87.1 keV) X-ray diffraction (XRD) experiments. The experimentally identified damage mechanisms were implemented into three-dimensional finite element simulations, which consider crystal plasticity. These simulations enable fatigue life predictions of real microstructures obtained for instance by means of, e.g. automated electron back scatter diffraction (EBSD) analysis. The simulations allow for determining whether microcracks (i) initiate in a microstructure, (ii) arrest in the midst of the first grain, (iii) are permanently, (iv) temporary or (v) not at all blocked by grain or phase boundaries. Moreover, this concept is capable to contribute to the concept of tailored microstructures for improved cyclic-loading behaviour.2

Influence of artificial seawater on the VHCF behavior of different steels

“…Up to an amplitude of σ a = 400 MPa, numerous run outs occur. 25 In comparison to the investigations in artificial seawater, the calculated quantile curves of the test data under air conditions by Dönges et al 35 are shifted to lower stress amplitudes at the same numbers of cycles to failure and the slope of the curves is flatter. However, the fatigue data of Dönges et al show a similar high logarithmic standard deviation of s logN,corr = 0.61.…”

“…Duplex stainless steels are generally known for their excellent corrosion resistance and good resistance to stress corrosion cracking while maintaining high mechanical properties. In addition to the test series determined in artificial seawater, a further SN curve is plotted for constant amplitude loading under air conditions, according to Dönges et al 35 (heat treatment: heating ‐ 1250°C ‐ 4 h; cooling ‐ 1050°C ‐ 3 h; quenching in water).…”

“…Fatigue test data of duplex stainless steel X2CrNiMoN22–5‐3 (1.4462) under seawater influence 25 compared to fatigue test data in air according to Dönges et al 35 [Colour figure can be viewed at wileyonlinelibrary.com]…”

Cyclic Deformation Induced Residual Stress Evolution and 3D Short Fatigue Crack Growth Investigated by Advanced Synchrotron Tomography Techniques

“… Relative change of lattice spacing d in a single grain during cyclic loading at a stress am-plitude of 380 MPa, using the example of another ferrite [031] reflection according to [ 8 ]. …”

“…In a previous study [ 8 ], the cyclic deformation-induced change of manufacturing-caused residual stress in the investigated austenitic–ferritic duplex stainless steel was experimentally characterized by means of high energy synchrotron radiation for up to one hundred million load cycles. Due to a higher thermal expansion coefficient of the austenite phase in comparison to the ferrite phase, tensile residual stress is generated in austenite grains and compressive residual stress is generated in ferrite grains as a result of prior quenching from 1050 °C to room temperature as the final part of the manufacturing process of the material.…”