Numerical experimental analysis of hybrid double lap aluminum-CFRP joints

Due to their reliability and ease of assembly, both the adhesively bonded and the mechanical joints are commonly used in different fields of modern industrial design and manufacturing, to joint composite materials or composites with metals.As it is well known, adhesively bonded joints are characterized by high stiffness and good fatigue life, although delamination phenomena localized near the free edges may limit their use, especially for applications where corrosive environments and/or moisture can lead to premature failure of the bonding. In these cases, a possible alternative is given by the well-known mechanical joints. On the contrary, these last joints (bolted, riveted) require a preliminary drilling of the elements to be joined, that may cause localized material damage and stress concentration, especially for anisotropic laminates characterized by high stress concentration factors and easy drilling damaging, with significant decrease of the load-carrying capacity of the joined elements. In order to exploit the advantages of the bonded joints and those of the mechanical joints, both industrial manufacturing and research activity have been focused recently on the so called hybrid joints, obtained by the superposition of a mechanical joint to a simple adhesively bonded joint.In order to give a contribution to the knowledge of the mechanical behavior of hybrid bonded/riveted joints, in the present work a numerical–experimental study of bonded/riveted double-lap joints between aluminum and carbon fiber reinforced polymer (CFRP) laminates, has been carried out. It has permitted to highlight both the static and the fatigue performance of such joints obtained by using aluminum and steel rivets, as well as to known the particular damage mechanisms related also to the premature localized delamination of the CFRP laminate due to the riveting process.     

搭接长度和铺层方式对CFRP复合材料层合板胶接结构连接性能和损伤行为的影响

针对不同搭接长度和铺层方式的碳纤维增强树脂（CFRP）复合材料层合板单搭胶接结构进行了拉伸试验,观察了试件的受力过程和失效形态,获得了载荷-位移曲线;同时基于连续损伤力学模型和三维Hashin失效准则模拟了CFRP复合材料层合板的层内损伤形成和演化,并利用内聚力模型来模拟层间及胶层的失效损伤,对CFRP复合材料层合板单搭胶接结构在拉伸作用下的失效强度和损伤机制进行了预测,通过对比验证了该数值方法的有效性;通过数值试验比较不同搭接长度和铺层方式的单搭胶接结构及双搭胶接结构的连接强度和损伤行为,并提出了一种优化的CFRP复合材料层合板胶接结构。结果表明:CFRP复合材料层合板胶接结构的极限失效载荷随着搭接长度的增大逐渐增加并趋于稳定值,且结构的失效形式逐渐从胶层自身剪切失效过渡到邻近胶层的层合板层间分层失效;CFRP复合材料层合板胶接结构的连接强度和损伤行为随着铺层方式的不同而改变,通过对3种铺层方式的对比和分析,得到性能最好的铺层方式是[0₃/90₃]_2S;在搭接长度为5~20 mm时,通过对搭接长度进行优化,得到单搭胶接结构的最优搭接长度是17 mm,双搭胶接结构的最优搭接长度是19.3 mm,与搭接长度为20 mm相比,单搭胶接结构和双搭胶接结构的连接强度分别提高了13.26%和0.43%。      

A study on the strength of adhesively bonded joints with different adherends

In this study, mechanical properties of adhesively bonded single-lap joint (SLJ) geometry with different configurations of lower and upper adherends under tensile loading were investigated experimentally and numerically. The adherends were AA2024-T3 aluminum and carbon/epoxy composite with 16 laminates while, the adhesive was a two-part liquid, structural adhesive DP 460. In experimental studies, four different types of single-lap joints were produced and used namely; composite–composite (Type-I) with lower and upper adherends of the same thicknesses and four different stacking sequences, composite–aluminum (Type-II) with lower and upper adherends of the same thicknesses and four different stacking sequences, composite–aluminum (Type-III) with lower adherend (composite) of the same thickness but upper adherend of three different thicknesses, aluminum–aluminum (Type-IV) with lower adherend of the same thickness but upper adherend of three different thicknesses, composite–composite (Type-V) with [0]₁₆ stacking sequences and three different overlap length, aluminum–aluminum (Type-VI) with three different overlap length. In the numerical analysis, the composite adherends were assumed to behave as linearly elastic materials while the adhesive layer and aluminum adherend were assumed to be nonlinear. The results obtained from experimental and numerical analyses showed that composite adherends with different fiber orientation sequence, different adherend thicknesses and overlap length affected the failure load of the joint and stress distributions in the SLJ.     

Waterproof characteristics of nanoclay/epoxy nanocomposite in adhesively bonded joints

When adhesively bonded joints are exposed to a moist environment, the tensile load capability of the joint is significantly decreased because moisture absorption weakens the mechanical properties of epoxy adhesive. In this paper, a nanoclay with excellent penetration resistance properties was used as a filler in epoxy adhesive in order to enhance adhesive strength in moist environments. The water absorption of the epoxy adhesive and the adhesive strength of the adhesively bonded joints were measured in water absorption experiments with respect to the weight fraction of the nanoclay and the moisture exposure time. These results showed that the tensile load capability of the nanoclay-filled adhesively bonded joint was greatly enhanced, even in a moist environment, because the nanoclay reduced water absorption into the epoxy adhesive as well as into the interface between the epoxy adhesive and the steel adherend and increased the strength of the epoxy adhesive itself.     

复合材料单搭接头的剪切强度对比实验研究

在常温和湿热高温的环境下,分别进行了三种不同的成碰工艺(RTM、引入缝纫的RTM和胶接)制备的单搭接头的剪切强度实验,研究了成型工艺对单搭接头的剪切强度的影响,并根据实验现象分析了单搭接头的剪切破坏机理.实验结果表明:引入缝纫的RTM成型试样的剪切强度性能最高,而胶接成型试样的剪切强度最低;环境温度对RTM和引入缝纫RTM成型试样的剪切强度影响不大,而对胶接成型试样的剪切强度有较大影响.     

Interfacial crack tip field in anisotropic/isotropic bimaterials

The co-cured joint is more efficient than the adhesively bonded joint for composite structures because of its several advantages. However, failure analysis of the co-cured joint has just a little been reported since the co-cured joining method was introduced. Observing based on the experimental results, failure starts at the interface corner of the co-cured joint. Therefore, it is important to consider stress intensities at the interface corner of the co-cured joint.

In this paper, the eigenvalue problem was used to determine asymptotic stress and displacement fields near the interface corner between composite and steel adherends. For obtaining stress intensities at the interface corner a path independent conservative line integral was used.     

Strength and damage tolerance of composite–composite joints with steel and titanium through the thickness reinforcements

Today’s aeronautic, automotive and marine industry is in demand of structurally efficient, low weight alternatives for composite–composite joints which combine the advantages of low weight input of adhesively bonded joints and high damage tolerance of through the thickness bolted joints. In the present work, composite–composite joints are reinforced through the thickness by thin metal inserts carrying cold metal transfer welded pins (CMT pins). The influence of pin alignment and type of pin on the damage tolerance of single lap shear (SLS) composite–composite joints is investigated. The use of titanium reinforcements is evaluated and compared to stainless steel reinforced, adhesively bonded and co-cured specimens. A detailed analysis of the stress–strain behavior is given and the stiffness and energy absorption of the SLS joints during tensile loading is assessed. The results show that joints reinforced with CMT pins absorb significantly higher amounts of energy, when compared to adhesively bonded and co-cured joints.     

Elastic analysis of hybrid bonded joints and bonded composite repairs

The authors extend the closed-form bonded joint linear elastic analysis method of Delale et al. [Delale F, Erdogan F, Aydinoglu MN. Stresses in adhesively bonded joints: a closed-form solution. J Compos Mater 1981;15:249–71] and Bigwood and Crocrombie [Bigwood DA, Crocombe AD. Elastic analysis and engineering design formulae for bonded joints. Int J Adhes Adhes 1989;9(4):229–42] to include the composite deformation mechanisms and the thermal residual strains that arise in hybrid metal-composite joints such as those presented by bonded composite repairs applied to metallic aircraft structures. The analytical predictions for the adhesive stresses and the compliance are compared to the results of a linear elastic finite element model that has itself been validated by comparison with experimental results. The results are applied to the problem of coupled linear extension and bending of a bonded composite repair applied to a cracked aluminum substrate. The resulting stress intensity factor and crack-opening displacement in the repaired plate are compared to the results of a three-dimensional finite element analysis, and also exhibit excellent results. Throughout the text, observations are made regarding the practical application of the results to failure prediction in hybrid joints, whereby the authors demonstrate the need for consistency in the analytical methods used to determine the fatigue and failure of composites from the coupon level to the analysis of the final structural details.     

End geometry and pin-hole effects on axially loaded adhesively bonded composite joints

An experimental investigation was performed to analyze the potential impacts of varying joint region geometries and adhesive filled pin holes on adhesively bonded composite structures. Tapers, especially half-length ones are observed to provide an anticipated progress in single lap joints. Besides, scarf joints with aligned adherends in the same plane exhibited enhanced stiffness and strength in consideration of single lap joints. In terms of the stiffness and strength, thickening of adherends was also found to be impressively efficient on composite single lap joints as well as scarf joints. Contrary to the expectation of that the hardened adhesive previously filled into the holes during adhesion would create a pin effect in load bearing, holey specimens exhibited poor performance and induced degradation in joint quality.     

Failure analysis of adhesively-bonded skin-to-stiffener joints: Metal–metal vs. composite–metal

The purpose of this research is to evaluate the performance of two adhesively bonded skin-to-stiffener connections: composite stiffener bonded to a Fiber Metal Laminate (FML) skin, representing a hybrid joint, and an Aluminium stiffener bonded to a FML skin, representative for a metal joint. The bonded joints were tested using Stiffener Pull-Off Tests (SPOT), which is a typical set-up used to simulate the structural behavior of full-scale components subject to out-of-plane loading, such as internal pressure of a fuselage or leading edge low pressure zone. In the hybrid joint, the damage initiates at the central noodle of the composite stiffener. Unstable delamination then propagates from the noodle to the tip of the stiffener foot, preferably through the stiffener foot plies (>90% of inter/intra-laminar failure) and, in limited areas, through the adhesive bond line (<10% of cohesive failure). In the metal joint, the failure starts at the tip of the stiffener foot at the adhesive bond line. Unstable debonding then propagates along the stiffeners foot. The complete failure occurs in the adhesive bond line (100% cohesive failure). The loads associated with >90% of inter/intra laminar failure of the composite stiffener (hybrid joint) are 40–60% lower than the ones associated with 100% cohesive failure (metal joint). This research identifies that in order to use the full capacity of adhesively bonded hybrid joints, the adhesion between carbon fibers of the composite laminate, ie intralaminar strength, must be improved. Otherwise, Aluminium stringers are still very competitive.     

Design of hybrid steel/composite circular plate cutting tool structures

Substituting composite structures for conventional metallic structures has many advantages because composite materials have both high specific stiffness and damping characteristics compared to conventional metallic materials. In this study, circular plate cutting tools which are used for rough machining of bearing sites in crankshafts or camshafts were designed with the fiber reinforced composite material to reduce tool mass and to improve the dynamic stiffness of circular plate cutting tools. The hybrid steel/composite circular plate cutting tool was analyzed by finite element method with respect to material types such as composite and foam, stacking angles of the composite, adhesive bonding thickness, and dimensions of the cutting tool. Also, the constrained damping characteristics of the tools were experimentally investigated with respect to the adhesive bonding thickness and material type such as composite and PVC foam. From the finite element analysis and experimental results, optimal design parameters for the hybrid steel/composite circular plate cutting tool were suggested.     

Design of the hybrid composite journal bearing assembled by interference fit

In this work, a hybrid composite journal bearing (HCJB) composed of carbon/phenolic laminated composite bush and steel housing was designed for marine vessels because the composite journal bearing reduces the possibility of the seizure problem between the journal and bearing. The two components of bearing were assembled by interference fit joining method and a series of durability tests were conducted using the laboratory bench with the lubricants of SAE 30 oil, water, and sea water. That the HCJB was found reliable under the interference fitting loads and environmental temperature change.     

A numerical methodology for optimizing the geometry of composite structural parts with regard to strength

The increased use of composite materials in lightweight structures has generated the need for optimizing the geometry of composite structural parts with regard to strength, weight and cost. Most existing optimization methodologies focus on weight and cost mainly due to the difficulties in predicting strength of composite materials. In this paper, a numerical methodology for optimizing the geometry of composite structural parts with regard to strength by maintaining the initial weight is proposed. The methodology is a combination of the optimization module of the ANSYS FE code and a progressive damage modeling module. Both modules and the interface between them were programmed using the ANSYS programming language, thus enabling the implementation of the methodology in a single step. The parametric design language involves two verifications tests: one of the progressive damage model against experiments and one of the global optimization methodology performed by comparing the strength of the initial and the optimum geometry. There were made two applications of the numerical optimization methodology, both on H-shaped adhesively bonded joints subjected to quasi-static load. In the first application, the H-shaped joining profile was made from non-crimp fabric composite material while in the second from a novel fully interlaced 3D woven composite material. In the optimization of the joint’s geometry, failure in the composite material as well as debonding between the assembled parts was considered. For both cases, the optimization led to a considerable increase in joint’s strength.     

嵌块长度对胶接接头应力分布影响的数值分析

利用弹塑性有限元方法研究了钢嵌块长度对单搭接接头应力分布的影响,结果表明,采用钢嵌块形成混合连接接头后,接头上的应力流线分布发生了变化,钢嵌块中存在的压缩轴向应力和负值剥离应力有利于提高接头的承载能力;嵌块长度的增加使胶层和被粘物中界面处的峰值应力显著增大,而嵌块中的应力变化不大,嵌块长度超过4mm后,胶层中的应力峰值急剧上升;在需用嵌块增强胶接接头时应注意采用适宜的嵌块长度.     

Damage monitoring and analysis of composite laminates with an open hole and adhesively bonded repairs using digital image correlation

High performance composite materials, such as Carbon–Fibre Reinforced Plastic (CFRP) composites, are being increasingly used in aerospace industry, such as fuselage primary structures in Boeing 787 or Airbus 350, where high strength and stiffness are required at minimum weight [1]. The design of composite structures frequently includes discontinuities such as cut-outs for access and fastener holes for joining and they become critical regions under thermo-mechanical loading. Understanding of notched specimen behaviour is necessary for the design of complex structures where parts are mostly connected with bolts and rivets [2]. The effect of these discontinuities on the behaviour of composite materials is an important topic because it causes a relatively large reduction in strength compared to the unnotched laminate [3]. In the first part of the current work, the assessment of the damage process taking place in notched (open-hole) specimens under uniaxial tensile loading was studied. Two-dimensional (2D) and three-dimensional (3D) Digital Image Correlation (DIC) techniques were employed to obtain full-field surface strain measurements in carbon–fibre/epoxy M21/T700 composite plates with different stacking sequences in the presence of an open circular hole. Penetrant enhanced X-ray radiographs were taken to identify damage location and extent after loading around the hole. DIC strain fields were compared to numerical predictions. In the second part of the study, DIC techniques were used to characterise damage and performance of adhesively bonded patch repairs in composite panels under tensile loading. This part of work relates to strength/stiffness restoration of damaged composite aircraft that becomes more important as composites are used more extensively in the construction of modern jet airliners. In the current work, external bonded patches have been studied. Adhesively bonded repairs are the most common type of repair carried out with composite materials [1], [4]. The behaviour of bonded patches under loading was monitored using DIC full-field strain measurements. Location and extent of damage identified by X-ray radiography correlates well with DIC strain results giving confidence to the technique for structural health monitoring of bonded patches.     

复合材料夹芯管胶接连接结构力学特性分析

针对在航空结构中广泛应用的复合材料蜂窝夹芯圆管中的接头这一最脆弱的部分,发展了一种分析复合材料蜂窝夹芯圆管胶粘接头力学特性的解析模型.该模型根据Gibson修正公式得到了蜂窝芯子的等效弹性参数,再运用经典的复合材料壳理论和线弹性理论得到管接头的控制方程,并通过状态空间法进行求解.运用本文模型,计算了管接头在扭矩和弯矩作用下胶层内的剪应力和剥离应力;同时采用有限元法对模型进行了数值模拟,并将模拟结果与模型计算结果进行了对比,最后分析了搭接长度对胶层内应力的影响.     

Design of a hybrid glass composite anchor for LNG cargo containment systems

Large cryogenic container for liquefied natural gas (LNG) should be joined to the inner hull of a ship. The minimum heat inflow, due to the large temperature difference between the LNG cargo container and inner hull, can induce considerable heat flux through the joined area.In this work, a hybrid composite anchor composed of two stainless steel caps and a glass fiber composite body was developed and optimized for strength, thermal stress and heat loss of the anchor. The stainless steel caps were adhesively bonded to the top and bottom areas of the glass composite body. The top stainless cap was then welded to the two stainless steel membranes, and the flange of the bottom stainless steel cap was bolt-fastened to the inner hull of a ship.The static adhesive strength and impact characteristics between the stainless steel caps and glass composite body of the hybrid glass composite anchor at the cryogenic temperature were investigated with respect to adhesive thickness. Finally, the optimal design for the hybrid glass composite anchor for the cryogenic containment systems of a LNG ship was suggested.     

Hybrid epoxy-silyl modified polymer adhesives for CFRP sheets bonded to a steel substrate

This paper presents an experimental study examining the interfacial behavior between a steel substrate and carbon fiber reinforced polymer (CFRP) sheets bonded with hybrid epoxy-silyl modified polymer (SMP) adhesives. The epoxy adhesive has high modulus and strength characteristics, while the SMP adhesive possesses a low modulus with permanent elastic nature. The hypothesis tested is that a combination of these two distinct materials can alleviate interfacial stresses along the bond line with maintaining adequate strength. Two types of double-lap tension tests are conducted to evaluate the bond-capacity of the epoxy and SMP adhesives and to study the effect of various hybrid bond schemes. Test results show that the specimens bonded with homogeneous epoxy demonstrate abrupt failure, whereas those with SMP exhibit gradual load-softening at failure. The load-carrying capacity and stiffness of the CFRP–steel interface are not influenced by hybrid bond configurations. The degree of CFRP-debonding is, however, affected by the hybrid bond scheme. Stress transfer from the steel substrate to the CFRP is well maintained along the hybrid bond line with significant local deformability of the interface layer. Analytical models report that shear stresses along the CFRP–steel interface are noticeably mitigated at geometric discontinuities and the proposed hybrid bond technique can be used for structure-level application.     

Numerical simulation study of fatigue crack growth behavior of cracked aluminum panels repaired with a FRP composite patch using combined BEM/FEM

A combined boundary element method and finite element method (BEM/FEM) is employed to investigate the fatigue crack growth behavior of cracked aluminum panels repaired with an adhesively bonded fiber-reinforced polymer (FRP) composite patch. Numerical simulation of crack growth process of a cracked aluminum panel repaired with a FRP composite patch under uniaxial cyclic loading has been carried out. The curve of crack length on unpatched side of the cracked panel versus the number of cyclic loading is determined by the numerical simulation, and it agrees well with experimental data. Furthermore, the crack front profiles of the cracked panel during fatigue crack growth and the distributions of stress intensity factors along crack fronts are also numerically simulated.     

Flexural behavior of steel beams strengthened by carbon fiber reinforced polymer plates – Three dimensional finite element simulation

Debonding, as a mode of failure, is one of the major limitations when using externally bonded carbon fiber reinforced polymer (CFRP) plates in strengthening of steel beams. In this work, mode of failure and flexural behavior of both steel and steel–concrete composite beams strengthened by different lengths of CFRP plates were numerically investigated. The effect of both splicing position (at mid-span and near supports) and CFRP plate ends configuration were studied. Three dimensional finite element analysis (3D FEA) was adopted to simulate the nonlinear behavior of these beams loaded under four point bending configuration. The present numerical analysis assisted by previously valuable experimental results found in the literature succeeded to predict the critical CFRP plate length at which, full efficiency of the adhesively bonded plate is achieved.