Issue 38

T. Morishita et alii, Frattura ed Integrità Strutturale, 38 (2016) 281-288; DOI: 10.3221/IGF-ESIS.38.38 282 reported that failure lives are reduced accompanying with an additional hardening due to non-proportional loading depending on both the strain path and the material [1-5]. The failure life under non-proportional loading at high temperature has been discussed in the previous study for the high chromium ferritic steel [6]. However, open questions of discussing creep-fatigue property still exist. Therefore, creep-fatigue test results and damage evaluation under multiaxial loading are required for reliable and safety design of strength for materials and components. In this study, creep-fatigue tests under a push-pull loading and a circle loading conditions with changing the strain rate were performed at 823K in air. The evaluation of failure life is also discussed from the obtained test results. E XPERIMENTAL PROCEDURE ultiaxial creep-fatigue tests under non-proportional loading with various waveforms were carried out at 823K in air. A hollow cylinder specimen which has a 9mm inner-diameter, a 12mm outer-diameter and a 12mm in a gauge length as shown in Fig. 1 was employed. The testing machine used was an electrical servo hydraulic tension and torsion fatigue testing machine. Strain paths and strain waveforms are shown in Fig. 2. They are the push-pull loading (PP) and the circle loading (CI). The PP test is the proportional strain loading test. The CI test is the non- proportional strain loading test in which axial strain and shear strain have 90 degree sinusoidal out-of-phase. Total axial strain and total shear strain ranges were the same ranges based on von Mises basis (  =  /  3). In these strain paths, three strain rates were employed. They are 0.2%/s, 0.01%/s and 0.002%/s based on von Mises basis which are indicated by ‘FF’, ‘SS’ and ‘SS * ’, respectively. Strain holdings are also employed, they are the tensile and the compressive holdings which are indicated by ‘TH’ and ‘CH’. The holding times used were 180s (3min.) and 600s (10min.). Number of cycles to failure (failure life) N f is defined as the cycles at which the axial or shear stress was reduced to 3/4 of the sudden decrease point. Figure 1 : Shape and dimensions of test specimen (mm). Axial strain ε Time t Axial strain Time t Strain rate 0.01 or 0.002%/s Strain rate 0.2%/s Axial strain ε Time t Holding time 3 or 10min Axial strain ε Time t Holding time 3 or 10min PP-FF PP-SS, PP-SS* PP-TH PP-CH t Axial strain ε Shear strain t Axial strain ε Shear strain t Axial strain ε Shear strain Holding time 3 or 10min CI-FF CI-SS, CI-SS* CI-TH Push-pull (PP) Circle (CI)  3  Strain rate 0.01 or 0.002%/s Strain rate 0.2%/s (a) Strain path (b) Strain waveform Figure 2 : Strain path and strain waveform. M