PSI - Issue 13

Daniele Rigon et al. / Procedia Structural Integrity 13 (2018) 1638–1643 D. Rigon et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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The specific heat loss per cycle can be evaluated in situ during a fatigue test by means of Eq.(1) (Meneghetti, (2007)): (1) where  is the material density, c is the material specific heat and f L is the load test frequency and ∂ T ∂ t ⁄ is the initial cooling gradient after having suddenly interrupted the fatigue test at the time t* (Fig. 1(b)). As highlighted in Meneghetti et al., (2013)(a), the specific heat loss is a material property for a given load ratio. It is interesting to investigate to which extent it depends on the stress state, the sole experimental outcome available up to date is that both uniaxial and torsional fatigue results are rationalized in a single scatter band, as reported in previous Fig. 1. In this contribution, the specific heat loss will be evaluated for the first time in multiaxial fatigue test of cold drawn AISI 304L stainless steel bars. A set of fatigue tests was carried out to investigate the effect of the biaxiality ratio  and of the phase shift angle  between bending and torsion loadings. The fatigue test results have been compared with the existing scatter band previously calibrated on uniaxial fatigue test results. 1. Materials and methods Starting from 25-mm-diameter, AISI 304L cold drawn bars, two sets of specimens were prepared. The first one consists of cylindrical hourglass-shaped specimens (specimen (a)), while the second includes thin-walled specimens with a net-wall thickness of 1.5 mm (specimen (b)) . The specimen’s geometries of the two sets and the mechanical properties of the cold drawn bars are reported in Fig. 2 and Table 1, respectively. L c T t f      Q =

Fig. 2. (a) Specimen’s geometry for bending - torsional fatigue tests; (b) specimen’s geometry for axial -torsional fatigue tests (dimensions are given in mm).

The experimental fatigue tests on the type (a) specimens were performed by using a flexible test bench consisting of two servo-hydraulic actuators equipped with 15 kN load cells and controlled by a digital controller MTS Flex Test 60, as reported in Fig. 3(a). Force controlled pure bending, pure torsion and combined in-phase and out-of-phase bending-torsion fatigue tests were performed according to Table 2, where  and  are the stress-based biaxiality ratio (  a /  a ) and the phase-shift angle, respectively. Table 1. Mechanical properties of AISI 304L stainless steel bars from technical datasheet. R p,0.2% [MPa] R m [MPa] KV [J] HB 475 679 176 227 Some dedicated fatigue tests on type (b) specimens were performed by using a MTS 809 servo-hydraulic biaxial testing machine (±100 kN, ±1100 Nm, ±75 mm/±55°) under load control. In particular fully reversed (R=-1) axial, torsional and combined in-phase (  = 0°) and out-of-phase (  = 90°) axial and torsional fatigue tests were performed according to Table 2. For type (a) specimen the surface temperature measurement was performed by means of copper-constantan thermocouples placed at the point of the net section where the maximum bending stress occurs. A silver-loaded conductive epoxy glue was used to fix the thermocouples at the relevant point. Temperature signal was acquired by means of a data logger Agilent Technologies HP 34970A operating at a sample frequency, f acq , of 22 Hz.