Stasyshyn I. Development of three-step phase-shifting interferometry method for monitoring the surface relief of structural elements

The new solution of the scientific and technical problem of monitoring the surfaces of materials and structural elements that work under cyclic loads and in working environment is proposed in the thesis. This solution is based on the analysis of the surface relief changes on nano- and microlevels. To solve this task, a three-step phase-shifting interferometry method and respective means of its implementation are developed that allow retrieving the surface relief in speed mode. It is proposed to analyze both the total retrieved surface relief and its components, in particular, roughness and waviness. A new algorithm of surface’s interferograms processing is elaborated for selection of these components. This allows reconstructing and analyzing surface reliefs of materials and structural elements with an error of no more than 5 nm. Based on the proposed method, recommendations for the creation and production of an optical 3D profilometer device have been developed. The created device possesses such characteristics as fast recording and retrieval of surface relief with 11-14 grades of finish according to ISO 25178-2:2012; extraction of the relief components, in particular, roughness and waviness, variable area of observation, determination of geometric parameters of roughness and waviness using arbitrary profile and (or) all observed surface area. The proposed method and corresponding equipment were used during the monitoring of objects subjected to fatigue cyclic loads. Due to this method and the created equipment, the fatigue process zone sizes near the stress concentrator were estimated. Additional analysis of the observed area roughness changes allows predicting the site of a fatigue crack initiation long before its appearance on the surface. Also with the help of the proposed interferometric means a number of experimental studies of the effect of hydrogen on the steel surfaces with different structures have been carried out. According to the results of experiments, it was found that the effect of hydrogen on steels with different structures is different. Blisters were created and the roughness increased on the surfaces of studied steels, however the blister sizes, their density and dynamics of changes were different for each material. Information on the time changes of the steel surfaces with different structures after the action of hydrogen on them is necessary to develop recommendations for materials to produce friction pairs and appropriate lubricants, as well as to predict the service life of structural elements operating in working environments. In order to predict the surface roughness parameters of titanium alloy details after their chemical-thermal treatment (nitriding and boriding), the influence of these technological processes on the change in roughness was studied. In particular, for titanium ВТ 1-0 it was found that the level of initial roughness and the temperature of the technological process also influenced on the change in roughness. The surface roughness of details before and after technological processes was estimated by non-contact method. It is established that the surface roughness after chemical-heat treatment increases on average by two classes.