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The present study investigates conventional and cryogenically turned specimens of metastable austenitic steel AISI 347 and stable austenitic steel AISI 904L in the VHCF regime. The cryogenic turning process includes cooling by CO2 snow and generates a surface layer on the specimens of metastable austenitic steel, which is characterized by a phase transformation from paramagnetic fcc - austenite to ferromagnetic bcc - martensite and grain refinement. The stable austenitic steel retains its purely austenitic structure after cryogenic turning, but also shows grain refinement in the surface layer. The specimens with different surface morphology were cyclically loaded at ambient temperature using an ultrasonic fatigue testing system developed and built at the authors’ institute. The testing machine operates at frequencies of approx. 20 kHz to achieve high numbers of load cycles within a reasonable time. To avoid self heating of the specimen, the tests were performed in pulse-pause mode. All specimens were tested with a load ratio of R = -1. During cyclic loading, the metastable austenitic steel partially transformed from paramagnetic fcc - austenite to ferromagnetic bcc - martensite, while no phase transformation could be detected in the stable austenitic steel.

This study presents the results of a detailed investigation of metastability and susceptibility to deformation induced α’-martensite formation of several austenitic steels (AISI 304, AISI 321, AISI 348 and two batches from AISI 347) in the solution-annealed state. Besides conventional characterization of metastability by calculating stacking-fault energy and threshold temperature (designated as MS and Md30), the present work introduced a new method for determining susceptibility to α’-martensite formation. The method was based on dynamically applied local plastic deformation and non-destructive micro-magnetic measurement of α’-martensite content. The parameter Iξ was established, which correlated very well with the grade of α’-martensite formation during cyclic loading. The cyclic deformation and phase transformation behavior of cyclically loaded specimens from different metastable austenitic steels were investigated in total-strain and stress controlled fatigue tests with load ratio R = -1 at ambient temperature. The influence of the strain rate on the cyclic deformation and phase transformation behavior was also examined. During the fatigue tests, besides stress-strain hysteresis and temperature measurement, in situ micro-magnetic measurements were performed. Using the compressive measured data, the influence of plastic induced self-heating of the specimen and the strain rate on α’-martensite formation was analyzed.

Cropland monitoring is important for ensuring food security in the context of global climate change and population growth. Freely available satellite data allow for the monitoring of large areas, while cloud-processing platforms enable a wide user community to apply remote sensing techniques. Remote sensing-based estimates of cropped area and crop types can thus assist sustainable land management in developing countries such as Ethiopia. In this study, we developed a method for cropland and crop type classification based on Sentinel-1 and Sentinel-2 time-series data using Google Earth Engine. Field data on 18 different crop types from three study areas in Ethiopia were available as reference for the years 2021 and 2022. First, a land use/land cover classification was performed to identify cropland areas. We then evaluated different input parameters derived from Sentinel-2 and Sentinel-1, and combinations thereof, for crop type classification. We assessed the accuracy and robustness of 33 supervised random forest models for classifying crop types for three study areas and two years. Our results showed that classification accuracies were highest when Sentinel-2 spectral bands were included. The addition of Sentinel-1 parameters only slightly improved the accuracy compared to Sentinel-2 parameters alone. The variant including S2 bands, EVI2, and NDRe2 from Sentinel-2 and VV, VH, and Diff from Sentinel-1 was finally applied for crop type classification. Investigation results of class-specific accuracies reinforced the importance of sufficient reference sample availability. The developed methods and classification results can assist regional experts in Ethiopia to support agricultural monitoring and land management.

The fatigue behavior of the two austenitic stainless steels AISI 904L and AISI 347 with different surface morphologies, (i) conventionally turned and finally polished, (ii) cryogenic turned using CO2 snow, as well as (iii) cryogenic turned and finally polished, was investigated using an ultrasonic fatigue testing system up to the very high cycle fatigue regime using an ultrasonic fatigue testing system. The AISI 904L is stable against deformation-induced phase formation while the AISI 347 is in the metastable state and shows martensite formation induced by cryogenic turning as well as mechanical loading. For the detailed characterization of the surface morphology, confocal microscopy, scanning electron microscopy, and X-ray diffraction methods were used. The specimens from stable austenite failed in the high cycle fatigue and very high cycle fatigue regime. Opposed to this, the metastable austenite achieved true fatigue limits up to load cycle N = 1 × 109 and failed only in the high cycle fatigue regime. Furthermore, due to surface modification, an increase of fatigue strength of metastable AISI 347 was observed.