Dynamic Testing of Lime-Tree (Tilia Europoea) and Pine (Pinaceae) for Wood Model Identification

The paper presents the results of dynamic testing of two wood species: lime-tree (Tilia europoea) and pine (Pinaceae). The dynamic compressive tests were carried out using the traditional Kolsky method in compression tests. The Kolsky method was modified for testing the specimen in a rigid limiting holder. In the first case, stress–strain diagrams for uniaxial stress state were obtained, while in the second, for uniaxial deformation. To create the load a gas gun was used. According to the results of the experiments, dynamic stress–strain diagrams were obtained. The limiting strength and deformation characteristics were determined. The fracture energy of lime and pine depending on the type of test was also obtained. The strain rates and stress growth rates were determined. The influence of the cutting angle of the specimens relative to the grain was noted. Based on the results obtained, the necessary parameters of the wood model were determined and their adequacy was assessed by using a special verification experiment.

DEVELOPMENT OF DIRECT IMPACT METHOD FOR DETERMINING DIAGRAMS OF DEFORMATION OF ELASTOPLASTIC MATERIALS AT LARGE DEFORMATIONS

The description and results of the analysis of the experimental method that implements the so-called direct impact scheme built on the basis of the measuring bar technique are presented. This method is used to determine the deformation diagrams of viscoplastic structural materials under conditions of high-speed compression at higher strain rates than in the traditional Kolsky method. Particular attention is paid to a variant of the method in which a striker of the same diameter as the measuring bar is used. In this case, it becomes possible to use equipment of the standard test scheme according to the Kolsky method (in particular, loading devices). Thus, a system of basic testing of materials in a wide range of strain rates is built on a single hardware platform. A numerical analysis of this method is performed. A comparison is made of the characteristics of the sample deformation calculated directly in the numerical simulation and determined indirectly from the information from the measuring bar available in the full-scale test. It is shown that the deformation of the sample is determined precisely only on the time interval corresponding to the double run of the elastic wave along the impactor. A modification of the algorithm for processing the information obtained in this experiment is proposed, taking into account the interference of elastic waves in the impactor and allowing expanding the strain range for which this experiment provides reliable information about the material deformation curve. The work of the technique is demonstrated by the example of testing a sample of M1 copper. For this material, using the direct impact method, a deformation curve is constructed at a strain rate of the order of 104 s-1. It is shown that the traditional experimental data processing scheme gives unphysical overestimated values of the sample deformation.

COMPREHENSIVE STUDY OF THE PROPERTIES OF FINE-GRAINED CONCRETE UNDER DYNAMIC TENSION AND SHEAR

An experimental study of the dynamic properties of fine-grained concrete under tensile and shear stresses has been carried out. A comprehensive study of the dynamic properties of concrete allows obtaining experimental data and constants. These data and constants are needed to equip dynamic strain and fracture models. The system of experiments is based on modifications of the Kolsky method. This system of experiments allows one to determine a wide range of strength and time characteristics of concrete under shock loading. The experimental setup according to the Kolsky method includes a system of measuring rods between which a test sample is installed, a gas gun for accelerating a cylindrical striker, strain gauges, a speed meter, recording and computing equipment with a software package. Conducted dynamic tests of concrete in a wide range of deformation rates (102–104 s–1). In experiments on direct tension and to determine the ultimate tensile stress of fine-grained concrete during splitting, two speed modes are implemented. In tests of shear strength, a modification of the Hopkinson split bar method was used to determine the dynamic shear strength, in which a sample made of the material under study is located in a rigid holder cut at an angle to the sample axis. The pulse recorded in the reference dipstick is used to plot the shear stress in the sample over time. Shear tests were carried out for three speeds. The results of the performed experiments on tensile, splitting and shearing are presented in the form of diagrams of stress versus deformation and stress versus time. The obtained characteristics can be used to equip computational experiments in the study of the strength of concrete structures subjected to dynamic influences.

Investigation of mechanical properties of limesand brick under dynamic loading

The results of experimental study of mechanical properties of samples of lime-sand brick under dynamic loading are presented. The tests were carried out using the traditional Kolsky method and its modification - dynamic splitting (the so-called «Brazilian test»). The laws of change in strength, strain, time properties and energy intensity of the investigated material are established in the strain rate range of 5·102-2.5·103 s-1 under compression and in the stress rate range of 2·101-3·102 GPa/s under tension.

Use of the Kolsky method for dynamic tests of brittle media

The dynamic test techniques used to obtain the mechanical properties of brittle materials are described. The techniques are based on the fundamental Kolsky method using the Split-Hopkinson Pressure Bar. Dynamic tests are characterized by high intensity and short duration and the influence of inertia on their results should be ruled out which is especially important for brittle media. The prerequisites and assumptions presented methods are described to justify the validity of the obtained data. The methods allow to obtain dynamic deformation diagrams at compression, splitting, indirect tension, shear and triaxial stress state and also to determine the ultimate strength, strain and time properties of brittle materials. The techniques are approved in the tests of ceramic bricks, finegrain concrete and fiber-reinforced concrete.

Numerical analysis of influence of length of the working part of specimen on dynamic diagrams of constructional materials obtained by the Kolsky method

It is known that experiments with intensive dynamic influences and high-speed deformation of specimens under tension are important in the analysis of deformations and strength of structures and their elements. The most reasonable and widely used method for obtaining dynamic stressstrain curves, characteristics of ultimate strength and deformations is the modified Kolsky method. It should be noted a small number of publications on the analysis of the influence of fillets and the length of the working part of the specimen on the most important mechanical characteristics obtained by this method. This work is aimed at increasing knowledge in this area. By numerical and experimental analysis it was established that when the working part of the specimen has a length of 5 mm, then the uniformity and one-dimensionality of the stressed state are seriously violated. When choosing the length of the specimen in the Kolsky method, there are important limitations: the length of the specimen must be much smaller than the length of the loading pulse. Therefore, in this study, two specimen lengths of 10 and 15 mm were analyzed. In this paper, we give experimental and calculated diagrams and their comparison with each other. It was shown, that for the above-mentioned lengths, the homogeneity and one-dimensionality of the stressed state is substantially improved.