Bio-inspired design of aerospace composite joints for improved damage tolerance

This paper uses a bio-inspired design strategy based on tree branch joints to improve the damage tolerance of co-cured composite T-joints. The design of tree branch joints at different length scales from the microstructural to the macro-length scale was investigated. X-ray computed tomography of a pine tree revealed three main features of tree branch joints which provide high structural efficiency and damage tolerance: integrated design with the branch embedded into the centre of the trunk; three-dimensional fibril lay-up in the principal stress directions; and variable fibril density to achieve iso-strain conditions through the joint connection. Research presented in this paper adapts the embedded structural feature of tree joints into a carbon/epoxy T-joint. The flange plies were embedded to 25%, 50% and 75% of the depth of the skin of the composite T-joint to mimic the design of tree branch joints. Experimental testing revealed that the bio-inspired T-joint design with integrated adherends had increased normalised inelastic strain energy (defined as ductility), increased normalised absorbed strain energy to failure, and higher load-carrying capacity following damage initiation (damage tolerance) compared to a conventionally bonded T-joint. However, these improvements were achieved at the expense of earlier onset of damage initiation in the T-joints.     

On design methods for bolted joints in composite aircraft structures

The problems related to the determination of the load distribution in a multirow fastener joint using the finite element method are discussed. Both simple and more advanced design methods used at Saab Military Aircraft are presented. The stress distributions obtained with an analytically based method and an FE-based method are compared. Results from failure predictions with a simple analytically based method and the more advanced FE-based method of multi-fastener tension and shear loaded test specimens are compared with experiments. Finally, complicating factors such as three-dimensional effects caused by secondary bending and fastener bending are discussed and suggestions for future research are given.     

Progressive damage analysis as a design tool for composite bonded joints

This paper discusses the application of progressive damage analysis (PDA) methods as a design tool. Two case studies are presented in which the effects of changing design features on the strength of bonded composite joints are evaluated. It is shown that the trends of parametric evaluations performed with full-featured PDA models can be unintuitive and the trends can be opposite to those obtained with traditional design criteria. The joint configurations that were tested exhibit multiple damage modes, requiring several different PDA tools to accurately predict the structural peak loads. For damage tolerant structures that exhibit complex sequences of multiple failure mechanisms, traditional failure prediction tools are insufficient. Parametric PDA models encompassing a bonded joint specimen's design space have the potential to reveal unintuitive and advantageous design changes.     

Corrosion of composite welded joints

The process of corrosion of composite welded joints of 20 steel and 08Kh18N9T stainless steel produced by manual argon-arc welding is investigated by electrochemical methods. It is shown that a contact galvanic couple is formed in the zone of the welded joint and, as a result, the surface of 20 steel in the vicinity of the weld turns into the anode and intensely corrodes. For this reason, composite welded joints should not be used in heart-power engineering. Odessa State Academy of Food Technologies, Odessa. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 6. pp. 33–36, November–December, 1996.     

Robust economic control chart design

Designing a control chart involves making fundamental decisions about the control chart parameters. Traditionally, practitioners select design parameters using ad hoc procedures. Academic research has been challenging this tradition by introducing a more rigorous criterion for selecting the design parameters based on economic criteria. However, there has been limited success in implementing economic designs. This research develops the concept of Robust Economic Design of control charts where multiple economic and process scenarios are considered in control chart design. By developing a robust optimization technique for economic control chart design, we hope to promote a better understanding of industrial implementation of economic designs of control charts. The effectiveness of this technique is illustrated through examples and a sensitivity analysis.     

Optimization design of control chart systems

This article proposes an algorithm to design an integrated control chart system consisting of several individual control charts, each of which is used to monitor a critical process stage in the manufacture of a product. The design algorithm considers all the charts within a system in an integrative and optimal manner. Consequently, the performance characteristics of the system as a whole can be significantly improved and the product quality will be further enhanced. Such an improvement is achieved without requiring additional cost and effort for on-line inspections. Furthermore, the floor operators can utilize and understand the optimal control chart system as easily as they do the existing system. Some useful guidelines have also been highlighted to aid the users to adjust the control limits of the control charts in a system.     

A review on the driving performance of FRP composite piles

Fibre composites have been a viable option in replacing traditional pile materials such as concrete, steel and timber in harsh environmental conditions. However, driving composite piles require more careful consideration due to their relatively low stiffness. Currently, there are no specific guidelines on the installation of composite piles which limits their acceptance in load-bearing applications. There is a need therefore to understand their behaviour during driving in order for composite piles to be safely and economically driven into the ground. This paper presents an overview on composite pile technologies and an examination on the different factors that affects their driving performance. Emphasis on the potential use of hollow fibre reinforced polymer (FRP) piles and the need for further study on their impact behaviour is highlighted. It is expected that the information provided in this paper will help researchers and engineers to develop efficient techniques and guidelines in driving composites piles.     

Application of different concepts for fatigue design of welded joints in rotating components in mechanical engineering

Several concepts are used for the fatigue design of welded joints. In this paper investigations are presented, which were carried out in a joint project between five research institutes [1]. The aim is to investigate currently applied fatigue concepts with respect to their limitations, compatibility and reliability, in order to improve the accuracy of lifetime estimation and to simplify the choice of the optimum fatigue concept. Here, the results of the investigation of welded joints in rotating universal joint shafts are shown [2]. In the critical weld, a structural steel and a quenched and tempered steel are joined. In practice, stresses result from rotating bending, torsion and also residual stresses are sometimes present. Several welding techniques, MAG, TIG and laser welding, and two seam geometries were investigated with regard to their influence on fatigue strength. Experiments were conducted with welded tube specimens representative of the actual component application and with derived flat specimens as detail specimens. The welded sheet thickness was 5.5 mm. Fatigue strength was investigated from 10⁴ to 10⁷ numbers of cycles. In numerical analyses, nominal stress, structural hot spot stress and elastic notch stress with reference radii of 0.3 mm and 0.05 mm were calculated. In the comparison of the concepts, their respective advantages and disadvantages have been demonstrated. A comparison of the results with the IIW recommendation for fatigue design of welded joints and components [3] has been carried out and improvements have been suggested.     

Fatigue behaviour of composite adhesive lap joints

This paper is concerned with a fatigue study of composite adhesive lap joints produced from a bi-directional woven E-glass fibres and polypropylene. The adhesive used was a Bostik 7452 (Rubber & Plastics Grade) ethyl cyanoacrylate type. The effects of layer orientation, lap joint length and water immersion on the fatigue behaviour were studied. The specimens were immersed in water during periods until ninety days for controlled temperatures of 20, 40 and 70 °C. The results are presented in the form of curves of stress amplitude versus number of cycles to failure and also in the form of number of cycles to failure against time to exposure in water for fixed stress amplitudes. The fatigue damage and failure mechanisms were analysed and discussed. The joint shows that creep deformation within the temperature range of this study was probably the mainly cause of the dynamic stiffness reduction observed.     

Bolted joints of pultruded sandwich composite laminates

This paper describes the tensile behavior of bolted joints of pultruded sandwich composite laminates. The pultruded sandwich laminates have a skin-core-skin structure. Joint strength of longitudinal specimens was independent of specimen width (w), whereas it increased with w in the case of transverse specimens. The joining efficiency of pultruded sandwich laminates was greater in the longitudinal direction than in the transverse direction. The core layer of longitudinal specimens failed by a combination of bearing and shear-out modes, independent of w. The failure mode of skin layers changed from net-tension to bearing mode with increasing w. In transverse specimens, the failure mode of core and skin layers changed from net-tension to bearing with increasing w. Finite element numerical analysis was carried out to predict the failure mode and joint strength. The numerical results were in good correlation with the experimental results.     

Multilens system design with an athermal chart

We have introduced an athermal chart that plots chromatic dispersive power and thermal dispersive power on a Cartesian coordinate, and we give the design method of a multilens system in contact that satisfies achromatism and athermalization. The advantages of this chart are (1) that the condition of achromatism and athermalization is clear and (2) that the approximate power of the lenses that compose the multilens system is easily found on the chart. Design indices are given through a few design examples with an athermal chart.     

Control chart design a review of standard practice

Many standard texts on SPC offer simple procedures for setting up control charts. Although the procedures work well in some situations, there are many instances where the procedures do not produce useful charts. The failure to sustain SPC initiatives is often due to an over simplistic model of process variability. This paper examines the widely used procedures for setting up control charts and illustrates how these may fail when the process variability has certain characteristics. A more robust alternative procedure is examined in some detail. This involves taking a closer look at process variability and incorporating this with control chart design.     

机械结合部动态刚度辨识与实验研究

机械结合部动态特性参数辨识最常用的方法是频响函数法.然而,实测频响函数数据即使经过降噪处理,仍受到噪声的污染,使得在数值运算时导致辨识结果出现不适定性.为解决这一问题,使得辨识结果能够真实反映结合部特性,提出采用条件数方法对实验数据进行处理之后,再进行参数辨识.为使条件数足够小,采取求平均值的办法,将条件数大于平均值的频率剔除,只保留状态值小于平均值的频率,将所保留下的频率进行比较,找出有交集的频率,并在这些频率下进行参数辨识,从而能够避免发生不适定问题.从仿真算例、实验研究结果表明,条件数法能够解决利用频响函数法辨识结合部参数时出现的不适定问题,从而提高了辨识精度.     

Particle fracture in metal-matrix composite friction joints

The influence of welding parameters, reinforcing particle chemistry and shape, matrix condition and silver interlayers on particle fracture during similar and dissimilar friction welding of aluminium-based metal-matrix composite (MMC) base material was investigated. Two composite base materials were examined, one containing Al₂O₃ particles and the other containing 72 wt% Al₂O₃–7 wt % Fe₂O₃–17 wt % SiO₂–3 wt % TiO₂ particles. The different material combinations comprised MMC/MMC, MMC/alloy 6061, MMC/AISI 304 stainless steel and MMC/1020 mild steel joints. Particle fracture was confined to a narrow region immediately adjacent to the dissimilar joint interface. The calculated normal pressure for fracture of Al₂O₃ particles ranges from 0.56–17.58 MPa and is in agreement with an experimentally measured pressure of 1.06 MPa found during sliding wear testing of aluminium-based composite base material. Because the lowest normal pressure applied during friction joining was 30 MPa, particle fracture occurs very early in the joining operation (immediately following contact between the two substrates). The application of a silver interlayer during dissimilar MMC/AISI 304 stainless steel joining decreased the particle fracture tendency. It is suggested that the presence of a silver interlayer decreased the coefficient of friction and lowered the stresses applied at the contact region. The particle fracture tendency was markedly increased when the MMC material contained blocky alumina particles. However, there was negligible particle fracture when the MMC base material contained spherical 72 wt % Al₂O₃–7 wt % Fe₂O₃–17 wt % SiO₂–3 wt % TiO₂ particles. This revised version was published online in November 2006 with corrections to the Cover Date.     

Optimum bolted joints for hybrid composite materials

The optimum bolted joints for hybrid composite materials composed of glass-epoxy and carbon-epoxy under tensile loading were investigated. The design parameters considered for the bolted joints were ply angle, stacking sequence, the ratio of glass-epoxy to carbon-epoxy, the outer diameters of washers and the clamping pressure. As bearing failure was desirable for bolted joints, the geometry of the bolted joint specimen was designed to undergo bearing failure only.

By inspecting the fracture surfaces of the specimens it was found that delamination on the loaded periphery of the holes and extensive damage on the edge region constrained by a washer occurred. To assess the delamination of the hybrid composite materials, three-dimensional stress analysis of the bolted joint was performed using a commercial finite-element software and compared with the experimental results.     

Quasi-static behaviour of composite joints with protruding-head bolts

An experimental study of the quasi-static behaviour of composite bolted joints under tensile and compressive loading has been performed. The joint plates made of quasi-isotropic and 0°-dominated lay-ups were fastened by two, four, or six titanium bolts. Two specimen configurations were tested: single lap and double lap. Strain gauges were applied in order to measure strain between the bolt rows and to calculate the load transfer of the bolt rows. Bolt-movement measurements were performed using extensometers to examine bolt behaviour under static loading. The obtained experimental results showed that bolted joints with four and six bolts perform better resistance to quasi-static loading than joints with two bolts. Strain distribution between two bolts is significantly affected by the bolt presence and their behaviour under loading. The load-transfer calculations showed that different bolt rows transfer slightly different amounts of load.     

Mixed-mode fracture of adhesively-bonded pultruded composite lap joints

The fracture behavior of adhesively-bonded double- and stepped-lap joints composed of pultruded glass fiber-reinforced polymer composite laminates and subjected to axial tension was experimentally investigated and numerically modeled. Two methods were used for the calculation of the strain energy release rate: the experimental compliance method and the virtual crack closure technique. Their results showed good agreement for stepped-lap joints, while significant deviations were observed for double-lap joints due to small stiffness changes. The experimental compliance method results were sensitive to these small changes and the virtual crack closure technique accuracy was affected by the inability of the finite element analysis to accurately model the behavior before visual crack initiation. The dominant fracture mode changed from Mode I to Mode II in the case of stepped-lap joints, while an almost constant mode ratio was retained for double-lap joints in the applied loading range. A non-convex mixed-mode fracture criterion was established for crack initiation and propagation based on the virtual crack closure technique results. Both the experimental compliance method and the virtual crack closure technique proved applicable for the interpretation of the fracture mechanics data of the structural joints examined, provided that stiffness degradation can be accurately described.     

On the prediction of bolted single-lap composite joints

A new set of failure criteria to predict composite failure in single-lap bolted-joints is proposed. The present failure criteria are an extension of Chang–Lessard criteria considering a three-dimensional stress field and including out-of-plane failure modes. The advantage with respect to other three-dimensional failure criteria is the consideration of non-linear shear stress–strain relationship. The failure criteria were implemented in a finite element model and validated through comparison with experiments in literature. Stresses were calculated by a non-linear finite element model developed in ABAQUS/Standard which considers material and geometric nonlinearities. A progressive damage model was implemented in a USDFLD subroutine. The model predicted the effect of secondary bending and tightening torque showing an excellent agreement with experimental results. Moreover, results were compared with those reported in literature using Hashin failure criteria. In addition, a parametric study was carried out to analyse the influence of friction coefficient and tightening torque.     

Economic design of a variable sampling rate EWMA chart

A Variable Sampling Rate (VSR) control chart is a control chart whose sampling scheme is to vary the sampling interval and the sample size for the next sample depending on the current chart statistic. A VSR EWMA chart is an EWMA chart with the VSR sampling scheme. An economic model, which was developed for a VSR chart, is also applied here to evaluate the efficiency of the VSR EWMA chart. The properties of the VSR EWMA chart are obtained by using a Markov chain approach. The model contains cost parameters which allow the specification of the costs associated with sampling, false alarms and operating off target as well as search and repair. This economic model can be used to quantify the cost saving that can be obtained by using a VSR chart instead of a Fixed Sampling Rate (FSR) chart and can also be used to gain insight into the way that a VSR chart should be designed to achieve optimal economic performance. It is shown that with some design parameter combinations the economically optimal VSR chart has a lower false alarm rate than the FSR chart.