13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 3. Material and experimental programme To investigate the influence of the size effect and the variable stress state on the fatigue behaviour, specimens with different geometries have been tested until complete fracture at EMPA-Dübendorf (Swiss Federal Material Testing and Research Laboratories). All specimens whose dimensions are given in table 1 and refer to Fig. 2 have been machined by the same manufacturer from rods of the aluminium alloy AlMgSi1 6082-T6 with chemical composition given in table 2. The d3 and d8 specimens were machined from rods of diameter 25 mm and the d22 specimens from rods with diameter 45 mm. The corresponding yield and ultimate strengths are given in table 2 as the mean of three values obtained from static strength tests using normalized specimens. Figure 2. Specimen geometry Table 1. Tested specimen geometries Name d0 [mm] L0 [mm] L1 [mm] R [mm] d3 3 0 22.4 24 d8 8 24 88.6 90 d22 22 240 385.0 245 Table 2. Chemical composition and strength of AlMgSi1 6082-T6 d [mm] Si [%] Fe [%] Cu [%] Mn [%] Mg [%] Cr [%] Zn [%] Ti [%] Pb [%] Rm [MPa] Rp0.2 [MPa] 25 1.002 0.499 0.091 0.749 0.831 0.032 0.186 0.063 0.026 410 402 45 0.850 0.280 0.060 0.430 0.710 0.050 0.050 0.030 N/A 369 350 Since the static strength of the specimens differs for the different rods, additionally micrographies were taken (one from each rod) as shown in Fig. 3. It can be observed that the microstructure happens to be quite similar for both samples, thus, justifying a direct comparison of the fatigue test data. The constant amplitude fatigue tests were carried out in pure tension with a stress ratio R=0.1. For the tests of the d3 and d8 specimens resonance frequency machines (Rumul) equipped with 5 kN and 100 kN load cells, respectively, were used, whereas the d22 specimens were tested by a servo-hydraulic machine (Schenck) with 630 kN load cell. The forces were calculated taking into account the measured diameters in the central section of each specimen and the stress ranges Δσ, to be applied in this section. For all specimens the difference between measured and nominal diameter was less than 0.03 mm. The predominant role of the surface flaws has been corroborated by ocular
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