Abstract This paper presents an analysis of the literature with the aim of defining basic criteria and developing a general model to describe joint strength. Two particular cases of the relationship: cosθ = f(γL) have been identified as prerequisites for further analysis of interfacial phenomena and conditions governing their existence were discussed. The fact has been pointed out, based on available experimental results, that for the most important case in practice where 0.6 ≤ cosθ ≤ 1.0, the relation cos θ = f(γL) can be treated as rectilinear. This finding will be utilized in the comprehensive development of criteria defining joint performance in Part II. Variability of the interaction factor Φ for various systems has been investigated in relation to cos θ, for the identified particular cases of the relationship cos θ = f(γL) A special value of the interaction factor, 0, was found. The importance of the rectilinear particular case of cos θ = f(γL) was shown, which involves constant factor Φ0 instead of...