The scientists at the Institute for Problems Mechanical Engineering of the Russian Academy of Sciences, together with colleagues, found out that grade 20 steel, which is widely used in industry, stops deforming irreversibly after a certain number of stretching cycles with complete removal of the load and begins to work almost like an elastic spring — returning to its original state after each pressing. The authors created a mathematical model that predicts accurately when this transition occurs.

The results of the research, supported by agrant from the Russian Science Foundation, are published in the scientific journal International Journal of Plasticity.

Many parts of cars, airplanes and other equipment operate under cyclic loading conditions — when the impact on them increases or decreases. The material deforms gradually: excessive stresses occur in it, which lead to cracks and subsequent destruction of parts. Therefore, when designing any equipment, engineers need to be able to predict in advance exactly how the metal will react to cyclic loads. However, until now, there has not been a reliable model that could mathematically describe this. Therefore, specialists had to put an additional margin of safety in the parts, making them heavier and spending more material on manufacturing.

Scientists at IPMash RAS and St. Petersburg State University determined how samples of grade 20 steel, widely used in mechanical engineering, react to repeated loads, and created a mathematical model describing the material's transition from a deformation mode to a stable state. The authors of the study conducted experiments with steel rods similar to an ordinary pencil and dumbbell-shaped samples (with a narrowed middle part). In the dumbbell samples, the deformation processes were concentrated at the narrowest point, and therefore they were easier to track.

Using a press, the researchers subjected the samples to cyclic loads, changing the intensity of exposure and its frequency. It turned out that at first, during the first hundreds of load cycles, the material is plastically deformed, that is, with each subsequent application of a tensile load, its shape and size, at least slightly, but irreversibly change.

However, after 800-5000 cycles, depending on the load, the steel enters an exceptionally stable state in which plastic deformations no longer occur in it.

This means that with the subsequent application of a tensile load, the metal, if deformed, returns to its original state like a spring that was first stretched and then released. For engineers, this means that after overcoming the «stabilization threshold», the part no longer wears out plastically — its further operation becomes predictable and safe, and the resource is limited only by fatigue processes, which develop much more slowly.

«Instead of laying down an excessive margin of safety, which leads to a heavier structure and waste of excess metal, with the help of the new model, specialists will be able to calculate in advance the actual service life of the part and replace it on time. This means fewer accidents and sudden breakdowns of machinery, saving materials and, ultimately, saving the health and lives of people who work on transport or near industrial equipment. In the future, we plan to develop the model so that it takes into account the mutual influence of plasticity and crack growth over the entire life of the part, as well as expand the range of loads from low—cycle to multi-cycle fatigue. Our goal is to create a versatile and simple tool for engineers working on several fundamental constants», — said Nina Selyutina, Senior Researcher at the Laboratory of Dynamics of Extreme States and Structural Transitions at IPMash RAS.