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

The use of deck fasteners as shear connectors for composite open web steel joists is studied. Results of thirty-six push-off tests are evaluated in terms of strength and behavior. Six types of deck fasteners are considered. They are: 1. #12-24 Teks/5 self-drilling, self-tapping screws. 2. 1/4-14 Teks/3 with a 1 1/4 in. stand-off sleeve. 3. 1/4-14 Teks/3 with a 1 3/4 in. stand-off sleeve. 4. 1/4-14 Teks/3 with a 2 1/4 in. stand-off sleeve. 5. 0.150 in. dia. air fired pins. 6. Puddle welds (5/8 in. and 3/4 in. diameter). All push-off tests utilize Vulcraft 1.5 VL, 22 gage, composite deck. Several modifications to the typical push-off test arrangement are made, which will permit the test to more closely model the top chord of an open web steel joist. It was found from the push-off test results that all of the deck fasteners, tested in this study, can obtain composite action for snort span open web steel joists with the exception of 0.150 in. diameter air fired pins. A description and the results of each push-off test is included. / Master of Science

A simple, design-oriented model of joints in vehicle structures is developed. This model accounts for the flexibility, the offsets of rotation centers of joint branches, and the coupling between rotations of a joint branch in different planes. The model parameters consist of torsional spring rates, the coordinates of the flexible hinges, and the orientations of planes in which the torsional springs are located. The model parameters are selected to be physically meaningful. In some cases, the behavior of joints can be accurately represented by using simpler models. The conditions under which the joint model can be simplified are discussed. A family of joint models with different levels of complexity are also defined. A probabilistic system identification is used to estimate the joint parameters by using the measured displacements. The parameters are estimated by minimizing the discrepancies between the measured and predicted displacements. Statistical tests which identify important parameters are also presented. These tests can be used to simplify the joint models without significantly reducing the accuracy in predicting structural responses. The identification methodology is applied to automotive structures with joints and also to isolated subassemblies consisting of joints and attached branches. / Ph. D.

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.

published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Civil Engineering / Master / Master of Philosophy

The strength and behavior of gusset plates in buckling is evaluated herein based on data from the experimental investigations conducted by other researchers and the analytical work presented herein. A set of design guidelines has been recommended through the review of the current practice. Representative single and double brace gusset plates normally adopted for connections with compressive bracing/diagonal members in braced frames and trusses, were modeled and analyzed using linear and nonlinear finite element methods to determine the buckling loads. The buckling analysis data along with the test data indicated the occurrence of inelastic buckling of the gusset plates. Current design practice and a set of formulas for determination of gusset plate thickness have been reviewed. A set of guidelines has been recommended for the design and evaluating gusset plate buckling loads.

The interference-fit joint is widely used in machine design to connect two cylindrical machine elements. Its popularity comes from the simplicity of the assembly and the low cost of the production process. Even so, no study has examined the boundary non-linearities in the assembly of these joints. Moreover, the contact stresses and the stress concentration factors in interference-fit joints under torsional loads have not yet been examined in detail. In addition, the interface stresses and stress concentration factors in interference-fit joints subjected to bending loads have not been approached theoretically before. The sugar-mill roller is one of the oldest and most important examples of the interference-fit joints. The frequent fatigue failure of the sugar-mill rollers under reversed bending causes costly emergency line-shut downs. The versatility of the finite element method and the capability of the point-matching technique in handling contact problems have been combined, in the present work, to produce a surface-matching technique. It has been found that the complete-cohesion contact assumption may be acceptable for coefficients of friction ≥ 0.2, which is the case for most of the normally machined surfaces. An approach combining the semi-inverse displacement finite element method and the surface-matching technique has been developed to perform the torsional analysis. It has been found that the governing non-dimensional quantities are the ratio of the cohesion-length to the interface-length and the coefficient of friction divided by the load-level. The load-level is the ratio between the angle of twist of the shaft per unit length when the torque acts on the shaft alone and the amount of the diametral interference divided by the shaft diameter. A finite element model, using 8-noded solid elements together with linear interface elements, has been employed to locate the cohesion and slippage-zones in the sugar-mill roller under bending. The contact pressure increases along the compression side and decreases along the tension side. On the contrary, the interface shear decreases along the compression side due to bending and increases along the tension side. Subsequently, a larger slippage-length has been detected along the tension side.

Embedded active and passive devices has been increasingly used by in order to integrate more functions inside the same or smaller size device and to meet the need for better electrical performance of the component assemblies. Solder joints have been used in the electronic industry as both structural and electrical interconnections between electronic packages and printed circuit boards (PCB). When solder joints are under thermal cyclic loading, mismatch in coefficients of thermal expansion (CTE) between the printed circuit boards and the solder balls creates thermal strains and stresses on the joints, which may finally result in cracking. Consequently, the mechanical interconnection is lost, leading to electrical failures which in turn causes malfunction of the circuit or whole system. When a die is embedded into a substrate, Young's modulus of the die is larger than one of the core of the substrate and the CTEs of the die is smaller than those of the substrate. As a result, mismatch in coefficients of thermal expansions (CTE) between the substrate with the embedded device and the solder balls may increase. In the present study, finite element method (FEM) is employed to find out the stress and strain distribution of ball grid array(BGA) solders under thermal cycling. The ANAND model for viscoplasticity is employed for this purpose.