Issue 48

F. V. Antunes et alii, Frattura ed Integrità Strutturale, 48 (2019) 676-692; DOI: 10.3221/IGF-ESIS.48.64 683 Figure 7 : Variation of Y II with  (x=52.5 mm, y=0). Furthermore, to evaluate the characteristics of mixed mode fatigue crack, it is necessary to introduce the comparative stress intensity factor K v considering mode I and mode II simultaneously. The exact relation between K v , K I and K II depends on the criterion employed. The criterion of Richard/Henn  18  proposes: ΔK 1 2 2 I ΔK ΔK 6 ΔK V I II 2 2    (8) The values of K II are in general much lower than the ones obtained for K I , therefore its influence on K v defined by eq. (8) is relatively low. The solutions presented here for Y I and Y II were used to obtain K v and this was compared with K v obtained directly from CosmosM results. Maximum and minimum differences were found to be +10.63 and -4.5% respectively, but the average difference is 1.01 %, which is excellent. E XPERIMENTAL FATIGUE RESULTS n experimental work was developed to study fatigue crack growth and fatigue crack closure under mixed mode loading in 6082-T6 aluminium alloy. The specimens were obtained in the transverse longitudinal (TL) direction from a laminated plate. The initial notch depth was 42.5 mm. Before testing the specimen surfaces were polished mechanically. The experiments were performed in a servo-hydraulic, closed-loop mechanical test machine with 100 kN load capacity, interfaced to a computer for machine control and data acquisition. All tests were conducted in air and at room temperature. The tests were performed under load control mode and the load ratio for all loading angles was kept constant at 0.05. The loads were applied with a sinusoidal waveform at a frequency of 20 Hz. Fatigue pre-cracking was introduced under mode-I loading to an a/W ratio of 0.51, where a and W are the crack length and width of the specimen, respectively. The crack length was measured in both x and y directions using a travelling microscope (45  ) with a resolution of 10  m. The specimen was painted ahead of the crack tip (Fig. 2) to enhance optical measurement of crack length. The maximum and minimum loads applied for each loading angle were chosen in order to have after fatigue pre-crack a comparative stress intensity factor,  K V , defined by eq. (8) of approximately 6 MPa.m 1/2 , where the mode-I and mode-II stress intensity factor ranges were calculated by the solutions presented in Appendix A. Tab. 3 summarizes the parameters of the experimental testes. Further details can be found elsewhere  3  . The K solutions developed here were applied to the treatment of the experimental results. The crack tip coordinates (x,y) and slope,  , were measured directly on specimen’s surface using Profilometer Rodenstok RM 6003D. However,  could have been obtained from (x,y) values using a procedure similar to the calculation of da/dN 0 1 2 0 0.2 0.4 0.6 0.8 1 1.2 abs(  )-t [º] Y II [-]      A