To address the limited understanding of the static stiffness characteristics of metal hoses in aerospace pipeline systems, axial stiffness tests was carried out on metal hoses.By designing axial stiffness tests, the tensile and compressive conditions under no internal pressure and 0.5 MPa internal pressure were considered, and the stiffness curves of two different specifications of metal hose specimens were obtained.The test results show that the axial stiffness of the hose exhibits high nonlinearity.In the low stiffness stage, the stiffness of the two specimens is 56.82 N/mm and 167.13 N/mm, respectively, while in the high stiffness stage, the stiffness of the two specimens is greater than 3 724.62 N/mm and 4 014.14 N/mm, respectively.Using digital image method, the braiding angle change and outer diameter change information of the net sleeve during axial tension were obtained, and the effects of geometric nonlinearity and boundary nonlinearity factors on the hose stiffness were analyzed.An explanation of nonlinear mechanical behavior based on contact was proposed for metal hoses.Using finite element simulation method, the nonlinear stiffness change of metal hoses during axial tension was reproduced, and the explanation of reliability of the nonlinear mechanical behavior based on contact for metal hoses was verified.The study provides a reference for further analysis of metal hose stiffness nonlinearity.

For the urgent demand of domestic automatic welding materials in 304LN stainless steel vessels, ER316LMnG austenitic stainless steel welding wire was developed.The deposited metal and 304LN butt joint of metal active gas arc welding（MAG） were studied.The results show that the microstructure of the as-welded and stress relieving heat treated deposited metal is austenite columnar crystal, without ferrite structure.The deposited metal has good strength and plasticity, especially the impact value at-196 ℃ reaches more than 80 J,and the impact fracture shows the characteristics of typical ductile fracture.In the case of welding repair, long-time or multiple stress relieving heat treatment has no obvious effect on the mechanical properties of the deposited metal; the welding quality of the butt joint obtained by the welding wire is good, and its mechanical properties such as tensile, impact and bending property meet the requirements of welding procedure qualification of pressure equipment, and the welded joint has strong intergranular corrosion resistance.The welding wire can be used for the welding of 304LN pressure vessel.

The hydrogen barrier performance of the polymer liner material of Type Ⅳ hydrogen storage cylinders used for high-pressure hydrogen storage plays a crucial role in the quality of the cylinders.The polymer liner materials-polyethylene（PE） and polyamide（PA6）,were used as the research objects to conduct molecular dynamics simulations and Monte Carlo simulations of hydrogen transport processes in PE and PA6 to calculate the solubility coefficients, diffusion coefficients and gas permeability coefficients of hydrogen in polymeric materials.The feasibility of molecular dynamics simulation of the barrier properties of the materials was demonstrated by gas permeation experiments.The results show that the dissolution coefficient of gases of PE and PA materials decreases and the gas diffusion coefficient increases as the temperature increases from 288 K to 328 K at 0.1 MPa.And the gas permeability coefficient of the materials increases by 1.0% and 80.3%,respectively, due to the predominance of diffusion.The same trend as above is also obtained for the operating conditions of 41.6 MPa, 52 MPa and 60 MPa.In addition, it is found that graphene and organic montmorillonite modification can reduce the diffusion coefficient of gas in the material by simulation.

Crack defect is one of the most dangerous defects in girth weld.Once pipelines encounter natural disasters, they will bear additional bending load in addition to internal pressure, and the bending deformation easily causes girth weld crack propagation and pipeline rupture, when oil and gas pipelines pass through areas with poor geological conditions.The engineering evaluation formula of stress intensity factor at crack tip was fitted based on a large number of finite element data of girth weld cracks.The submodel analysis method can obtain the finite element result data quickly without losing the accuracy, and it is an efficient method to obtain the fitting data of crack stress intensity factor in full-scale bending pipeline.In combination with the theoretical formula of stress intensity factor for circumferential crack provided in API 579,the basic form of stress intensity factor was given.The crack size range was 0.03＜a/c＜0.2 and 0.27＜a/t＜0.54,and the corresponding maximum error was 4.86%.Through full-scale bending tests, it is verified that the critical displacement accuracy of the corresponding formula during crack propagation can reach 5.49%,and the critical far-field strain accuracy of the formula can reach 4.95%.

To investigate the effects of structural parameters and inlet Reynolds number on the performance of the asymmetric airfoil fin printed circuit heat exchanger（PCHE）,a three-dimensional numerical simulation based on computational fluid dynamics（CFD） was carried out, and the results were fitted to obtain the empirical criterion formulas of Nusselt number（Nu） with deviation less than 10% and friction factor（f） with deviation less than 13%.Then, a one-dimensional analysis method was used to calculate the volume, heat transfer rate, and total pressure drop of the full-size PCHE（without simplifying the heat exchanger model）.Finally, a multi-objective optimization design using the NSGA-Ⅱ algorithm was carried out by using the Q/V and Δp as the objective functions, and using the transverse spacing, fin thickness, and inlet Reynolds number as the design variables.The research results show that, compared with the transverse fin spacing of 4.8 mm, when the transverse spacing equals to 3 mm, the values of Q/V can be increased by up to 88.7%,and the values of Δp can be increased by up to 178.6%.Compared with the fin thickness of 0.6 mm, when the fin thickness equals to 2.4 mm, the values of Q/V can be increased by up to 76.5%,and the values of Δp can be increased by up to 93.9%.The NACA8540 airfoil fin PCHE has the best performance compared with the other asymmetric airfoil fin PCHE with different structural parameters.The conclusions obtained herein have certain guiding significance for the application of PCHE with asymmetric airfoil fins.

The ultra-high pressure tubular reactor is composed of autofrettage thick-walled straight tubes and thick-walled bent tubes.The autofrettage stress of the thick-walled bend tube is difficult to be obtained by theoretical analysis.The finite element method was adopted to analyze the stress of the autofrettage thick-walled bend tube.Moreover, the analysis result was compared to that of the autofrettage thick-walled straight tube based on the plane strain model.The results show that the bending radius of autofrettage thick-walled bend tube has a significant effect on its stress condition.The greater the bending radius, the closer the stress distribution of thick-walled bend tube is to that of thick-walled straight tube.Taking the equivalent stress amplitude of the inner surface of the thick-walled bend tube under working pressure as the benchmark, it can be concluded that the fatigue performance of the thick-walled bend tube is basically equal to that of the thick-walled straight tube when the rotation radius increases to a certain value.For thick-walled autofrettage bend tube of the specifications commonly used in LDPE/EVA units, if 10% of the equivalent stress amplitude on the inner surface of the thick-walled straight tube is taken as the maximum deviation of the equivalent stress amplitude of the thick-walled bend tube, then the value of the radius of rotation R should be greater than 7D_o at least.When the deviation is 5%,the radius of rotation R needs to be greater than 10D_o.The research results are consistent with the design parameters of the ultra-high pressure tubular reactor imported from abroad.

In order to prevent the welding residual stress from causing the corrosion cracking of the J-shaped welds on the head cover of reactor pressure vessel, the J-shaped weld structure of the large-diameter and large-angle head cover penetration was designed and optimized by increasing the weld joint area and reducing the structural discontinuity surface.The residual stress on the outer wall of the penetration was measured using X-ray diffraction method, and the residual stress on the inner wall of the penetration was measured using blind hole method.The residual stress value and distribution law on the inner and outer walls were obtained through the residual stress test.It is confirmed that the optimized weld structure can reduce the residual stress to about 70% of the lower limit of acceptable value.The structural integrity and safety margin of reactor pressure vessel are effectively improved.The study also provides guidance for the optimization direction of subsequent structural design.

The stress of the compensator designed with reducer pipe structure was analyzed by the finite element analysis software, and the strength check was carried out according to the design-by-analysis method.At the same time, the influence of the size of the semi-apex angle and the flanged arc R on the structural force was analyzed.The analysis results show that the thickness of the reducer pipe can refer to the calculation method of the thickness of the flanged conical shell under internal pressure in GB/T 150.3—2011,however, considering the large axial force borne by the compensator used for preheating pipelines, it is suggested to further check by finite element analysis according to the design by analysis to improve the safety of the product.When the structure of the compensator used for preheating pipelines is designed with a reducer pipe, von Mises stress decreases somewhat with the increase of arc R,but the decrease is not large, and the equivalent stress decreases from R110 to R190 by about 5%.The von Mises stress increases with the increase of the semi-apex angle, and the change is nearly linear.It is suggested that the smaller semi-apex angle can be preferred in the design of the compensator used for preheating pipelines, and the appropriate arc R can be determined according to the forming process.

The calculation method of tubesheets of floating-head heat exchanger in Appendix I of JB 4732—1995 did not consider the additional bending moment at cylinder edge, which needs to be improved.The modified calculation formula was given by derivation of the tubesheet calculation method, and the influence of the additional bending moment at the edge of the cylinder not included in JB 4732—1995 on the calculation results of three structures of tubesheet, namely b-type, c-type and d-type, was discussed and analyzed.The results show that the tubesheet calculation results without considering additional bending moment is unsafe under some loading conditions, so the calculation method in JB 4732—1995 needs to be improved.

Joint is a weak link prone to harmful defects which are easy to exist on polyethylene gas pipeline.Therefore, it is necessary to carry out regular testing in this weak area to ensure the safe operation of polyethylene gas pipelines.In order to improve the image recognition ability of polyethylene gas pipeline joint defects, an improved convolutional neural network recognition algorithm based on the ResNet network model was proposed.Firstly, the method of the Laplacian operator and median filter was used to preprocess the defect image of the PE gas pipeline joint; The dropout layer and the ELU function were then added to the ResNet34 network model to build the image recognition model.Finally, an improved ResNet34 network model was used to train and test the data set containing six types of hot melt defects through experiments.The experimental results show that the improved ResNet34 network model can achieve 97.3% accuracy in training defect images, and has a higher accuracy than the original ResNet34 network model and DenseNet network model, which verifies the effectiveness of this model for the recognition of hot-melt joint defect images.

In order to solve the problems of surfacing at the weld of Q345R+Incoloy 825 alloy composite steel plate and surfacing on the inner wall of the nozzle, the welding process evaluation of electrode arc welding and tungsten argon arc welding was carried out on low alloy steel Q345R by selecting reasonable welding process parameters and welding methods according to the welding characteristics of nickel-based alloy.The types of defects that are easy to occur in the welding process and the welding points that need to be mastered were analyzed, and the quality control measures of the welding process were defined to effectively avoid harmful defects in the welding process and ensure the smooth manufacturing of Q345R+Incoloy 825 alloy composite steel plate pressure vessel, which provides a reference for the subsequent manufacturing of the like material products.