Issue34

P. Gallo et alii, Frattura ed Integrità Strutturale, 34 (2015) 180-189; DOI: 10.3221/IGF-ESIS.34.19 188 This allows us to take into account the notch sensitivity of this material at temperatures higher than 500°C:   n 2 2 t,n c w 0 K Δσ R ΔW = c Q T L(f / f )F(2α)×H(2α, )× ρ E (3) Q(T) is the notch sensitivity function at a specific temperature T. This function has to be set (as a function of the temperature T) by equating at high cycle fatigue (10 6 cycles) the SED value from plain specimens and those from notched specimens; f is the test frequency of notched specimens at high temperature and f 0 the test frequency of unnotched specimens at the same temperature. L is a function related to the sensitivity of the material to the load frequency and depends on the ratio f/f 0 . Function L is required to be equal to 1.0 if f=f 0 , a condition respected in all tests of the present analysis. The critical radius R c is kept constant and equal to that obtained at room temperature (R c =0.05 mm). Dealing with our specific case Q(T=650°C) = 0.18. Eq. (3) can be re-written by substituting the numerical value of each function: n n 2 2 2 2 t,n t,n K Δσ K Δσ ΔW = 1.0×0.18×1.0×0.7049×0.5627 = 0.07139 E E (4) where, as said above, K t,n =3.84. By considering Eq. (4) applied to notched specimens and Eq. (1) applied to plain specimens, the SED master curve for 40CrMoV13.9 at 650°C has been obtained. The fatigue data from tests at 650°C are plotted in terms of averaged strain energy density range over a control volume in Fig. 9-b, considering the critical radius previously derived at room temperature. It is possible to observe that the scatter band is quite narrow, with the scatter index being T w = 2.56 that results in T σ =1.60 in terms of equivalent local stress range. The inverse slope of the scatterband is equal to 1.43. Thanks to the SED approach it is possible to summarise in a single scatterband all the fatigue data at the same temperature, regardless of the specimen geometry. C ONCLUSIONS he present paper addresses experimentally the high temperature fatigue of 40CrMoV13.9 steel and the effect of surface roughness on fatigue strength and cracks initiation. In order to completely characterize the high temperature behaviour of this steel, firstly uniaxial-tension load controlled fatigue tests have been conducted at different temperatures up to 650°C. Two geometries have been considered in this phase: plain specimens and plates weakened by symmetric V-notches. Subsequently, with the aim to investigate the influence of the roughness on the high temperature behaviour and the cracks initiation, uniaxial-tension load controlled fatigue tests have been conducted on plate with central holes at the service temperature of 650°C, with different surface roughness. This geometry simulates the cooling channels of rolls for hot-rolling of metals, which are the most stressed zone of the rolls. Finally, fatigue data from un-notched and notched specimens are re-analysed by means of the mean value of the Strain Energy Density (SED) extended at high temperature. The main results can be summarized as follows:  The tested alloy exhibits good high temperature fatigue behaviour up to 500°C. Until that temperature no reduction in the fatigue strength with respect to the room temperature has been detected. Above 500°C, instead, a significant reduction in fatigue strength is shown both for plain and V-notched specimens.  Data from tests carried at room temperature up to 500°C are summarized in terms of mean SED over a control volume with R c =0.05 mm. A sound agreement in terms of SED has been found between the present results and those recently obtained from a large bulk of multiaxial tests performed at room temperature on the same material.  A specific master curve based on SED has been proposed for the considered steel tested at T=650°C. The scatter band makes possible to summarize together data from plain and notched specimens. Dealing with notched specimens an empirical expression has been also proposed for the SED calculation. The equation can be directly employed for practical applications of 40CrMoV13.9 steel at 650°C.  The roughness influences the fatigue strength. When the quality of the surface finish is improved, we can see some enhancements on the fatigue behaviour. The more evident improvement is registered for the value of R a =0.15μm. Comparing this value with that of the starting poor roughness, the stress range at one million cycles increases of 44%. T