Quasi-static tensile behavior and constitutive modeling of large diameter thin-walled commercial pure titanium tube

Large diameter thin-walled (LDTW) commercial pure titanium (CP-Ti) bent tubes are widely used in the pneumatic system of commercial airplane. Understanding and modeling the temperature and strain rate dependent quasi-static tensile behaviors are fundamentals for the improvement of bending formability of LDTW CP-Ti tubes. With the LDTW CP-Ti tube of 76.2 mm×1.07 mm (D×t, D-outer diameter, t-wall thickness, D/t=71.2) as the objective, uniaxial tensile tests were conducted under various temperatures (298–873 K) and different strain rates (10−3–10−2 s−1). The major results show that: (1) The flow stress of LDTW CP-Ti tube becomes less temperature dependent at 473–573 K, and the positive effect of strain rate on the flow stress is not obvious at 523–773 K. These results are interpreted in terms of dynamic strain ageing. (2) The fracture elongation of LDTW CP-Ti tube can be greatly improved at 873 K, but the ductility decreases with the increase of temperature at 573–773 K, which is considered as “blue brittle” phenomenon. (3) The strain hardening exponent of the LDTW CP-Ti tube increases from 0.073 to 0.155 as the temperature increases from 373 K to 573 K, and the increased strain hardening exponent has positive effect on the improvement of bending formability of the LDTW CP-Ti tube. Then, at the temperature range of 298–573 K, by introducing a quadratic function of the reciprocal of temperature and the semi-log scale of strain rate, a group of new equations for strain hardening exponent, strain rate sensitivity parameter and strength coefficient were proposed based on Fields–Bachofen(FB) equation; comparing the predicted results with the experimental data, the proposed constitutive models present a good estimate of the quasi-static flow stress for the CP-Ti tube, and the largest mean error is 3.66%.