Comparison of application results of two speckle methods for study multi-cycle fatigue of structural steel

The new method of time-averaged speckle images and the well-known method of speckle-field interference studied the development of plastic deformations occurring during multicycle fatigue of structural steel. The correctness of strain determination by the new method is evaluated by comparing the data obtained by the two methods. Two optical systems including laser modules with different wavelengths were investigated. The proposed optical system allows to determine the components Δuy, Δuz of the relative displacement vector of two surface points located at a distance (measurement base) Δs = 66 μm. The known scheme makes it possible to describe deformations by the traditional method on the base of 470 μm. The object of study was a flat specimen made of 09Г2С steel with two side notches. Fatigue tests were carried out on a resonance-type machine at different cycle amplitudes. It is shown that at all cycle amplitudes the development of plastic deformations occurs by the mechanism of cyclic creep. There is a good correlation between the data obtained by two different speckle methods. At the same time, the strain estimated by the new method is in some cases an order of magnitude higher than the strain calculated by the known method. Obviously, this is due to the existence of local smallsized (of the order of 101 μm) strain areas which cannot be measured by conventional methods. The ultimate tensile strain Δuy/Δs calculated by the new method is of the order of 10–1, which coincides with the similar strain occurring in tensile testing of standard specimens. The results obtained by the new method justify the need of developing sensors and nondestructive testing devices of a new generation, allowing to estimate the time to fatigue crack initiation by the rate of change of physical quantities and by their limit values.