Multiscale Mechanical Performance of Wood: From Nano- to Macro-Scale across Structure Hierarchy and Size Effects

Abstract

This review describes methods and results of studying the mechanical properties of wood at all scales: from nano- to macro-scale. The connection between the mechanical properties of material and its structure at all these levels is explored. It is shown that the existing size effects in the mechanical properties of wood, in a range of the characteristic sizes of the structure of about six orders of magnitude, correspond to the empirical Hall-Petch relation. This “law” was revealed more than 60 years ago in metals and alloys and later in other materials. The nature, as well as the particular type of the size dependences in different classes of materials can vary, but the general trend, “the smaller the stronger”, remains true both for wood and for other cellulose-containing materials. The possible mechanisms of the size effects in wood are being discussed. The correlations between the mechanical and thermophysical properties of wood are described. Several examples are used to demonstrate the possibility to forecast the macromechanical properties of wood by means of contactless thermographic express methods based on measuring temperature diffusivity. The research technique for dendrochronological and dendroclimatological studies by means of the analysis of microhardness and Young’s modulus radial dependences in annual growth rings is described.

A Comparative Study of Indentation Size Effect Models for Different Materials

“…where k0 is required load of producing the initial plastic deformation, and k1 is the load independent constant. Combining Equations ( 1)- (3) and hardness H = P/A, the H-K model is obtained as:…”

“…However, for service equipment, small size materials and expensive materials in aerospace, nuclear power, microelectronics, medicine and other fields, it is difficult to obtain the mechanical properties parameters by the traditional mechanical test. Due to the "no sample" and "micro damage", the indentation test has been proposed by Oliver and Pharr, and widely employed to calculate the elasticity modulus and flow stress of different materials [1][2][3][4][5]. The phenomenon of indentation size effect (ISE), which the measured hardness changes with indentation depth, have been observed in various materials such as metallic, ceramics, fibers and thin films [6][7][8][9][10].…”

Discoloration and Surface Changes in Spruce Wood after Accelerated Aging

“…Diversely shaped and assembled anatomical elements of wood represent a heterogeneous porous system. Besides this, species-dependent differences in pore shape, size, mutual linking, and arrangement are obvious [5,7]. The porous nature of wood results in a large interior area and high surface roughness-mostly negative in the context of aging.…”

“…Wood degradation caused by the effects of various environmental factors is a very intricate and effort-demanding issue that needs a comprehensive approach. For a long time, the research on wood resistance against aging has been oriented on the variability of wood surface structure and properties, on degradation causal agents, on possibilities of how to avoid the degradation, and on the study of a range of factors supposed to influence the aging of wood surfaces [7][8][9][10][11][12][13][14][15]. The results of several works dealing with this problem have been summarized in [1,2].…”