Juan Li

Juan Li, M.Sc.

Fachgebiet Organische Baustoffe und Holzwerkstoffe

Hopfengarten 20

38102 Braunschweig

Telefon: 0531/2207734

Forschungsvorhaben:

Ageing of solid wood in structural applications-effect of environmental parameters on wood and wood-environment interface

Wood has been extensively used in constructions for structural and nonstructural applications for its excellent mechanical properties, attractive appearance and abundant resources. Recently, the world is experiencing a renaissance of usage of wood due to its environmentally friendly nature.

Ageing of wood is a complex phenomenon occurring in material with time and is known as a function of ambient environment, i.e. temperature, moisture, solar radiation …. Moisture and temperature stress are often critical. The term "ageing" is defined as "changes" in this work. It is quite general and can be categorized by different properties or functional focuses, e.g., biotic decay by fungi; surface properties related to colour stability, water performance and sorption behavior; mechanical properties, etc. Overlapping fields of study include researches on weathering, degradation, durability, service life prediction, etc. All these efforts aim to determine the limit of overall performance of wood.

Many definitions of ageing of wood can be found in the literature. Unger et al. (2001) suggested ageing is the irreversible change of properties of a material with time. Kranitz (2015) adopted this definition and studied wood properties under aerobic and anaerobic natural aging conditions. Some other researchers considered wood ageing as a mild thermal oxidation at room temperature and the mechanism of wood ageing can be investigated by heat treatment of wood (Stamm 1956; Millett and Gerhards 1972; Matsuo etc. 2011). Ganne-Chédeville et al. (2011) also agreed that the slow deterioration of objects of cultural heritage is called ageing and artificial ageing by means of hydrothermal treatment was the preferred method. All the definitions focus on the changes of material properties over time.

As a construction material, the mechanical properties, such as tensile, compressive, flexural shear capabilities, etc., are of vital importance for wood applications. Various experimental methods are used to describe these properties, both destructive and non-destructive. Destructive tests have been used to get strength-related properties for very long time. However, destructive tests only permit one-time measurements of a specimen, so variabilities between different specimens need to be considered.

Solid wood materials are heterogeneous. Enormously variations exist among species from hardwood to softwood due to different types of cells and arrangements and wood properties varies greatly even within the same species because of different density, moisture content, and ratios of earlywood to latewood. Specimens cut from the same bulk volume of wood can have very different properties considering the above-mentioned factors. Measurements of non-destructive properties allow repeatable tests on the same specimens, upon which comparisons of changes are able to be made along the time scale.

Wood, as a porous hygroscopic polymeric system, is susceptible to variations of temperature and relative humidity (RH) of ambient environment with diurnal and seasonal variations under natural conditions. The hygric, thermal or hygrothermal behavior of wood material has gained extensive research using climate chambers. On the other hand, in-situ experimental studies have also been conducted to evaluate the hygric, thermal or hygrothermal performance of materials. The objective of some studies is to compare the performance of tested material before and after certain conditions, but to describe material properties. While some studies focus on the mechanisms behand observed wood performance. For example, the effect of temperature on mechanical properties was determined by comparisons of tensile, compressive and flexural tests at various temperatures.

My research tasks:

Perform thorough literature review of the subject

Define (precisely) the gaps in knowledge and problems to be solved

Formulate the research hypotheses

Define materials and methods

  1. Use wood as a "model material" and generalize the findings to other similar lignocellulosic materials such as plant-based fibers or yarns

  2. Use the analytical methods such that the expected changes in material properties can be related to changes in microscopic or chemical scales (remember, not all materials properties are relevant to be studied and you will need to select those most important)

  3. Develop analytical models that will allow reasonable prediction of the material behavior based on various environmental variables and loading (including random loads)

  4. For structural applications, consider the random character of loading (magnitude and time)