Issue 35

R.A. Cardoso et alii, Frattura ed Integrità Strutturale, 35 (2016) 405-413; DOI: 10.3221/IGF-ESIS.35.46 408 always has the same magnitude in two orthogonal planes, the chosen plane for crack propagation direction is the one that experiences the maximum Δ σ n (less frictional energy is lost). Figure 4 : Plane of the minimum shear stress at the crack tip. A VAILABLE DATA o evaluate the accuracy from the aforementioned methodologies experimental data available from tests under fretting conditions will be assessed. All the tests considered are from cylinder-on-plane configuration. Tests were conducted on steel alloy 35NCD16 and AISI 1034 specimens. In both cases the pads were made from different materials, steel 100C6 and chromium 52100 steel, respectively. The mechanical properties are presented on Tab. 1, and the experiments have been reported and discussed in detail in [14] and [15], hence, only essential information necessary to carry out the analyses are briefly presented here. Material E (GPa) v σ Y (MPa) σ u (MPa) σ -1 (MPa) τ -1 (MPa) Δ K th (MPa.m 1/2 ) AISI 1034 200 0.3 350 600 270 - 7 52100 210 0.3 1700 2000 - - - 35NCD16 200 0.3 1127 1270 590 460 - 100C6 195 0.3 1500 - - - - Table 1 : Material properties. The load history applied in the tests are as follows. Firstly, a constant normal load, P , was applied to the fretting pad and held constant. Then an oscillatory shear force varying between ± Q a was applied. In the tests analysed here there is absence of a bulk load. All the tests were designed to run in partial slip regime, i.e., Q a /P<f where f is the coefficient of friction in the slip zones. One test will be analysed for AISI 1034 specimen and another for steel alloy 35NCD16. For AISI 1034 the tests analysed presented short crack arrest, hence, this data will be used only to assess the methodologies to estimate crack initiation path. The data from 35NCD16 will be used to assess methodologies in both the initiation and propagation stages. The most important parameters in the load history to carry out the analyses are reported in Tab. 2. Specimen P (N/mm) Q a (N/mm) R (mm) f AISI 1034 227 169 40 0.9 35NCD16 1000 500 80 0.9 Table 2 : Parameters of the test; R is the pad radius. In both test programs after interrupting the experiment the crack path and its extension were observed in an optical microscope by cutting the samples in the centre of the contact zone [16]. In case of AISI 1034 the tests were conducted until 10 6 cycles, for 35NCD16 three tests were performed and each of them were interrupted after a different number of cycles and analysed. The crack extension and profile are thus obtained from information collected from different tests. T