AN INVESTIGATION OF THE EFFECTS OF STRESS RATIO AND CRACK CLOSURE ON THE MICROMECHANISMS OF FATIGUE-CRACK GROWTH IN TI-6AL-4V

The effects of positive stress ratios on the fatigue crack growth beha vior of a forged, mill-annealed Ti-6Al-4V alloy (with a duplex alpha beta microstructure) are discussed. Differences between fatigue crack growth rates at low and high stress ratios are shown to be due largel y to crack closure. A 1% offset procedure is shown to collapse ''closu re corrected'' low stress ratio data with the ''closure-free'' high st ress ratio data. A three parameter multiple regression model is develo ped for the prediction of the fatigue crack growth rate as a function of stress ratio, crack closure and stress intensity factor range for T i-6Al-4V. The micromechanisms of fatigue crack growth in the near-thre shold and Paris regimes are elucidated via crack tip transmission elec tron microscopy (TEM) and scanning electron microscopy (SEM). Crack ti p TEM shows that Stage II fatigue crack growth in Ti-6Al-4V is crystal lographic in nature. Stage II cracks are shown to unzip along intersec ting slip bands that are induced as a result of shear localization. Th e unzipping crack growth mechanism observed in polycrystalline Ti-6Al- 4V is shown to be consistent with Neumann's ''alternating slip'' model for the information of striations in single crystals. (C) 1997 Acta M etallurgica Inc.