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SUMMARYViscoelastic line spectra are identified from creep or relaxation data of static experiments with different numerical methods, which may or may not depend on additional informations, to be provided by the user, about the unknown parameters. If the least square method is applied, a non-linear optimization problem with non-negative constraints on the parameters has to be solved. Its solution can be achieved directly by using a gradient-based optimization algorithm like the projected Newton method of Bertsekas. However, appropriate starting values for the unknown parameters must be chosen. The problem can be alleviated by dividing the identification task into three successive steps, based on the Tschebyscheff approximation and the quadratic optimization method by Wolfe. Alternatively, the identification task can be reduced to a quadratic optimization problem, if the user provides additional informations about the distribution of the respondance times of the spectra. The windowing-method of Emri and Tschoegl is based on this assumption. If the line spectrum is assumed to have equally distributed spectrum lines on the logarithmic axis, the identification problem can also be solved by standard regularization techniques, like the truncated singular value decomposition or the Tikhonov regularization.The choice of qualified respondance times as additional information requires some experience with the identification task at hand. Its results may be improved after several reruns of the algorithms. Various applications of the methods to test and experimental data are given and a comparison of their performance is discussed.

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Delamination in layered composites and debonding in adhesive joints are modeled and analyzed using interfacial mechanics, consisting of interface elements for the kinematical assumption and tractionseparation equations for the constitutive model. Material equations are presented for the inelastic behavior of pure and ductile-modified epoxy resins, used for the matrix phase of the composite and in a chemically modified form for the adhesive in bonded structures.Two different modeling approaches are proposed. The first is a brittle fracture model with a stressbased failure criterion and rate-dependent strength parameters together with a mixed-mode energy criterion for the interaction of the three different modes of failure. The second makes use of an elastic-plastic approach with a traction-separation equation for ductile materials and rate-dependent yield stresses.Standard tests for the delamination of layered composites under various modes of failure are simulated by making use of the interface element for bonding/debonding. The model for the inelastic behavior of a thin layer of the structural adhesive is validated up to fracture for various modes of failure due to pure and combined loading in the normal and shear directions. Therefore, the relevant part of the experimental setup for the testing is meshed with finite elements and the results of the simulation are compared to the corresponding test data.