Finite element analysis of residual stress in the welded zone of a high strength steel

The distribution of the residual stress in the weld joint of HQ130 grade high strength steel was investigated by means of finite element method (FEM) using ANSYS software. Welding was carried out using gas shielded arc welding with a heat input of 16 kJ/cm. The FEM analysis on the weld joint reveals that there is a stress gradient around the fusion zone of weld joint. The instantaneous residual stress on the weld surface goes up to 800 ∼ 1000 MPa and it is 500 ∼ 600 MPa, below the weld. The stress gradient near the fusion zone is higher than any other location in the surrounding area. This is attributed as one of the significant reasons for the development of cold cracks at the fusion zone in the high strength steel. In order to avoid such welding cracks, the thermal stress in the weld joint has to be minimized by controlling the weld heat input.     

含雷击热-力耦合损伤复合材料层压板拉伸剩余强度预测

为了对含雷击热-力耦合损伤复合材料层压板的剩余强度进行预测,基于连续介质损伤力学法(CDM)和唯象分析法,建立了表征复合材料雷击热-力耦合损伤的刚度矩阵渐进损伤退化模型。基于该模型,通过ABAQUS有限元仿真软件,建立了含雷击热-力耦合损伤的复合材料层压板结构三维模型。结合UMAT子程序,完成了拉伸载荷下的剩余强度预测。结果表明:通过与试验对比,仿真结果与试验结果取得了良好的一致性。本文所建立模型,能够有效进行含雷击热-力耦合损伤复合材料层压板结构拉伸剩余强度预测。     

Relationship between transverse residual stress and viscoelasticity of resin in aramid-aluminium laminate

In this paper, the fibre/resin layers in unidirectional aramid aluminium laminate was considered as viscoelastic layers and the modulus was expressed by the function of temperature and time. In order to determine the constant parameters in the function, dynamic mechanical property of the fiber/resin layers was tested under dynamic load with different frequency. The test temperature was range from room temperature to 200°C. After the parameters in the function was determined, the transvers residual stress in the laminate was calculated. In order to examine the calculated results, strain gages were embedded in the unidirectional aramid aluminium laminates to measure the transverse residual stresses. The calculated results are in good agreement with the experimental values.     

Effect of fibre/matrix adhesion on residual strength of notched composite laminates

Effects of fibre/matrix adhesion and residual strength of notched polymer matrix composite laminates (PMCLs) and fibre reinforced metal laminates (FRMLs) were investigated. Two different levels of adhesion between fibre and matrix were achieved by using the same carbon fibres with or without surface treatments. After conducting short-beam shear and transverse tension tests for fibre/matrix interface characterisation, residual strength tests were performed for PMCLs and FRMLs containing a circular hole/sharp notch for the two composite systems. It was found that laminates with poor interfacial adhesion between fibre and matrix exhibit higher residual strength than those with strong fibre/matrix adhesion. Major failure mechanisms and modes in two composite systems were studied using SEM fractography. The effective crack growth model (ECGM) was also applied to simulate the residual strength and damage growth of notched composite laminates with different fibre/matrix adhesion. Predictions from the ECGM were well correlated with experimental data.     

More on the effects of thermal residual and hydrostatic stresses on yielding behavior of unidirectional composites

The influence of manufacturing process thermal residual stresses and hydrostatic stresses on yielding behavior of unidirectional fiber reinforced composites has been investigated when subsequently subjected to various mechanical loadings. Three-dimensional finite element micro-mechanical models have been used. The results of this study reveal that the size of the initial yield surface is highly affected by the thermal residual and hydrostatic stresses. It was also found that effects of a uniform temperature change on the initial yield surface in the composite stress space is not equivalent to a solid translation of the surface in the direction of the hydrostatic stress axis. At the micro-level, magnitudes of various stress components within the matrix due to the thermal residual and hydrostatic stresses are different. However, at a macro-level, both temperature change and hydrostatic loading of composites show similar effects on the initial yield surface in the composite stress space. In an agreement with experimental data, results also show that residual stresses are responsible for asymmetric behavior of composites in uniaxial tension/compression in the fiber direction. This asymmetric behavior suggests that the existing quadratic yield criteria need modification to include thermal residual stress effects.     

圆板状SiC/C功能梯度材料残余热应力特征有限元分析

功能梯度材料残余热应力的大小及分布对其性能有效发挥及长期稳定使用有着较大的负面影响,为了尽可能充分发挥材料性能,增加材料的使用寿命,需尽可能减小残余应力以及使其合理分布.本文采用ANSYS有限元分析软件对不同叠层工艺参数的等离子体第一壁候选材料--SiC/C功能梯度材料(FGM)的残余热应力进行了数值模拟,获得了使热应力有效缓和的较适宜的工艺参数,对实际研发制备目标材料也可提供一些理论参照.相关结果表明,适量增加梯度叠层数及中间梯度层厚度可逐步有效缓和残余热应力,同时,针对本文今后应用的仍以炭材料为主体的炭基陶瓷保护层复合SiC/C FGM而言,纯SiC层厚度应取较小值,而叠层成分分布指数应取0.8～1.0为宜.     

Influence of thermal residual stresses on the composite macroscopic behavior

The influence of the thermal residual stress on the deformation behavior of a composite has been analyzed with a new micromechanical method. The method is based on secant moduli approximation and a new homogenized effective stress to characterize the plastic state of the matrix. It is found that the generated thermal residual stresses after cooling and their influence on the subsequent deformation behavior depends significantly on the aspect ratio of the inclusions. With prolate inclusions, the presence of thermal residual stresses generate a higher compressive hardening curves of the composite, but it is reversed with oblate inclusions. For particle reinforced composite, thermal residual stresses induce a tensile hardening curve higher than the compressive one and this is in agreement with experimental observations.     

Measurement and modelling of residual stress effects on cracks

Defects that form by mechanisms such as fatigue and stress corrosion cracking are influenced both by external loads on engineering structures and internal, residual stresses that are generated during the manufacture and operation. This paper describes a programme of experimental and analytical work on a high‐strength, low‐toughness aluminium alloy (AL2024‐T351) to assess the influence of residual stress on crack opening displacement (COD) and crack‐driving force (CDF) for a range of fatigue crack lengths in compact tension (CT) specimens containing a mechanically induced residual stress field. Comparison of experimentally measured and numerically predicted CODs, at the mid‐plane and surface of CT specimens, show generally good agreement for cracks introduced into the finite‐element model in a progressive, element‐by‐element manner. Cracks introduced in a simultaneous manner give larger than observed CODs. The CDFs for the progressively introduced crack are always smaller than for simultaneously introduced. These results have implications for the assessment of initiation for slowly growing cracks.     

Numerical investigation on welding residual stresses in a PWR pressurizer safety/relief nozzle

This paper presents finite element simulation results of residual stresses in dissimilar metal welds of a PWR pressurizer safety/relief nozzle. The present results are believed to be significant in two aspects. The first one is to consider the effect of the presence of similar metal welds on resulting residual stresses. The second one is the mitigation effect of the overlay welding thickness on residual stresses. After dissimilar metal welding, tensile residual stresses are present both at the inner surface and at the outer surface of dissimilar metal welds. Adjacent similar metal welding, however, decreases residual stresses to compressive ones at the inner surface of dissimilar metal welds, possibly due to the bending mechanism caused radial contraction of the weld. At the outer surface of dissimilar metal welds, similar metal welding increases residual stresses. Overlay welding further decreases residual stresses at the inner surface of dissimilar and similar metal welds, but increases slightly residual stresses at the outer surface.     

Development of residual strength evaluation tool based on stress-strain approximation

To allow for a safe design of metallic structures, it must be considered that, as a result of the manufacturing and processing operations, cracks or flaws below the NDI (Non-Destructive Inspection) detectability level always exist in the component after inspection. Further problems originate from the defect geometry. Failure of engineering structural components and structures have been mostly traced to surface cracks. Especially for a surface crack containing structure that is thin, the limit collapse of a ligament is usually the main cause of structure rupture. Study of the evaluation procedure relating to the limit load of the surface crack ligament is, therefore, an important project for conventional fracture assessment, particularly for pressure vessel (LBB) assessment. For this purpose, novel more accurate residual strength prediction method based on the Elasto-Plastic Fracture Mechanics (EPFM) has been developed and tested. Laboratory tests on tensile plate specimens with surface cracks were performed considering two different materials. In the final part of the work, effort was directed toward the verification and justification of selected analytical methods by adequate component testing. The most significant results of this work deal with residual strength evaluation for the thin wall pressure containing components. The important finding is that there is a potential for improvement in comparison to the current methods that may be used to increase payoff of the lightweight structures. The presented very robust analysis method and the useful structure integrity evaluation procedure should significantly contribute to the state-of-the-art structure optimisation and being applied to the design of the light-weight structures should ease the effort of the structure engineer to develop the successful and reliable hardware and to keep in place with advancing technologies. This revised version was published online in August 2006 with corrections to the Cover Date.     

The effect of a residual stress field on fracture initiation in RPV steel

Whether flaws in structures containing residual (secondary) stresses will extend under particular operational (primary) loads depends on the extent to which the residual stress field affects: (a) the nature and distribution of initiators; (b) the combined (primary + secondary) stresses and strains experienced by potential initiators. This paper compares fractographic data from specimens loaded by only a primary stress with data from specimens also containing a tensile residual stress field. Three‐dimensional elastic–plastic finite element calculations are used to characterize the stress–strain conditions at the initiation sites at the onset of brittle fracture. The introduction of a residual stress changes the dominant stage in fracture nucleation from microcrack extension to particle cracking. This offsets some of the decrease in fracture toughness expected when the residual stress field increases specimen constraint.     

含冲击损伤缝合层板剩余压缩强度

通过对含冲击损伤缝合复合材料层板进行压缩实验,揭示了含损伤缝合层板在压缩载荷下的破坏模式和破坏机制.将冲击损伤等效为圆孔,利用杂交单元计算冲击后缝合层板的应力分布,采用基于特征曲线概念的点应力判据预测了含损伤缝合层板的剩余压缩强度.研究结果表明:冲击损伤近似为圆形;缝合和未缝合层板冲击后压缩的损伤模式不同;采用开口等效法可以有效分析缝合层板的剩余压缩强度;特征距离、损伤面积是影响计算结果的主要因素.     

Measurement of thermal stress in graphite intercalated with bromine

The thermal stress of graphite intercalated with bromine was found to increase from zero at about 100°C to about 1.3 MPa at 200°C. The effect was reversible with hysteresis. The thermal stress increase had a sharp temperature dependence due to its association with the exfoliation phase transition.Paper presented at the Ninth International Thermal Expansion Symposium, December 8–10, 1986, Pittsburgh, Pennsylvania, U.S.A.     

真空镀Al/PVC膜残余应力的研究

利用X射线衍射研究了在塑料表面真空生成的铝膜残余应力.结果表明,残余应力在研究范围内为压应力,初步认识了铝薄膜的附着机理.     

Fatigue resistance and residual strength of riveted joints in FML

Static and fatigue tests have been carried out on 190-mm-wide fibre metal laminate (FML) riveted lap joints. The specimens were made with a Glare 3-3/2-0.3 material, i.e. a laminate composed of three layers of 2024-T3 aluminium alloy, thickness 0.3 mm and two double layers (0/90) of pre-preg FM-94-27%-S2 glass fibre. Seven configurations were tested which differed in the number of rivet rows (two or three), in the rolling direction of metal layers (perpendicular or parallel to the load direction), in the rivet type (solid or special, such as Hi-Lok or Lock-Bolt), in the rivet material for solid rivets (7050-T73 or 2017-T3), in the rivet diameter (4.8 or 5.5 mm) and in the presence of interlaminar doublers in the overlap area (titanium, aluminium, glass fibre). An additional difference was in the pre-formed rivet head: solid rivets had countersunk head, while Hi-Loks and Lock-Bolts had protruding head. The fatigue tests demonstrated the efficiency of a selective local reinforcement in the overlap area; in some cases, the fatigue resistance was so high that fatigue cracks nucleated in the laminates, rather than in the overlap area, as commonly expected.     

Measurement of residual stresses in thick composite cylinders by the radial-cut-cylinder-bending method

Thick fabric composite cylinders for nozzle parts in solid rocket motors should be designed to endure the extreme temperature and pressure of combustion gas. As the thickness of the composite cylinder increases, fabricational residual stresses due to the anisotropic thermal expansion or shrinkage of fabric composites also increase, which induces inter-laminar failures. Therefore, the accurate estimation of the residual stresses is indispensable for the development of thick fabric composite cylinders.

In this paper, the residual stresses in thick cylinders made of carbon fabric phenolic composites were measured by a new radial-cut-cylinder-bending method. To obtain the residual stresses from the measured relative strains during the radial-cut operation, a bending test of the cylinder with the radial-cut was performed instead of measuring the material properties with respect to radial positions. The thermal residual stresses were also calculated by finite element method considering shear deformation of fabric layers, and compared with the measured residual stresses by the new method, from which it was found that the new simple method estimated the residual stresses pretty well. Also the inter-laminar tensile strength at the position of maximum radial residual stress could be obtained from the bending test.     

Effect of thermal annealing on the microstructure,mechanical properties and residual stress relaxation of pure titanium after deep rolling treatment

The aim of this paper was to investigate the effect of thermal annealing on the microstructure, mechanical properties, and residual stress relaxation of deep rolled pure titanium. The microstructure and mechanical properties of the surface modified layer were analyzed by metallographic microscopy, transmission electron microscope and in-situ tensile testing. The results showed that the annealed near-surface layer with fine recrystallized grains had increased ductility but decreased strength after annealing below the recrystallization temperature, where the tensile strength was still higher than that of the substrate. After annealing at the recrystallization temperature, the recrystallized near-surface layer had smaller grain size, similar tensile strength, and higher proportional limit, comparable to those of the substrate. Moreover, the residual stress relaxation showed evidently different mechanisms at three different temperature regions: low temperature (T?≤?0.2?T_m), medium temperature (T?≈?(0.2?0.3)?T_m), and high temperature (T?≥?0.3?T_m). Furthermore, a prediction model was proposed in terms of modification of Zener-Wert-Avrami model, which showed promise in characterizing the residual stress relaxation in commercial pure Ti during deep rolling at elevated temperature.     

A fracture mechanics approach to estimate the fatigue endurance of welded t-joints including residual stress effects

Residual stresses and weld defects play a major role in the fatigue behaviour of welded structures, so these effects need to be accounted for in a theoretical analysis. A simplified engineering procedure based on linear‐elastic fracture mechanics is applied to estimate the fatigue behaviour, particularly the limit of endurance. Local geometrical irregularities and pre‐existing flaws, which are typical for this kind of weld, are covered by an overall notch intensity factor instead of a specific stress intensity factor, so the initial flaw size is not needed explicitly in the analysis. The effect of residual stresses can be easily included. The cut‐compliance method was applied to measure the residual stress distribution on the cross‐section of the weld. A welded T‐joint was used as a benchmark. Unexpectedly, compressive residual stresses were found to prevail in the root region. According to the analysis, they contribute to the endurance limit of the considered joint by about 50%. This result was confirmed by fatigue tests where a significant decrease in the fatigue strength after a post‐weld stress relieving heat treatment was observed.     

Synchrotron diffraction investigation of the distribution and influence of residual stresses in fatigue

This paper presents some results obtained from synchrotron diffraction investigations into two somewhat related areas of interest to the fatigue community. Firstly, the influence of fatigue cycling on the distribution and magnitude of residual strains and stresses and, secondly, the residual strains and stresses engendered around a growing fatigue crack. Its main premise is that modern tools such as automated synchrotron strain scanning offer the potential for more complete insight into the distribution of residual strains and stresses and their influence on fatigue performance. The first part of the work was accomplished using friction‐stir welded (FSW) and metal‐inert gas (MIG) welded specimens. The particular interest in these specimens was obtaining detailed knowledge regarding as‐welded variation in residual stresses between specimens, the location of peak values relative to local microstructure and stress concentrations, and of their modification during fatigue cycling. Such information may indicate a route forward to the selection of welding process parameters for optimised fatigue performance. The second part of the work considered an established fatigue crack in a compact tension (CT) specimen and examined the ability of synchrotron diffraction to characterize the stresses associated with the plastic enclave around a fatigue crack. This work is of interest in the context of better knowledge of crack‐tip shielding by plasticity‐induced closure and its incorporation into life prediction methodologies.     

Evaluation of interfacial shear strength and residual stress of sol-gel derived fluoridated hydroxyapatite coatings on Ti6Al4V substrates

Interfacial shear strength is one of the critical properties in bioceramic coatings on metal implants because it directly affects the success of implantation and long-term stability. In this study, shear strain lag method was employed to evaluate the interfacial shear strength of sol-gel derived fluoridated hydroxyapatite (FHA) coatings on Ti6Al4V substrates. The residual stresses were measured using the “wafer curvature method”. The resultant interfacial shear strength increased from pure HA’s ∼393-459 MPa as fluorine was increased to 1.96 at% and further increased to ∼572 MPa as fluorine increased to 3.29 at%. The residual stresses in the coating also decreased from pure HA’s ∼273-190 MPa and further to ∼137 MPa as fluorine composition in the coating increased. The reduction in the residual stress mainly comes from the reduction in the difference in coefficient of thermal expansion between the coating and the titanium alloy substrate.