| Elbow is commonly used in directly buried heating pipe and it is also the weak link.Stress calculation methods and strength-checking principles are always the urgent problems need to solve.Currently,the most frequently used method for calculating stress is elastic flexure-resisting hinge analysis.Domestic researchers have done many researches and found that elastic flexure-resisting hinge analysis can be used to calculate the stress of large diameter elbow despite that there are restrictions on diameter and elastic arm of elbow.However,few researchers analyzed stress calculation of elbow with short elastic arm.This thesis uses finite element simulation method and offers the stress calculation method for elbow with short elastic elbow.The impact of curvature radius and expansion mat on elbow stress is analyzed.This thesis is divided into six chapters.Chapter One introduces the background and significance of this thesis.Current research is introduced and analyzed.A brief introduction to research methods and contents is also given in Chapter One.Chapter Two analyzes functions and features of different kinds of load of directly buried heating-pipe and classify the stress and failure forms that may appear.Two forms of failure are pointed clearly,one is plastic deformation,the other is low cycle fatigue.Chapter Three compares stress calculation methods at home and abroad.These methods are used to calculate the stress when the elbow is in the state of infinite plastic deformation and low cycle fatigue.Maximum principal shear stress theory is used to derive the calculation methods for theoretical wall thickness of elbow.At the same time,European calculation method for theoretical wall thickness is introduced.After explaining the hypothesis model of elastic flexure-resisting hinge analysis and calculation methods,readers will find that domestic formulas for theoretical wall thickness conform to foreign ones.Although restrictions on diameter and load effects on calculation of elbow stress are considered,results still show that when the elastic arm meets length requirement,elastic flexure-resisting hinge analysis can be used to calculate stress of large diameter heating-elbow.After explaining the European standards,the author uses Palmgren-Miner formula and S—N curve to calculate the critical stress of elbow.Based on the calculation,one can know whether the elbow can meet the safety requirement of low cycle fatigue.Chapter Four uses ANSYS finite element software to exert displacement load,temperature load,pressure load on the thin wall model.The result shows that displacement load can replace temperature load.The sum of displacement loads on both ends of elbow equals to elbow stress.There is a linear relation between displacement load on the single side and elbow stress.Elbow should meet the linear superposition relation of pressure load,displacement load(temperature load).The formula of elbow with short elastic arm length is given.To increase the curvature radius is another way to lower the elbow stress.Chapter five uses two engineering cases to prove the module,the ways how loads are applied,and the stress calculation method for elbow with short elastic arm are correct and reasonable.Chapter six summarizes the research results and puts forward the research direction in the future. |
