| Steel is widely used in transportation,pressure vessels,high-rise buildings and other fields.It has been an important material in national economic construction and national defense construction.However,during the installation and actual service,steel will be affected by the temperature and the plastic deformation.And in low temperature environment,brittle cleavage fracture will occur easily.Cleavage fracture is a random event that can lead to catastrophic structural failures.For example,the sinking of the Titanic,the overturning of Norwegian drilling rig and other major accidents,resulted in heavy losses.How to predict the fracture behavior of specimens and structures and further carry out fracture prevention design have always been the focus and difficulty of academic research.For more than 30 years,scholars have been actively promoting a micro-mechanical-based probabilistic approach,also known as the local approach,to evaluate the cleavage fracture behavior of materials so as to avoid sudden catastrophic structural failure.The key point in this approach is the characteristic parameters calibration of the local approach model for the cleavage fracture of the material,that is,the calibration of the Weibull parameters.Based on the proposed new local approach model for cleavage fracture,this paper studies and applies the parameter calibration of the new local approach model using 20 Mn Mo Ni55 ferritic steel and domestic pressure vessel steel A508-Ⅲ.In this paper,the finite element analysis software ABAQUS was employed to model,mesh and numerically simulate the two types of pressure vessel steels.And the corresponding stress distribution results were obtained.Then combined with the new local approach model,the research and discussion were carried out.The main research contents are as follows:(1)The elastic-plastic finite element method was used to simulate the both 10 groups of samples of 20 Mn Mo Ni55 steel at two temperatures(123K and 223K).The stress distributions of the two kinds of specimens at different temperatures and loads were obtained.Combined with the formula of the new local approach model,the parameters calibration of the samples at two temperatures was carried out.Four different threshold coefficients ? of fracture process zone were adopted to carry out parameters calibration and result analysis,respectively.A simplified method for Weibull parameters calibration when using the new local approach model was proposed.(2)The cleavage fracture behavior of 20 Mn Mo Ni55 steel was predicted.Based on the new local approach model and the calibration results of the Weibull parameters of 20 Mn Mo Ni55 steel,the relationship between the cleavage fracture of 20 Mn Mo Ni55 steel and the external loads of the structure was predicted.The mathematical derivation of the formula of the new local approach model of cleavage fracture was carried out.Based on the probability density function and the maximum principle,two important cleavage fracture probability calculation formulas were proposed.These two important formulae for calculating the cleavage fracture probability were applied to 20 Mn Mo Ni55 steel.And two important cleavage fracture probability values and corresponding load values were proposed.Loads that are most likely to cause structural fracture at both temperatures were predicted.(3)This paper investigated whether the Weibull parameters calibration of the new local approach model for cleavage fracture is affected by the geometric size of the specimens.According to the fracture experimental data of A508-Ⅲ steel,three-dimensional modeling,meshing and numerical simulation of low temperature round notched bar tensile specimens of A508-Ⅲ steel with two different notch sizes were carried out by finite element analysis software ABAQUS.The stress distributions of the two kinds of specimens were obtained.Combined with the new local approach model of cleavage fracture,the Weibull parameters of two notched size samples were calibrated by linear regression method.The results show that the Weibull parameters of samples with different notched sizes are different when using the new model to calibrate the model parameters,which indicates that the parameters calibration of the new local approach model is dependent on the notch size. |
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