An investigation into the influence of cracks in bonded specimens is conducted. Photoelastic specimens containing a bondline are subjected to a constant displacement boundary condition created by bonded end grips. Specimens containing various crack orientations are analyzed to determine stress intensity factors at the induced crack tips. Specimens containing interface and sub-interface cracks were investigated. Two global geometries were used in this investigation, square and rectangular. The constant displacement boundary condition was induced on the specimen through dead weights hung from bonded aluminum end grips. Stress intensity factors were determined using photoelastic techniques. The stress intensity factors were examined to determine trends in the results as a function of changes in geometry. The effects of the induced boundary condition, the specimen geometry, and the bondline were investigated. The results from this investigation were compared to known solutions with a similar specimen geometry. These tests exhibited influences from the bondline, the boundary conditions, and the specimen geometry. The bondline tended to decrease the stress intensity factor for specimens with small crack lengths and tended to increase the stress intensity factor for specimens containing long crack lengths. As the crack length increased so too did the stress intensity factor. A reduction in the bondline to crack distance with sub-interface crack specimens caused a reduction in the stress intensity factor. A reduction in the global height of the specimen caused a reduction in the stress intensity factor also. The results from this investigation will aid in the understanding of the influence of interface and sub-interface cracks in bonded specimens. / Master of Science

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.

The main objective of the project was to establish a fracture-mechanics energy-based approach for the design of structural adhesive joints under cyclic loading. This required understanding how an adhesive system behaved near its fatigue threshold, and how the key factors affected this behavior in a fresh undegraded joint. The investigated factors were mode ratio (phase angle), substrate material, surface treatment and surface roughness (both affecting the bond strength), bondline thickness and load ratio. It was first required to understand how the adhesive system behaved under quasi-static loading by examining a fracture mechanics-based design approach for adhesive systems with different substrate materials and geometries. Experiments were initially performed to characterize the strength of aluminum and steel adhesive systems based on the fracture envelope, critical strain energy release rate as a function of the mode ratio. Ultimate failure loads of aluminum and steel adhesive joints, having different overlap end conditions and different geometries were then experimentally measured. These values were compared with the failure loads extracted from the fracture envelope. Considering the toughening behavior of the adhesive in the fracture mechanics analyses, a very good agreement (average of 6%) was achieved between the predictions and experiments for all types of overlap end conditions and geometries. Different fatigue threshold testing approaches, which are commonly used in the literature or suggested by the ASTM standard, were evaluated for the cracked and intact fillet joints. Based on the experimental and analytical studies, the most appropriate technique for fatigue testing and characterization of adhesive systems was suggested. Comparing the mixed-mode near-threshold behavior of different adhesive systems with the fracture behavior and fatigue mode-I and mixed-mode high crack growth rates showed the high sensitivity of the mixed-mode near-threshold fatigue to the subtle changes in the interfacial bond strength. In order to make a baseline for the design of adhesive joints under cyclic loading, similar to the previous fracture tests and following the energy-based approach, fatigue behavior was characterized as a function of the loading mode ratio for aluminum and steel adhesive joints. The effect of substrate material, surface treatment, bondline thickness, surface roughness and fatigue testing load ratio on the near-threshold fatigue behavior of adhesives joints was evaluated experimentally. The experimental observations were then explained using finite element modeling. To generalize the conclusions, the majority of experiments and studies covered a broad range of crack growth rates, as low as fatigue threshold and as high as 10-2 mm/cycle. Having understood the significant testing and design parameters, an adhesive system can be designed based on a safe cyclic load that produces an insignificant (for automotive industry) or reasonably low but known crack growth rate (for aerospace industry).

The main objective of the project was to establish a fracture-mechanics energy-based approach for the design of structural adhesive joints under cyclic loading. This required understanding how an adhesive system behaved near its fatigue threshold, and how the key factors affected this behavior in a fresh undegraded joint. The investigated factors were mode ratio (phase angle), substrate material, surface treatment and surface roughness (both affecting the bond strength), bondline thickness and load ratio. It was first required to understand how the adhesive system behaved under quasi-static loading by examining a fracture mechanics-based design approach for adhesive systems with different substrate materials and geometries. Experiments were initially performed to characterize the strength of aluminum and steel adhesive systems based on the fracture envelope, critical strain energy release rate as a function of the mode ratio. Ultimate failure loads of aluminum and steel adhesive joints, having different overlap end conditions and different geometries were then experimentally measured. These values were compared with the failure loads extracted from the fracture envelope. Considering the toughening behavior of the adhesive in the fracture mechanics analyses, a very good agreement (average of 6%) was achieved between the predictions and experiments for all types of overlap end conditions and geometries. Different fatigue threshold testing approaches, which are commonly used in the literature or suggested by the ASTM standard, were evaluated for the cracked and intact fillet joints. Based on the experimental and analytical studies, the most appropriate technique for fatigue testing and characterization of adhesive systems was suggested. Comparing the mixed-mode near-threshold behavior of different adhesive systems with the fracture behavior and fatigue mode-I and mixed-mode high crack growth rates showed the high sensitivity of the mixed-mode near-threshold fatigue to the subtle changes in the interfacial bond strength. In order to make a baseline for the design of adhesive joints under cyclic loading, similar to the previous fracture tests and following the energy-based approach, fatigue behavior was characterized as a function of the loading mode ratio for aluminum and steel adhesive joints. The effect of substrate material, surface treatment, bondline thickness, surface roughness and fatigue testing load ratio on the near-threshold fatigue behavior of adhesives joints was evaluated experimentally. The experimental observations were then explained using finite element modeling. To generalize the conclusions, the majority of experiments and studies covered a broad range of crack growth rates, as low as fatigue threshold and as high as 10-2 mm/cycle. Having understood the significant testing and design parameters, an adhesive system can be designed based on a safe cyclic load that produces an insignificant (for automotive industry) or reasonably low but known crack growth rate (for aerospace industry).

The study of the influence of viscoelastic adhesives on timber-glass composite beams addresses the development of new and innovative load bearing structures. Hybrid timber-glass beams, comprising of timber flanges and a glass web, were considered. The solutions proposed in this study are based on utilizing viscoelastic adhesive bond lines to obtain optimal structural interaction between timber and glass. For hybrid timber-glass beams, numerical simulations have been developed with Abaqus, Finite element software which are verified by analytical methods according to the Eurocode 5 (EN 1995). In this study, three different bond-line geometries and three adhesives (epoxy, silicone and acrylate) have been investigated. The beams were analyzed under four-point bending. This report summarizes theoretical investigations, background studies, numerical modelling and analytical solution that have been performed. Guidance is presented which can be used for the design of timber-glass composite beams. The study has shown that the viscoelastic properties of the adhesive had no significant influence for the selected epoxy adhesive, but it can slightly affect the beam behavior with the silicone and acrylate adhesives used. From the three geometries studied, the design with an epoxy bond-line on both sides and on top of glass is regarded the best way, leading to good load-bearing structural elements where the response of the adhesive was highly influenced by its near incompressible behavior.

In this time are for design of wooden elements available simplified procedures for fire modeling which defines ČSN EN 1991-1-2. But it is not experimentally tested the impact of adhesive on the final value of fire resistance of laminated panels. This diploma thesis solves basic normative practices that could explain the durability of glue bond against increased temperatures. in the practical part will be undertaken to experimentally assess the impact of various kinds of adhesive-based (epoxy, formaldehyde, PUR and EPI) on the glued bond durability against increased temperature. Glued joints that will be subjected to gradual temperature exposure in order to assemble the dependence of the temperature influence on the final strength of the joint according to the type of adhesive.

The aim of my diploma thesis are methods which are able to describe durability aspects of glued wooden joints. The main objective are methods which primarily includes hygrothermal stress acting in the glued joint of wooden adherend and determine the service life of the specific glued joint by modelling based on Arrhenius relation and the other methods. Other part of this work is aimed to shear tests of the samples exposed to different thermal and humidity conditions and for individual sets of exposures determining the glue penetration depth into the wooden adherend by EPI fluorescent analysis. These analytic methods are used for quality check of the glued joints, which are mostly based on strength, chemical or structural parameters of the tested sample.