Issue34

S. Ackemrann et alii, Frattura ed Integrità Strutturale, 34 (2015) 580-589; DOI: 10.3221/IGF-ESIS.34.64 583 respectively. The earliest onset and the highest secondary hardening was found for the  = -1 case. An analogous trend was observed for the fatigue lifetimes: the smallest was found for  = -0.1 and -0.5 and the highest for  = -1. The authors conclude that shear loading (  = -1) supported yielding and resulted in lower maximum axial forces, as reported also for cast TRIP steel [9]. Moreover, shear loading showed the most pronounced secondary hardening due to the earliest onset of hardening and the highest fatigue lifetime. The von Mises type force amplitude correlated the cyclic deformation curves of positive strain ratios well, but was not appropriate for negative strain ratios, in particular shear loading. The cross sectional areas in tests with different strain ratios were not equivalent as assumed for the von Mises type force amplitude. Further experiments revealed that the onset of secondary hardening was shifted to lower number of cycles and the magnitude of secondary hardening increased with increasing von Mises strain amplitude Δ  vM /2 in the range of 0.3 to 0.6 · 10 -2 due to higher plastic deformation. The cyclic hardening depends on the plastic strain amplitude. The investigated steel PM 16-7-6 showed a martensitic phase transformation during cyclic deformation. Fig. 2b presents the  ’-martensite volume fraction vs. number of cycles of an uniaxial reference test and the biaxial tests at a von Mises equivalent strain amplitude Δ  vM /2 =0.4 · 10 -2 . A pronounced increase in  ’-martensite content was observed after an incubation period and correlated with the onset of secondary hardening. The latest onset of martensite formation was found for  = 0.5 and  = 1. The other states of strain (  = -0.1, -0.5, -1, uniaxial) showed similar courses. The  ’-martensite volume fraction at fatigue failure was the lowest for  = 0.5 (about 5 vol.%) and the highest for  = -1 as well as uniaxial loading (up to 45 vol.%). The authors conclude that  ’-martensite formation depended on plastic strain amplitude and caused the secondary hardening which is in good agreement to observations reported for uniaxial tests in literature [11–13]. Moreover, shear and uniaxial loading promoted  ’-martensite formation, whereas the  ’-martensite content was significantly lower in all other investigated biaxial states of strain. Figure 2 : a) Cyclic deformation curves and b)  ’-martensite evolutions of TRIP steel PM 16-7-6 at von Mises equivalent strain amplitude Δ  vM /2 = 0.4 · 10 -2 for different strain ratios  .  Fig. 3 presents the  ’-martensite volume fraction at fatigue failure of the steel PM 16-7-6 plotted vs. the von Mises equivalent strain amplitudes Δ  vM /2 and vs. the number of cycles to failure N f of uniaxial and biaxial tests. A small amount of martensite (< 8 vol.%) was observed at a von Mises equivalent strain amplitude Δ  vM /2 of about 0.3 · 10 -2 , see Fig. 3a. Higher strain amplitudes resulted in higher  ’-martensite contents due to higher plastic deformation. The highest  ’-martensite content up to 85 vol.% was observed at Δ  vM /2 = 0.6 · 10 -2 for shear loading. Under uniaxial loading similar martensite volume fractions were observed as under shear loading. The martensite contents were considerably lower than those under biaxial shear loading at strain ratios  > -1. An analogous trend was observed for the cyclic hardening. Fig. 3b shows an increase in fatigue lifetime with decreasing  ’-martensite content (and decreasing Δ  vM /2) for the investigated uniaxial and biaxial tests (  = 1 and -1) which is in good agreement to uniaxial findings reported elsewhere [13]. Regarding the loading conditions the following trend was observed: The lowest  ’-martensite volume fractions were observed for  = -0.5, -0.1, 0.5 followed by  = 1 and uniaxial loading. The highest  ’-martensite contents developed in