Neutron imaging inspections of composite honeycomb adhesive bonds

Numerous commercial and military aircraft, including the Canadian Forces CF188 Hornet, use composite honeycomb structures in the design of their flight control surfaces (FCS). These structures provide excellent strength to weight ratios, but are often susceptible to degradation from moisture ingress. Once inside the honeycomb structure moisture causes the structural adhesive bonds to weaken, which can lead to complete failure of the FCS in flight. There are two critical structural adhesive bonds: the node bond and the filet bond. The node bond is integral to the honeycomb portion of the composite core and is located between the honeycomb cells. The filet bond is the adhesive bond located between the skin and the core. In order to asses overall structural degradation and develop repair procedures, it is important to determine the degree of degradation in each type of bond. Neutron radiography and tomography of the adhesive bonds was conducted at the Royal Military College (RMC) and FRM-II. Honeycomb samples were manufactured from FCS with in-service water ingress. The radiographs and tomograms provided important information about the degree of degradation in the core as well as about which adhesive bonds are more susceptible. The information obtained from this study will help to develop repair techniques and assess the flight worthiness of FCS.     

Design of load-bearing antenna structures by embedding technology of microstrip antenna in composite sandwich structure

Electrically and structurally effective antenna structure is developed for the next generation of surface technology for communication, in which the structural surface itself becomes an antenna. The basic design concept is a sandwich structure composed of composite laminates and Nomex honeycomb, with which microstrip antenna is integrated. Composite materials with high electrical loss must not reduce antenna efficiency. Stacked-patch microstrip antenna is preferred for wideband performance. An open condition that defines a position of outer facesheet is exploited in order to allow an antenna into the sandwich structure without loss of antenna performances. Measured electrical performances of fabricated structure show that the gain is more improved than original antenna and the bandwidth is as wide as specified in our requirements. With the open condition, wideband antenna can be integrated with mechanical structures without reducing any electrical performances, as confirmed experimentally here.     

A study of microwave reaction rate enhancement effect in adhesive bonding of polymers and composites

Microwaves have been investigated as an efficient alternative energy source for polymers and composites processing. In this paper, microwaves are applied in bonding two polymer composites with an epoxy-based adhesive. Results were compared with thermal process. Microwaves reduced the bonding time dramatically, which resulted from faster curing of the adhesives. The literature disagreed on whether localized superheating of functional groups or specific non-thermal effects are the main reason of microwave enhancement of reaction rate. In this study, the mechanism of microwave fast curing was investigated and the experimental results supported localized superheating of the functional groups as the main mechanism of reaction rate enhancement of microwave energy.     

Smart cure cycles for the adhesive joint of composite structures at cryogenic temperatures

Adhesive joints are employed for composite structures used at the cryogenic temperatures such as LNG (liquefied natural gas) insulating tanks and satellite structures. The strength of the adhesive joints at the cryogenic temperatures is influenced by the property variation of adhesive and the thermal residual stress generated due to the large temperature difference (ΔT) from the adhesive bonding process to the operating temperature. Therefore, in this work, the strength and thermal residual stress of the epoxy adhesive at cryogenic temperatures were measured with respect to cure cycle. Also, the cure cycles composed of gradual heating, rapid cooling and reheating steps were applied to the adhesive joints to reduce the thermal residual stress in the adhesive joints with short curing time. Finally, a smart cure method was developed to improve the adhesive joint strength and to reduce the cure time for the composite sandwich structures at cryogenic temperatures.     

Effect of surface pretreatment on adhesive properties of aluminum alloys

The lap-shear strength and durability of adhesive bonded AI alloy joints with different pretreatments were studied by the lap-shear test and wedge test. The results indicate that the maximum lap-shear strength and durability of the bonding joints pretreated by different processes are influenced by the grade of abrasive papers and can be obviously improved by phosphoric acid anodizing. Alkali etching can obviously improve the durability of bonding joints although it slightly influences the maximum lap-shear strength. The process which is composed of grit-finishing, acetone degreasing, alkali etching and phosphoric acid anodizing, provides a better adhesive bonding property of Al alloy.     

An evaluation of the performance of advanced melded composite joints

Melding is an efficient three step composite joining process that involves the selective cure of composite adherends before the final adhesive joint is created using the adherends own resin system. Melding does not require many of the processes and compromises associated with conventional techniques like adhesive bonding and mechanical fastening.     

Failure of carbon composite-to-aluminum joints with combined mechanical fastening and adhesive bonding

Composite-to-aluminum double lap joints were tested to obtain the failure loads and modes for three types of joints: adhesive bonding, bolt fastening and adhesive-bolt hybrid joining. A film type adhesive FM73 and a paste type adhesive EA9394S were used for aluminum and composite bonding. A digital microscope camcorder was used to monitor the failure of the joints. It was found that hybrid joining improves joint strength when the mechanical fastening is stronger than the bonding, as when the paste type adhesive is used. On the other hand, when the strength of the bolted joint is lower than that of the bonded joint, as when the film type adhesive is used, bolt joining contributes little to the strength of the hybrid joint.     

Effect of processing conditions and electrode characteristics on the electrical properties of structural composite capacitors

Structural capacitors are manufactured from glass fabric/epoxy prepreg dielectrics and metalized polymer film electrodes. The electrical breakdown strengths of these multifunctional materials are investigated across a wide range of electrode constructions and processing parameters. The results show that electrode selection and materials processing have a significant impact on the energy that the device can store. Also, this careful consideration of processing parameters and electrode construction has led to the development of a structural capacitor with an energy density exceeding 0.90 J/cm³, the highest yet reported.     

复合催化剂对聚酰胺纳滤膜结构和性能的影响

以聚砜(PSf)为基膜,间苯二胺(MPD)和均苯三甲酰氯(TMC)为反应单体,通过界面聚合制备聚酰胺复合纳滤膜.考察了复合催化剂三乙胺(TEA)和樟脑磺酸(CSA)及反应条件对纳滤膜功能层结构和性能的影响.结果表明:在反应体系中,TEA和CSA的质量比为1/2时,随着复合催化剂中TEA的质量分数从0.5％增加到3％,纳滤膜功能层密度增大,表面粗糙度和水接触角下降,膜通量明显升高,但纳滤膜的截留率及其对盐的选择顺序基本不变.在TEA质量分数为2％、反应时间40 s、热处理温度80℃和热处理时间3 min的最优条件下,所得复合纳滤膜对2 g/L MgSO4溶液的截留率为93.2％,通量为16 L/(m2·h).在0.2～1.0 MPa的操作压力下,聚酰胺复合纳滤膜分离性能稳定.     

Buckling and debond growth of partial debonds in adhesively bonded composite splice joints

The strains at which buckling and debond growth occur in adhesively bonded composite flanges containing an initial debond were experimentally measured. Test parameters including initial debond geometry, flange material stiffness, and the adhesive critical strain energy release rate (G_c) were investigated. Debond growth was found to be strongly dependent on initial debond length but weakly dependent on flange width; i.e., debonding resistance did not increase in direct proportion with the bonded overlap dimension. Flanges having higher bending stiffness exhibited significantly lower debonding strain. Finally, the effect of G_c was evaluated at three levels by controlling the adhesive cure temperature and bondline thickness. Lower values of G_c (207 and 552 J/m²) allowed debond growth to occur while at the highest value of G_c (1500 J/m²), alternate failure modes occurred prior to debond growth. Ultrasonic C-scans revealed that debond growth occurred along a curved front, as dictated by the post-buckling deformation of the flanges.     

Heavily loaded bonded composite structure: design, manufacture and test of ‘I' beam specimens

The use of adhesive bonding in advanced composite structures offers the potential for considerable weight and cost saving compared to the use of mechanical fasteners. However, confidence must be developed in the ability to design and analyse such structures and in their manufacture to suitable quality standards. The programme of work described here was carried out as a step towards building such confidence. Secondarily bonded ‘I' beams were designed to carry an ultimate shear flow of 1712 Nm m⁻¹. They were manufactured under factory conditions and predictions were made of their failure loads and modes. The beams were then tested in three- and four-point bending and the outcomes of these tests were compared to predictions. The beams failed at loads comfortably in excess of the target, a shear flow of 2828 Nm m⁻¹ being carried in four-point bending. Failure was a very energetic process leading to a great deal of damage within the test structures. Despite this an attempt was made to inspect the failed beams in an attempt to determine the quality of the bondlines and identify the site of failure.     

Sensitivity of embedded fibre optic Bragg grating sensors to disbonds in bonded composite ship joints

The potential application of embedded fibre optic Bragg grating strain sensors for the health monitoring of adhesively bonded composite ship joints is investigated in this paper. Bragg grating sensors were embedded at various locations along the interface of adhesively bonded glass-reinforced plastic composite joints with artificially introduced disbonds to assess their capability to detect bond-line damage under in-plane shear and through-thickness tension. Finite element (FE) models indicated that the presence of the disbond significantly altered the bond-line strain distribution under such loads. The embedded sensors successfully detected this effect, and the sensor measurements compared well to FE predictions. However, the experimental measurements of the magnitude of the strain at the tips of the disbonds showed significant variations, presumably due to its high sensitivity to defect edge conditions which could not be experimentally controlled with a high degree of repeatability. Both the FE models and the experimental results showed that the effect of disbond damage was localised, more so under in-plane shear than through-thickness tension. This would necessitate the use of an optimised sensor array and pattern recognition algorithm for the reliable detection of an arbitrary disbond.     

球磨工艺对B_4C-Al复合粉末性能的影响

利用高能球磨干混的方法制备B4C-Al复合粉末,研究球磨过程中转速、球磨时间对粉末粒度、B4C颗粒均匀性及界面结合的影响。结果表明,球磨转速和球磨时间是影响增强体颗粒分布均匀性以及与基体粉末界面结合的主要因素,存在一个最佳球磨转速和球磨时间使增强体颗粒分布均匀且与基体粉末的界面结合最佳。低转速球磨6h和中转速球磨3h时,增强体颗粒以偏聚的形式分布于基体粉末间隙之间,界面结合率很低。中转速球磨6h时,增强体颗粒均匀地分布于基体材料中,界面结合率较高。中转速球磨12h和高转速球磨6h时,增强体颗粒分布较均匀,界面结合率很高,但是容易形成焊合。     

Enhancement of ceramic-to-metal adhesive bonding by excimer laser surface treatment

A silicon nitride (Si₃N₄) based ceramic cutting material and a structural alloy steel (SAE 4340) were surface-treated using a 800 mJ KrF excimer laser with an aim to improve the ceramic-to-metal bond strength. For these two materials, the effects of laser energy density and the number of pulses upon the surface morphologies of the laser-treated surfaces to be joined were examined. Conical surface microstructures were generally observed on the laser-treated surfaces of the Si₃N₄ material, and the number of conical features was found to be significantly influenced by the laser energy density. Moreover, the results of XPS have shown that the surface chemistry of the ceramic was altered after being laser treated. On the other hand, excimer laser treatment had caused the alloy steel surfaces be melted and resulted in some “beach-mark” features. However, the laser energy density has little effect on the overall surface morphology and the roughness of the treated surfaces. Shear tests were performed on adhesive bonded samples of the laser-treated ceramic and alloy steel. Significant improvement in adhesion shear strength was obtained for the laser-treated samples as compared with those tested in the as-received and mechanically abraded conditions. The required laser operation condition for achieving good shear joint strength was discussed.     

Damage resistance of single lap adhesive composite joints by transverse ice impact

This paper focuses specifically on the high velocity transverse impact of composite joints by hailstones. Impact tests with ice spheres onto composite lap joint specimens were conducted to determine the failure threshold energy describing damage initiation, and to investigate the modes of damage. The damage areas imaged by ultrasonic scanning were quantitatively measured and the specimens were also sectioned and observed with optical microscopy to determine the exact location of damage. The damage area versus impact kinetic energy was found to increase dramatically for impacts beyond the failure threshold. Delamination of the composite originated at the bond overlap termination facing away from the impact side. The damage usually occurred at specific ply locations and a transition of the delamination to other ply locations was also observed. Numerical simulation of the impact was conducted and the results show that the plies where delaminations were observed to occur have the highest peel and shear stresses.     

含量变化对银-二硫化钼复合材料电磨损性能的影响

采用粉末冶金法制备银-二硫化钼复合材料, 通过改变材料中MoS2与Ag的含量,研究含量变化对复合材料电磨损性能的影响.结果表明,随着材料中MoS2加入量的增加,电刷与换向器之间接触电压降呈上升趋势,复合材料的摩擦系数与表面粗糙度先降低后趋于稳定,磨损率先下降后上升,在MoS2含量为15%时磨损率最低.     

相对冲击位置和补片层数对胶接修理CFRP复合材料层合板抗冲击性能的影响

本文针对碳纤维增强聚合物（CFRP）复合材料修补结构,基于连续损伤力学和粘结单元模型,在ABAQUS软件中对低速冲击载荷下不同冲击位置和补片层数的CFRP复合材料层合板内部和层间损伤进行了数值分析,并与试验结果进行了对比。选择相对冲击位置为0 mm、10 mm、20 mm、30 mm和40 mm时对应的五种修补结构,通过数值计算和试验,获得了修补结构在低速冲击过程中的冲击力、冲击能量等数据。在保持补片单层厚度不变的前提下,使补片层数从1层增加到5层,计算获得了修补结构的低速冲击响应。研究结果表明:冲头接触修补结构时会对补片造成较大的损伤,补片可以提高含孔损伤母板的抗冲击性能;冲击点离修补结构损伤孔越近,结构受冲击所产生的分层损伤越严重;增加补片的层数可以提高修补结构的抗冲击性能;通过对补片层数进行优化,得到优化层数为2,其对应的修补结构与无修补结构相比分层损伤面积减少了19.9%,较好地提升了母板的抗冲击性能。      

Analysis of adhesive bonded composite lap joints with transverse stitching

The effect of transverse stitching on the stresses in the adhesive is investigated using an adhesive sandwich model with nonlinear adhesive properties and a transverse stitching model for adhesive bonded composite single-lap and double-lap joints. Numerical results indicate that, among all stitching parameters, thread pretension and stitch density have significant effect on the peel stresses in the adhesive; increase in the thread pretension and the stitch density leads to a decrease in peel stress in the adhesive, while an increase in other parameters generally results in a negligible reduction in peel stress. The effect of stitching was found to be negligible on the shear stresses in the adhesive. Thus it is concluded that stitching is effective for the joints where peel stresses are critical and ineffective for those where shear stresses are critical.     

Dynamic analysis of walls strengthened with composite materials

The bidirectional dynamic behavior of walls strengthened with composite materials is studied. For that purpose, a multi-layered high order finite element is developed. The finite element accounts for the bidirectional (plate-type) dynamic behavior and for the interfacial interaction between the adhesively bonded components. The formulation uses a viscoelastic first order shear deformation orthotropic plate theory for the independent modeling of the existing wall and the composite layers and a high order theory for the displacement fields of the adhesive layers. The Finite element framework simplifies the coupling with adjacent structural elements and the use of standard computational procedures. The convergence of the formulation and two numerical examples are studied. The first case studies the response of a strengthened wall to a step base acceleration. The second case studies a wall built in a surrounding frame and strengthened on the outer face. The numerical study examines the capabilities of the model and reveals some of the unique aspect of the dynamic response, including the effects of the orthotropy and orientation of the strengthening system. It also highlights the potential of the high order finite element to become a platform for the modeling and dynamic analysis of the strengthened wall.