Canadian machinery vibration association EFFECT OF RESIDUAL STRESSES ON MODAL PARAMETERS OF WELDED STRUCTURES.

In the production of welded structure, residual thermo-elastic stresses are generated with peak level over 200 Mpa. The stress level and spatial distribution depends on growth speed, wall thickness, and temperature gradients present during growth. Hole drilling technique, contour method, X-ray diffraction has been used with some success for measuring residual stresses. As a result there exists a need for non-destructive in-line or off-line monitoring of residual stress. In this paper, we present a new experimental method based on modal parameters measurement to diagnose residual stress. The natural frequencies and modes are measured before and after the welding process and after the application of a relaxation technique. It is shown that vibration data correlates reasonably well with the stress data, accordingly with the considered modes shapes. The method has been applied in industry by measuring the first three natural frequencies of an hydraulic turbine during it’s production before and after thermal treatment (600 C degrees). Residual stresses have been recorded during this process by using suitable welded strain gages