Study of the structure and mechanical properties of annual rings of pedunculate oak (Quercus robur) using nanoindentation and scratch test methods

Abstract

Optical methods and optical properties are usually used to research the structure of wood and its ring structure. However, these properties are not directly related to its mechanical and other physical characteristics. To study them, methods of x-ray densitometry, synchrotron radiation, nuclear magnetic resonance, etc., which are not very common in wood science, are used. These methods are quite labor-intensive and require expensive equipment. In this regard, there is a need to develop simple and convenient means and methods for studying the micromechanical properties of wood. The main goal of the work is to develop such an approach using nanoindentation and digital scratching of a cross section of wood and to identify its potential in the further development of dendrochronology and related disciplines. Using the NI method, radial dependences of hardness H and Young's modulus E were obtained for eleven consecutive annual pedunculate oak (Quercus robur L.) wood rings for 3 different loads Pmax = 2, 100 and 500 mN. The values of H in the range from 70 to 340 MPa and Young's modulus E in the range from 2 to 10 GPa were determined for the corresponding loads and early (EW) and late wood (LW). Using the scratch test method, profiles of the normal force Fn and the corresponding hardness HS (in the range from 53 to 225 MPa) were obtained for the period 2007-2020. According to both methods, the widths of annual rings were determined; the discrepancy between the values and the optical method was < 3 %.