A method for determining deformation causing failure of a ductile structural material subjected to various stress states and temperatures, using an assembly of an instrument and a specimen, in which a specimen of given material is subjected to twisting in a predetermined temperature, and further comprising the following steps: producing a specimen of a material to be tested; the specimen having an axis of symmetry and comprising a measuring part and opposite end parts and provided with a linear marker parallel to the specimen's axis of symmetry on a surface of the measuring part; placing the specimen in the instrument with one end part of the specimen secured immovable relative to the instrument and the other, opposite end part of the specimen, mounted in a rotatable manner and applying torque that induces twisting of the measured portion of the specimen, and fixing the instrument with the specimen with external retaining means; bringing the specimen to a predetermined test temperature; inducing in the specimen a state of pure shear by twisting it, when the predetermined temperature has been obtained and until fracture; measuring the angle of twist φ of the specimen's measuring portion corresponding to the specimen's fracture plane, in relation to displacement of the specimen's linear marker in the fracture plane and referring to the initial state before torsional deformation, determining the value of angle of shear γ 12 based on the angle of twist φ of the specimen's measuring portion, determining the value of coefficient a based on the value of angle of shear γ 12, according to the equation: 1 3 ⋅ γ 12 = a determining the value of limit failure strain ε f, corresponding to failure of the tested material in various stress states and at given temperature at which the specimen was subjected to twisting strain based on the equation: ε f = ae − 3 2 η where η is stress triaxiality coefficient. producing a specimen of a material to be tested; the specimen having an axis of symmetry and comprising a measuring part and opposite end parts and provided with a linear marker parallel to the specimen's axis of symmetry on a surface of the measuring part; placing the specimen in the instrument with one end part of the specimen secured immovable relative to the instrument and the other, opposite end part of the specimen, mounted in a rotatable manner and applying torque that induces twisting of the measured portion of the specimen, and fixing the instrument with the specimen with external retaining means; bringing the specimen to a predetermined test temperature; inducing in the specimen a state of pure shear by twisting it, when the predetermined temperature has been obtained and until fracture; measuring the angle of twist φ of the specimen's measuring portion corresponding to the specimen's fracture plane, in relation to displacement of the specimen's linear marker in the fracture plane and referring to the initial state before torsional deformation, determining the value of angle of shear γ 12 based on the angle of twist φ of the specimen's measuring portion, determining the value of coefficient a based on the value of angle of shear γ 12, according to the equation: 1 3 ⋅ γ 12 = a determining the value of limit failure strain ε f, corresponding to failure of the tested material in various stress states and at given temperature at which the specimen was subjected to twisting strain based on the equation: ε f = ae − 3 2 η where η is stress triaxiality coefficient.