Mechanical properties moisture changes

Unfortunately, not much attention has been paid to these types of effects, and long-term performance has usually been treated only from the point of view of durability related to mechanical properties. Dimensional changes due to moisture... 

The mechanical properties of composites are mainly influenced by the adhesion between matrix and fibers of the composite. As it is known from glass fibers, the adhesion properties could be changed by pretreatments of fibers. So special process, chemical and physical modification methods were developed. Moisture repel-lency, resistance to environmental effects, and, not at least, the mechanical properties are improved by these treatments. Various applications for natural fibers as reinforcement in plastics are encouraged. 

Wood is a material that has evolved to fulfil a number of structural roles in supporting the tree canopy, such as resisting wind loads, and so on. These mechanical properties can be exploited and used in various structural roles by mankind. It is, however, important to note that wood, as it performs in the natural environment, is fully saturated with water. When used in man-made structures, wood is invariably dried to a relatively low moisture content (below the FSP) and as a consequence exhibits physical properties that are subject to change as the moisture content of the cell wall changes. 

The dynamic viscoelastic properties of acetylated wood have been determined and compared with other wood treatments in a number of studies. Both the specific dynamic Young s modulus (E /j) and tan S are lower in acetylated wood compared with unmodified wood (Akitsu etal., 1991, 1992, 1993a,b Korai and Suzuki, 1995 Chang etal., 2000). Acetylation also reduces mechanosorptive creep deformation of the modified wood (Norimoto etal., 1992 Yano etal, 1993). In a study of the dynamic mechanical properties of acetylated wood under conditions of varying humidity, it was concluded that the rate of diffusion of moisture into the wood samples was not affected by acetylation (Ebrahimzadeh, 1998). 

Moisute acts as a plasticizer and influences the mechanical properties of powdered materials for tablet compression. In the case of microcrystalline cellulose, at moisture levels above 5% the material exhibits significant changes consistent with a transition from the glassy state to the rubbery state [17]. The possible influence of moisture on the compaction behavior of powders was also analyzed by Gupta et al. [18]. This work evaluates the effect of variation in the ambient moisture on the compaction behavior of microcrystalline cellulose powder. 

Changes in the moisture content of the wood cell wall have a major effect on the mechanical properties of wood [5]. At moisture contents from oven-dry (OD) to the fiber saturation point (FSP), water accumulates in the wood cell wall (bound water). Above the FSP, water accumulates in the wood cell cavity (free water) and there is no tangible strength effect associated with a change in free water content. However, at moisture contents between OD and the FSP, water does affect strength. Increased amounts of bound water interfere with and reduce hydrogen bonding between the polymers of the cell... 

Table 1 shows that fiber stress at proportional limit, work to proportional limit, and maximum crushing strength are the mechanical properties most affected by changing moisture content by only + 1 % below the FSP. The change in mechanical properties from green to ovendry are shown in Table 2. 

Table 1 Approximate Change in the Mechanical Properties of Clear Wood When Subjected to Change in Moisture Content...

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