Multiaxial Creep Behavior Study Of Materials And Components Based On Time-hardening Model

Creep is the phenomenon that the strain increases with time under constant loading.For the complexity of structure geometry and stress state,the structural components are often in the multi-axial stress state when they are running.Thus,the creep deformations in actual situation are often multi-axial creep.At present,the research on uniaxial creep is relatively mature,but the research of multi-axial creep is not enough systematic and perfect for both experimental and theoretical research.The systematic study of multi-axial creep behavior of materials has important engineering application value and academic research significance,which needs to be further developed.The multi-axial stress field will form near hole edge of a plate specimen with a circular hole and the notch of a notched bar when the specimen is drew.In this paper,the multi-axial creep of materials was studied experimentally and simulated by finite element method under multi-axial stress state.Furthermore,the multi-axial creep behavior of elbow structure at high temperature was simulated and predicted.The following main works were carried out in this paper:1.DIC measurement technology and GOM Correlate image analysis software were used to analyze the strain during uniaxial tensile creep of plate and tensile creep test with circular orifice plate for vulcanized rubber.The distribution of strain during creep and its evolution with time were obtained.The time hardening creep model was used to simulate the multi-axial creep test of circular orifice plates.The simulation results were in good agreement with the experimental results in the distribution trend of strain field and the strain value.2.A new method for predicting the multi-axial creep life of notched bars was proposed.Based on the Time-Hardening creep model and the “skeletal point stress” method,the stress distribution at the notch during creep was simulated.The position where stress did not change with time on the minimum section of notch during creep was defined as “skeletal point”.Then,the multi-axial creep life of notched bar specimens was predicted by using the life-stress relationship of uniaxial creep test of smooth bar specimens and the equivalent stress of the “skeletal point” position.The multi-axial creep lives of different notched bars of P92 steel were predicted by this method and European multi-axial creep code respectively.The results show that the precision of predicted creep lives of two kinds of method are equivalent.Furthermore,the forecasting method proposed by this paper is rather simple and convenient for engineering applications.In addition,the multi-axial creep of notched bars with different notch radius was simulated by finite element method.It was found that the position of “skeletal point” would move towards the notch surface with the decrease of notch radius.3.Using Time-hardening creep model to simulate and analyze the multi-axial creep of straight pipe and elbow with different wall thickness subjected to 20 MPa internal pressure at 625?.It was found that the maximum stress concentration of the elbow was located at the inner arch of the 45° section of the elbow.When the wall thickness was increased,the wall thickness reduction of the inner arch was gradually reduced,but the wall thickness reduction of the outer arch was increased.In the case of a certain diameter,the wall thickness increases,and the “skeletal point” stress of the outer arch increases.For elbow pipes,the creep life calculated according to the “bone point” stress of multi-axial creep was quite different from the creep life calculated according to the maximum equivalent stress at the inner arch for uniaxial creep.In design,it should be paid sufficient attention to the difference between uniaxial creep and multi-axial creep.