Issue34

S. Ackemrann et alii, Frattura ed Integrità Strutturale, 34 (2015) 580-589; DOI: 10.3221/IGF-ESIS.34.64 581 maximum principal strain which correspond to crack mode I. Parsons and Pascoe [2] published observations of surface cracks in cruciform specimens after different biaxial low cycle fatigue loading conditions. Mostly stage I cracks were found under plane strain and shear tests before fatigue failure. But in the later fatigue lifetime regime cracks also bifurcated at each crack tips from stage I cracks to pairs of two stage II cracks in shear tests of steel AISI 304. Itoh et al. [3] investigated the fatigue behavior of steel AISI 304 at 650 °C and found stage I cracks only in shear tests, whereas other strain ratios showed stage II cracks after fatigue failure. Brown and Miller [1] proposed two types of cracks which are characterized by different crack growth directions of stage I and II cracks in dependence on the load ratio  =  2 /  1 between the principal stresses  1 and  2 . Crack type A occurred under  = -1 loading which corresponds to pure shear loading and propagated along the surface. Cracks of type B formed under equibiaxial  = 1 loading and grew through the specimen thickness which is critical for plane stress states. Various models have been published to assess fatigue failure in complex stress-strain conditions besides the widely used von Mises equivalent strain amplitude. For example, the  -plane theory of Brown and Miller [1] considers maximum shear strain and tensile strain on maximum shear plane, and the COD strain of Sakane et al. [3] is based on crack opening displacement (COD). Biaxial fatigue has been studied by biaxial-planar tests using cruciform specimens. Principal stress and strain directions are fixed in biaxial-planar tests due to fixed perpendicular loading axes, even under biaxial loading with a phase shift  between the controlled axial strains. Thus, no rotation of stress and strain directions occurs, in contrast to non-proportional axial- torsional tests on thin-walled tubular specimens [4]. Using cruciform specimens different states of strain can be achieved by varying phase shift  between 0° and 180° as well as strain ratio  =  2 /  1 between -1 and 1, where  1 and  2 are principal strains [3, 5]. Loading conditions with  = -1 and 1 correspond to pure shear and equibiaxial loading, respectively, whereas  = -0.5 corresponds to uniaxial loading since Poisson’s ratio tends to 0.5 under plastic straining. It is known from literature that shear loading yields the highest fatigue lives compared to other biaxial loading conditions [3–9]. Itoh et al. [3] investigated strain ratios  of -1, -0.5, 0, 0.5, and 1 for austenitic steel AISI 304 at 650 °C. The fatigue lives ranged between those of equibiaxial (  = 1) and shear (  = -1) loading. In the present study the biaxial fatigue behavior of a metastable austenitic steel is investigated for strain ratios  in the range of -1 to 1 at room temperature. The material was produced from powder by hot-pressing technique and is compared to a previously studied cast variant. The steel shows martensitic phase transformation from austenite via  -martensite into  ’- martensite during cyclic deformation, e.g. [10], causing cyclic hardening, see e.g. [11–13]. The aim of the present study is to clarify the influence of different states of strain on cyclic deformation behavior, crack propagation, and fatigue failure. Therefore, surface cracks were investigated by using electron monitoring with an electron beam universal system and scanning electron microscopy (SEM). M ATERIAL AND EXPERIMENTAL DETAILS he investigated material was a metastable austenitic TRIP steel (TRansformation Induced Plasticity) with 15.3–16.3 %Cr, 5.4–7.2 %Mn, 5.8–6.5 %Ni, 0.04–0.07 %C and 0.05–0.08 %N (wt.%). The material was processed as sintered steel powder (called PM 16-7-6 in the following). Sintered discs were produced by using hot-pressing technique at 1250 °C with 30 MPa for 30 minutes. Thus, a fine grained, mainly austenitic microstructure with  -ferrite volume fraction less than 2 % and an average grain size of 20  m was achieved, see Fig. 1a. An austenitic TRIP cast steel 16-6-6 (%Cr-%Mn- %Ni) with similar chemical composition and a grain size up to 1.5 mm was used as reference material. Sintered discs (diameter 140 mm) were welded into hot-rolled plates of austenitic steel AISI 304 by electron beam welding. The welds had a good connection to both materials without defects and cracks. The position of the weld was put on a position where it is uncritical for the fatigue tests, see Fig. 1b. Fig. 1b shows the cruciform specimen geometry which is in accordance to [5, 14]. The specimen center is planar (diameter 15 mm) and of reduced thickness (1.5 mm). The cyclic tests were carried out on a servo hydraulic biaxial-planar tension- compression machine (Instron 8800) with four actuators of 250 kN capacity. The  ’-martensite volume fraction was measured by using a ferrite sensor (Fischer-scope MMS PC) due to ferromagnetic behavior of  ’-martensite. The sensor was attached to the polished sample surface for in situ measuring within a volume of about 3.14 mm² x 1.5 mm. A biaxial orthogonal extensometer with four ceramic arms and a gauge length of 13 mm was used to measure axial strains  A and  B in the specimen center on the sample surface. In biaxial-planar tests axial strains  A and  B correspond to principal strains in terms of the assumption  1 >  2 . T