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The flexibility of welded joints is an important issue in design of car bodies. Two generic, 3-D, design-oriented models (simple and complex) are developed to represent the compliant behavior of multibranch flexible joints. The simple model consists of torsional springs restraining the relative rotation of the joint branches in the three planes, while all branches are assumed to be rigidly connected in translation. Coupling between motions in different planes is neglected. The complex model accounts for such coupling. A statistical system identification method is proposed for inferring the model parameters from the static response of the structure. The method is demonstrated by applying it to a simple cube frame structure and a car body. Finally, the two models are compared in terms of their ability to predict static response. / Ph. D.

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.

In this study, general models of the capacity of threaded fastener joints were developed from extensive experimental tests. One study objective was to develop a general model of threaded fastener withdrawal strength applicable to joints containing fasteners with widely varying thread geometries. A total of 419 tests of joints using six different fasteners and five species were tested. A multiplicative model containing wood specific gravity and the wood volume contained within the fastener threads provided very accurate predictions of withdrawal strength. A second study objective was to assess the accuracy of existing design criteria for threaded fastener joints subject to combined axial withdrawal and lateral shear loading. A total of 321 joints using 3 different fasteners and two species were tested at five angles between 0° and 90° to fastener axis. Little to no interaction was found between the lateral and withdrawal force components on joint capacity. Current design philosophy in the National Design Specification generally yielded conservative predictions of actual joint capacity. Improved design criteria suggested by experimental results were derived. A pilot study was also conducted to assess the influence of wood desorption on the withdrawal resistance of tapping screws. Forty joints of two moisture conditions, green and dry at insertion and two species were tested in withdrawal. In general, maximum and proportional limit loads were not affected by desorption whereas stiffness was significantly reduced for joints which desorbed after insertion. / Master of Science

A critical assessment of the current design theory for bolted joints which is based on a linear, one-dimensional stiffness analysis is presented. A detailed nonlinear finite element analysis of a bolted joint conforming to ANSI standards was performed. The finite element results arc presented in the classical bolted joint diagram and compared with the linear theory. The results revealed that the joint stiffness is highly dependent on the magnitude of the applied load. The joint stiffness changes continuously from extremely high for small applied loads to extremely low for large applied loads, contrary to the constant joint stiffness of the linear theory. The linear theory is shown to be extremely inadequate in characterizing the joint stiffness. The significance of the results in terms of the failure of bolted joints is discussed. Straight-forward analytical procedures are proposed for establishing estimates of the nonlinear stiffness description and the associate bolt loading in fatigue environments. The linear theory should be discarded and the more accurate nonlinear joint description be used. These results also provide the finite element community an improved model for the interconnection of substructures. The two-dimensional, axisymmetric finite element model includes bilinear gap elements to model the interfaces. Special orthotropic elements were used to model the bolt/nut thread interaction. A free-body-diagram approach was taken by applying loads to the outer diameter of the joint model which correspond to internal, uniformly distributed line-shear and line-moment loads in the joint. A number of convergence studies were performed to validate the solution. / Ph. D.

Call number: LD2668 .T4 1939 D32 / Master of Science

Call number: LD2668 .T4 1939 F72 / Master of Science

LD2668 .T4 1940 M66 / Master of Science