This paper presents the results on the effects of die-casting process on the strength parameters of castings of the aluminium AlSi9Cu3 alloy

belonging to the group of EN AB-46000, made on renovated high pressure die-casting machine. Specimens for quality testing were taken

from the places of the casting most loaded during the service. The aim of a research was to prove how the new die-casting process control

capabilities influence on the tensile strength of the cast material defined as a value of the breaking force of the specimens. It has been

found that it is possible to specify a set of recommended settings valves of second (II) and third (III) phase, which are responsible for

filling the metal mould on die-casting pressure machine. From the point of view of the finished cast element, it was noticed that exceeding

the prescribed values of valve settings does not bring further benefits and even causes unnecessary overload and reduce the durability of

the mold. Moreover, it was noticed that reduction of the predetermined setting of the second phase (II) valve leads to the formation of

casting defects again.

In this work, the author implements concepts and methods of analysis of nonlinear elasticity theory in a simplified description of elastic-plastic properties of materials. Taking the principle of conservation of energy as the theoretical basis, the author formulates a criterion that makes it possible to examine the stability of internal equilibrium in deformed material whose nonlinear properties are defined by strain energy density function. The formulae allowing for assessment of complex states of strain in the aspect of material strength were derived on the assumption of small deformation. These formulae can replace mathematical relationships traditionally known as strength hypotheses. The example included in the paper presents the method of determining, in the space of strain state components, the areas where permanent deformation or destruction of material is possible because of strain state stability. Characteristic parameters used in the example are obtained in a static tensile test on specimen 01· constructional carbon steel of ordinary grade. The results of the analysis, based on the formulated strength hypothesis on stability of strain state, are compared with those resulting from the Huber's hypothesis on energy of non-dilatational strain.