The dissertation work provides solution to an important scientific and practical problem of predicting the strength of adhesive joints of thermally modified ash wood with unmodified pine wood glued with polyvinyl acetate adhesives with durability class D4, which is important for gluing wood and predicting the strength of the adhesive joints. Problems in gluing such constructions arise due to physical and mechanical properties, which are not the same for thermally modified wood and unmodified wood. To be exact, the increased temperature during the modification of ash wood leads to the destruction of the main components, namely cellulose and hemicellulose and to an increase in the amount of lignin. In addition, at elevated temperatures, furfural is formed in modified wood, which reduces the adherency of adhesive materials to the surface of such wood. To study changes in the strength of such adhesive joints, a long-term method and an accelerated research method are used. The long-term method allows obtaining real results and evaluating the change in the strength of wood adhesive joints during their operational use, taking into account the influence of atmospheric factors. The accelerated method makes it possible, in a short period of time, to investigate the effect of the ambient temperature and humidity on the strength of adhesive joints and to confirm the reliability of the results obtained using the long-term method. For these studies, two types of wood were used, namely: thermally modified ash wood and unmodified pine wood, and thermoplastic polyvinyl acetate adhesive with durability class D4 was used. According to the results of long-term experimental studies, it was found that the strength of thermoplastic polyvinyl acetate adhesive joints between thermally modified ash wood and unmodified pine wood decreased by 27.9% over two years of testing. In real terms, the change in strength decreased from 7.12 MPa to 5.13 MPa. Namely, after 3 months - by 11%, after 6 months - by 7.8%, after 9 months - by 15%, after 12 months - 18%, after 15 months - 17.5%, after 18 months - 16 .7%, after 21 months – 21%, and after 24 months – by 27.9% , of the strength of the control specimens. As a result of accelerated experimental studies, it was found that the strength of adhesive joints satisfies the standard requirements according to which these experiments were carried out. Namely, the strength decreased from 7.12 MPa (control specimens) to 3.90 MPa after cyclic temperature-humidity tests. Based on the results of long-term experimental studies, a mathematical model was built for predicting the strength of adhesive joints between thermally modified ash wood and unmodified pine wood. Prediction with using a mathematical model is the most common and promising predicting method, since it takes into account the variable of elastic-deformation processes in the adhesive joint during exposure to humidity and temperature. By using this mathematical model, it is possible to predict the strength of such adhesive joints depending on their operating conditions, namely, the ambient humidity and temperature. Predicting was carried out for twenty-four months, according to the duration of the experimental studies.
