This work offers the fundamentals of a new approach to the nature of strength and failure of metallic materials under the conditions of stress–strain state (SSS) of a general type, which occurs within the local zones of non-uniform force fields (NFF) due to the stress raisers (SR). Basic foundation of the physical and mechanical concept of structural strength of metals and alloys, σ_NF, is considered based on the comparison of the proposed strength margin (SM) parameter for two cases: ΔS for uniform force fields (UFF) and Δσ_N for non-uniform force fields (NFF) near the SR. Exhaustion of each type of SM means a transition to brittle fracture, which, for SM at NFF, manifests itself in a steep fall in structural strength: σ_NF < σ_0.2. Exhaustion of strength margins ΔS and Δσ_N may occur due to both physical (alloy structure, temperature conditions) factors and purely mechanical ones (type of SSS, SR), which cause embrittlement of metals. A special mechanical characteristic is proposed—break resistance b_r (basic one for the uniform SSS) and b_rN (structural one for the non-uniform SSS), the ratio between which governs the regularities of influence of strength σ_0.2 (conditional yield strength) and strength margin ΔS on such manifestations of brittleness of metals as cold brittleness, when the temperature is decreased, or ‘mechanical brittleness’ due to the SR at room temperature T_К. Examples of the application of parameters ΔS, Δσ_N and b_r, b_rN are given for the analysis of experimental evidence published by various authors. A conclusion is made about the effectiveness of using structural strength parameters in investigation of the nature of brittleness and embrittlement of metal alloys caused by their extra hardening, the temperature decrease, or the SR effect (cracks, etc.).