Issue 41

M. A. Meggiolaro et alii, Frattura ed Integrità Strutturale, 41 (2017) 98-105; DOI: 10.3221/IGF-ESIS.41.14 104 The individual amplitudes of each  x and  xy /  3 strain component are 0.6% for all six experiments. The first four experiments involve basic load path shapes: cross, diamond, circle and square, see Fig. 5. The last two involve combinations of the previous four: the square/cross path load blocks consisting of one square cycle followed by one cross cycle, while each load block of the square/circle/diamond path involved one square, one circle and one diamond cycle, in that order. All the tests were carried out until a small crack was detected on the surface by visual inspection. In all specimens, the initiated crack was later confirmed to have surface widths between 1 and 2 mm. This variability contributes to the uncertainties in the experimental data, even though it can be inferred that the number of growth cycles between 1 and 2mm should be relatively small, since the visual inspection was carried out on a frequent basis. Tab. 1 shows the observed fatigue lives in number of blocks, where each block consists of a full load period. Figure 4 : Tension-torsion testing machine and extensometer mounted on a tubular specimen. Figure 5 : Applied periodic  x ×  xy /  3 strain paths on six tension-torsion tubular specimens, all of them with normal and effective shear amplitudes 0.6% . Tension-Torsion path observed life (blocks) Miner’s prediction (blocks) Cross 1535 - Diamond 976 - Circle 837 - Square 772 - Square/Cross 342 514 Square/Circle/Diamond 288 285 Table 1 : Observed initiation lives, in number of blocks, for each applied tension-torsion path, and Miner’s predictions.