An interdisciplinary study concerning the durability of adhesively elastomer/metal bonded joints in marine environments is reported. The generation of OH⁻ ions at the bondline due to an imposed cathodic current from an external source is suspected to be the predominant cause of failure. A surface analysis study was performed early in this research for the purpose of identifying the cause(s) of failure. Characterization of the rubber and the metal failure surfaces with XPS (X-ray Photoelectron Spectroscopy) showed similar composition on both sides and to that of the bulk degraded primer component of the adhesive. Saponification of the adhesive and the leaching of chlorine (forming HCI that attacks the oxide) are identified as two possible failure mechanisms. The locus of failure is believed to be very close to the . adhesive/oxide interface. The exposure of bulk adhesive free-standing films to different environments showed that the hydroxyl is detrimental in the environmental durability of these specimens judging by the percentage of net mass uptake. An alternate interfacial failure mechanism is also presented where the neutralization of the adhesion-promoting attachment sites (A.S.) at the interface leads to de-adhesion and whereby OH⁻ ions chemically break-up the -COOH-Fe bond forming a non-operative activated complex at the degraded crack tip. Debond, or loss of adhesion, can exist in two modes. Weakening denotes debond growth which takes place undetected to the naked eye, and is governed by a diffusion-control degradation process that gives a straight line when plotted against the square root of time. Delamination, on the other hand, is a "post weakening" process., Debond rates in this mode are influenced by the applied total strain energy release rate, G_T, and by the environment and can be described by an exponential function in G_T. The effect of shear stress on debond acceleration was determined to be minimal. While compressive stresses seem to be beneficial in slowing the ingression of the bulk hydroxyl into the bondline, no noticeable improvement was detected when an imposed current was used. The use of zinc phosphate-coated steel substrates is shown to improve bond durability significantly at low voltages. Similar trends are observed when silane (γ-aminopropyltrimethoxy) modified primer were used in bonding. Two approaches are used in order to model debonding: empirical and analytical. Statistical Analysis System (SAS) is used to fit the empirical model which draws heavily on the functional dependencies of debond rates on the accelerating parameters, i.e., temperature, stress, and applied voltage. An Arrhenius relationship is shown to model the effect of temperature very well. Also, voltage effect is correlated with the corresponding current densities which, in turn, were found to obey an exponential relationship with debond rates. SAS fits of the experimental data are shown to model the process accurately and could be utilized for life predictions. Integration of delamination rates in real time is a feasible method to predict durability as well. A generalized analytical model for debonding is also developed, and it draws on the similarities between this application and corrosion fatigue of metals. The model is based on the conservation of mass of the involved species and is composed of a system of partial equations and their associated boundary conditions. Furthermore, temperature and voltage-dependent diffusion coefficients and reaction rate constants were used. The resulting boundary value problem amounts to a diffusion-chemical reaction mechanism into which a mechano-chemical failure mechanism is incorporated. A simplified version of the full scale analytical model is solved numerically and some interesting conclusions concerning the failure criterion are drawn. The model also simulates the weakening and delamination behavior and allows for temperature and voltage treatment as well. Delay times are also predicted as a function of the applied voltage and temperature. A particularly important conclusion is that the "marching boundary" phenomena seems to account for most of the accelerating influence of applied G. / Ph. D.

An investigation of polysulfone-Al/Li alloy interaction involved single lap shear joints and wedge samples following an FPL etch, sulfuric acid anodization (SAA) and phosphoric acid anodization (PAA). The study of the Al/Li surfaces involved the determination of the elemental composition and morphological features of the pretreated adherend before bonding and following failure. When thermoplastic polysulfone (PSF) was either thermally pressed or primed onto the microporous surface, the PSF indeed penetrated into the porous oxide and thereby provided a mechanical means of adhesion. The wedge test results for the adherend pretreated by PAA and SAA were superior to those for the FPL etched adherend. The failure path for the FPL etched samples was at the adhesive/oxide interface whereas the failure path for the SAA and PAA samples was within the adhesive but with occasional divergence of the crack into the oxide. The porous oxides on Al/Li alloy formed after PAA and SAA treatment were shown to undergo dramatic changes in morphology on short term (<90 hrs) exposure to 71 C and 100% R.H. environment. The mechanism of failure was due to moisture which caused slight hydration of the Al/Li oxide and subsequent debonding of the PSF from the oxide layer. Lithium was not concentrated at the surface in the PAA treated Al/Li alloy as shown by AES depth profiling. The effect of lithium on the durability of the bonded alloy is considered minimal. Along these lines, cyclic loading, use of primers, and infrared spectroscopy studies have been carried out. The mode of moisture intrusion into the polysulfone-Al/Li oxide interphase region is discussed. Specifically, water molecules diffuse into the polysulfone rather than transporting along the interface. Moisture then attacks the oxide interface. Thin polysulfone coatings on pretreated aluminum surfaces were characterized utilizing dynamic mechanical thermal analysis (DMTA), and dielectric thermal analysis (DETA) to detect changes in the molecular motions and structural transitions in the polysulfone-aluminum interphase. The order of the loss peak temperature of the polysulfone is, PSF coating on a porous Al > PSF coating on a smooth Al > neat PSF film. The activation energy of relaxation is also lower for neat PSF when compared to the thin film cast onto a smooth Al or a porous PAA Al substrate. The loss peak temperature shift and the higher activation energy associated with the coated films can be explained by the entropy being reduced when the chains are laid down in two dimensions. Thus studies of polymer properties in the interphase region will contribute to the understanding of the adhesive-adherend interaction. / Ph. D.

M.S.

Active damage control is a new technique which mimics the self-repairing capability of biological beings and can be used to greatly increase the structural integrity and fatigue life. This thesis describes two approaches used in active damage control: direct stress cancellation and indirect stress cancellation. Direct stress cancellation is illustrated using an example of active crack damage control with embedded shape memory alloy actuators. Both experimental and theoretical investigations have been conducted to analyze the damage control mechanism. The approach of indirect stress cancellation is demonstrated with active fatigue damage control of adhesive joints. The damage and active control mechanisms of the adhesive joints have been experimentally and theoretically investigated. / Master of Science

This study reports on the development and application of the peninsula blister test to quantitatively measure the adhesion of various adhesively bonded joints. Analytical results reveal that this peninsula-like geometry benefits from both a constant strain energy release rate over the major portion of the debond length and a high strain energy release rate at any given pressure. Applications of this technique to several adhesion systems were conducted. Although some of these systems haven't been successfully tested due to various reasons, experimental results from systems of PSA tapes and thin polyimide films bonded on aluminum substrates were promising. The agreement of the bond strength in terms of strain energy release rates obtained from both experimental and analytical methods from the last two systems indicates the feasibility of this technique. Primary studies on the stress analysis for several thin film adhesion tests suggest that the high ratio of strain energy release rate to applied pressure offered by this modified blister geometry may not be able to overcome the tensile strength limitations of thin film adhesion testing. Further studies need to be conducted in order to understand if the strain energy release rate can be raised without the increase of membrane stresses by altering specimen geometries. In conclusion, although this modified blister is not an universal adhesion test for every adhesion system, the attractive nature of the constant strain energy release rate produced by the peninsula blister specimen warrants further investigations and wider applications. / Master of Science

Water diffusion through the adhesive is the rate controlling factor for the durability of many metal-to-polymer bonds exposed to moist environments. A methodology is proposed, to relate the diffusion coefficient of water in polymers to temperature, strain and penetrant concentration. The approach used is based on well known free volume theories. In the rubbery state, it is assumed that the transport kinetics is governed by the constant redistribution of the free volume, caused by the segmental motions of the polymeric chains. An expression for the diffusion coefficient is inferred from the temperature, strain and penetrant concentration of the free volume. lt is shown that the free volume treatment can be extended to the glassy range by introducing a few additional features in the model. The stress dependence of solubility as well as the non-fickian driving forces contributing to mass transport are predicted from the Flory-Huggins theory. Experimental validation of the concentration dependence and temperature dependence of the diffusion coefficient is shown. The effect of mechanical strain on diffusivity and solubility in the glassy state is also investigated experimentally, using both the permeation and sorption techniques. Good agreement with theory is generally found. The coupling mechanisms between the diffusion process and the viscoelastic response of the adhesive are explained. A numerical scheme for fully coupled solutions is implemented in a two- dimensional finite element code. A few numerical solutions are shown. In the case of bonds undergoing unusually harsh environmental exposure however, alternative methods must be sought for durability characterization and prediction. This is illustrated with the case of rubber-to-steel joints exposed to a cathodic potential in seawater. The mechanical analysis of a durability specimen is presented and a procedure for debond prediction is suggested. / Ph. D.

No description available.

The use and implementation of adhesive joints for space structures is necessary for incorporating fiber-reinforced composite materials. Correct modeling and design of cylindrical adhesive joints can increase the dimensional stability of space structures. The few analytical models for cylindrical adhesive joints do not fully describe the displacement or stress field of the joint. A two-dimensional axisymmetric finite element model for the design and analysis of adhesive joints was developed. The model was developed solely for the analysis of cylindrical adhesive joints, but the energy techniques used to develop the model can be applied to other types of joints as well. A numerical program was written to solve the system of equations [K]{d}={R} for the unknown displacements {d}. The displacements found from the program are used to design cylindrical adhesive joints based on dimensional stability. Stresses were calculated from the displacements for comparison with analytical models. The cylindrical joints were assumed to remain within the linear elastic region and no failure criteria was taken into account. The design process for cylindrical joints was developed based on dimensional stability. The nodal displacements found from the finite element model were used in the optimization of geometric parameters of cylindrical joints. The stacking sequence of the composite, the bond length, and the bond thickness were found to have the greatest impact on dimensional stability. Other factors that were found to further reduce the maximum displacements are the implementation of 0° and 90° laminas, the isotropic cylinder thickness, tapering of the isotropic cylinder, and the inside radius of the cylindrical joint. This axisymmetric finite element model is beneficial in that a cylindrical joint can be designed before any testing is performed. The results and cases in this thesis are generalized in order to show how the design process works. The model can be used in conjunction with design requirements for a specific joint to reduce the maximum displacements below any specified operating requirements. The joint is dimensionally stable if the overall displacements meet specific design requirements.

No description available.

The effect of the element length is examined in modelling crack growth in fatigue loading of an adhesive joint. This is done for a cohesive element using an expression for the damage evolution developed at the University of Skövde which is implemented using the UMAT subroutine in the FE-solver Abaqus. These analyses are done for pure mode I loading by analysing a DCB-specimen loaded by a pure moment. An expression is developed in which the critical element length is dependent on the geometry of the specimen (in the form of the wave number of the adhesive joint), the element length, the material properties of the adhesive (in form of the material parameters , , ), the load applied (in form of the stress in the crack tip), the time step used in the analysis and the crack growth rate. It is shown that the results converge by decreasing the element length and the time step used. Therefore an expression for the crack growth rate as a function of the remaining parameters can be determined. Another expression is thereafter developed for the element length needed in order to get a crack growth rate within a certain range of the critical element length. The results show a regular pattern but are not monotone. Therefor two different definitions of the critical element length are tested, either by defining the critical element length as the point where the error is greater than an arbitrary boundary of 1 % of a converged result or where a least square approximation of the error is within 1 % of the converged results. The first method shows a highly irregular result which makes it difficult to develop an expression out of these results. The second method on the other hand gives results that are predictable enough to develop a function out of them. This is done using a regression analysis with all parameters of a third order expression in order to get an expression.