Issue34

S. Ackemrann et alii, Frattura ed Integrità Strutturale, 34 (2015) 580-589; DOI: 10.3221/IGF-ESIS.34.64 582 Cyclic deformation tests were carried out under total strain control of axial strains  A and  B at strain rates of about 0.004 s -1 . Different states of strain were investigated by using strain ratios  =  B /  A of 1 (equibiaxial loading), 0.5, -0.1, -0.5 and -1 (pure shear loading). Fig. 1c shows the triangular signals of axial strains  A and  B of in-phase tests. Both axial strain functions had equal frequency. Axial strain amplitudes for different strain ratios were determined to achieve comparable von Mises equivalent strain amplitudes  vM /2 which were calculated according to [15]. Biaxial in-phase tests were performed at  vM /2 in the range of 0.3 – 0.6 · 10 -2 . Biaxial reference tests on a cast TRIP steel 16-6-6 were performed under in-phase loading (  = 1, -1) and out-of-phase loading. Out-of-phase loading was set by a phase shift  between the strain functions of the two axes A and B. Thus, strain ratio  changes continuously between 1 and -1 within one cycle. However, phase shifted loading by means of a cruciform specimen causes no rotating principal stress and strain directions due to fixed perpendicular loading axes, in contrast to non-proportional axial-torsional tests [4]. Fig. 1d shows axial strain courses for phase shifts  of 45°, 90° and 135° as examples. Uniaxial reference tests were carried out on a conventional 250 kN servo hydraulic testing system (MTS). In the present study a von Mises type force amplitude is used to characterize the cyclic deformation behavior because the cross sectional area in cruciform specimens is unknown and numerical simulation like in [3] was not available. The von Mises type force was calculated according to [9] by using the axial forces F A and F B at maximum principal strain in axis A, see Eq. (1). It is assumed that yielding and hardening of the material take place in a defined cross sectional area and maximum stresses and strains occur in the specimen center (Ø 15 mm) with homogenous distributions.     2 2 vM A B A B F F F F F    (1) Figure 1 : a) Initial austenitic microstructure of the investigated TRIP steel PM 16-7-6. b) Cruciform specimen and position of the weld in case of PM 16-7-6. Schematic courses of control signals of axial strains  A and  B for different c) strain ratios  (in-phase tests) and d) phase shifts  (out-of-phase tests) at a von Mises equivalent strain amplitude  vM /2 = 0.4 · 10 -2 . R ESULTS AND DISCUSSION Cyclic deformation behavior and martensitic transformation he cyclic deformation behavior of the investigated steel was studied by means of von Mises type force amplitude  F vM /2. Fig. 2a presents the cyclic deformation curves for strain ratios  of 1 (equibiaxial), 0.5, -0.1, -0.5 and -1 (shear) at a von Mises equivalent strain amplitude Δ  vM /2 of 0.4 · 10 -2 . Three stages are visible independent of the state of strain: i) primary hardening, ii) softening and iii) secondary hardening. This behavior is known from uniaxial reference tests and literature on TRIP steels [11 – 13]. The course of  = 0.5 had nearly the same evolution as the course of  = 1. The level of the curves of negative strain ratios declined with decreasing strain ratio from -0.1 to -1. Furthermore, the onset and the magnitude of secondary hardening were the latest and the smallest for  = -0.1 and -0.5 loading, T