Issue 38

T. Morishita et alii, Frattura ed Integrità Strutturale, 38 (2016) 289-295; DOI: 10.3221/IGF-ESIS.38.39 290 and a material constant related to the additional hardening due to non-proportional loading [4;10-12]. However; there is few studies discussing failure life in high cycle region and fatigue strength under non-proportional loading [13;14]. In order to ensure reliability and safety of machinery; evaluating models for non-proportional loading including the high cycle region is required. In this study; multiaxial fatigue tests under proportional and non-proportional loading conditions were carried out in the low stress level to discuss fatigue strength. For evaluation of failure life in the high cycle region; an applicability of equivalent stress and strain ranges based on von Mises and Δε NP is discussed and Δε NP is also modified to be suitable strain range for life evaluation. T EST MATERIAL AND EXPERIMENTAL PROCEDURE aterial tested was rolled steel for general structure; type SS400 steel (A283 GRADE D for ASTM; St 44-2 for DIN). A hollow cylinder specimen with 12mm outer-diameter; 9mm inner-diameter in a gauge part is used. An electrical servo controlled hydraulic fatigue testing machine for push-pull and reversed torsion loadings of which maximum push-pull loading and torque are ±50 kN and ±500 N·m was employed as testing machine. Load controlled fatigue tests were carried out at room temperature. Stress paths were a push-pull; a reversed torsion (rev. torsion) and a circle loading. Fig. 1 shows the stress paths and the stress waveforms. The push-pull and the rev. torsion loading tests are proportional loading tests in which principal directions of stress and strain are fixed. The circle loading test is non-proportional loading test in which axial stress and shear stress have 90 degrees sinusoidal out-of-phase difference. In the circle loading test; axial and shear stress ranges are the same value based on von Mises; Δσ= 3 Δτ. Number of cycles to failure (failure life) N f was determined as the cycle at which a crack occurred on the surface of test specimen. The crack size is big enough to be checked by looking and this test is controlled by loading. Therefore; N f can be considered as the cycle at which test specimen ruptured. (a) Stress path (b) Stress waveform Figure 1: Stress path and stress waveform. E XPERIMENTAL RESULTS AND DISCUSSION Evaluation of Failure Life with Equivalent Stress and Strain ig. 2 shows a correlation of failure life with an equivalent stress amplitude based on von Mises Δσ eq /2. Failure life can be correlated by a unique line independent of loading path in the stress region over the fatigue strength σ w . In the load controlled test; failure life in the circle loading test tends to be longer than those in the push-pull and rev. torsion loading tests. The strain range in the circle loading test becomes smaller in comparison with that in the push-pull loading test at the same stress range because of additional hardening caused by non-proportional loading. In addition; it is known that failure life in the circle loading test is smaller than that in the push-pull loading test at same strain range due to M F