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• The principle of virtual energy (PVE) is established based on the modern thermodynamics for the accurate prediction of material strength. • Some examples are presented to show the universality and feasibility of the principle, demonstrating its excellent prospects for all engineering applications. • It is found that material strength may not be an intrinsic property of the material but rather a property of the material specimen. As a matter of safety, the reliable prediction of the structural failure of materials is the most important key issue for various types of engineered structures. Traditional theories of material strength mainly rely on empirical formulas. Although there are a variety of strength theories available in the literature, they are largely limited by the materials and working conditions of their application. To ensure safety, engineering structures are often designed with large safety factors, resulting in a significant use of material resources. However, this still cannot eliminate the occurrence of catastrophic accidents. Recently, the author developed a thermodynamic framework, referred to as the thermodynamic strength theory (TST) [Eng. Fract. Mech., 2021, 254: 107936; Adv. in Mech., 2023, 53: 693–712.], to accurately predict the failure strength of engineering material structures. In this paper, the principle of virtual energy (PVE) is established based on the modern thermodynamics for the practical application in engineering. For a given imaginary small change in the key state variables of a material system, one can establish a failure strength criterion for the loaded material structure by investigating the induced change in the free energy. Some examples are presented to show the universality and feasibility of the principle, which demonstrates its excellent prospects for all engineering applications. [ABSTRACT FROM AUTHOR]