Issue 27

P. Hou et alii, Frattura ed Integrità Strutturale, 27 (2014) 21-27; DOI: 10.3221/IGF-ESIS.27.03 24 R ESULTS AND DISCUSSION Qualitative identification of the fatigue damage emperature evolution is mainly attributed to the thermo-elastic effect, inelastic effect and heat conduction effect. The fluctuating temperature signals are attributed to the thermo-elastic effect. The cyclic temperature amplitude increases with the increasing cyclic loading amplitude. However, the thermo-elastic effect has no contribution to the average temperature increment [20]. Fig.3 presents the relative temperature evolution at different stress levels. Figure 3 : Temperature increment at different stress amplitude. The elastic stress controls the mechanical responses of the tested specimens when the stress level is lower than the fatigue limit of Ti-6Al-4V alloy. During this period, the specimen mainly takes place elastic deformation, and the temperature signal variation is mainly induced by the non-plastic effects, i.e. viscosity effect etc. If the stress level is higher than the fatigue limit, micro-cracks would initiate from the specimen boundaries, such as the corner and the free surface. The fatigue cracks initiated along the axial direction of 45 o , and then coalesced to form a main macro-crack. The main crack propagated perpendicular to the principal stress direction. The plastic strain energy continues to accumulate during the fatigue evolution process. Most of the mechanical energy was dissipated as heat energy heating up the specimen. Fig.4 shows variations of the stress standard deviation of the material. There is no obvious plastic deformation when the applied stress is lower. However, if the stress is higher than the fatigue limit, it sets to increase sharply in the localized zone due to the stress concentration effect. The break point indicates that the mechanisms related to fatigue failure have changed into plastic effect. Local stress concentration, due to pores and impurities, governs the stress distribution. The damage status can be qualitatively identified by the standard deviation to avoid the sudden fracture. From Fig.4, the standard deviation of the Ti-6Al-4V alloy is small when the fatigue stress amplitude is lower than the fatigue limit. However, the standard deviation of the Ti-6Al-4V continues increasing with the increasing stress. Figure 4 : Standard deviation of stress at different stress amplitude. T