Tham khảo tài liệu 'advances in gas turbine technology part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 320 Advances in Gas Turbine Technology Fig. 3. The effect of orientation on the strain-life of Ti6-4. 1500 z 1000 500 0 -500 RD max stress RD min stress TD max stress TD min stress . Ị ị -1000 Normalised cycles Fig. 4. The effect of orientation of the stress response of Ti6-4 under strain control loading max R 0 . Notched specimen behaviour In considering notched specimen behaviour it is important to acknowledge the requirement for a predictive methodology to enable designers to extrapolate to conditions for which reliable test data does not exist. Previous work has shown that the Walker strain approach Titanium in the Gas Turbine Engine 321 Walker 1970 is an appropriate method for these types of predictions. The Walker strain relationship is an empirical method for correlating R values and involves correlating strain control data of different R ratios allowing for the derivation of a master curve . As stated earlier the material at a notch root is assumed to experience strain control type conditions due to restraint from material surrounding the critically stressed volume of material. Through application of Neuber s rule Neuber 1968 that the product of stress and strain is a constant conditions at the notch root can be approximated allowing for the calculation of the individual Walker strain value for that specimen. Subsequently a predicted life can be inferred from the master curve based on the strain control data. This approach has been found to be accurate for similar titanium alloys to Ti6-4 Whittaker et. al. 2007 but has not previously been tested on a textured alloy. During the course of the work two notched specimen geometries were tested both with cylindrical notches the first was a V shaped cylindrical notch VCN which has a stress concentration factor Kt of the second a round cylindrical notch RCN with a Kt of . Initially apparent from Figure 5 is the fact that no orientation effect appears to exist in the RCN
