Method for analyzing residual stress of polyethylene pipe welded joint

Abstract

The invention provides a method for analyzing residual stress of a polyethylene pipe welded joint. The method comprises the following steps: carrying out a unidirectional stretching experiment and a stress relaxation experiment to obtain a tangent modulus and a yield limit value of a base material sample and a stress loading function curve of the base material sample under unit strain; establishing a viscoelastic constitutive model based on a generalized Maxwell equation and Pluronic series, and fitting the curve by adopting the model to obtain parameters of the model; establishing a basic material database of the polyethylene material according to the parameters, the thermophysical parameters and the thermodynamic parameters; establishing a geometric model, dividing grids, inputting thermal boundary conditions and force boundary conditions of each welding stage, and setting load steps and convergence parameters to obtain instantaneous temperature field data; and taking the instantaneous temperature field data as a preheating load, and adjusting input parameters by adopting a sequential coupling method to obtain the distribution condition of the residual stress. According to the method for analyzing residual stress of a polyethylene pipe welded joint, nondestructive analysis of the residual stress of the joint after the welding process is accurately and quickly realized.

Description

technical field [0001] The invention relates to the field of residual stress analysis, in particular to an analysis method for residual stress of welded joints of polyethylene pipes. Background technique [0002] In the safety classification of key nuclear power equipment, it is generally divided into one, two, three safety levels and non-safety levels according to the safety functions performed by each pressure-bearing equipment. Among them, the water supply circulation system widely used in nuclear power equipment also involves safety. Grade and non-safety grade, and it uses traditional metal pipes such as nickel-based alloy steel and austenitic stainless steel, but these pipes have poor anti-corrosion performance, serious fouling phenomenon, and are easily affected by erosion-corrosion, often not reaching the rated design It has already failed and destroyed during its lifespan, thus posing a great threat to the safe work of nuclear power plants. [0003] Due to its excel...

AI Technical Summary

Problems solved by technology

In the prior art, power-law constitutive or elastic-plastic constitutive models are usually used to describe the mechanical behavior of polyethylene materials. Traditional power-law constitutive and elastic-plastic constitutive models both emphasize the elastic-plastic properties of materials, especially metal materials will behave However, the application history of polyethylene materials is not as good as that of metal materials, and there are relatively few research results. Therefore, the initial research on the mechanical constitutive properties of polyethylene materials was based on metal materials. From the perspective of materials, because polyethylene polymers have different mechanical properties and constitutive models than metals, the traditional elastic-plastic constitutive or power-law constitutive models cannot accurately express the mechanical behavior of polyethylene materials.

For example, the power law model, the calculation formula used is: ε=Aσ n t m , the strain ε is simply related to the stress σ and time t through the fitting parameters A, n, m, there is no essential material behavior description, only intuitively starting from the obtained experimental data, and does not explain the polyethylene material from the mechanism It has creep and relaxation characteristics, lacks physical meaning and its parameter fitting steps are more complicated

For another example, the elastic-plastic constitutive model is only from the perspective of studying metal materials, ignoring the essential viscoelastic properties of polyethylene materials, so it is often not suitable for polyethylene materials; and the way to obtain the material parameters of the viscoelastic constitutive model is as follows: For example, the corresponding material parameters are obtained by proposing the viscoelastic constitutive creep expression, or the viscoelastic constitutive parameters of the material are obtained by Fourier series fitting, but the fitting process of the above methods is relatively Complicated, with many parameters, small application area, low accuracy and applicability

[0005] In addition, in the existing technology, the reliability evaluation of welded joints of polyethylene pipes is often realized by destructive measurement technology, but because the welding process takes a long time, the process is complicated, and there are many influencing factors, only the destructive measurement method is used to verify the welded joints. Integrity and reliability will consume a lot of resources and time, and polymers do not have a complete lattice structure, and their properties such as yield strength and fracture strength are not as good as metal materials, resulting in relatively small residual stress values ​​generated by welding, which is difficult to Accurately measure with general methods

In addition, due to the use of destructive measurement technology in the existing technology, the distribution and characteristics of the residual stress of polyethylene pipe welded joints cannot be systematically analyzed, especially for the area where harmful residual tensile stress exists. The effect of stress will cause damage to the welded joints, and even rapid failure

Images