Adhesives and adhesively bonded structures are being considered as a viable alternative to conventional fastening methods. In order to gain wider acceptance, it is essential to address the issue of the mechanical characterization of adhesive materials and its implementation in the design of bonded joints. While measuring the in-situ properties of the adhesive in a joint is a difficult task, characterizing its bulk properties is a relatively simpler undertaking. The objective of this study was to propose and verify an experimental procedure that would allow the analytical prediction of the viscoelastic behaviour of a bonded joint, using bulk adhesive properties. The Arcan joint geometry was chosen because of the simple state of stress within the adhesive. / Master of Science

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

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Siew-Hoon Lee. / M.S.

Introduction of a composition gradient at a join between two materials of different elastic properties should reduce the stress concentrating effect of the interfacial discontinuity. A crack oriented perpendicular to this elasticity gradient will experience mode-mixity, and possible subsequent crack deflection. Explicit analytical solutions for the stress state at the tip of an angled crack in a graded material of a given finite geometry do not exist, and ongoing crack path development in such a gradient has not been characterised. An infiltration processing technique is developed which allows two materials to be joined through a region of graded composition, of tailored width and composition profile. Composition discontinuities at layer interfaces in a stepped gradient can be tolerated due to the resulting interpenetrating network structured (INS) microstructure. Firing stresses were found to be a limitation of the processing technique, overcome by limiting the steepness of the elastic gradient. Alumina and epoxy resin graded composites were produced and tested under monotonic loading, resulting in stable crack path evolution. Stress-field asymmetry at the tip of a crack oriented perpendicular to an elastic gradient was demonstrated, followed by subsequent crack deflection. Stress intensity factor and deflection angle increase with increasing gradient steepness. Rising R-curve behaviour was demonstrated for all compositions of the INS composite, with initiation and plateau toughness decreasing with increasing epoxy content. Evidence of crack bridging by intact ligaments of the epoxy phase in the crack wake explains this behaviour. Crack deflection towards the epoxy region was anticipated and demonstrated for all gradient configurations. An increase in relative crack depth was seen to increase mode-mixity at the crack-tip and subsequent crack deflection, up to a relative depth of ~0.5. No conclusive evidence was found for the influence of crack bridging on crack deflection. Toughness was shown to increase with the inclusion of a microstructural gradient. Measured toughness within graded samples was shown to be controlled by both the local composition and the volume of bridging ligaments in the crack wake. The optimum gradient should ??? extend over the widest region practical, ??? encompass the widest composition range possible, and ??? demonstrate extrinsic crack extension toughening.

Adhesives have presently reached a stage where they have become part of everyday life both in a professional sense as well as for household applications. They offer advantages that in many respects surpass other joining processes such as bonding of large areas, joining a wide range and dissimilar materials; and without the need for special tooling or operator training, that is often required by many other joining processes. They are of course not a panacea to all fastening applications, but they can easily be described as the most versatile and most widely used joining method at present. Engineering applications have also benefited from the advantages offered by adhesives, but they are not as liberally used due to the severe consequences that may result from bond failure. Although adhesives can demonstrate their ability to fulfil the joining strength requirements under laboratory conditions, their application in industry proved to be not as reliable as expected. A number of parameters that can easily be controlled under laboratory conditions such as temperature, humidity, surface preparation and uniform adhesive application are not as easily observed in industry. Quality assurance during manufacturing can achieve excellent results; however even in these cases the probability of having adhesive bond defects is still present. Therefore, there is a need for post process inspection of adhesive bonds where risk levels require higher reliability than what is offered though process quality control. Adhesive bond inspection is a well researched area with respectable outcomes. Non destructive inspection techniques such as x-ray, thermal, and ultrasonic are well utilised in the inspection of adhesive bonds. However, despite all the effort in this area for more than forty years, there is still no singular technique that can achieve the confidence level required in some engineering applications. Therefore, the need for continuing research in the area of non-destructive evaluation of adhesive bonds is as necessary today as it�s ever been. The research presented in this thesis, continues in the same endeavour as many other researchers; that of achieving the ultimate technique in adhesive bond inspection, capable of reaching the confidence level required for all engineering applications. The research in the thesis commenced with coverage of adhesives used for engineering applications and a study of the adhesion science that was considered necessary to enable an informed approach to the problem. Adhesive bond failure is also analysed through a literature survey as well as experimental tests on standard specimens. At the completion of the literature survey and preliminary tests, a decision was taken to follow the ultrasonic path of non-destructive testing of adhesive bonds. The reasons for this, are clearly outlined in the main body of this thesis but in summary, the literature has shown that ultrasonic evaluation is the most widely used technique by industry. Therefore, improvements on data analysis using existing techniques that exploit ultrasonic inspection have the potential to reach the widest spectrum of industrial applications. Ultrasonic inspection equipment was sourced that was capable of achieving experimental results to the accuracy level required in this research. A precision test rig was designed and constructed that was subsequently calibrated using computer based statistical techniques to ensure the validity of all results. Other ancillary equipment, such as a portable tensile testing device were also designed and constructed during the research as it became necessary. Research concentrated on techniques found to be inadequately researched in this domain. The first technique evaluated was to measure bond quality through the stress distribution in adherent and adhesive. Computer based Finite Element Analysis showed that the ability to detect variation in stress distribution at the adhesion interface is capable of revealing the local bond strength. Having found that there is no technique available at present that can measure the stress distribution at the interface, a different direction was taken that showed potential in achieving excellent quantitative results in the analysis of ultrasonic signals from adhesive bonds. This technique was rigorously evaluated and the results are systematically reported in this work.

published_or_final_version / Industrial Engineering / Master / Master of Philosophy

Introduction of a composition gradient at a join between two materials of different elastic properties should reduce the stress concentrating effect of the interfacial discontinuity. A crack oriented perpendicular to this elasticity gradient will experience mode-mixity, and possible subsequent crack deflection. Explicit analytical solutions for the stress state at the tip of an angled crack in a graded material of a given finite geometry do not exist, and ongoing crack path development in such a gradient has not been characterised. An infiltration processing technique is developed which allows two materials to be joined through a region of graded composition, of tailored width and composition profile. Composition discontinuities at layer interfaces in a stepped gradient can be tolerated due to the resulting interpenetrating network structured (INS) microstructure. Firing stresses were found to be a limitation of the processing technique, overcome by limiting the steepness of the elastic gradient. Alumina and epoxy resin graded composites were produced and tested under monotonic loading, resulting in stable crack path evolution. Stress-field asymmetry at the tip of a crack oriented perpendicular to an elastic gradient was demonstrated, followed by subsequent crack deflection. Stress intensity factor and deflection angle increase with increasing gradient steepness. Rising R-curve behaviour was demonstrated for all compositions of the INS composite, with initiation and plateau toughness decreasing with increasing epoxy content. Evidence of crack bridging by intact ligaments of the epoxy phase in the crack wake explains this behaviour. Crack deflection towards the epoxy region was anticipated and demonstrated for all gradient configurations. An increase in relative crack depth was seen to increase mode-mixity at the crack-tip and subsequent crack deflection, up to a relative depth of ~0.5. No conclusive evidence was found for the influence of crack bridging on crack deflection. Toughness was shown to increase with the inclusion of a microstructural gradient. Measured toughness within graded samples was shown to be controlled by both the local composition and the volume of bridging ligaments in the crack wake. The optimum gradient should ??? extend over the widest region practical, ??? encompass the widest composition range possible, and ??? demonstrate extrinsic crack extension toughening.

Thesis (PhD) - Swinburne University of Technology, Faculty of Engineering and Industrial Sciences, 2005. / Thesis submitted to Swinburne University of Technology, for the degree of Doctor of Philosophy, 2005. Typescript. Bibliography p. 144-149.

Thesis (Ph.D.)--University of Wollongong, 2007. / Typescript. Includes bibliographical references.

Thesis (MEng) - Swinburne University of Technology, Industrial Research Institute Swinburne - 2007. / A thesis submitted for fulfillment of the requirement for the degree of Master of Engineering, Industrial Research Institute, Swinburne University of Technology - 2007. Typescript. Includes bibliographical references (p. 168-193).