Yield phenomena during the tensile testing of pure titanium sheets of 0.2 mm and 0.4 mm in thickness were investigated in detail focusing
on the effects of grain size (d), thickness (t), t/d ratio and tensile direction. With decreasing grain size, the yielding behavior changed from
continuous yielding to one accompanied with yield point drop. Upper and lower yield stresses and 0.2ÿ–proof stress followed the Hall–Petch
relationship; however, coarse–grained specimens (d ÿ 20 μm) showed larger scatter in 0.2ÿ–proof stress than the others. Consequently, the
Hall–Petch coefficient (k) and friction stress (ÿ0) derived from 0.2ÿ–proof stress are not accurate enough. The values of k and ÿ0 derived from
various yield stresses and tensile directions were in the range of 250–600 MPa·μm0.5 and 30–180 MPa, respectively. Therefore, the validity of
stress for yield stress was a concern, and the combination of the lower yield stress in the fine–grain range (d ÿ 20 μm) and 0.2ÿ–proof stress
excluding work–hardening in the coarse–grain range (d > 20 μm) was suggested to obtain reliable values of k and ÿ0, resulting in values of
370– 460 MPa·μm0.5 and ÿ0 65–140 MPa, respectively. Moreover, it was revealed that k decreased and ÿ0 increased with the increasing angle
of tensile direction to the rolling direction, regardless of thickness. The anisotropy of k is presumed to be affected by the grain boundary
character rather than the Schmid factor, and neither rigidity nor the length of the Burgers vector are responsible. Meanwhile, the anisotropy of
ÿ0 is verified to be affected by Schmid factors. Furthermore, it was clarified that the t/d ratio hardly affects upper and lower yield stresses (t/d >
14) nor 0.2ÿ– proof stress (1.5 ÿ t/d ÿ 14).