Influence of Stress Concentration on Steel Deformation 20 Under Cyclic Loads

Abstract

The effect of stress concentration on fatigue behavior of steel 20 is studied. Experimental study is carried out on tubular smooth and notched specimens under uniaxial cyclic loading and alternating torsion. Load and strain controlled fatigue tests were carried out at room temperature using servohydraulic machine with independent control of push-pull and torsion loads with frequency of 0.5-3 Hz. One basic geometry for two different types of specimens shown in Figure 1 with 1.1 mm wall thickness, 22 mm inside diameter, and 40 mm gauge length was used in this work. One type was a tubular smooth thin-walled specimens, the other type was the same thin-walled specimen with 3.4 mm circular through-thickness hole at the middle of gauge length. The 5% load drop for uniaxial strain control tests and 5% strain and rotation angle increment for uniaxial and torsion load control tests respectively, as compared to midlife stable cycle for smooth specimens were considered as a small crack initiation life. The criterion of maximum principal stress was used as a control parameter of the test program. Torque loading was accompanied by a significant decrease in durability compared to uniaxial loading due to conservative methodology. Although maximum principal stress criterion could correlate axial and torsion constant amplitude data of notched specimens with a factor of 4, it could not correlate axial with torsion data of smooth specimens. It is found that the prediction of fatigue life can be successfully performed according to the shear form of Fatemi-Soci criterion by taking into account the mechanism of destruction of samples. The shear strain-life curve was generated based on von Mises criterion. The FS parameter was associated with local stress-strain condition based on FE analysis results.