INFLUENCE OF INTERNAL DEFECTS ON FATIGUE BEHAVIOUR OF ALPHA-TITANIUM

The influence of internal preformed cracks, introduced through controlled hydride precipitate fracture, on the fatigue behaviour of three α-titanium alloys containing 0.085, 0.125 and 0.27 wt.% oxygen, and designated I.M.I. Ti 115, Ti 130 and Ti 160, respectively, has been examined. Specimens of the three alloys, with similar grain sizes (~0.25 mm), were fatigued in tension-compression about a zero mean load at a frequency of ~140 cycles sec-1 at room temperature. The presence of the preformed cracks reduced the stress level that could be sustained during cyclic loading. Cracks were also introduced during cyclic loading in Ti 115 and Ti 130 as a consequence of damage associated with {1121} twins; this effect was progressively reduced with increasing oxygen content. The influence of preformed and twin associated cracks of varying lengths, and the different fatigue stress levels in the three alloys at which failure did not occur after 107 cycles, are rationalised in terms of stress intensities associated with those cracks for which the growth rate was diminishingly small. © 1969.