Issue 17

B. Atzori et alii, Frattura ed Integrità Strutturale, 17 (2011) 15-22 ; DOI: 10.3221/IGF-ESIS.17.02 21 block of cycles were definitely higher than those observed during constant amplitude fatigue tests at the same stress level on the undamaged material. On the contrary, specimens 3 and 4 exceeded 10 millions of cycles once loaded at the second stress level. In fact for these specimens the Q values measured during the second block of cycles were very close to those detected during constant amplitude fatigue tests at 190 MPa on undamaged material. Then, as far as the experimental data analysed in the present paper are concerned, it can be concluded that the parameter Q is sensitive to the fatigue damage accumulated prior to the second block of fatigue cycles. Finally, by considering only the two specimens that failed, the damage D was evaluated on the basis of the Q-N curve shown by Fig. 7, according to the Miner’s rule (Eq. 6): 2 1 2 1 1 2 i i i n n n D N N N      (6) where n i is the number of cycles carried out at  a,i stress amplitude and N i represents the corresponding number of cycles to failure. Figure 10 compares the experimental results obtained for specimen 1 and 2 with Miner’s hypothesis (D=1): the horizontal and the vertical axis represent the fraction of fatigue life spent during the first block (n 1 /N 1 ) and during the second block (n 2 /N 2 ) of load, respectively. The comparison allowed us to notice that the Miner’s hypothesis, based on the Q-N curve, is in good agreement with the experimental results. Figure 10 : Comparison between the experimental data and Miner’s rule applied in terms of Q . C ONCLUSIONS n this paper the fatigue behaviour of a stainless steel AISI 304 L was analysed in terms of energy released to the surroundings as heat by a unit volume of material per cycle, Q . After evaluating the material constant amplitude fatigue limit under completely reversed axial stress, the fatigue data were analysed in terms of both stress amplitude and energy parameter Q , according to [4]. Then some specimens were fatigued in variable amplitude, two load levels to investigate the sensitivity of the Q parameter to prior fatigue damage. The main results can be summarised as follows:  the Wöhler curve of material generated from constant amplitude fatigue tests presents a knee point at 165800 cycles;  the average curve which synthesises the fatigue data in terms of the energy parameter Q presents a constant slope from 10 4 to 10 7 cycles;  the data dispersion in terms of fatigue lives is reduced by a factor greater than two if the experimental data are processed by using the energy parameter Q rather than the applied stress amplitude;  the observed energy parameter Q increases by a factor of about seven if specimens are tested just 13 MPa above the fatigue limit with respect to the values measured at or below the fatigue limit itself;  it can be concluded that the parameter Q is a fatigue indicator sensitive to the damage accumulated during the prior load history;  Miner’s hypothesis applied on the basis of the Q-N curve is in good agreement with the experimental data available. As a final remark, additional work is needed to further support the experimental findings presented in this paper. 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Specimen 1 Specimen 2 Miner's rule 1 1 N n 2 2 N n 1 N n N n D 2 2 1 1    I