Experimental Research On Tension-Compression Low Cycle Fatigue Of U-Shape Notched Specimens

Due to stress concentration or strain concentration, fatigue life and intensity of common notched or similar notched component control the fatigue life and strength of structures in practical structure engineering. Hence the research for notched component is an important link in structure fatigue analysis. There were many scholars had studied fatigue problem of notch component. However, it involves many complicated factors and remains to be further investigated. This thesis mainly used the method of experiment associated with numerical simulation to discuss the classical local stress strain method.The Q235 steel plate samples were taken as the research object. At first, the fatigue experiment of smooth plate samples were conducted in different strain amplitude (0.005,0.006 and 0.008) on MTS tension-torsion fatigue testing machine. Using Chaboche combination model of ABAQUS finite element software to describe the material plastic behavior under cyclic loading, the unit cell model was used to calibrate the model parameters through comparing with the hysteresis loops of the smooth plate samples under cyclic loading. The fatigue experiments under tension-compression loading were conducted and the U-shape notched specimens were loaded through controlling the displacement of specimen gage section, which were calculated by finite element method (FEM) prior experiments. Then the local stress-strain method was adopted to analyze the fatigue lives by test. According to the results, we find that:(1) whether FEM or modified Neuber method is taken to calculate stress and strain of notch root region, for fatigue life assessment the local stress-strain method is applicable only to the specimens with larger notch radius; (2) the estimated life values obviously lower than tests for specimens with smaller notch radius, and more serious for that the strain in notch root evaluated by FEM than by modified Neuber equation.On this basis, the influence of strain gradient in notch root region of specimen was studied. The famous Taylor model was applied to estimate the raise of flow stress due to the strain gradient in notch root region, by which the strain distribution of specimen was recalculated. The fatigue life estimation taken the influence of strain gradient into account was found improved for all notch specimens. When considering the material hardening caused by strain gradient, we found that the elements selected by Scheme 2 were best suited for the material hardening analysis through the comparison of three schemes.