DYNAMIC COMPRESSIBILITY OF BIRCH AT DIFFERENT TYPES OF STRESS-STRAINED STATE

The results of dynamic tests for compression across the fibers at room temperature of birch samples with air humidity are presented. Dynamic tests were carried out on a setup with a split Hopkinson bar according to the Kolsky method at a strain rate of ~2000 s–1. To assess the effect of the type of stress-strain state on the behavior of the material, in addition to specimens in the form of cylinders with its free expansion during loading (uniaxial stress state condition), specimens were tested in a rigid casing that prevents the radial expansion of the specimen (uniaxial deformation condition), as well as local compressive tests of rectangular board fragments. In the latter case, the material surrounding the loading zone plays the role of a compliant confining casing. In this case, a certain intermediate stress-strain state is realized in the sample. For these three types of stress-strain state, dynamic deformation diagrams were obtained with registration of additional loading cycles. Comparison of obtained deformation diagrams shows a significant effect of the type of stress-strain state on the behavior of the material under study. In the case of free expansion of the specimen in the radial direction, the absence of strain hardening is observed in the first loading cycle. In subsequent cycles, hardening is negligible. The deformation diagrams of specimens in the casing as well as board fragments are characterized by a noticeable increase in the modulus of the hardening branch with increasing deformation. In this case, it can be noted that the behavior of the material in the case of testing a piece of board is intermediate between the cases of uniaxial stress state and uniaxial strain state. Some mechanical characteristics of the material are determined using the diagrams obtained. The obtained experimental results can serve as the basis for the subsequent identification of the model of deformation and destruction of wood.