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The primary objective of this research is to conduct further evaluation of ultrasonic digital image correlation (UT-DIC) on strain map analysis and defect detection of adhesive joints with embedded speckle pattern. UT-DIC is a non-destructive evaluation method that utilized ultrasonic C-scan images for whole field strain and displacement analysis. Acrylic glass, epoxy resin system and metal shavings with high acoustic impedance of a specific weight were used to create the samples which were loaded under tension. Defects with varying shapes and sizes were implemented by surface preparation to understand the limits of this approach. UT-DIC and optical DIC strain map results were compared, and it was found that each approach detected certain shapes of defect better than the other.

Joining of uniformly-curved composite sandwich panel segments, typical in state of the art aerospace launch vehicles, should be mass-efficient. Adhesively bonded joints can provide increased mass-efficiency over mechanically-fastened joints. But, due to manufacturing sensitivities and certification requirements, conventional bonded joints can be improved upon by introducing structural redundancy. A longitudinal, durable redundant joint (DRJ) architecture featuring multiple adhesive load-paths, via a novel composite preform insert, was proposed to join composite sandwich panel segments of the interstage element for NASA's Ares V launch vehicle. A series of twenty-five static linear-elastic finite element models with plane strain solutions were developed to assess certain characteristics of a joint's structural response when subjected to a simplified circumferential hoop loading convention. Shear and normal stress distributions at the adherend-adhesive interface along the splice plate bondline of the DRJ are compared with those from a conventional splice joint (CSJ) configuration for a series of linearly increasing bondlines thicknesses and joint overlap lengths. The parameter studies indicate the DRJ configuration's adhesive peak stresses are independent of the joint overlap length at the joint edges. Also, simulated bonding defects, in the form of local adhesive gaps, due to manufacturing processes are investigated to determine the load path redistribution for the DRJ and CSJ configurations. Results for pristine versions of both configurations are included. The defective CSJ joint exhibits severe overloading of certain laminates, while the defective DRJ load redistributions are relatively mild. Between the two primary types of bondline gaps considered for the DRJ configuration, the gap corresponding to the splice plate, a more mature manufacturing operation and also a more easily inspected location than the insert-to-face sheet interface, is noted to be more severe. A direct joint-to-joint mass-comparison reveals a 164% increase in mass, per unit thickness, between the CSJ and DRJ. To put this in perspective, a second comparison is made using a four-segment sandwich panel barrel. A 3.51% increase in mass is observed between the CSJ and DRJ-based cylinders. Also, for a simplified sizing philosophy, based solely on the peak stresses in the adhesive domain, a CSJ may require a 1.5-inch longer joint overlap than a DRJ. The mass-estimate is recomputed, and the mass percent-increase of the segmented cylinder is reduced to 2.61% over a CSJ configuration. / Master of Science

Adhesive joining is an attractive alternative to conventional joining methods, such as welding and mechanical fastening. The benefits of adhesive bonding include: the ability to form lightweight, high stiffness structures; joining of different types of materials; better fatigue performance, and reduction in the stress concentrations or the effects of the heat associated with welding. However, concerns about the durability of adhesive joints still hinder their widespread use in structural applications. Moisture has been identified as one of the major factors affecting joint durability. This is especially important in applications where joints are exposed to varying moisture conditions throughout their useful life. The aim of this research is to develop models to predict degradation in adhesive joints under varying moisture conditions. This was achieved by a combination of experimental and numerical methods. Experiments were carried out to characterise the moisture uptake and mechanical properties of the single part epoxide adhesive, FM73-M. Single lap joints were manufactured from aluminium alloy 2024 in heat treated (T3) and non heat treated (O) states using the FM73-M, BR127 adhesive-primer system. Tensile testing of the single lap joints was carried out after the joints had been exposed to hot-wet conditioning environments. Models were developed for predicting moisture concentration in the adhesive under cyclic moisture absorption and desorption conditions. A finite element based methodology incorporating moisture history was developed to predict the cyclic moisture concentration. In the next step, a novel finite element based methodology, which was based on moisture history effects, was developed to determine stresses in bonded joints after curing, conditioning and tensile testing. In the final step, a moisture history dependent cohesive zone element based damage and failure criterion was introduced to predict damage initiation, crack growth and failure under variable moisture and temperature conditions. The methodology proposed in this work and its implementation by finite element method provides a systematic approach for determining the degradation in adhesive joints under varying environmental conditions and accomplishes the aim of this research.

Este trabalho tem como objetivo estudar a resistência mecânica de junções adesivas. A influência das condições de superfície e de cura na resistência de junções adesivas, unidas por adesivo acrílico, foi quantificada através da realização de ensaios de cisalhamento de chapas sobrepostas, com resultados processados através do Planejamento Fatorial 2k. Sobreposição simples de chapas também foi utilizada nos ensaios que permitiram analisar outro fator, o comportamento da resistência em função do comprimento de sobreposição das chapas. Esclarecendo contradição apresentada em diferentes publicações. Um dispositivo modificado de Arcan foi utilizado na realização de experimentos, os quais tornaram possível analisar o comportamento de junções adesivas submetidas a esforços combinados, com diferentes velocidades no deslocamento. Este estudo contribui em projetos que envolvem junções adesivas, apresentando informações de alto nível de importância, a serem utilizadas para determinar a melhor condição de trabalho destas junções. / The aim of this work is to study the mechanical strength of adhesively bonded joints. The influence of surface and cure conditions on the strength were taken in consideration. Such influences were sized through shear tests of single lap joints, with results processed by Factorial Design 2k. Single lap joints were also used in tests, which made possible to analyze another factor, the influence of overlap length on the joint mechanical strength. A modified Arcan Device was used, with specific specimens, to analyze the joints behavior, when submitted to combined strengths. This study provided important informations, which are able to be applied to adhesively bonded joints design.

To support the trust in the design development of adhesively bonded joints, it is important to precisely predict their mechanical failure load. A numerical simulation model with a two-dimensional linear elastic cohesive zone model using a combination of a soft and a stiff adhesive is developed to optimize the strength of a lap-joint. Separation under mixed-mode conditions (normal and shear direction) is considered. By varying the length of the adhesives, the fracture load is optimized. The results obtained from the numerical experiments show an improvement in strength.

In order to incorporate fiber-reinforced composite materials in space structures, adhesive joining techniques are required. Because analytical models have a hard time capturing the complex stress state inherent to adhesively joining dissimilar materials, a different modeling technique was deemed necessary. A two-dimensional axisymmetric finite element model capable of capturing the three-dimensional stress state of cylindrical adhesive joints was developed. In order to rigorously validate the model, testing was undergone to ensure the model accurately predicted joint displacements. Displacement data was acquired via an Epsilon axial extensometer. Load data was taken simultaneously via the load cell incorporated in the Tinius Olsen tensile test machine used. The measured force vs. displacement data was found to agree with the model’s predicted displacement for a given load. Displacement data was also taken, again with the extensometer, as the joints were rapidly cooled to liquid nitrogen temperature. It was found that the joints behave much like laminated plates in that after the first several cycles they “settle down”. The term “settle down”, in this context, means that after the first several cycles the displacements of the joints when placed from a room temperature environment to a cryogenic environment become consistent and smooth. This result allows for the joints to be modeled. The finite element model was shown to accurately predict the settled down displacement given the prescribed temperature change. The joints were also shown to maintain structural integrity post thermal cycling. Transient temperature tensile tests were performed until sample failure. One result with major design implication coming from this test was that the material properties do not change significantly enough over the temperature range tested to affect the joint’s behavior. The same properties used in the room temperature model were used to model the measured data of the transient temperature data and were found to match satisfactorily. Having validated that the developed axisymmetric finite element model accurately predicts cylindrical joint displacement fields, the model becomes an invaluable tool in design. The model can now be used in confidence, in conjunction with design requirements for a specific joint, to reduce the maximum displacements below any specified operating requirements.

Increasing use of composite materials in industry brings the need for newer and more practical methods to evaluate them. Widespread use of composite materials heavily depends on the manufacturer’s ability to unquestionably ensure its safety, given how much the user trusts them. Non-Destructive Evaluation (NDE) can be used to evaluate adhesive bondline health. This thesis employs Digital Image Correlation (DIC) method, one of the known methods in NDE, and combines it with an embedded speckle pattern in order to obtain valuable information from within the adhesive bondline. By recording the movement of the speckles and analyzing their behavior according to DIC algorithms, a strain map of the adhesive is drawn. An adhesive strain map helps find defects that might be out of sight using conventional NDE methods. This thesis discusses different possible materials to be used as the speckle pattern and chooses the one shows better results based on different criteria. Then employing the material, it records the speckle pattern using optical and ultrasonic methods to draw a strain map. By analyzing the obtained strain maps, defects within the bondline are revealed.

Submitted by Lucienne Costa (lucienneferreira@ufcg.edu.br) on 2018-04-10T19:51:47Z No. of bitstreams: 1 THIAGO TAKUMI TAN – DISSERTAÇÃO (PPGEM) 2017.pdf: 9018003 bytes, checksum: 45bd9458239c0c274dabe5c7aee2f165 (MD5) / Made available in DSpace on 2018-04-10T19:51:47Z (GMT). No. of bitstreams: 1 THIAGO TAKUMI TAN – DISSERTAÇÃO (PPGEM) 2017.pdf: 9018003 bytes, checksum: 45bd9458239c0c274dabe5c7aee2f165 (MD5) Previous issue date: 2017-03-13 / O objetivo desta dissertação foi desenvolver dispositivos que possam ser utilizados para ensaios de fratura interlaminar modo-I e modo-II em materiais dissimilares, para aplicação de cargas dinâmicas. Para o desenvolvimento dos dispositivos foi utilizada uma adaptação da metodologia de projeto apresentada por Maribondo (2000), que consiste em projeto informacional, projeto conceitual, projeto preliminar, projeto detalhado, fabricação e testes. A escolha dos ensaios que podem ser utilizados para obter tais modos de fratura foi realizada com base no estudo de trabalhos acadêmicos, que apontou o ensaio double cantilever beam (DCB), como sendo o mais utilizado para estudo de fratura interlaminar modo-I, e também foi observado o crescimento do uso do ensaio end-loaded split (ELS), para estudo de fratura interlaminar modo-II. Os dispositivos desenvolvidos foram testados em uma máquina de impacto por queda de peso, onde foram utilizados corpos de prova de junta adesiva de alumínio/epóxi. Realizaram-se testes modo-I e modo-II bem-sucedidos, analisando o comportamento dos dispositivos e dos corpos de prova durante o ensaio e após o mesmo. / The main objective of this dissertation for the devices that can be used for mode-I and mode-II interlaminar fracture test in dissimilar materials. For the development of devices for design methodology that projects in informational design, conceptual design, preliminary design, detailed design, fabrication and testing. The choice of the tests that can be used to obtain such fracture modes was made based on the study of academic papers, which pointed to the double beam cantilever (DCB) test, as being the most used for the study of interlaminar mode-I, and end-loaded split (ELS) for the study of mode-II interlaminar fracture. The developed devices were tested in a weight drop impact machine, where aluminum / epoxy adhesive joints were used. Testing and analysis of the behavior of the devices and test bodies were carried out during and after the test.

Este trabalho tem como objetivo estudar a resistência mecânica de junções adesivas. A influência das condições de superfície e de cura na resistência de junções adesivas, unidas por adesivo acrílico, foi quantificada através da realização de ensaios de cisalhamento de chapas sobrepostas, com resultados processados através do Planejamento Fatorial 2k. Sobreposição simples de chapas também foi utilizada nos ensaios que permitiram analisar outro fator, o comportamento da resistência em função do comprimento de sobreposição das chapas. Esclarecendo contradição apresentada em diferentes publicações. Um dispositivo modificado de Arcan foi utilizado na realização de experimentos, os quais tornaram possível analisar o comportamento de junções adesivas submetidas a esforços combinados, com diferentes velocidades no deslocamento. Este estudo contribui em projetos que envolvem junções adesivas, apresentando informações de alto nível de importância, a serem utilizadas para determinar a melhor condição de trabalho destas junções. / The aim of this work is to study the mechanical strength of adhesively bonded joints. The influence of surface and cure conditions on the strength were taken in consideration. Such influences were sized through shear tests of single lap joints, with results processed by Factorial Design 2k. Single lap joints were also used in tests, which made possible to analyze another factor, the influence of overlap length on the joint mechanical strength. A modified Arcan Device was used, with specific specimens, to analyze the joints behavior, when submitted to combined strengths. This study provided important informations, which are able to be applied to adhesively bonded joints design.