A crucial feature in health monitoring of already existing structures is to be seen particularly in identifying their topical internal structural parameters and controlling their remaining bearing capacity in the course of ageing processes. This is commonly carried out by measuring the deformations/strains caused by test-loading and calculating the parameters on the basis of the metered data.

In the case of elastic response of materials, the information on the parameters is directly related to the time of measurement; in the case of visco-elastic response, however, the history of the time-depending structural response during the period between initial loading and initiating the test-measurements is generally unknown. The problem exists, then, to separate the superimposed strains due to the existing state and to the test-load. For solving the problem, at first the relevant relations between stress/strain and the visco-elastic parameters are considered. Then a procedure will be described how to determine the strain state owing to the test-load only and to calculate the relevant parameters as functions of time. According to the principle of time-shift invariance, the results describe the time-depending response of the viscoelastic material, no matter at which time the loads are applied.

The presented method will be illustrated by two simple but instructive examples.

The observations on Myzus persicae and Macrosiphum euphorbiae development were carried out on boxthorn (Lycium halimifolium) growing in various places of the city ofWroclaw in 1999-2001. They concerned the following issues: aphid population dynamics and aphid age and morph diversity in population structure. Aphids infested only certain boxthorn bushes and other bushes were free of these insects. M. persicae infested more bushes than M. euphorbiae did. The first species appeared 7-10 days earlier on boxthorn than the second one. Time of the maximum population density of both aphid species was similar. Age structure of M. persicae and M. euphorbiae populations was similar. The youngest larvae (L,-L,) predominated and the least numerous were alatae females. Coccinellids were the most numerous aphid predators on boxthorn.