Issue 17

B. Atzori et alii, Frattura ed Integrità Strutturale, 17 (2011) 15-22; DOI: 10.3221/IGF-ESIS.17.02 16 were carried out in order to investigate the sensitivity of Q parameter as damage indicator in the case of variable amplitude fatigue. T HEORETICAL MODEL TO ESTIMATE THE Q PARAMETER he experimental technique used to evaluate the Q parameter is based on a theoretical model presented elsewhere [4], so that only the main features will be presented here. Let us consider a control volume dV of material under fatigue loading conditions, as shown in Fig. 1. The first law of thermodynamics applied to the control volume can be written in terms of power as:   dV cd cv ir p T W dV H H H dV c E t                     (1) where W is the expended mechanical power in a unit volume; H cd , H cv , H ir represent the thermal power dissipated in a unit volume due to conduction, convection and radiation, respectively; the last term in the second member is the rate of variation of the internal energy, which is related to the material density  , the specific heat c, the time variation of the temperature T and to the time variation of energy absorbed by the material p E  . The term p E  represents the rate of accumulation of plastic hysteresis energy, i.e. fatigue damage. dV W Q U Figure 1 : First law of thermodynamics applied to a control volume of material undergoing a fatigue test. Usually the surface temperature of material rapidly increases during the first part of fatigue test and then reaches a stationary value which depends on the applied stress level [3, 4]. In steady state conditions Eq. (1) becomes   cd cv ir p W H H H E      (2) By considering a sudden stop of fatigue test, the terms W and p E  become zero and then from Eq. (1):   cd cv ir T c H H H t          (3) is possible to evaluate the thermal power H dissipated in steady state conditions (Eq.(2)) by measuring the time derivative of temperature (see Eq.(3)). Finally, the energy released to the surroundings as heat by a unit volume of material per cycle, Q , can be calculated as: HQ f  (4) where f is the test frequency. M ATERIAL , SPECIMEN GEOMETRY AND TEST PROCEDURE he experimental tests were carried out on specimens prepared from 6-mm-thick AISI 304L stainless steel sheets. The specimen geometries used for static and fatigue tests are shown in Fig. 2a and Fig. 2b, respectively. Tests were carried out at room temperature on a Schenck Hydropuls PSA 100 servo-hydraulic test machine, T