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[References: 20 tit.]

Cleanliness of the test surface is of particular importance in liquid penetrant testing. The surface cleanliness implies both the quality of surface cleaning from polluting substances, for example, oil, and the quality of its treatment, i.e. surface roughness. The surface roughness parameter increased to more than Rz = 20 ?m can lead to unreliable test results since each groove on a rough surface will be a capillary for liquid spreading. Therefore, it will be difficult to completely remove the penetrant from the surface of the test object while removing its excess, which causes an indicator background of the surface and complicates defect detection. The study aims to determine the effect of liquid wettability on liquid penetrant testing results depending on the surface roughness. The paper proposes a mathematical model of liquid spreading on the rough surface depending on physicochemical properties of the liquid itself and material properties of the test object. The numerical modelling of the process is performed. The modelling results are confirmed experimentally, and statistical processing of the experimental results is performed.