Improving the Fatigue Behaviour of PURMAL S-70 Polyurethane Paint

Abstract

Oxidation is one of the main degradation mechanisms that affects most industrial components and occurs as a result of a chemical reaction by which an oxygen atom is added to the molecule of an organic substance or compound. Light, in particular the ultraviolet (UV) component, activates the process of degradation of organic materials, favouring a series of oxidation reactions that occur when the component is in contact with oxygen in the air or water (especially salt water).Given that a number of components work in variable mechanical stress conditions (vibrations caused by the technological process itself or generated by repeated starts / shutdowns of equipment), the protective coatings aim to improve the physico-chemical properties of the surfaces to reduce the degradation, as well as to improve the fatigue behaviour, knowing that the fatigue cracks are initiated from local stress concentrators caused by the surface imperfections of the components. Paper presents the experimental results on fatigue behaviour of the deposited layers, after exposure to UV radiation. In the experimental program, a 2 mm thick AlMg3 sheet (EN AW-5754) was used as substrate. The Aluminium oxide layer naturally created on the metal surface, prevents the effect of oxygen and atmospheric pollutants, but this layer is unstable in corrosive environments. Thus, for a qualitative protection, protective organic layers are used. In experiments, the polyurethane paint PURMAL S 70, RAL 9010 (PUR) and the same paint but aditivate with graphene oxide (PUR + GO) were compared. Prior to fatigue testing, the painted specimens were subjected to accelerated artificial UV-aging process with ultraviolet radiation for 72 hours. Axial fatigue tests were performed with a frequency of 40 Hz, in pulsating mode, with R = 0.053 and with decreasing loads, starting from 0.85% of the tensile strength (Rm), up to 0.5% Rm. Analysis of S‑N curves revealed that, in the field of oligocyclic fatigue there are no significant differences between the fatigue results obtained on PUR-protected specimens, compared to those protected with PUR + GO, instead in the case of polycyclic fatigue, the paint with addition of 1% graphene oxide has improved fatigue resistance.