The Charpy impact tests were carried out at different temperatures and loading rates. The temperature dependences of crack initiation and propagation in carbon steels 45 and St. 3 under impact testing were determined from the obtained force variation plots. The effect of the impact velocity in the range from 1 to 4.4 m/s on the fracture toughness temperature dependence is estimated.Keywords: carbon steels, Charpy specimen, instrumented impact testing machine, fracture toughness, crack initiation and propagation energy, ductile-brittle transition temperature.Introduction. One of the main characteristics of materials in the assessment of the strength and reliability of structures and constructions is the ductile-brittle transition temperature (DBTT). In order to determine it, various testing methods are used, of which the Charpy impact testing is the most simple and widespread. Usually, as a result of such testing, the temperature dependences of fracture toughness are determined, and by their processing, there are determined the ductile-brittle transition temperatures [1][2][3]. In so doing, it should be taken into account that the fracture toughness is an integrated characteristic that includes the specimen deformation energy, crack initiation and propagation energy.The use of instrumented impact testing machines makes it possible to record force versus time plots during testing and thus to get much more information on the material behavior [1,[4][5][6][7]. The use of modern high-speed digital recording systems increases considerably the capabilities of processing of the obtained data. These data permit analyzing various approaches to determination of the ductile-brittle transition temperature and the energy spent at different stages of deformation and fracture of Charpy specimens under impact testing [2].The goal of the present work was to study the behavior of carbon steels, such as St. 3 and 45, at different temperatures and loading rates using an instrumented vertical impact testing machine.Testing Procedure and Processing of the Results. Impact testing of steels was carried out using an instrumented vertical impact testing machine equipped with a multichannel system for high-speed recording of forces and strains (the sampling frequency is 20 MHz) and a system for cooling and heating specimens in the temperature range T = from −150 to 400°C [8]. Standard Charpy specimens of steels St. 3 and 45 of 55 10 10 × × mm in size served

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