A review of pure viscoelastic creep and mechano-sorptive creep of wood

Wood, as one kind of fibre-composed material, consists of various cell types. The layer structure of cell wall and the chemical compositions of cell are diversity. Under constant stress, the deformation degree of wood increases with increasing time, i.e., wood creep. Wood creep plays an important role in the quality of wood products and engineering components, which is also the main design parameter for wood structures. According to the state of moisture, creep phenomenon could be divided into "pure viscoelastic creep"[under constant moisture content(MC) conditions] and "mechano-sorptive(MS) creep"(under varied MC conditions). This paper reviewed the phenomena and characteristics of pure viscoelastic creep and MS creep, respectively. For pure viscoelastic creep, the emphases were on the effects of anatomy structure, moisture content, temperature and stress level as well. For MS creep, the emphases were on microfibril angle, chemical composition, moisture content as well as temperature. In addition, the main mechanisms of MS creep were interpreted from the perspectives of molecular level, cell wall layer structure and physical ageing, etc. Stress-strain constitutive equation and Kelvin-Voigt model were also summarized in this paper. Additlonally, the numerical simulation of pure viscoelastic creep and MS creep were introduced, respectively. Wood creep mainly depends on anatomy structure and ambient conditions, and even the interactions among these factors, which lead to tremendous space for improvement. In the future, our main work should focus on the following four topics: 1) Depending on orthotropy properties of wood to characterize orthotropic creep behavior combined with loading directions and deformation modes. 2) To clarify the effect mechanism of moisture, temperature and their interactions on chemical component. Moreover, the combined technology of real-time measurements on creep compliance and Poisson's ratio should also be conducted. 3) Creep behavior of single wood fiber under cyclic humidity is essential to investigation, which is helpful to illuminate the roles of hemicellulose on MS creep, and the hydrogen bonding effect mechanism at molecular level. 4) To establish models to demonstrate the mechanism of pure viscoelastic or MS creep in wood. © 2016, Editorial Department of Scientia Silvae Sinicae. All right reserved.