The application of optical interferometry to time dependent unbonding

Improved methods of adhesive joining for constructing structural elements have led to an increase in the use of bonding. Bonding is already widely used in the aerospace industry. It allows potential for greater weight and manufacturing cost savings when compared to mechanical fastening as well as providing better fatigue resistant capabilities. More accurate design or life prediction based on a better understanding of adhesive joint failure will allow far more extensive and safer use of adhesively bonded structures. The possibility of time dependent failure is introduced in bonding,by the fact that many adhesives are polymers and measurably viscoelastic under many circumstances. Failure can occur by unbonding at the adherend-adhesive interface or totally within the adhesive. Here the time dependent failure by unbonding at the interface is addressed as one initial investigation of the whole problem. In many engineering problems the modelling of failure has been based on linear fracture mechanics. The extent to which the use of the linear theory can be justified is often questionable in view of the nonlinearity of the local crack front deformation field. One aspect of the current investigation is therefore the measurement of the local crack front deformation to examine the limits of the validity for the linear theory and to provide a basis for future modelling, Within this framework, particular attention is paid to the aspects of local failure mode interaction and the time dependence of the unbonding process. The small displacements in the crack front region require a resolution on the order of a wavelength of light. The experimental techniques used are therefore based on optical interferometry. The displacements applied to the adherends are controlled to the same order by a novel, thermally actuated servoloading device. The thesis is divided into three distinct parts. The first part deals with the development of the experimental techniques used to measure the unbound profiles as well as to measure and control the displacements applied to the adherends. For stationary cracks, the second part describes experiments which examine the extent of the nonlinearity in the crack front deformation field while fracture criteria are developed and compared for unbonding rates in steadily propagating cracks. Finally, proposals for improvement of the experimental techniques and further experiments are discussed in the third part