Bond properties of rubber modified epoxy

In this paper, the bond integrity of unmodified and rubber-modified epoxy used for bonding the carbon fibre sheets to the hosting steel surface was investigated. The rigidity of the bonding agent is one of the factors that have a significant role in the premature failure (debonding) of this application. In order to overcome this issue, a series of experiments were conducted on the steel plates using the epoxy resin modified by CTBN and ATBN reactive liquid polymers, in addition to the unmodified epoxy resin. The interface between the carbon fibre matrix and the hosting surface is subjected to a longitudinal shear force for which the corresponding displacement is recorded. The shear stress-strain relationship for the tested specimen is plotted. The result shows that, the bond behaviour of modified epoxy using CTBN and ATBN reactive liquid polymers was improved in terms of ductility and toughness

Role of contamination on the bondline integrity of composite structures

Adhesively bonding composite structures have many applications in aerospace, automotive and submarine industries. The adhesive bonding joints have substantial advantage over the traditional metallic mechanical bonding joints, such as rivet and welding. However, the adhesive bonding joints require additional steps of surface preparation and cleaning to ensure consistent bond strength. In application, the adhesively bonded joints are exposed to environmental degradation and industrial solvent contaminates. Accordingly, the assurance of reliability of bonded composite structures requires detailed investigation of the role of contaminates on bondline integrity. This dissertation focuses on assessing the contaminates effect on the adhesive bondline integrity. A combined experimental and numerical framework is developed to study the contamination effect on the adhesive mechanical properties and adhesive joint strength. The bondline integrity were examined for a system of adhesive (EA9394) and the carbon-fiber system (Hexply IM7/8552), after being subjected to different level of exposures to aviation hydraulic fluids and mold cleaning agents. A testing protocol based on nanoindentation for initial screening is used to predict the interfacial fracture characteristics after exposure to contamination. It is found the adhesive modulus and stiffness dropped by up to 10% for the hydraulic fluid contaminates, suggesting increase of the plastic dissipation within the bondline. However, the trend for the cleaning agent was not clear since the modulus drop while its hardness increased. Detailed measurements of interfacial fracture toughness are carried out via standard tests of double cantilever beam specimens, exposed to varying level of contamination. The tests were carried out in a computer controlled Instron universal testing frame. An optical based crack propagation measurement technique is developed to in situ monitor the crack extensions with micrometer resolution. It is found that even at the trace level of 3 micro-gram/cm2, the interfacial fracture toughness is reduced by more than 35%. The surface topography of the fractured interfaces is further examined by surface profilometer. A clear transition from very rough fractured surface with fiber/matrix pull out, to very smooth fractured surface with interface failure is observed with the increased level of contamination. This transition of fracture surface topography testified the proposed cohesive model. Finite element analysis with cohesive zone model is used to rationalize the experimental results and understanding the mechanism of contamination degradation. Double cantilever beam model with various adhesive bonding parameters were tested. The interfacial cohesive parameters, the adhesive properties and the thickness of the adhesive layer were examined. The results show the parameters effect on the process zone propagation and the adhesive bonding toughness. The relation of process zone size and the bondline parameters were examined and compared with the existing double cantilever beam results. The finite element work showed that the contamination-induced degradation of the interfacial adhesion strength is the primary effect in mode I fracture. To fully understand the contaminations effect on adhesively bonded joints, mode II fracture test is conduced. Single shear lap test shows the contaminations has softening effect could strengthen the adhesive bonding initiation force. Further simulation work shows the detailed process zone propagation. It is shown that the contaminates effect on the adhesive matrix hardness becomes the primary effect for the adhesive debonding

Enhancement of Local Buckling Behaviour of Steel Structures Retrofitted Through Bonding GFRP Plates

An effective technique involving the use of glass fiber reinforced polymer (GFRP) plates to enhance the local buckling behaviour of steel plates, beams, and moment resisting frames is presented in this Thesis. The enhancement in buckling capacity is achieved by bonding GFRP plates to the steel sections. These steel/GFRP joints have the advantages of ease of application, low cost, high strength-to-weight ratio, and resistance to corrosion. An interface element that simulates the behaviour of the adhesive bonding the steel and GFRP elements is developed and is implemented into an in-house developed finite element model to represent steel/GFRP joints. The model is based on a powerful nonlinear shell element that is capable of simulating both thin and thick-walled structures. The strength and stiffness of both the GFRP and the adhesive used in the model are based on values obtained from previously conducted tests. The enhancement in buckling capacity of retrofitted steel/GFRP plates is studied by bonding GFRP plates to steel plates having different aspect and slenderness ratios. The study also considers the effect of initial geometric imperfections on both the elastic and inelastic buckling capacities of retrofitted plates. Better improvement in load capacity is predicted for slender steel plates. The strength of the adhesive is shown to play an important role in defining the mode of failure and in determining the capacity of the retrofitted plates. The improvement in buckling behaviour of retrofitted steel/GFRP beams is then studied considering various thicknesses of GFRP plates. The conducted analysis covers a range of slenderness ratios of steel beams and assesses the effect of plastic modulus of steel, initial geometric imperfection, and residual stresses of the steel section on the load-deflection behaviour of steel beams. The lateral behaviour of moment resisting steel frames retrofitted with GFRP plates is studied to assess their capacity improvement in seismic regions. Nonlinear static pushover analyses are carried out for frames retrofitted at their beams’ flanges with different thickness of GFRP plates. The global capacity curves for the retrofitted frames are compared with their corresponding original frames to assess the improvement in seismic performance of the frames. Finally, an experimental investigation is carried out to assess the strength and stiffness properties of adhesively bonded steel/GFRP joints under cyclic loading. A number of shear lap tests are conducted and the obtained results are used to determine the characteristics of spring systems that simulate the shear and peel behaviour of the adhesive. Comparison is made between the stiffness and strength capacity under cyclic loading to the corresponding values under monotonic loading

Modelling the bond slip behaviour of FRP externally bonded to timber

Recent studies and applications have demonstrated that Fibre Reinforced Polymer (FRP) has become a mainstream technology for the strengthening and/ or rehabilitation of ageing and deteriorated structures. However, one of the main problems which limit the full utilisation of the FRP material strength is the premature failure due to debonding. This research study presents 1) a review of available FRP-to-timber and FRP-to-concrete bonded interface models, and 2) investigates factors affecting bond strength. A stepwise regression method has then been employed to evaluate the influence of potential factors on the bond strength. The proposed stepwise regression model is based on 195 experimental results of FRP-to-timber bonded interfaces. Results of this stepwise regression analysis are then assessed with results of pull-out tests and satisfactory comparisons are achieved between measured failure loads (R2=0.59) and the predicted loads (R2=0.71, P<0.0001)

Application of SH and Lamb Wave Emat’s for Evaluation of Adhesive Joint in Thin Plate

The applicability of SH wave for the evaluation of adhesive joint in thin plate was studied. The advantages of the SH guided wave is that its displacement and stress are oriented parallel to the adhesive-adherent interface, and therefore, it can be used to evaluate interface properties. The experimental studies are focused on the relations between acoustic parameters and geometry conditions, the comparison of ultrasonic data and strength data of the joint, and the attenuation in different cases. The experiments were done on lap-shear samples and long samples. For excitation and reception of SH waves, non-contact electromagnetic transducers were used. An additional investigation was carried out using Lamb waves for the same sample parameters

Quantitative peel test for thin films/layers based on a coupled parametric and statistical study

The adhesion strength of thin films is critical to the durability of micro and nanofabricated devices. However, current testing methods are imprecise and do not produce quantitative results necessary for design specifications. The most common testing methods involve the manual application and removal of unspecified tape. This overcome many of the challenges of connecting to thin films to test their adhesion properties but different tapes, variation in manual application, and poorly controlled removal of tape can result in wide variation in resultant forces. Furthermore, the most common tests result in a qualitative ranking of film survival, not a measurement with scientific units. This paper presents a study into application and peeling parameters that can cause variation in the peeling force generated by tapes. The results of this study were then used to design a test methodology that would control the key parameters and produced repeatable quantitative measurements. Testing using the resulting method showed significant improvement over more standard methods, producing measured results with reduced variation. The new method was tested on peeling a layer of paint from a PTFE backing and was found to be sensitive enough to register variation in force due to differing peeling mechanisms within a single test

Bonding Evaluation of Graphene-Oxide Layers on Flexible Substrates

Flexible electronics are getting interest in development of field effects transducers such as biomedical health screening tools, structural health monitoring in infrastructures, aerospace, vehicular industries and cell phones. As a promising candidate for flexible electronics, graphene-based devices have been developed through exceptional electrochemical and thermomechanical properties of graphene. Reducing the graphene oxide enables creation of large scale devices through its high manufacturability. Same as other types of electronics, the bonding of sensing units to the substrate is significantly dependent to the deposition method used for the fabrication of device. In this study, the mechanical strength of reduced graphene oxide (rGO) layers on the polymeric substrates is evaluated while the rGO layers are deposited by drop casting on the substrate. The tape test is adopted to measure the failure strength at the interface of rGO layers and substrate. To achieve a consistent and repeatable measurement of peel force, a new design of peel test fixture is suggested to control effective parameters on the peel test and keep constant the peel rate and angle. The new design of peel test has shown low coefficient of variation of about 8% for peel force measurement, which is much lower than 37% reported by ASTM standard for the tape test. Employing an image processing technique, a geometric analysis is conducted to identify the contributions of cohesive and adhesive failures in overall peel force. A mathematical method is developed to connect the geometric analysis result from the image processing to the experimental peel force measure. As a result of mathematical method, the magnitude of cohesive and adhesive energies are identified. Performing analysis of variation (ANOVA) on the bonding energies, the significant parameters of thermal processing on the bonding strength of rGO layers and substrates are determined so that the concentration of GO solution has illustrated as the most significant factor. The surface treatment duration for GO and substrates are the next priorities of significant factors. In this study, the mechanical strength and performance of rGO-based electronics were evaluated based on a new methodology for the peel test. The Kapton has demonstrated the best performance and is served as the best candidate for the fabricating of rGO-based electronics based on thermal processing. The PDMS showed high potential for being a suitable candidate for graphene-based electronics. Considering low surface energy of Teflon (PTFE), it would be viable candidate for transfer printing of graphene-based sensors. Despite the rGO layers showed very low adhesion boding to the Teflon substrate, the rGO layers on the Teflon had shown good uniformity itself

Modeling the mechanics and failure of discrete viscoelastic fiber networks

University of Minnesota Ph.D. dissertation. October 2018. Major: Mechanical Engineering. Advisor: Victor Barocas. 1 computer file (PDF); ix, 152 pages.Network problems arise in all aspects of bioengineering, including biomechanics. For decades, the mechanical importance of highly interconnected networks of macromolecular fibers, especially collagen fibers, has been recognized, but models at any scale that explicitly incorporate fiber-fiber interactions into a mechanical description of the tissue have only started to emerge more recently. The mechanical response of networks shows an inherent non-linearity arising from the network architecture, and the non-affine deformations occurring within it. Thus, the overarching goal of this dissertation was to model the steady-state and time-dependent behaviors of discrete fiber networks to understand better how the behavior of an individual fiber differs from that of a network, and to study the effect of a network’s structure on its mechanics. First, viscoelastic relaxation of networks composed of linear viscoelastic fibers was analyzed, throwing light on two different contributions to the network re- laxation process: a material contribution due to the intrinsic viscoelasticity of the fibers, and a kinematic contribution due to the structure of the network. The effect of network composition on its relaxation spectrum was also analyzed revealing a constant evolution of structure-dependent characteristic relaxation times with changing composition. Next, network fatigue behavior was modeled using a fiber-based cumulative damage model to obtain stress-life (SN) curves for the network, and to compare fatigue behaviors of different network structures. Finally, the network model was used in a multiscale finite element approach to model actin-myosin motor-driven cell cytoskeletal contraction. The multiscale model was also used to highlight the importance of the choice of microstructure in predicting tissue pre-failure and failure behaviors