影响胶接表面特性的诸因素分析

讨论了影响胶表面特性的三种因素和磷酸阳极化表面处理方法。     

Cohesive/adhesive failure interaction in ductile adhesive joints Part II: Quasi-static and fatigue analysis of double lap-joint specimens subjected to through-thickness compressive loading

This paper proposes a new methodology for the finite element (FE) modelling of failure in adhesively bonded joints. Cohesive and adhesive failure are treated separately which allows accurate failure predictions for adhesive joints of different thicknesses using a single set of material parameters. In a companion paper (part I), a new smeared-crack model for adhesive joint cohesive failure was proposed and validated. The present contribution gives an in depth investigation into the interaction among plasticity, cohesive failure and adhesive failure, with application to structural joints. Quasi-static FE analyses of double lap-joint specimens with different thicknesses and under different levels of hydrostatic pressure were performed and compared to experimental results. In all the cases studied, the numerical analysis correctly predicts the driving mechanisms and the specimens’ final failure. Accurate fatigue life predictions are made with the addition of a Paris based damage law to the interface elements used to model the adhesive failure.     

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.     

Predicting the strength of adhesively bonded joints of variable thickness using a cohesive element approach

One major characteristic of bonded structures is the highly localised nature of deformation near sharp corners, ply-terminations, and ends of joints where load transfer occurs. This paper presents an investigation of the use of a cohesive zone model in predicting the strong effects of stress concentration due to varying adherend thickness on the pull-off strength measured by the Pneumatic Adhesion Tensile Testing Instrument. A comparison is made with the point-strain-at-a-distance criterion, where the plastic deformation of the adhesive is analysed using a modified Drücker–Prager/cap plasticity material model. The fracture properties of the cohesive zone model were determined using double-cantilever and end-notch flexural specimens, and the cohesive strengths were measured using tensile and lap shear tests. Comparisons with experimental results reveal that the cohesive zone model with perfectly plastic (or non-strain-softening) cohesive law provides accurate predictions of joint strengths.     

The measurement of damage initiation,particle adhesion and cohesive strength from traction displacement curves of a nano-toughened epoxy

In this work the deformation behaviour of a nano-toughened epoxy adhesive is measured at different levels of stress triaxiality. The test method consists of a notched axisymmetric adhesive layer loaded in tension. The recorded traction displacement curves were analysed numerically and it was found that the measured peak stress corresponds to the intrinsic cohesive strength, σ_max of the material. This method allows experimental measurement of σ_max for use in cohesive zone models of fracture. Additional features of the traction displacement curves include a kink that corresponds to particle debonding at a critical hydrostatic stress. By application of the Mori–Tanaka model, the relationship between the experimental measurements and particle/matrix adhesion is described.     

Shear Strength of SMR—Based Pressure-Sensitive Adhesives

The shear strength of Standard Malaysian Rubber (SMR)-based pressure-sensitive adhesives was studied using coumarone-indene resin as the tackifier resin. Three grades of SMR, i.e., SMR L, SMR 10, and SMR 20 were used as the elastomers. The concentration of tackifier resin was varied from 0–80 parts per hundred parts of rubber (phr). Toluene was used as the solvent throughout the experiment to prepare the pressure-sensitive adhesives. A SHEEN hand coater was used to coat the adhesive on the polyethylene terephthalate substrate to give a coating thickness of 30, 60, 90, and 120 µm. Shear strength of the adhesive was determined by using a Texture Analyzer. Results indicate that for a fixed coating thickness, shear strength decreases gradually with increasing resin content for all the rubbers studied. This observation is attributed to the decreasing cohesive strength of adhesive as resin loading is increased. However, for fixed resin content, shear strength increases with increasing coating thickness suggesting that shear strength is thickness-dependent. SMR L consistently shows higher shear strength than SMR 10 and SMR 20 for all coating thickness, an observation, which is attributed to higher purity of SMR L, compared with the latter two rubbers. The shear strength passes through a maximum at 5 min of mastication time, after which it decreases gradually with further mastication.     

胶粘耐磨涂层的研制及其磨损机理分析

本文介绍了ＪＮＴ９０系列涂层的研制过程及计算机辅助涂层配方优化方法，分析了影响胶粘胶耐磨涂层耐磨性的主要因素，并通过扫描电镜对粘涂层的结构形貌，磨损机理进行了研究，提出了提高涂层性能折途径和见解。     

Influence of Layer Thickness on Cohesive Properties of an Epoxy-Based Adhesive—An Experimental Study

Cohesive laws are determined for different layer thicknesses of an engineering adhesive. The shape of the cohesive law depends on the adhesive layer thickness. Of the two parameters of the cohesive law—the fracture energy and the strength—the fracture energy is more sensitive to thickness variation than the strength. The fracture energy in peel mode (Mode I) increases monotonically as the thickness is increased from 0.1 to about 1.0 mm. At an adhesive thickness of 1.5 mm, the fracture energy is slightly lower than for a 1.0 mm adhesive thickness, indicating a maximum between 1.0 and 1.5 mm. In shear mode (Mode II), the thickness dependence is not as strong, but an increasing trend in fracture energy with increasing adhesive thickness is evident. A slight decrease in strength with increasing adhesive thickness is found in both loading modes.     

Cohesive properties of refractory castable at 600 °C: Effect of sintering and testing temperature

This paper aims to evaluate cohesive properties for an alumina refractory with mullite-zirconia aggregates from wedge splitting tests (WST), and to assess their sensitivity to sintering and testing temperature. Five experiments were analyzed of which four were performed at 600^°C. The sought parameters were determined via weighted finite element model updating. The cohesive strength and the fracture energy were successfully calibrated and resulted in simulated data close to their experimental counterparts (i.e., between 4 and 11 times the measurement uncertainty). Increasing the sintering temperature from 1400^°C to 1450^°C enhanced the cohesion between the mullite-zirconia aggregates and the alumina matrix (20 % increase of the fracture energy and of the fracture process zone length). When the WSTs were performed at 600^°C, the cohesive strength was 10 % smaller while the fracture energy was 70 % higher than that at room temperature.     

粘附性颗粒流态化聚团尺寸的预测模型

在大量实验的基础上,通过对细颗粒碰撞聚团过程进行受力分析,提出了一种流化床中粘附性颗粒聚团和破碎的力学模型．通过模型计算,求出了不同类型粘附性颗粒流态化时聚团的大小．模型预测结果与实验测定基本一致．     

Non-destructive Inspection of Adhesively-bonded Joints

The objective of any system of non-destructively examining an adhesive joint must be to obtain a direct correlation between the strength of the joint and some mechanical, physical or chemical parameter which can readily be measured without causing damage. Faults or defects are defined as anything which adversely affect the short or long term strength of a joint. There are two basic areas for examination, the cohesive strength of the polymeric adhesive, and the adhesive strength of the bond between polymer and substrate.

Adhesive strength is very difficult to measure since it is an interfacial phenomenon involving a very thin layer of material, thin even in comparison with bond-line dimensions. Effectively, it would be necessary to assess intermolecular forces and this is not readily possible with existing techniques. This aspect of quality control is usually reduced to assessing the nature of the adherend surfaces prior to bonding.

The cohesive strength of the adhesive is really the only parameter which can be estimated with any degree of confidence, and it is this which features most on destructive tests of bonded joints.

In this paper, defects including porosity, surface un-bonds, zero-volume unbonds, poor cure and so on are discussed, together with the various methods currently used (and some new methods) for physical non-destructive testing.     

Simulation of adhesive joints using the superimposed finite element method and a cohesive zone model

Adhesive joints have been widely used in various fields because they are lighter than mechanical joints and show a more uniform stress distribution if compared with traditional joining techniques. Also they are appropriate to be used with composite materials. Therefore, several studies were performed for the simulation of the bonded joints mechanical behavior. In general for adhesive joints, there is a scale difference between the adhesive and the substrate in geometry. Thus, mesh generation for an analysis is difficult and a manual mesh technique is needed. This task is not efficient and sometimes some errors can be introduced. Also, element quality gets worse.In this paper, the superimposed finite element method is introduced to overcome this problem. The superimposed finite element method is one of the local mesh refinement methods. In this method, a fine mesh is generated by overlaying the patch of the local mesh on the existing mesh called the global mesh. Thus, re-meshing is not required.Elements in the substrate are generated. Then, the local refinement using the superimposed finite element method is performed near the interface between the substrate and the adhesive layer considering the shape of the element, the element size of the adhesive layer and the quality of the generated elements. After performing the local refinement, cohesive elements are generated automatically using the interface nodes. Consequently, a manual meshing process is not required and a fine mesh is generated in the adhesive layer without the need for any re-meshing process. Thus, the total mesh generation time is reduced and the element quality is improved. The proposed method is applied to several examples.     

Non-destructive Inspection of Adhesively-bonded Joints

The objective of any system of non-destructively examining an adhesive joint must be to obtain a direct correlation between the strength of the joint and some mechanical, physical or chemical parameter which can readily be measured without causing damage. Faults or defects are defined as anything which adversely affect the short or long term strength of a joint. There are two basic areas for examination, the cohesive strength of the polymeric adhesive, and the adhesive strength of the bond between polymer and substrate.

Adhesive strength is very difficult to measure since it is an interfacial phenomenon involving a very thin layer of material, thin even in comparison with bond-line dimensions. Effectively, it would be necessary to assess intermolecular forces and this is not readily possible with existing techniques. This aspect of quality control is usually reduced to assessing the nature of the adherend surfaces prior to bonding.

The cohesive strength of the adhesive is really the only parameter which can be estimated with any degree of confidence, and it is this which features most on destructive tests of bonded joints.

In this paper, defects including porosity, surface un-bonds, zero-volume unbonds, poor cure and so on are discussed, together with the various methods currently used (and some new methods) for physical non-destructive testing.     

接管高应变区断裂评定工程方法及失效评定曲线的研究

采用三段幂次法则和ＥＰＲＩ弹塑性Ｊ积分工程估算方法，推导得到了１６ＭｎＲ材料的接管高应变区二维模型分段弹塑性Ｊ积分工程估算公式，同时运用有限元方法进行了计算，在此基础上建立了高应变区的失效评定曲线ＦＡＣ，得到了该曲线的３大特征，并指出选择１曲线的不足。     

Calibration of cohesive parameters for a castable refractory using 4D tomographic data and realistic crack path from in-situ wedge splitting test

Crack propagation in an alumina castable refractory with mullite-zirconia aggregates was investigated in-situ using a wedge splitting test performed inside a laboratory tomograph. Four-dimensional (i.e., 3D space and time) data from digital volume correlation were used to investigate the influence of a realistic crack path on the simulation of the fracture process. A cohesive law was chosen, since toughening mechanisms were present, and calibrated via finite element model updating. When a straight crack path was assumed instead of the experimental crack path, a 10% higher fracture energy and a 35% higher cohesive strength were calibrated. Although the force alone could be used in the minimized cost function, the kinematic information gives valuable insight into the trustworthiness of the geometrical hypotheses assumed in the finite element model. Such framework can be applied to study nonlinear fracture processes for different materials with complex toughening mechanisms such as crack deflection or branching.     

Effect of Adhesive Thickness on Joint Strength: A Molecular Dynamics Perspective

Recent studies suggest that adhesion in thin joints depends on several factors including temperature, interface toughness, strain rate, surface roughness of adherends, bondline thickness of adhesives, and many others. Influence of thickness on joint properties is surprising but experimentally well documented without reasonable explanations. In this study, we attempt to address the mechanical behavior of polymer adhesives by molecular dynamics (MD) simulation. We show that interfacial strength of the joints in tensile, shear, or combined loading significantly depends on the coupling strength between adhesives and adherends. Failure of joints is always at the interface when coupling strength is weaker. With stronger interfaces, cohesive failure occurs by cavitation or by bulk shear depending on the loading condition. When joints are loaded in tension, it requires an exceedingly stronger interface to realize pure shear failure, otherwise failure is through interface slip. Under a mixed mode condition, interface slip is difficult to avoid. As long as failure is not at the interface alone, the yield strength of joints improves significantly with the reduction of thickness. Increase in bulk density and change in polymer configurations with the reduction of adhesive thickness are believed to be the two key factors in improving mechanical behavior of adhesives.