Contribution to Evaluation of Mechanical Properties during Impact Loading

The behaviour and properties of a material during deformation is, among others, influenced by external factors, which also include the strain rate. The strain rate has a significant effect on the material behaviour during the deformation (forming) process 1,2 , as well as on the final properties of products 3,4,5 . Dynamic loading has also a significant effect on the degradation of the properties of products during their service life 6,7 . The investigation of the influence of the impact rate on the properties and behaviour of materials is experimentally very demanding. Therefore the literature has paid a great attention to a possibility to replace the strain rate with the temperature in the form of a relationship where a material property is a function of the temperature and the strain rate 8 . However, it appears that this approach makes it possible to predict the influence of the impact rate on the material properties only partially. This is mainly due to the non-homogeneous distribution of stress and strain in a product, which increases with an increasing impact rate. This non-homogeneity during impact loading is due to the effect of additional inertial-force fields, which are formed by accelerating or decelerating the mass of a structural unit. In case of tests, it is the mass and the grade of the test specimen material, the toughness of the test equipment, etc. that makes the interpretation of the test results very difficult. The literature 9 analyzes the temperature dependence of the impact toughness KCV during static (v=2.10 ms) and dynamic (v=5 ms) loading. In the subtransitional area, the KCV during static loading of the S 315MC grade was 20% lower than during dynamic loading and for the S 460MC grade it was 5% lower than during dynamic loading; this indicates that the grade of a material influences, besides its sensitivity to the strain rate, also the amount of energy lost during dynamic loading. The paper aims to analyze the behaviour of selected materials at various loading rates in terms of energy lost during impact loading, their sensitivity on the strain rate, and the final strength properties.