Fatigue behaviour of riveted Glare lap joints

Fibre metal laminates, such as Glare, are a family of materials with very interesting properties for fatigue-critical applications. This article describes the results of a research programme carried out to evaluate the fatigue and damage tolerance characteristics of riveted Glare lap joints, representative of fuselage longitudinal joints. The comparison with the behaviour of metallic joints shows that different contributions are made to the total fatigue life, with the crack propagation life being by far the longer one for the Glare material whereas the crack nucleation life covers almost the entire fatigue life for metallic joints. Design rules should take this peculiar behaviour into consideration, to achieve the maximum benefit in the use of this class of materials in fatigue-critical applications.     

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.     

Cruciform fillet welded joint fatigue strength improvements by weld metal phase transformations

Arc welding typically generates residual tensile stresses in welded joints, leading to deteriorated fatigue performance of these joints. Volume expansion of the weld metal at high temperatures followed by contraction during cooling induces a local tensile residual stress state. A new type of welding wire capable of inducing a local compressive residual stress state by means of controlled martensitic transformation at relatively low temperatures has been studied, and the effects of the transformation temperature and residual stresses on fatigue strength are discussed. In this study, several LTTW (Low Transformation‐Temperature Welding) wires have been developed and investigated to better characterize the effect of phase transformation on residual stress management in welded joints. Non‐load‐carrying cruciform fillet welded joints were prepared for measurement of residual stresses and fatigue testing. The measurement of the residual stresses of the three designed wires reveals a compressive residual stress near the weld toe. The fatigue properties of the new wires are enhanced compared to a commercially available wire.     

钎缝厚度对陶瓷连接接头强度的影响

采用不同厚度的Ag-Cu-Ti钎料连接Si3N4陶瓷材料,研究了钎缝厚度对连接强度的影响规律,利用扫描电镜观察了接头微观结构.试验发现,在一定钎缝厚度范围内,随着钎缝厚度的增加,接头强度随之提高.研究表明:钎缝厚度影响连接强度的主要原因在于对残余应力的缓解,但钎缝厚度过厚时,对应力的缓解作用变化不大,而钎缝的拘束作用减弱,接头强度有下降的趋势;在相同连接规范下,钎缝厚度与反应层厚度之间也存在一定的关系.     

New Developed Welding Electrode for Improving the Fatigue Strength of Welded Joints

A new welding electrode, low transformation temperature electrode (LTTE), was introduced in this paper. It was described in design principle, mechanics, chemical compositions of their deposited metal and manufacturing methods. It was proved that the best transformation starting temperature from austenite to martensite of the deposited metal of LTTE was at about 191℃ and it was obtained by adding alloying elements such as Cr, Ni, Mn and Mo. The microstructure of the weld metal of the LTTE was low carbon martensite and residual austenite. The compressive residual stress was induced around the weld of the LTTE and the -145 MPa in compression could be obtained in middle of weld metal. The fatigue tests showed that the fatigue strength of the longitudinal welded joints welded with the LTTE at 2×106 cycles was improved by 59% compared with that of the same type of welded joints welded with conventional E5015 and the fatigue life was increased by 47 times at 162 MPa. It is a very valuable method to improve th     

泡沫金属三明治结构压印-粘接复合接头剥离性能分析

为研究三明治结构压印-粘接复合接头的抗剥离性能,选取AA5052铝合金板以及泡沫镍、泡沫铜以及泡沫铁镍进行压印-粘接复合连接,对接头进行拉伸剪切试验,采用扫描电镜对失效断口进行了观察,分析了接头的失效形式、失效载荷、能量吸收值以及失效机理。结果表明:夹层的存在会使接头中胶层的失效过程更加稳定,夹层对于压印-粘接复合接头的承载能力并无显著影响,但是会降低其失效过程中的能量吸收值。     

Effect of adhesive thickness on fatigue and fracture of toughened epoxy joints – Part I: Experiments

The effect of bondline thickness, from 130 μm to 790 μm, on the fatigue and quasi-static fracture behavior of aluminum joints bonded using a toughened epoxy adhesive was studied experimentally under mode-I (DCB) and mixed-mode (ADCB) loading. Under mode-I loading, it was found that the fatigue threshold energy release rate, G_th, decreased for very thin bondlines, while under mixed-mode loading, the G_th changed very little with bondline thickness. In both cases, the effect of bondline thickness was more pronounced at higher crack growth rates. For quasi-static fracture, the effect of adhesive thickness on the energy release rate for the onset of fracture from the fatigue threshold, G_c0, was similar to that found for the fatigue threshold; however, the steady-state energy release rate, , increased linearly with increasing bondline thickness.     

Analysis of automotive mixed‐adhesive joints weakened by moist conditions: Experimental characterization and numerical simulation using cohesive zone model

This paper is mostly concerned with an experimental and numerical study to clarify the behaviour and failure in the mono‐adhesive joints and mixed‐adhesive joints under different environmental conditions (dry [E₀], 75.3% relative humidity [E₁], 84.2% relative humidity [E₂], and submerged in tap water [E₃] at 25°C) and different strain rates (1 and 100 mm/min). Experimental investigations are compared with the numerical analysis, which is carried out by bilinear cohesive zone model (CZM). Through this work, degradations of cohesive parameters are calculated by using open‐faced double cantilever beam (DCB) and end notch flexure (ENF) specimens. The experimental data show that the cohesive parameters of Araldite 2015 have negative correlation to moisture content. Although Araldite AV138 parameters experience a decrease in mode I, in mode II, its cohesive parameter increases. First, it is found that the decrease in experimental failure load of mixed‐adhesive joints with regard to the dry condition is recorded as 32.6%, 54%, and 59.1% for E₁, E₂, and E₃ conditions, respectively. Following this further, single lap joint (SLJ) that has non‐uniform moisture content distribution is modelled by engaging specific CZM parameters related to the specific moisture content. The results show a good agreement between experimental and numerical data.     

残余应力对Si3N4/S45C连接体强度的影响

本文主要就Si3N4／S45C连接体在插入软金属及附加热循环的情况下，使残余应力发生变化，而讨论其对抗弯强度的影响．其结果为：（1）Si3N4表面边缘处的拉伸应力是影响连接体强度的主要原因．（2）抗弯强度和残余应力具有σ0＝-1．63σ＋658．1的线性关系．（2）周期性热循环对连接体的残余应力及抗弯强度有较大的影响；即发生加工硬化等使残余应力上升，产生裂纹等导致强度下降．     

Fatigue design of welded double‐sided T‐joints and double‐sided cruciform joints in steel marine structures: A total stress concept

Fatigue is a governing design limit state for marine structures. Welded joints are important in that respect. The weld notch stress (intensity) distributions contain essential information and formulations have been established to obtain a total stress fatigue damage criterion and corresponding fatigue resistance curve; a total stress concept. However, the involved weld load carrying stress model does not provide the required estimates and trends for varying geometry dimensions and loading & response combinations. A new one has been developed and performance evaluation for T‐joints and cruciform joints in steel marine structures shows that in comparison with the nominal stress, hot spot structural stress and effective notch stress concept based results up to 50% more accurate fatigue design life time estimates can be obtained. Taking advantage of the weld notch stress formulations, the effective notch stress concept performance has improved adopting a stress‐averaged criterion rather than a fictitious notch radius‐based one.     

A new filler metal with low contents of Cu for high strength aluminum alloy brazed joints

The effect of Cu with low contents of 10, 12, 15 wt.% on the microstructure and melting point of Al–Si–Cu–Ni alloy has been investigated. Results showed that low-melting-point properties of Al–Si–Cu–Ni alloys with low contents of Cu were attributed to disappearance of Al–Si binary eutectic reaction and introduction of Al–Si–Cu–Ni quaternary reaction. With raising Cu content from 10 to 15 wt.%, the amount of complex eutectic phases formed during low temperature reactions (Al–Cu, Al–Si–Cu and Al–Si–Cu–Ni alloy reactions) increased and the melting temperature of Al–Si–Cu–Ni filler metals declined. Brazing of 6061 aluminum alloy with Al–10Si–15Cu–4Ni (all in wt.%) filler metal of a melting temperature range from 519.3 to 540.2 °C has been carried out successfully at 570 °C. Sound joints can be obtained with Al–10Si–15Cu–4Ni filler metal when brazed at 570 °C for holding time of 60 min or more, and achieved high shear strength up to 144.4 MPa.     

Fatigue life prediction for butt‐welded joints considering weld‐induced residual stresses and initial damage,relaxation of residual stress,and elasto‐plastic fatigue damage

Fatigue damage of butt‐welded joints is investigated by a damage mechanics method. First, the weld‐induced residual stresses are determined by using a sequentially coupled thermo‐mechanical finite element analysis. The plastic damage of material is then calculated with the use of Lemaitre's plastic damage model. Second, during the subsequent fatigue damage analysis, the residual stresses are superimposed on the fatigue loading, and the weld‐induced plastic damage is considered as the initial damage via an elasto‐plastic fatigue damage model. Finally, the fatigue damage evolution, the relaxation of residual stress, and the fatigue lives of the joints are evaluated using a numerical implementation. The predicted results agree well with the experimental data.     

热残余应力对C/SiC复合材料界面剪切强度影响的有限元分析

根据C/SiC复合材料的属性,建立单纤维顶出的二维轴对称模型,采用有限元法对C/SiC复合材料的界面剪切强度进行数值研究,分析中考虑材料制备过程中的残余应力对界面剪切强度的影响,在细观力学层面上系统分析纤维顶出过程的界面剪应力及其相关影响因素。分析得出,残余应力会对界面造成损伤,降低界面脱粘载荷。材料的界面承受能力与热膨胀系数呈正相关,与固化温度呈负相关。     

Error estimation of stress intensity factors for mixed‐mode cracks

This paper presents an a posteriori error estimator for mixed‐mode stress intensity factors in plane linear elasticity. A surface integral over an arbitrary crown is used for the separate calculation of the combined mode's stress intensity factors. The error in the quantity of interest is based on goal‐oriented error measures and estimated through an error in the constitutive relation. Copyright © 2006 John Wiley & Sons, Ltd.     

Effects of direct metal deposition combined with intermediate and final milling on part distortion

ABSTRACT

Combining direct metal deposition and milling in one machine promises the additive fabrication of complex parts with a high surface quality and dimensional accuracy. However, residual stress induced by the additive process can impair the final part shape after finishing. Undercuts and inaccessible areas are particularly prone to distortion since they require intermediate milling steps during buildup. Herein, strategies to reduce residual stress by process optimisation are discussed and demonstrated. The effects of intermediate and final milling on dimensional accuracy are analysed for the fabrication of a distortion-critical beam from stainless steel. 3D scans reveal that additive buildup on a semi-finished part causes local warpage of milled surfaces, resulting in deviations in length that are by factor 10 higher than the milling accuracy. Global distortion of the substrate plate is significant, but the milling sequence itself has finally no considerable influence.     

考虑冲压残余应力和厚度变化的车身结构模态分析与优化

车身零件冲压成形过程中产生的残余应力和厚度变化对其结构模态和固有频率具有重要的影响,而目前常用的有限元模态分析方法中,常常忽略了成形因素所带来的影响,导致分析结果与实际情况不相符。采用有限元网格映射算法,在车身结构模态分析中引入成形信息,并将成形残余应力作为预应力施加,研究成形因素对车身结构模态和固有频率的影响。研究结果表明,考虑成形比未考虑成形影响的模态分析结果更接近实验结果。最后,进行了基于成形因素的车身结构灵敏度分析与优化。     

Effect of adhesive thickness on fatigue and fracture of toughened epoxy joints – Part II: Analysis and finite element modeling

The effect of bondline thickness on the fatigue and fracture of aluminum adhesive joints bonded using a rubber-toughened epoxy adhesive was studied using finite element analysis. The fatigue data of Part I examined the dependence of the fatigue threshold and cyclic crack growth rate on the adhesive thickness under both mode-I and mixed-mode loading. The fracture data of Part I illustrated the relation between the adhesive thickness and the quasi-static crack initiation and steady-state critical strain energy release rates. These experimental trends are explained in terms of the effects of the adhesive thickness and the applied strain energy release rate on the stress distribution in the bondline, the stress triaxiality at the crack tip, and the plastic zone size in the adhesive layer.     

Effects of fibre/matrix adhesion on carbon-fibre-reinforced metal laminates—II. impact behaviour

Quasi-static, low-hanging and high-velocity impact tests have been conducted in order to study the effect of fibre/matrix adhesion on the impact properties of fibre-reinforced metal laminates. Differences in fibre/matrix adhesion were achieved by using treated or untreated carbon fibres in an epoxy resin system. Chemical removal of the aluminium layers and a sectioning technique were applied to examine and characterize the impact damage in the laminates. The results show that the laminates with the weaker fibre/matrix adhesion exhibit larger damage zones, although the back face crack length and permanent indentation after impact are smaller for a given impact energy. Residual tensile strength after impact is also higher for the untreated fibre laminates due to increased fibre/matrix splitting in the composite layer.     

Diffusion bonding of TiAl based alloy to Ti–6Al–4V alloy using amorphous interlayer

Powder metallurgical TiAl based alloy and Ti–6Al–4V (TC4) alloy were diffusion bonded at 915 °C for 1 h under a pressure of 80 MPa. Single Ti‐based amorphous or Ni‐based amorphous foil was utilized as an interlayer during diffusion bonding process. The tensile mechanical properties of the diffusion bonded joints between TiAl based alloy and TC4 alloy were tested. The fracture surface and microstructure characteristic of these joints were also observed carefully by scanning electron microscope. The TiAl based alloy with a fully lamellar microstructure is more suitable for diffusion bonding to TC4 alloy. Adding a Ti‐based amorphous interlayer is more conducive to the element diffusion, giving rise to the improvement in the mechanical properties of the diffusion bonded joints. Results also show that the diffusion bonded joints form a metallurgical bond and fracture in a brittle manner.     

Fatigue strength assessment of partial and full‐penetration steel and aluminium butt‐welded joints according to the peak stress method

In fatigue design of welded joints, the local approach based on the notch stress intensity factors (NSIFs) assumes that the weld toe profile is a sharp V‐notch having a tip radius equal to zero, while the root side is a pre‐crack in the structure. The peak stress method (PSM) is an engineering, FE‐oriented application of the NSIF approach to fatigue design of welded joints, which takes advantage of the elastic peak stresses from FE analyses carried out by using a given mesh pattern, where the element type is kept constant and the average element size can be chosen arbitrarily within a given range. The meshes required for the PSM application are rather coarse if compared with those necessary to evaluate the NSIFs from the local stress distributions. In this paper, the PSM is extended for the first time to butt‐welded joints in steel as well as in aluminium alloys, by comparing a number of experimental data taken from the literature with the design scatter bands previously calibrated on results relevant only to fillet‐welded joints. A major problem in the case of butt‐welded joints is to define the weld bead geometry with reasonable accuracy. Only in few cases such geometrical data were available, and this fact made the application of the local approaches more difficult. Provided the local geometry is defined, the PSM can be easily applied: a properly defined design stress, that is, the equivalent peak stress, is shown (i) to single out the crack initiation point in cases where competition between root and toe failure exists and (ii) to correlate with good approximation all analysed experimental data.