The increasing use of composite materials in many industries such as aerospace, automotive, and civil industries has increased the need for the development of effective techniques to detect defects in the bondlines of adhesive joints in composite structures. Currently, composite structures used in commercial applications such as modern aircraft use mechanical fasteners in redundancy to adhesive bonds to ensure structural integrity due to a lack of methods to reliably detect defects in the bondline of composite structure. As such, this thesis facilitates the development of nondestructive evaluation techniques for detecting bondline defects by using finite element (FE) modeling to simulate the effects of disbond defects caused by contamination of the bondline. These models were developed for single-lap joint specimens made of metal, composite, and dissimilar materials (metal bonded with composite) with contamination induced disbonds. The created FE models were used to generate whole-field strain data for single-lap joints under tensile loading. This generated strain data was then used to provide a model for evaluating and interpreting experimental strain measurements captured by digital image correlation (DIC). Finally, conclusions were drawn outlining the observed capability of strain measurement in the evaluation of bondline contamination in single-lap joints.

L'optimisation des structures aéronautiques fabriquées en composite a mené EADS-IW à développer une technique d’assemblage par cloutage qui a pour objectif la fabrication à coût réduit de sous ensembles structuraux avec un fort niveau d'intégration. L'objectif de ce travail de thèse est d'étudier le comportement mécanique des assemblages cloutés et de proposer des modélisations associées. Dans le cadre d'une démarche multi-niveau, le comportement des liaisons clou/résine et clou stratifié a d'abord été étudié. Des campagnes expérimentales ont montré l'influence du diamètre du clou et de la profondeur d'enfoncement sur la tenue en arrachement ainsi que des similitudes entre le comportement du clou noyé dans de la résine et implanté dans le stratifié. Une modélisation capable d'estimer la tenue en arrachement d'un clou a été développée. Au niveau éprouvettes technologiques, des éprouvettes cloutées représentatives de structures aéronautiques de type « L » ou « T » ont été testé en sollicitations statiques montrant l'influence des paramètres de conception et les possibles avantages du cloutage. Finalement, des modélisations basées sur les études élémentaires ont permis de simuler le comportement de ces éprouvettes cloutées retrouvé en essai et notamment de prédire leur tenue sous sollicitations différentes validant ainsi la démarche multi-niveau.

Ett sätt att öka mängden trä i byggnader är att ersätta stålförband i takstolar med ett trädymlingsförband. Det bör dock undersökas om trädymlingsförbandet har tillräckligt högt hållfasthetsvärde. Denna studie undersöker hållfastheten på träförband förankrade med trädymlingar. Dymlingarna har tre olika dimensioner (8, 10 och 12 mm) och förankras i förbanden genom svällning. Teoretiska beräkningar utförs med formler från Eurokod 5 (2004) för stålskruvar. Formlerna modifieras för att kunna tillämpas på trä istället för stål. Trycktester utförs på tio tvåplansskjuvnings-modeller per dymlingsdimension. Studien anses vara statistisk och sammanställs i normalfördelningskurvor. Av data från testerna räknas karakteristiska värden för trädymlingarna med säkerhetsmarginalen 95 %. Karakteristiskt hållfasthetsvärdet för trädymlingsförbanden blev 2100 N för 8 mm, 2300 Nför 10 mm och 4400 N för 12 mm. / One way to increase the amount of wood in buildings is to replace framing tie with timber dowel joints. It has to be investigated if timber dowel joints strength value is high enough. This study investigates the strength of timber joints anchored with a dowel. The dowels have three different dimensions and were anchored in the joint by swelling. Theoretical calculations were made with formulas from Eurokod 5 (2004) for steel screws. The formulas were modified to be applied to wood instead of steel. Pressure tests wereperformed on ten double shear models per dowel dimension. The tests are considered tobe statistically and compiled in a normal distribution curve. Characteristic strength with thesafety margin of 95 % was calculated from the data from the tests. Characteristic strength for the timber dowel joints were 2100 N for 8 mm, 2300 N for 10mm and 4400 N for 12 mm.

This study demonstrates an experimental investigation into the efficacy of a novel surface pre-treatment technique to improve the toughness and energy dissipation of composite CFRP T-joints. This novel technique optimizes CO2 laser irradiations to remove surface contaminations and modify the surface morphology of CFRP T-joint adherents. Pull-off tests were performed on T-joints that experienced peel-ply (PP) treatment and to those that were ablated with 10% (LC) and 30% (LA) laser power respectively. A further developed alternative pattern between LA and LC surface pre-treatment was examined. Two different quasi-isotropic stacking sequences have been studied by having surface fibers aligned in 0° and 45° direction. A series of surface roughness analysis, optical microscopy, SEM, CT scan and pictorial findings have been carried out to characterize the surface morphologies and failure modes prior to and after the failure. The patterning technique promoted non-local damage mechanisms which resulted in large improvements in the toughness and energy dissipation as compared to the other pre-treatment techniques. Up to ~12 times higher energy dissipation compared to peel-ply pre-treated T-joint were achieved with patterned T-joint structures that are stacked with a 0° surface fiber direction.

ABSTRACT The task was to design and assess the supporting structure of the lookout tower. Material steel and wood. The lookout tower is 27.22 m high and decreases in height from the base of the srka. Construction system with continuous columns and horizontal reinforcement.

The purpose of this research was to identify the types of joints available for use on concrete bridge decks and to investigate the performance characteristics of each type, including primary functions and movement ranges. Eleven reports on joint performance published by state departments of transportation and universities nationwide were analyzed in order to obtain information on joint performance problems typically encountered by state transportation agencies. In addition, test methods and specifications provided by the American Society for Testing and Materials (ASTM) were reviewed for application by bridge engineers to ensure the adequacy of deck joints. The research indicates that compression seals should be used to accommodate movements less than 2 in., while strip seals should be used for movements up to 4 in. A lubricant conforming to ASTM D 4070, Standard Specification for Adhesive Lubricant for Installation of Preformed Elastomeric Bridge Compression Seals in Concrete Structures, should be applied during installation of compression and strip seals. Finger joints with troughs should be used instead of reinforced elastomeric joints and modular elastomeric joints for movements greater than 4 in. To maximize the performance of finger joints, ensuring adequate structural properties of the finger plates and proper installation of the troughs is necessary. When Utah Department of Transportation (UDOT) engineers conduct in-house experiments on bridge deck joints in the future, they should be more consistent and provide more information about the bridge structures in reports, including, for example, the anticipated deck movements, average daily traffic, and design loads for the bridges. Also, UDOT should establish a consistent evaluation program for investigating joint products during the approval process. The program should include quantitative measurements including, but not limited to, debris accumulation, adhesion and cohesion of the joint material, condition of anchorages and header materials, watertightness of the joints, condition of the concrete edges of the deck, deterioration of substructures, ride quality, noise level under travel, and general appearance of the joints. These experimental data should then be thoroughly documented in the resulting reports.

Fracture path transition, from interfacial fracture to a pulled button, in peel tests of spot welds in SAE 1039 steel was evaluated to determine the controlling material properties. Welds were tested in the as welded condition and after tempering at various temperatures to develop a range of hardness and strength in the weld metal. Two transitions were found, from complete interfacial fracture to partial interfacial and from partial interfacial fracture to a pulled button. Samples tempered at less than 350 °C exhibited complete interfacial fracture while those tempered at 500 °C or higher pulled full buttons. Each transition was accompanied by a large increase in the energy absorbed during fracture. Both partial and complete interfacial fracture occurred by intergranular fracture along prior austenite grain boundaries. Optical microscopy utilizing a tint etch indicated that austenite existed as films on prior austenite grain boundaries of samples tempered at less than 500 °C and TEM confirmed that these films were austenite. Weld metal toughness was found to control the fracture path. If fracture initiation was delayed until the applied load caused plastic deformation of the coupons pulled button fracture occurred. Fracture initiation prior to plastic deformation of the coupons resulted in interfacial or partial interfacial fractures. A small secondary hardening peak was observed in samples tempered at 450 °C. Secondary hardening, which normally results from alloy carbide precipitation, was due to AlN precipitation in this aluminum killed plain carbon steel. / Ph. D.

Prediction of the performance of adhesively bonded joints is essential to the acceptance of this mode of fastening. In ideal situations where the bonding is uniform throughout the joint the stress distribution depends on the material properties and the joint geometry. Knowledge of the bond properties and the stress distribution and magnitude can then be used to determine failure initiation, damage growth, subsequent stress distributions, and final failure. However, few bonds can be characterized as "ideal", as the bonding is generally not uniform throughout the joint and even the properties of the adhesive may vary. This paper describes work that addresses this situation. Acoustic microscopy has been used to provide a detailed image of the bonded joint, while acousto-ultrasonic measurements have been used to evaluate the stress transfer capability of modified lap shear joints. Knowledge of the stress distribution in the joint has provided a means of identifying critical areas of interest in the joint. The adhesively bonded composite specimens were mechanically tested and performance correlated with NDE results. Wave mode filtering is offered as a phenomena describing the basis for the correlation. / Master of Science

An analytical and experimental investigation was conducted to study the six bolt flush moment end-plate connection configuration which is used in steel frame construction. The limit states of plate yielding and bolt fracture were analyzed using yield-line theory to predict endplate thicknesses and a split-tee analogy to develop a method to predict bolt forces. Five experimental tests were conducted on four configurations within a matrix of geometric parameters. The predicted ultimate moment showed good . correlation to the yield moment obtained from the experimental deflection plots. The experimental bolt forces correlated well with the predicted bolt forces when plotted versus the applied moment. Additionally, an equation to model the moment-rotation relationship was developed from a regression analysis to determine the construction type suitable for a given connection configuration. Finally, a method of designing the six-bolt flush end-plate configuration is presented and an example given. / Master of Science

Large scale structures such as those in aerospace flight vehicles are made in parts and assembled. Joints are inevitable in these systems and they are potential threats to the structural integrity of the flight vehicles. Fastener and bonded joints are the most commonly used methods of joining in these structures. Among these, adhesive bonding has become more popular with the advent of composite structures, due to the presence of less number of points of stress concentration and the resulting benefit for static strength and fatigue life. In modern aircraft in which maximum percentage of composite materials are being employed due to several benefits, designers are contemplating to replace discrete joints with adhesively bonded joints wherever possible. A detailed literature survey shows that the field of adhesively bonded joints has been extensively studied in the past. Initial publications appeared in late 1950’s and early 1960's, but many of the initial attempts were based on one dimensional (1-D) approximation of the adherents due to lack of computing power. With the current day emphasis on safety and damage tolerance, there is a definite need to study these joints with 2-D and 3-D idealization. In spite of valuable contributions in the literature from several researchers in past 4-5 decades, one finds that there are gaps to be filled, in particular, with reference to static strength prediction and de-bond growth to failure under fatigue loading. This thesis is intended as a modest contribution in this direction covering the methods of strength prediction and also correlations between de-bond growth and fracture parameters. Most commonly used bonded joints are single lap joints. The primary issue in their analysis is the geometric nonlinearity resulting in large deformations due to eccentricity of load path between the adherents. Further, adhesives have very low yield strength and plastic deformation in thin adhesives could affect the mechanics of load transfer. The current work is initiated by carrying out geometric and material nonlinear analysis of adhesively bonded single lap joints between metal-metal (aluminum-aluminum) adherents using standard NASTRAN finite element software. Modified Newton-Raphson iterative technique has been used to economize the computer time and also achieve fast convergence. A convergence study has been conducted to determine the order of mesh size required. Preliminary results are obtained on configurations analysed by earlier workers and the current results are compared with their results. Later, extensive experimental and numerical studies have been taken up on the numerical strength prediction of these joints correlating them to the experimental values. Cohesive failure along the centre line of the adhesive is assumed under both static and fatigue loading. The bonded joints are studied with both 2-D plane stress and plane strain nonlinear FE analysis. The issue in this type of analysis is the presence of theoretical elastic singularity at the ends of the lap length. The normally used maximum stress criterion can not be used in such circumstances. There were attempts in the past to use point stress or average stress criteria for this purpose. In point stress criterion the shear stress (or von-Mises stress) is picked at a characteristic distance away from the ends of the lap length and compared with the corresponding strength value to predict failure. In the average stress criterion the stresses are averaged over a characteristic distance from the ends of lap length and this is compared with the corresponding strength to predict failure. Determination of the characteristic distance in both the cases needs extensive experimental results on static strength of joints. The static strength data is to be correlated with numerical results to determine the characteristic distance in various specimens. In the current thesis a series of specimens with aluminum-aluminum, aluminum-CFRP composite and CFRP-CFRP composite adherents were tested to determine the static strength. In all the specimens the adhesive used was Redux 319 A. These experimental strength data was used to determine characteristic distance using point stress criterion. The consistency of estimates of the characteristic distances in all the specimens shows that the approach is capable of predicting the static strength. The above approaches are capable of predicting the strength of joints with linear material and nonlinear geometric analysis. But when the adhesive yield strength is low, a novel approach is required to predict the static strength. Numerical analysis is conducted using a combined material and geometric nonlinear analysis in NASTRAN software. The plastic zone size from the ends of the lap length is determined at different load levels. Combining the numerical results with experimental failure load data, a failure criterion based on plastic zone size (PZS) is proposed in this thesis and validated. It has been observed that the validation is with limited testing carried out and further experimental programs are required to complete the validation. To the best of the knowledge of the author PZS criterion is used for the first time for failure prediction of bonded joints. The structural integrity of the joints also requires a study of de-bond growth and damage tolerance assurances in the presence of de-bond type of defects. The first step in this direction is to estimate the fracture parameters at the tips of de-bond in the adhesive of lap joints between various adherents. Modified virtual crack closure integral (MVCCI) technique has been developed in the past for estimating Strain Energy Release Rates (SERR) in several crack problems. Large contributions for developing this technique have come from the group where the author has worked. This technique is simple and has the ability to estimate individual SERR components GI and GII in cases of mixed mode fracture. It is seen clearly that the de-bond growth in bonded joint is one of mixed mode. The mode-II component is because of shear stresses transferring the load across the adherents and mode-I component is due to peel stresses developed during the deformation. The mode I SERR component is primarily responsible for de-bond growth and the effect of mode II component on de-bond growth is insignificant. The mesh details for accurately estimating the SERR components are evaluated and those meshes are used to estimate these values for the cases of aluminum-aluminum, aluminum-CFRP composite and composite-composite joints. Obviously, when the adherents are dissimilar, mode I SERR components are the highest and assist faster de-bond growth. Painstaking fatigue de-bond growth experiments were conducted and de-bond growth rate with number of cycles of fatigue loading was determined. MVCCI method is used to estimate SERR components at maximum load and zero load in the fatigue cycle, to determine the SERR range in the fatigue cycle. Since the stress ratio, R of the loading cycle is -1, the minimum load for estimating SERR components is taken as zero. From the experimental data and numerical estimates, a Paris type of equation was developed for the de-bond growth. The thesis concludes with a summary of the achievements in the current work with respect to the structural integrity of adhesively bonded joints and also with suggestions for future work.