激光冲击处理对焊接接头力学性能的影响（Ⅱ）

上文对GH3 0、1Cr18Ni9Ti焊缝进行了激光冲击处理 ,试验发现激光冲击处理提高GH3 0焊接接头强度 12 % ,但对疲劳寿命影响不明显 ;冲击提高 1Cr18Ni9Ti焊接接头强度仅为 5 % ,但提高疲劳寿命 3 0 0 %以上。为作进一步分析 ,本文测试了两种金属焊缝在有无激光冲击处理条件下的表面显微硬度分布和残余应力状态 ,并对GH3 0试件疲劳断口进行扫描电镜分析。经分析发现 ,激光冲击处理能明显提高GH3 0焊缝表层的显微硬度 ,也可以提高抗拉强度并获得较高的表面残余压应力 ,因此强化区域能抑制疲劳裂纹的萌生和扩展 ,降低裂纹扩展速率 ;但由于焊缝较宽 ,激光冲击光斑未能完全覆盖焊缝及热影响区 ,部分未冲击区域影响了疲劳寿命的提高。 1Cr18Ni9Ti在等离子焊接过程中产生相变马氏体 ,减弱了激光冲击处理产生形变马氏体对提高显微硬度的作用 ,因此对抗拉强度影响不大 ,但激光冲击处理可以使焊缝表面获得较高的表面残余压应力 ,因此能明显提高疲劳寿命。     

16Mn钢埋弧焊接头疲劳裂纹扩展速率的研究

对16Mn钢双面埋弧焊CT试样的焊缝、热影响区、母材及垂直焊缝方向的疲劳裂纹扩展速率进行了研究。结果表明,焊接接头的不同部位疲劳裂纹扩展速率不同,平均应力、焊接残余应力、金相组织对疲劳裂纹的扩展速率都有一定的影响。     

热真空低温环境实验台研制

为满足低温实验的环境要求,建设了液氮温度级别(80 K)的热真空冷阱低温环境实验台,可进行低温实验中压力与压差、温度与温差、流量与热负荷的测量.该实验台采用附加液氮冷阱的真空多层绝热结构,冷阱温度最低可达80 K,无负载时冷箱真空度可达0.000 03 Pa;在采用外循环工质时,测试压力范围为0-1 MPa、压差范围为...     

A procedure for the simulation of fatigue crack growth in adhesively bonded joints based on the cohesive zone model and different mixed-mode propagation criteria

This work deals with the simulation of the fatigue crack growth (FCG) in bonded joints. In particular a cohesive damage model is implemented in the commercial software Abaqus, in order to take into account for the damage produced by fatigue loading. The crack growth rate is evaluated with different Paris-like power laws expressed in terms of strain energy release rate. The crack growth rate is then translated into a variation of the damage distribution over the cohesive zone setting an equivalence between the increment of crack length and the increment of damage. The model takes also into account mixed mode I/II conditions. In this work the validity of the model is tested by comparison with theoretical trends for conditions of pure mode I, pure mode II and mixed mode loading. In the case of mixed mode conditions, different models are implemented for the crack growth rate computation. The results of the model are in very good agreement with the expected trends, therefore the model is adequate to simulate the fatigue crack growth behaviour of bonded joint.     

Analysis of the S–N curves of welded joints enhanced by ultrasonic peening treatment

Contrast fatigue tests were carried out on T-shape tubular joints of 20 steel in three conditions: as welded, treated by ultrasonic peening treatment (UPT) before loading and UPT under loading. Results are: (1) Dispersity of test results measured by nominal stress is much larger than that measured by hot spot stress. After UPT before loading, fatigue strength of 20 steel tubular joints measured by hot spot stress increases by 67% and fatigue life is prolonged by 22–45 times. (2) Under low stress ratio R, UPT before loading can improve the fatigue performance of welded tubular joints significantly. (3) Under high stress ratio R, UPT under loading (static load) is recommended to improve the fatigue performance of welded tubular joints. UPT under loading not only enhances the fatigue properties at low stress level, but also at high stress level. (4) The general rule of S–N curves of welded joints treated by UPT is commonly effected by external load (static load) and self release of residual stress.     

Numerical and experimental analysis of stresses and failure in T-bolt joints

T-bolt joints are widely used in the wind turbine industry. This paper describes the constitution and function of the different components of the joint, showing the different parameters that affect its performance, and presents a parametric analysis, useful for joint design and optimization, relating laminate thickness, bolt and cylinder diameters to pitch distance, and pretension of the bolt with external load. In order to provide information on the influence of the T-bolt geometry on the strength of the laminate in the net tension and bearing zones, tests for two different glass fiber – epoxy laminates, under different pitch/diameter ratios have been performed, forcing either bearing or tension failure modes. Stress and strain concentrations in the laminate have been estimated trough a three-dimensional finite element model that takes into account pretension effects and contact nonlinearities. How experimental results correlate with simple and finite element model analysis is discussed. Stress Point Criteria may be applied for joint tension failure with a characteristic distance of d₀ = 0.9 mm.     

Fatigue crack propagation in API 5L X‐70 pipeline steel longitudinal welded joints under constant and variable amplitudes

Fatigue crack propagation (FCP) under constant and variable amplitude loading in base metal (BM), weld metal (WM) and heat affected zone (HAZ) of longitudinal welded joints of an API X‐70 pipeline steel was investigated. Constant amplitude loading tests were performed at R = 0.1 and 0.5, whereas for variable amplitude testing single peak tensile overloads (OLs) alternating between 75 and 100% of maximum load were applied at 2.5 mm intervals in crack growth. Results of SE(B) specimens tested under constant and variable amplitude loading revealed that BM, WM and HAZ regions subjected to R = 0.5 and low ΔK‐values presented the highest crack growth rates. At higher ΔK values FCP rates in all the studied regions were similar and the R effect on FCP rate was no more observed. Crack growth retardation due to OLs was observed at the three studied regions, showing a decrease on the FCP delay with a decreasing on ΔK.     

Interface between Sn-Sb-Cu solder and copper substrate

Influence of antimony and copper in Sn-Sb-Cu solder on the melting and solidification temperatures and on the microstructure of the interface between the solder and copper substrate after wetting the substrate at 623 K for 1800 s were studied. Microstructure of the interface between the solder and copper substrates in Cu-solder-Cu joints prepared at the same temperature for 1800 s was observed and shear strength of the joints was measured. Influence of the atmosphere - air with the flux and deoxidising N₂ + 10H₂ gas - was taken into account. Thermal stability and microstructure were studied by differential scanning calorimetry (DSC), light microscopy, scanning electron microscopy (SEM) with energy-dispersive spectrometry (EDS) and X-ray diffraction (XRD). Melting and solidification temperatures of the solders were determined. An interfacial transition zone was formed by diffusion reaction between solid copper and liquid solder. At the interface Cu₃Sn and Cu₆Sn₅ phases arise. Cu₃Sn is adjacent to the Cu substrate and its thickness decreases with increasing the amount of copper in solder. Scallop Cu₆Sn₅ phase is formed also inside the solder drop. The solid solution Sn(Sb) and SbSn phase compose the interior of the solder drop. Shear strength of the joints measured by push-off method decreases with increasing Sb concentration. Copper in the solder shows even bigger negative effect on the strength.     

Elastoplastic Finite Element Analysis and Strength Evaluation of Adhesive Butt Joints of Similar and Dissimilar Hollow Shafts Subjected to External Bending Moments

Stress distributions and deformation of adhesive butt joints are analyzed by an elastoplastic finite element method when the joints of similar and dissimilar shafts are subjected to external bending moments. The effects of the ratio of Young's modulus for the adherends to that for an adhesive and the effects of the adhesive thickness on the interface stress distribution are investigated. Joint strength is predicted by using the elastoplastic interface stress distributions. It is found that the singular stress at the edge of the interfaces increases with an increase of the ratio of Young's modulus. Measurement of strains in joints and experiments on joint strength were conducted. The numerical results are in fairly good agreement with the experimental results. It is observed that the joint strength for dissimilar shafts are smaller than those for similar shafts. A fracture of dissimilar adhesive up-bonded shafts occurred from the interface of the adherend with smaller Young's modulus. It is seen that joint strength increases as the adhesive thickness increases.     

Relationship between Viscoelastic and Peeling Properties of Model Adhesives. Part 1. Cohesive Fracture

The viscoelastic and peeling properties of polybutadiene/tackifying resin compatible blends have been studied in detail. Viscoelastic properties have been described through the variations of the complex shear modulus, G*(ω), as a function of frequency, ω and peeling properties through the variations of peeling force (F) as a function of peeling rate (V).

After showing the objective character of the peeling curves obtained, the variations of the peeling force and peeling geometry have been studied as a function of volume fraction of the tackifying resin.

In this first paper, the analysis is focused on the first domain of the peeling curves, i.e. the cohesive fracture region. In this region, the peeling properties have been related to the viscoelastic properties in the terminal region of relaxation. It is shown that the longest relaxation time, τ_o, is a reducing parameter of the peeling curves, so a peeling master curve-which is independent of temperature, resin volume fraction and polymer molecular weight-may be defined. Furthermore, the variations of the test geometry as a function of peeling rate have been investigated: the variations of the radius of curvature of the aluminium foil have been analyzed with respect to the viscoelastic behavior of the adhesive, which in fact governs the test geometry.

A detailed analysis of all these features leads to a model which allows one to calculate the peeling curves in the cohesive domain from the adhesive formulation.