This paper reviews new perspectives in polymer adhesion mechanisms. Both reptation theory and related experiments have demonstrated the existence of either self-diffusion or interdiffusion of rubbery polymers at the interfaces. Thus, adhesive strength, green strength, and tack of elastomeric adhesives can be at least partially explained on the basis of the diffusion mechanism. Besides diffusion, the adsorption mechanism plays a major role in achieving molecular contact. Without molecular contact, polymer molecules can not attain adequate diffusion or physical adsorption at the interface through van der Waals attraction. When the molecular distance is less than 4 angstroms, chemisorption can take place due to molecular interaction. Thus, both physisorption and chemisorption are important in enhancing adhesion. In addition to the above mechanisms, we shall mention briefly the mechanisms of mechanical interlocking, chemical bonding, and electronic adhesion.

Optical Designers seem to enjoy designing doublet lenses with one lens much thicker or physically stronger than the other. For manufacturers, that can cause distortion problems when the two dissimilar materials are cemented. We measure this pull or distortion by using our Zygo interferometer. In other words we can detect the distortion in the elements by observing the fringe pattern on one of the outside surfaces. The usual element we choose to observe is the thinnest element. We regard a thin optical element as anyone with a thickness to diameter ratio of over eight to one. This is a discussion on how our company solved this problem of distortion when using an ultra violet lens cement and avoid pulling without reticulation. The goal is always to have less than 1/4 fringes or less distortion.

Ultraviolet curing adhesives are widely used in optical lamination of precision optical lenses, safety windows, holographic displays, and flat panel displays. In most of these applications they are being used in relatively thin films from 3 microns to 40 mils, because most formulations will only allow uv light to penetrate to a certain depth. If the formulation does cure in thicker sections, the stress due to shrinkage typically becomes a problem. U.V. adhesives have been developed which can be cured in relatively thick sections with minimum stress. In this paper, a number of formulations are compared for variations in stress when cured in thick sections and the conditions and mechanism to minimize stress are characterized.

Loctite Cold Bloc^TM 3791, a new uv-curing 'cold' blocking adhesive enables optical manufacturers to produce lens surfaces that are practically distortion free and easily debonded using a variety of debonding agents and techniques. The adhesive is a significant advance in the lens blocking process, as it eliminates heat induced blocking strain; the most significant problem encountered with current hot pitch blocking methods. In addition, this blocking process reduces costly processing time, is compatible with existing tooling, utilizes convenient ready-to-use packaging and virtually eliminates the environmental concerns (solvents) of the current technique.

The military environment imposes harsh conditions on adhesives. These conditions differ both qualitatively and quantitatively from typical civilian environments. Military systems must withstand exposure to moisture, temperature extremes, sunlight/ultraviolet radiation and other climatic stresses that are far in excess of what would be expected for commercial applications. Additionally, civilian products rarely consider issues such as fungus susceptibility, resistance to jet fuels and de-icing solvents, or resistance to chemical warfare agents and their decontaminants. The effect of military environments on both the optical and mechanical properties of optical adhesives are discussed for avionic display applications.

Outgassing of adhesives and coatings can cause problems in optics used in space applications. The actual test NASA runs on candidate materials is very expensive and time consuming. End users will typically test every batch of material they receive and reject any that do no pass the requirements. This paper discusses a test which acts as an indicator to measure the relative performance and determine if the manufactured material will pass the outgassing requirement without going through the actual ASTM test. This is not a substitute for the final test. Its purpose is for screening materials being used in manufacturing or research.

The performance stability of a contacting butt-joint type of fiber attachment method, which is commonly used in the fiber optic industry, is evaluated in this paper. The particular butt joint design in question is a glass fiber bonded to a lithium niobate Mach-Zehnder modulator using UV curing optical adhesive. The joint is susceptible to optical loss levels that can increase with repeated thermal cycles, unless the fiber attachment method is carefully carried out. Difficulties have occurred during the warm portions of thermal cycles and subsequent microscopic analyses of failed devices suggested the presence of adhesive delamination at fiber-to-modulator joints.

This paper describes a systematic problem solving approach used to determine causes for separation that may occur in bonded optical assemblies when using an ultraviolet curing optical adhesive. Many interrelated factors in the bonding process can influence the integrity of the bond. By characterizing and diagramming these factors, separation causes can be identified and eliminated thus reducing problem solving time. The U.V. adhesive bonding process characterization can also be used as a guide for process analysis and optimization.

Electronic still cameras capture images with CCDs instead of silver halide film. However, the laws of optics continue to apply, and the active area of the CCD must be located in the focal plane of the camera optics. Typically, this means that the CCD is actively aligned during camera assembly. It is desirable to avoid this alignment operation. One way of circumventing alignment is to assemble the CCD in a package with a reference surface that is mechanically coupled to the surface of the CCD. This reference surface can then be mated to the camera optics, and alignment is automatic. We have developed a Glass-on-Chip package that uses optical adhesives to bond a shaped piece of glass directly to the surface of a CCD after wire bonding. If the glass thickness is controlled, this package meets the requirements for passive alignment of CCD to optics. Properties of the adhesive are critical to the success of this approach. The adhesive must be optically transparent, heat curable and compatible with the polymeric color filters already on the CCD.

A Vicker M55 inverted microscope (Vickers Instrument, U.K.) with a stage upward at the top is refitted for normal indentation and scratch indentation tests of epoxy coating debonding induced by indenters of tungsten carbide (WC) spheres and cones on the soda-lime glass substrates. A special designed apparatus assembled on the microscope stage can load the indenter quantitatively onto the coating and the amounts entering into the coating of indenter can be read out by an alignment microscope. The model system of the coating/substrate combination between the indenter and the objectives of Vicker microscope can be slide by handle and motor with velocity of 0.8 mm/min. Lots of experiments are done with satisfactory results by the use of the refitted microscope.

Moisture-assisted crack growth at polymer/glass interfaces was measured as a function of applied stain energy release rate using a four-point flexure apparatus coupled with an inverted microscope. The specimens consisted of two glass plates bonded together with a thin layer of epoxy or epoxy acrylate adhesive. Of particular importance in this study was the growth of cracks on both interfaces of the sample. Finite element analysis was carried out to gain a fundamental understanding of the observed crack growth behavior.

The optical method of caustics has been utilized to study the problem of a crack lying along the interface of adhesively bonded aluminum structures. The basic principles of the method, the mathematical description and the derivation of caustic evaluation formulas are presented for homogeneous and dissimilar materials. Optically flat mirror surfaces were achieved by developing a 'coating adhesion' technique. Predicted caustics using the derived equations were compared with the caustics obtained experimentally which show good agreement.

A modified edge notched flexural (MENF) specimen in a three point loading configuration is proposed for testing crack initiation toughness of bimaterial interfaces. An interface with a relatively large stiffness mismatch between two dissimilar materials, Aluminum-PMMA, is considered. The deformations near interface cracks are mapped in real time using a full-field optical method called Coherent Gradient Sensing (CGS). The homogeneous counterparts of MENF specimens have produced a fairly wide range of mode-mixities in our previous studies. This paper presents details regarding the extension of the model to bimaterial interface crack tip measurements and fracture toughness testing. The CGS measurements are used to extract both crack tip stress intensity factors as well as mode mixities using plane elastostatic interface crack tip fields. The results are compared with the ones from a beam model. The load data corresponding to crack initiation is used for evaluating fracture toughness of the interface as a function of mode mixity.

The use of loaded epoxy adhesives in the microelectronics industry is widespread. Integrated circuit (IC) chips are often adhesively bonded to die-pads, substrates, leadframes, and/or heatspreaders. In addition to silicon, a variety of adherends including IC passivation coatings (such as polyimide), copper and copper alloys (plated with precious metals and non-plated), aluminum, and ceramics (such as alumina (Al₂O₃)) are encountered in situations such as these. The adhesion strength as well as the mechanical properties of the adhesives used in these configurations are unfortunately not well known. In order to obtain an estimate of the values of the parameters that are of interest it is necessary to experimentally investigate the adhesives in the configuration under consideration. This was done for a number of commercially available loaded epoxy adhesives. A full factorial experimental study was conducted, using double lap shear test specimens (DLSTSs), in order to determine the effect of moisture and temperature on the adhesion strength of these adhesives. An Instron mechanical test system was used to generate force-displacement (F-d) curves for each of the adhesives that were studied. The adhesives that were examined consisted of both 'low stress' and high strength materials and they were obtained from four different vendors. The experimental results were also used in conjunction with a Finite Element (FE) model of the DLSTS in order to determine the non-linear Young's moduli of the adhesives as a function of strain, moisture, and temperature.

Ice-solid interfaces are of considerable interest in electrical and civil engineering and aviation. Efforts have been made to investigate the strength of ice-solid interfaces in terms of tensile, shear, and impact. This paper examines the fracture energy of ice/metal interfaces, in an attempt to utilize fracture mechanics to characterize the failure of ice/solid adhesion. A four point bending delamination specimen was used to measure the fracture energy of ice/Al and ice/steel joints. The fracture energy of an ice/steel interface was found to be lower than that of an ice/Al interface. The fracture mode was a mixture of cohesive and adhesive failure. The effects of impurities on interface fracture energy was significant.

One of the objectives of structural adhesive characterization is to provide material property data for analytical design purposes. However, the adhesive properties obtained from bulk testing may often prove unrealistic when transferred to a bonded joint in the presence of interfaces between the adhesive and the adherends. As a result, researchers are developing novel experimental procedures that can measure the in situ adhesive properties. A three-point bend test of a bonded beam to measure the in-situ adhesive shear modulus is a case in point. The adhesive shear modulus obtained from the three-point bend test was found to be up to an order of magnitude lower than that obtained from tensile test on bulk adhesive. In order to resolve this issue, a finite element analysis was performed to numerically simulate the test results and to conduct a sensitivity study to determine the dependence of beam deflection on various material parameters. The objective of this paper is to demonstrate that an approach that combines the use of a computational procedure such as the finite element method with a new test methodology leads to a better understanding of the test results while providing a consistency check. The finite element code used for this purpose, called NOVA-3D, is specifically designed for the analysis of adhesively bonded joints. Some of the unique capabilities of the code with regard to stress analysis and life-prediction of a bonded joint are described in this paper. An application of this special purpose finite element code to study the influence of environmental effect on adhesive response is also discussed.

In an effort to develop simple model geometries which could be used to characterize fracture at higher mode mixities, the plain strain fiber pullout specimen has been analyzed. The specimen has been calibrated for the independent application of mechanical axial loads, and processing thermal loads. Coupled solutions were obtained via the principle of linear superposition. Trends in the energy release rate and associated near-tip phase angle have been obtained both analytically and numerically via the method of finite elements. The specimen has been found to be dominated by slightly higher mode II than mode I conditions with phase angles in the range of 50 degree(s) - 60 degree(s). Further phase angle increases were obtained by introducing symmetric stiffening layers. The solutions indicate that the plain strain pullout specimen offers an attractive experimental alternative for mixed mode fracture toughness calculations. Mode II interfacial fracture data can be obtained using the same specimen with either unabridged or bridge stiffening layers.

Die attach voids and bonding defects, including insufficient and excessive adhesive, as well as porosity (resin starvation) are imaged with Reverse Geometry X-ray^TM technology. HIgh contrast sensitivity (0.2%) is obtained in a large area, real-time/near real-time system. X-rays originate in the source, which is modelled upon the cathode ray tube. A computer synchronizes the beam sweep, the detector readout rate, and the monitor sweep. Digital output from the detector unit allows the system to display first generation images, without recourse to an image intensifier or vidicon. A small scintillating crystal detector is placed from 7 centimeters to 3 meters from the source/object.

The holographic interferometry is a tool, to detect the displacement on the surface of a specimen, with an accuracy of less than the wavelength of the used laser. The Finite-Element- Method can be used, for instance to model problems of the structural mechanics. To verify the model with the behavior of the real specimen, there must be an interface. Two investigations were done on overlap-adhesive-bonds. First the inhomogeneous surface displacement over an adhesive defect under thermal load could be explained and second the material properties of the adhesive-layer could be determined, both with the surface deformation as interface between model and experiment.

This paper concerns the problems of dynamic response of the semi-infinite stationary mode III cracks in bonded materials subjected to dynamic load. Two kinds of semi-infinite cracks are considered that either lies along or perpendicularly terminates the interface of two half-plane dissimilar materials perfectly bonded together. The stress fields near the crack tips are analyzed in which the propagations of the different stress waves are illustrated. The results show that the standard square root singularity of stress distribution is valid for the interface crack but untenable for the crack perpendicular to the interface.

Using the method of images, the stress field of a screw dislocation in a multi-layer material is determined by non-local elasticity. The general solution holds two particular cases: one is metals with surface film, the other is bimedium materials. All classical singularities of the stresses and of the forces on the screw dislocation are eliminated.

The experimental methods of dynamic photoelasticity, optical caustics and the numerical method of finite element technique are utilized to study the transient response and the debonding failure of a cantilever beam of bimaterial with an edge crack in the center part of the main beam terminating at the interface with the reinforced beam. The isochromatic patterns are observed to investigate the bending wave propagation of elastodynamic stresses in the composite beam and the shadow patterns of optical caustics are recorded to obtain the response of the dynamic stress intensity factors of the crack tip in the main beam to illustrate the effect of the reinforcement. The stress distribution at the interface is analyzed to interpret the bond rupture of the adhesive layer of the composite structure. The experimental results and the numerical analysis show clearly the dynamic behavior of the composite beam before and after the interface debonding.

Numerous authors have investigated the state of stress in the adhesive of adhesively bonded joints. They have made various assumptions concerning the behavior of the adhesive and adherends to yield tractable differential equations which remove the stress singularities which occur at the edges of the bi-material interfaces. By examining several test problems, this paper investigates the effect of these assumptions on predicted adhesive stress.

Nonlinear von Karman plate theory is used to analyze circular, island and peninsula blister specimens on a consistent basis in order to examine their suitability as interfacial fracture toughness specimens for thin films on substrates. The peninsula specimen was found to be the most promising.

Joints are important load transferring members in large assembled structures. In joining similar and dissimilar materials, the use of adhesives offers many advantages when compared to other conventional methods. Most commonly used adhesives are the polymers, which exhibit nonlinear behavior. Finite element analysis with paired nodes along the crack path is employed to predict the crack initiation and growth leading to failure. The bond strength is predicted by investigating the possibility of propagation of a crack at the interface of adherend and adhesive. Paired nodes are opened in a sequence, modelling the crack growth. The adhesive is treated to be elasto-plastic for its response. Effect of the parameters such as, stacking sequences in composite adherend, crack growth locations, bond length, bond thicknesses and adhesive stiffnesses on the failure load is studied. The growth of plastic zone as the crack propagates is also examined. On the basis of this study optimal geometrical and material parameters are suggested. The elasto-plastic analysis predicts higher failure loads as compared to linear elastic analysis. The computed bond strength assuming elastic behavior for the adhesive shows satisfactory comparison with experimental results.

Holographic Interferometry techniques have been successfully employed to characterize and test the behavior of structures under stress. These techniques are especially suited to investigating the efficacy of bonds at the interfaces of different materials in complex laminated structures. Complex bonded structures of highly dissimilar materials are often difficult to characterize with radiographic, ultrasonic, or other 'conventional' techniques. Such structures can present great variations in density and extremes in bond strength, as well as tremendous dimensional differences which may foil such non-destructive evaluation. These are ideal requisites, however, for analysis using advanced real-time and multi-exposure holographic methods.