Initially symmetric structural elements or units are widely used in machines and engineering objects (reinforced members of thin-walled structures, sections of shafts or pipelines, etc.). Damages, which can appear in such structures during operation, disturb their initial symmetry. This paper considers damage diagnostic procedures based on utilization of vibration effects caused by the distortions of system’s initial symmetry due to appearance of defects. Object of the study is a uniform viscoelastic fixed beam (e.g. span of a pipe) which is initially symmetric relative to the central section. In order to find possible defects, forced vibrations of the beam are excited with the aid of test harmonic force Psinωt applied in the middle section. Damage is simulated as a local reduction of beam’s bending rigidity in corresponding cross-section. The goal of the research is to find such vibration diagnostic signs, which will make it possible to detect damage, its approximate size and location with the highest sensitivity. Dynamics of the system has been analyzed using two different methods: modeling on the specialized analogue-digital computer system developed in Riga Technical University and numerical simulation with program ANSYS. New diagnostic procedures based on distortions of vibration flexural modes and frequency spectrums due to arising of defect are proposed. The main advantage of this approach lies in the more high detection sensitivity in comparison with traditional resonant frequency methods. It is shown, that further rise of detection sensitivity can be achieved by insertion of additional nonlinear element into the structure of testing object.