Issue 30

G. Meneghetti et alii, Frattura ed Integrità Strutturale, 30 (2014) 191-200; DOI: 10.3221/IGF-ESIS.30.25 196 obtained by carrying out fatigue tests at different load ratios cannot be rationalised in a single scatter band by using the Q parameter. 0.1 1 0 0.25 0.5 0.75 1 N/N f R=-1 R=0.1 R=0.5 Q [MJ/(m 3 ·cycle)] 10 0.01 Figure 5 : Measured Q trends against the number of cycles normalised with respect to the number of cycles to failure, N f . 0.1 1 10 10 3 10 4 10 5 10 6 10 7 N f , number of cycles to failure R=-1: k=1.98; Q A,50% =0.133 MJ/(m 3 ·cycle); T Q =2.99; T N,Q =8.75 R=0.1: k=3.08; Q A,50% =0.026 MJ/(m 3 ·cycle); T Q =2.88; T N,Q =26.0 Q [MJ/(m 3 ·cycle)] 0.01 R=0.5: k=5.51; Q A,50% =0.012 MJ/(m 3 ·cycle) N A Figure 6 : Fatigue data analysed in terms of energy released as heat by a unit volume of material per cycle. Scatter bands are defined for 10 and 90% survival probabilities. E XPERIMENTAL EVALUATION OF THE THERMOELASTIC CONSTANT o experimentally evaluate the thermoelastic constant K m (see Eq. 4), load-controlled ramps at different load rates were carried out, aiming at the evaluation of the minimum stress-rate   required to achieve adiabatic test conditions. Five stress rates were applied, namely   = 5, 19, 37, 54 and 73 MPa/s. The tests were conduced by applying an initial compressive stress equal to –150 MPa, followed by a ramp up to 150 MPa. The value of 150 MPa was chosen to guarantee the linear elastic behaviour of the analysed material. The adopted procedure can be summarised as follows: - application of a compressive stress equal to –150 MPa applied to the specimen; - holding of the compressive stress to allow for material thermal equilibrium with the surroundings (reference temperature T 0 ); - execution of a tensile ramp up to 150 MPa with given   value and measurement of the corresponding temperature drop. Since T 0 is the reference temperature at a stress of –150 MPa, the thermoelastic temperature T the reached at the end of the test will be referred to a stress of 300 MPa. Fig. 7 shows as an example the results of a test conduced at   = 54 MPa/s. Within the time window (t f -t i ), it can be observed that the temperature decreases from the initial value T 0 =301.25 K. The stress range adopted to evaluate K m corresponds to the stress variation in the time window  t=t f -t i, (  =  (t f ) -  (t i )), that is 297 MPa in Fig. 7. T