Application of Cohesive Zone Modeling to Composite Bonded Repairs

Cohesive zone models can play an important role in the definition of repair strategies. These models allow the prediction of damage initiation and propagation. They are based on a softening relationship between stresses and relative displacements between crack faces, thus simulating a gradual degradation of material properties. Typically, stress-based and energetic fracture mechanics criteria are used to simulate damage initiation and growth, respectively. Those elements are placed at the planes where damage is prone to occur which, in the case of bonded repairs, is usually easy to identify a priori. Taking this into consideration, cohesive mixed-mode damage models based on interface finite elements were used with the objective of optimizing the repair efficiency. The determination of the cohesive pure mode softening laws is a key aspect of these models, and the direct method is the most accurate process to do it. The models were validated and applied to two different cases involving repairs. Several geometrical aspects influencing the single-strap repair strength were analyzed as well as the evolution of the maximum load and alteration of damage mechanism as a function of the angle used in scarf joints. It was verified that CZM are able to predict with accuracy the damage mechanisms and strength of composite bonded repairs, thus constituting a powerful tool in its design.     

On the Certification of Bonded Repairs to Primary Composite Aircraft Components

Airworthiness certification is required when bonded repairs are made to primary composite structure in situations where damage has reduced or has the potential to reduce residual strength to below the design ultimate strength. Generally, certification of bonded primary structure poses many difficulties. As most repairs are one-off events meeting these certification requirements is especially challenging since demonstration by testing will generally not be possible or cost-effective. This paper discusses options for addressing the two key issues relating to certification: (a) how to validate initial and enduring bond strength of adhesive bonds, mainly given the inability of conventional non-destructive inspection to provide this assurance and (b) how to develop acceptable generic design allowables for bonded repairs which represent actual failure modes – especially for cyclic loading, since validation by testing of simulated repairs will generally be infeasible. It is concluded that proof testing of bonded repair coupons is a promising approach for validating bond strength and fatigue testing of representative bond joint specimen can provide generic allowables for patch design. For hidden structure or very high value repairs structural health monitoring of repairs based on a strain-transfer approach offers considerable promise.     

不同波长激光能量对碳纤维复合材料损伤实验研究

本文利用YAG激光器及CO2激光器对固体火箭发动机壳体用碳纤维材料试件进行了两种典型波长（10.6μm和1.06μm）的高能量密度激光能量作用下的损伤实验研究,分析了波长对碳纤维试件损伤效果及损伤方式的影响。结果表明：在相同的功率密度条件下,YAG激光能量除造成表面树脂的分解碳化外,还会直接造成纤维的断裂,CO2激光能量对碳纤维材料的损伤则主要表现在内部的树脂分解;此外,YAG作用的试件单位面积内平均质量损失为28.64mg／cm^2,小于CO2激光作用时的平均值40.33mg／cm^2,约是其71％;平均每焦耳激光能量下的质量损失YAG为15.0mg／J,小于C02的试验结果21.4mg／J;从烧蚀热看,损失相同质量的条件下,需要YAG激光能量大于CO2激光能量。     

Some Effects of Heat Processing on Adhesively Bonded Automotive Steel

During a program to develop new structural adhesives that would meet the processing requirements of the current automotive “high heat” bake cycles, substantial differences in performance were noted between the “standard” cold rolled steel (CRS) of SAE1008 and certain Drawing Quality steels (DQSK).

In parallel tests, certain DQSK test specimens (bonded with 200°C heat cycle) failed consistently in the interfacial region, while the CRS samples failed center of bond.

The surface characteristics of the steels and failed adhesive specimens were examined with ESCA, AUGER, and ISS spectroscopic methods. The metal failure surfaces of the DQSK samples were shown to contain relatively high levels of silicon and oxygen, and smaller amounts of boron, with a lower concentration of iron, as compared to CRS which shows iron surface with minor contaminations.

In subsequent testing, with other samples of DQSK SAE1008, this effect was not observed. These samples did not exhibit the same levels of contamination. It is suggested that certain DQSK processes may involve processing steps that are detrimental to the surface properties of the rolled sheet stock.     

Three-Dimensional Finite Element Analysis of the Stress Distribution in Bi-Adhesive Bonded Joints

A 3-D elastic finite element model was developed to investigate the stresses distribution of bi-adhesive bonded joints (i.e., the bond line of joints filled with two adhesives of dissimilar toughness). The effects of the loading mode on the stress distribution of joints, including the single-lap joints under tensile loading (i.e., single-lap joints) and the butt joints under cleavage loading (i.e., cleavage joints), were also studied in detail. Results showed that higher stress, distributed at the contact position of the dissimilar adhesives placed along the bond line of bi-adhesive bonded joints. Also, the maximum stress of the adhesive layer decreased when the length ratios and bonding sequence along the bond line, filled with two dissimilar adhesives, was appropriately designed. At the same time, stress convergence in the adhesive layer of bi-adhesive joints was also obviously reduced in contrast to the mono-adhesive joints. The numerical investigation shows that it is necessary to take into account the change of loading modes when optimizing the bi-adhesive joint design, because of the uneven and complex loading modes of the adhesive bonding structure in the engineering applications.     

The Effect of Surface Preparation and Exposure Environment on the Bond Failure Processes in Adhesively Bonded Sheet Molded Composite (SMC)

The durability of adhesively-bonded composites has been investigated using a wedge-type specimen. Polyester-resin, fiberglass sheet molded composite (SMC) was bonded with a commercial two-part poly-urethane adhesive. The SMC composite received one of four different surface preparations: no treatment, abrasion, priming, or abrasion and priming. The wedge test was used to study the durability of the samples which were exposed to air and to the vapor above water, concentrated ammonium hydroxide, or methanol at 60[ddot]C. The crack length was measured during the experiments. The crack growth rate as a function of surface treatment varied in the manner: untreated ≈ abraded > primed ≈ abraded and primed. The crack growth rate as a function of vapor changed in the manner: methanol > ammonium hydroxide > water ≈ air. The samples were removed at the conclusion of the test and the failure mode was determined visually, by scanning electron microscopy (SEM), and by X-ray photoelectron spectros-copy (XPS). Initial insertion of the wedge resulted in substrate failure (delamination of the composite). Exposure of untreated and abraded samples under stress to the test vapors promoted adhesive failure. Primed and abraded/primed samples under stress and exposed to methanol vapor debonded via cohesive processes.     

Investigation of accumulated laser fluence and bondline thickness effects on adhesive joint performance of CFRP composites

In the present study, both the effects of accumulated laser fluence as surface treatment and bondline thickness on adhesive bonding of carbon fiber reinforced polymer (CFRP) composite materials were investigated. Proper CFRP composite surfaces for adhesive bonding were obtained by a laser treatment process using pulsed CO₂ laser. Laser treatments were obtained with different accumulated laser fluences and then surfaces were analyzed with roughness and contact angle measurements. Adhesive bonding was performed with various bondline thicknesses ranged between 30–500 µm using two component structural epoxy based paste adhesive (Loctite Hysol ® EA 9396^TM). Adhesive bonding strength of bonded samples was determined with single lap shear tests. It is worthy to note that if the accumulated laser fluence which has significant effect on shear strength does not optimize, it causes ineffective adhesion.     

Effect of Temperature on the Quasi-static Strength and Fatigue Resistance of Bonded Composite Double Lap Joints

Fibre reinforced polymer composites (FRP's) are often used to reduce the weight of a structure. Traditionally the composite parts are bolted together; however, increased weight savings can often be achieved by adhesive bonding or co-curing the parts. The reason that these methods are often not used for structural applications is due to the lack of trusted design methods and concerns about long-term performance. The authors have attempted to address these issues by studying the effects of fatigue loading, test environment and pre-conditioning on bonded composite joints. Previous work centered on the lap-strap joint which was representative of the long-overlap joints common in aerospace structures. However, it was recognised that in some applications short-overlap joints will be used and these joints might behave quite differently. In this work, double-lap joints were tested both quasi-statically and in fatigue across the temperature range experienced by a jet aircraft. Two variants on the double-lap joint sample were used for the testing, one with multidirectional (MD) CFRP adherends and the other with unidirectional (UD) CFRP adherends. Finite element analysis was used to analyse stresses in the joints. It was seen that as temperature increased both the quasi-static strength and fatigue resistance decreased. The MD joints were stronger at low temperatures and the UD joints stronger at high temperatures. It was proposed that this was because at low temperature the strength was determined by the peak stresses in the joints, whereas, at high temperatures, strength is controlled by creep of the joints which is determined by the minimum stresses in the joint. This argument was supported by the stress analysis.     

Effect of Temperature on the Quasi-static Strength and Fatigue Resistance of Bonded Composite Double Lap Joints

Fibre reinforced polymer composites (FRP's) are often used to reduce the weight of a structure. Traditionally the composite parts are bolted together; however, increased weight savings can often be achieved by adhesive bonding or co-curing the parts. The reason that these methods are often not used for structural applications is due to the lack of trusted design methods and concerns about long-term performance. The authors have attempted to address these issues by studying the effects of fatigue loading, test environment and pre-conditioning on bonded composite joints. Previous work centered on the lap-strap joint which was representative of the long-overlap joints common in aerospace structures. However, it was recognised that in some applications short-overlap joints will be used and these joints might behave quite differently. In this work, double-lap joints were tested both quasi-statically and in fatigue across the temperature range experienced by a jet aircraft. Two variants on the double-lap joint sample were used for the testing, one with multidirectional (MD) CFRP adherends and the other with unidirectional (UD) CFRP adherends. Finite element analysis was used to analyse stresses in the joints. It was seen that as temperature increased both the quasi-static strength and fatigue resistance decreased. The MD joints were stronger at low temperatures and the UD joints stronger at high temperatures. It was proposed that this was because at low temperature the strength was determined by the peak stresses in the joints, whereas, at high temperatures, strength is controlled by creep of the joints which is determined by the minimum stresses in the joint. This argument was supported by the stress analysis.     

生物无机/有机复合多孔材料的制备与应用

综述了生物有机/无机复合多孔材料的几种制备方法,包括粒子致孔法、气体发泡法、相分离法、烧结颗粒法、静电纺丝法、快速成型技术等,从孔隙率、孔的尺寸以及对促进细胞和组织生长等方面对上述方法进行了比较,并对此类材料目前的应用和今后的发展方向进行了描述.     

表面处理对可生物降解黄麻/PLA复合材料结构和性能的影响

采用注塑成型法制备了生物降解黄麻短纤维增强PLA复合材料,通过力学性能测试及SEM,探讨了碱处理、碱和硅烷偶联剂KH550同时处理对复合材料结构和性能的影响。结果表明:两种处理方法均能够增加黄麻纤维的表面粗糙度,但碱和偶联剂KH550同时处理的效果要优于碱处理,且KH550改善了黄麻短纤维与PLA树脂之间的界面黏结性能提,高了黄麻/PLA复合材料的拉伸强度和弯曲强度。     

Photoelastic Determination of Flaw Size and Distribution Effects on Adhesively Bonded Lap Joints

It is well known that the load carrying capacity of adhesively bonded lap joints can be influenced by the presence of flaw-like defects which are often created during its bonding process. To design an effective adhesive joint containing possible bonding defects, adequate knowledge and understanding of the shear stress distribution along the entire lap joint are necessary.

This paper describes an investigation into the effects of internal adhesive flaw size and distribution on the fracture behaviour of adhesively bonded lap joints. Photoelasticity is used to gain a quantitative understanding of the localized shear stress concentrations due to the presence of the internal flaws along the bonding layer. It is observed that a 20% increase in the maximum shear stress may be induced when an isolated central flaw of S. O mm was extended to 37.5 mm representing a flaw size of 75% of the lap length. For the presence of multiple flaws along the bonding line, there is no significant effect of the flaw separation distance on the maximum shear stresses. There is, however, a marked increase in the maximum shear stress up to about 45% when a flaw size is increased from 2.5 mm to 7.5 mm.     

Fabrication and Mechanical Characterization of Calcium Alginate Fiber-Reinforced Polyvinyl Alcohol Based Composites

Calcium alginate fibers were prepared from sodium alginate by extruding aqueous sodium alginate solution (4% by weight) into a calcium chloride (2% by weight) bath. Water uptake and mechanical properties of the calcium alginate fiber were investigated. Water uptake tests of calcium alginate showed that it absorbed 50% of water within a minute and indicated strong hydrophilic nature. Polyvinyl alcohol (PVA)-based calcium alginate fiber reinforced unidirectional composites (10% fiber by weight) were fabricated by compression molding. Tensile strength (TS), tensile modulus (TM), bending strength (BS), bending modulus (BM) and impact strength (IS) of the PVA matrix and the composite were evaluated. TS, BS, TM, and BM of the PVA matrix were found 10, 18, 320 and 532 MPa, respectively. TS and BS of the PVA based composite were found to be 16 and 27 MPa, respectively, which were 60 and 50% higher than that of the PVA matrix. TM and BM of the composite were found to be 620 and 1056 MPa, respectively, which were improved by 94 and 98% over the matrix material. Degradation tests of the composites were performed for up to 2 months in soil medium and found that composites lost almost 50% of its original mechanical properties. The interfacial properties of the composite were also investigated by using the single fiber fragmentation test (SFFT).     

Fabrication of nanometer-sized carbon electrodes by the controllable electrochemical deposition

A new and simple method for fabricating controllable insulted nanometer-sized carbon electrodes is presented. Electrochemical etching of carbon fibers is employed, and then a repeated process of cycle voltammetric deposition of electrophoretic paint and heat cure for film shrink is followed which controls effectively the size of nanoelectrodes. This technique allows complete insulation of the whole body of the carbon fiber etched except for the very tip with the film shrink during heat curing of the film, leaving an electrochemical active area with effective diameters of nanometers. The process overcomes the pinhole formation resulted from the electrophoretic paint deposition process. The fabricated electrodes show ideal steady-state voltammetric behaviors from which the effective areas of nanoelectrodes are measured. The effective radius of the nanoelectrodes prepared ranges from tens of nanometer to hundreds of nanometer.     

碳纤维增强聚醚醚酮复合材料的研究及应用

综述了碳纤维增强聚醚醚酮（PEEK）复合材料在机械力学性能、摩擦磨损机理、生物相容性、微观结构等方面的研究进展。     

Influence of Thermal Spikes on the Ageing of Adhesively Bonded Structures - A Dielectric Study

Dielectric measurements are reported on epoxy bonded aluminium joints exposed to moisture at 75°C. The observed changes in the dielectric spectrum that occur with time of exposure are correlated with the variation in the mechanical strength. Certain joints were subjected to periodic cooling to -20°C to simulate the shock of aircraft flight. The effects of dehydration were examined for joints that had been exposed to moisture for a prolonged period of time. The data indicate that the initial hydration process lowers the glass transition temperature and the stress released creates micro-voids. Further ageing leads to hydroxide formation in the interfacial layer. Freezing of water during the thermal spike experiments induces cracking in the adhesive, and leads to greater water uptake. Plasticisation of the resin appears to be to a large extent reversible and dehydration allows some recovery of the bond strength. The shock cooling of the joints has only a relatively small effect on the ageing of the joints but does, however, produce differences in the dielectric data. This study illustrates the power of the dielectric technique for the assessment of ageing in adhesively bonded structures.     

螺杆钻具定子注胶工艺对橡胶与金属粘合强度的影响

从定子管材的表面处理、粘结剂涂刷方式、注胶方式和硫化工艺等方面阐述影响橡胶与定子粘合强度的因素。金属的表面处理应根据定子热处理和机加工过程中存在的油污的类别选择合适的处理方式及清洗剂;粘合工艺的重点是根据配方选择合适的粘结剂、涂刷方式、涂刷厚度和隔绝空气污染;注胶方面应考虑注胶压力、速度、温度及方式,减少胶料对粘结剂的冲刷;硫化工艺方面,控制好硫化罐内温度、压力的均匀性等。     

碳纤维在机械设备和建筑物上的应用

碳纤维具有轻质高强、耐高温、导电、导热、耐腐蚀等诸多优异特性，在机械设备、建筑物上诸多领域具有应用前景；本文主要介绍碳纤维在机械设备和建筑物上以及其它领域的应用。     

Effect of Adherend Thickness and Mixed Mode Loading on Debond Growth in Adhesively Bonded Composite Joints

Symmetric and unsymmetric double cantilever beam (DCB) specimens were tested and analyzed to assess the effect of (1) adherend thickness and (2) a predominantly mode I mixed mode loading on cyclic debond growth and static fracture toughness. The specimens were made of unidirectional composite (T300/5208) adherends bonded together with EC3445 structural adhesive. The thickness was 8, 16 or 24 plies. The experimental results indicated that the static fracture toughness increases and the cyclic debond growth rate decreases with increasing adherend thickness. This behavior was related to the length of the plastic zone ahead of the debond tip. For the symmetric DCB specimens, it was further found that displacement control tests resulted in higher debond growth rates than did load control tests. While the symmetric DCB tests always resulted in cohesive failures in the bondline, the unsymmetric DCB tests resulted in the debond growing into the thinner adherend and the damage progressing as delamination in that adherend. This behavior resulted in much lower fracture toughness and damage growth rates than found in the symmetric DCB tests.     

碳纤维及其复合材料在国外军民领域的应用

介绍了碳纤维（CF：carbon fiber）及碳纤维增强复合材料（CFRP：carbon fiber minfomed plastics）在国外发达国家军事领域和民用领域的应用。