Issue34

S. Ackemrann et alii, Frattura ed Integrità Strutturale, 34 (2015) 580-589; DOI: 10.3221/IGF-ESIS.34.64 588 By using EBSD measurements large areas of  ’-martensite and  -martensite were observed after shear loading and only small  ’-martensite and  -martensite regions were found after equibiaxial cycling. The specimen loaded under  = 0.5 fatigue showed the most intensive  -martensite formation, although the  ’-martensite regions were small. The fatigue lives of  of 0.5, -0.1 and -0.5 tests were the lowest of all investigated loading conditions by using von Mises equivalent strain hypothesis, but ranged in the scatter band of the fatigue lives of uniaxial and equibiaxial (  = 1) tests. Thus, the uniaxial Basquin–Manson-Coffin relationship is conservative for biaxial loading of the investigated TRIP steel. The highest fatigue lives were observed under shear loading in comparison to uniaxial and other investigated biaxial loading conditions which is in good agreement to the literature of biaxial-planar and torsional tests. The COD strain proposed by Sakane et al. [3] based on crack opening displacement gave a good correlation of all investigated fatigue lives including those for shear. The fatigue lives for the TRIP steel PM 16-7-6 were higher than those of the cast steel 16-6-6 unser the same loading conditions. This effect was smaller under shear loading than under equibiaxial loading. Furthermore, the factor between fatigue lives of shear and those of equibiaxial loading was lower for PM 16-7-6 than for cast 16-6-6. Different defects in the materials due to production processes were observed by scanning electron microscopy. Observations of surface cracks after fatigue failure revealed that most of the major and minor cracks had mode I direction which corresponds to stage II crack propagation in the plane of maximum principal strain. However, the major cracks of two shear tests showed mode II (stage I) crack propagation in the plane of maximum shear strain. Subsequently, the stage I major cracks bifurcated into two pairs of mode I cracks (stage II). These observations are in good agreement to the literature. The same behavior was observed for a few minor cracks with length between 0.05 and 0.13 mm. Furthermore, the investigated minor cracks after shear loading had much larger length than minor cracks after equibiaxial loading. The results support the assumption that the period of stage I (mode II) crack propagation is much longer under shear loading than under other biaxial conditions due to absence of crack opening stresses. A CKNOWLEDGEMENT his study is part of the Collaborative Research Centre 799 ‘‘TRIP-Matrix-Composite’’ and supported by the German Research Foundation (DFG), subproject B4. The authors thank all of the involved staff of the CRC 799, of the Institute of Materials Engineering and of the Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Dresden. In particular, thanks to Dr. Weigelt and Mr. Räthel for material supply and sintering, Mr. Büttner, Mrs. Geißler and Mr. Halbauer for specimen manufacturing, Mr. Kortmann and Mrs. Zuber for metallographical specimen preparation, Mr. Droste for assistance with uniaxial fatigue tests as well as Dr. Weidner and Mrs. Fischer for SEM investigations. R EFERENCES [1] Brown, M.W., Miller, K.J., A Theory for Fatigue Failure under Multiaxial Stress-Strain Conditions, Proc. Inst. Mech. Eng., 187 (1973) 745–55. DOI: 10.1243/PIME_PROC_1973_187_161_02. 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