Issue34

N.R. Gates et alii, Frattura ed Integrità Strutturale, 34 (2015) 27-41; DOI: 10.3221/IGF-ESIS.34.03 32 crack orientation data from pure torsion, torsion with static axial stress, and in-phase axial-torsion fatigue tests of smooth specimens. Nominally applied loadings are given along with shear stress amplitude and maximum normal stress on the maximum shear plane. Specimen suffixes indicate the loading conditions as follows: ST are pure torsion tests, STSA are torsion tests with static axial stress, and SC are in-phase axial-torsion tests. The table is sorted based on decreasing shear stress amplitude on the maximum shear plane. Within each shear stress level, tests are sorted by decreasing level of maximum normal stress on the maximum shear plane. Approximate crack plane orientation is then listed for different ranges of overall tip-to-tip crack length, 2 c (in mm), as well as the total number of cycles to failure, N f , at which point the final crack lengths were approximately 15-20 mm. The dependence of crack growth direction on loading level is easy to see from Tab. 1. Above a shear stress amplitude of 188 MPa, all significant ranges of crack growth for all loading conditions took place in mode II on planes of maximum shear. Below this level, cracks begin to transition to mode I growth shortly after initiation and continue growing in mode I until near failure. The only exception to this trend comes when a tensile normal stress exists on the maximum shear plane. In these cases, cracks spend more time in mode II growth than specimens tested at the same shear stress amplitude without a tensile normal stress. In all but one case, both crack initiation and final failure were observed to be shear-mode processes. Figure 2 : Superimposed crack paths for precracked specimens (black) and smooth specimens (green) tested under identical loading conditions for (a) higher load level pure torsion, (b) lower load level pure torsion, (c) higher load level in-phase axial torsion, and (d) lower load level in-phase axial-torsion tests. Relative load level is indicated by the ratio of von Mises equivalent stress to yield strength. (a) (b) (c) (d)