Ratchetting Of Metal Materials

The ratchetting of materials corresponds to progressive inelastic deformation, under cyclic loading. The harm of rechetting has arose abroad recognition by international scholars and the research of it has been hot research direction. The known research emphasizes the work of description of ratchetting constitutive relationships under cyclic loading. Traditional constitutive models are hard to develop and complex in application. These models have many constants needed defined and it is difficult to fit for diverse loading conditions. So constitutive models described ratchetting need to be made further progress.The system experimental study for uniaxial ratchetting of 304 stainless steel and T225NG alloy were carried out. The emphasis of study was evolution characters of ratchetting and influential factors that included mean stress, stress amplitude, loading rate,previous cyclic loading history and so on. The following research findings were obtained.(1)The system experimental study was carried out for the influence of temperature,mean stress, stress amplitude, loading rate and previous cyclic loading history for uniaxial ratchetting of 304 stainless steel and T225NG alloy at diverse temperature.(2) The results of uniaxial ratchetting experiments for 304 stainless steel and T225NG alloy at elevated temperature were analysed. It is shown that ratchetting strain is mainly controlled by peak stress and previous lower stress loading history has little influence on saturated ratchetting strain. The more high the temperature is,the less cycles materials need to reach saturated state. At room temperature T225NG reaches to saturated state after more than 10,000 cycles,but less than 1,000 cycles at 350 ? . But temperature has little influence on the shape of curve between saturated ratchetting strain and peak true stress. The creep before ratchetting affects evolution of ratchetting only and has little influence on saturated ratchetting.(3) Base on the unitary stress parameter theory, the differentSaturated Ratchetting Models with Temperature effect (SRMT) were presented to predict saturated ratchetting strain of 304 stainless steel and T225NG alloy respectively. The predicting results are rather exact comparing with experimental results at different temperature.(4) Base on saturated ratchetting characters of materials,the single-specimen method was presented to fix constants in SRMT by loading to saturated ratchetting step by step with single specimen. Contraposing experiment phenomenon of T225NG, the improved single-specimen method with tiny increasing of peak stress in a cycle was presented to fix constants in SRMT of T225NG. The improved single-specimen method can shorten experiment time and avoid fatigue after too many cycles.(4) A new separate visco-plastic constitutive model was presented. A memory surface for maximum plastic strain was introduced and different plastic strain flow rules and evolution rules of back stress were applied according to the memory surface. The monotonic tensile behavior and cyclic behavior of the material were separately described. That made the process of fixing constants in the model simple. The model was applied to simulation for uniaxial ratchetting behavior of 1Crl8Ni9Ti stainless steel and T225NG alloy respectively. It is shown that the simulative results agree well with the experimental ones.(5) Based on MTS TestStar II system, a set of software called MTSCL for biaxial complex path experiment was developed. The software can realize complex path experiment conveniently. The MTSCL software has many virtues which include simple usage, few input parameters,high reliability and preview fuction.The software can get real-time data which is computed with original data. That can simplify the process of dealing with data and increase the experiment efficiency.The software was applied to interrelated experiments successfully.