Interaction between mechanosorptive and viscoelastic response of wood at high humidity level

To clarify the contribution of mechanosorptive e ﬀ ect and compliance evolution according to moisture content during adsorption under load creep tests on longitudinal spruce specimens in variable environment were performed. By comparing di ﬀ erent loading history we observe that creep increase during the ﬁrst adsorption ( ++ e ﬀ ect) mostly results from the increase of compliance induced by higher moisture content. We also discuss the viscoelasticity and mechanosorptive interaction during creep.


Introduction
Since the first-published work in the 50's the time dependent behaviour raised an increasing interest in wood material research. Creep research is critical for in timber structures where the prediction of total deflection is necessary for durable structural design. The first approach to quantitative description of mechanosorptive creep was made by Ranta-Maunus [1] in 1975 based on plywood experimental results. He defined "hydroviscoelastic constants" to quantify the evolution of elastic compliance J induced by change of moisture content w. Three notations has been created representing the slope dJ/dw during first adsorption "a ++ ", any subsequent desorption "a − " or adsorption "a + ". In 1984 according to new experimental results Hunt [2] observed that these constants depend on strain and suggested a limit to the amount of mechanosorptive creep approached asymptotically by repeated humidity cycles. a e-mail: montero@lmgc.univ-montp2.fr In this study we propose to clarify the contribution of mechanosorptive effect and compliance evolution according to moisture content in the "++" effect. We also discuss on the viscoelasticity and mechanosorptive creep interaction.

Wood material
Matched spruce specimens were used in this study. Specimens have been 100 mm in longitudinal direction, 10 mm radial and 2 mm tangential directions. This geometry have been made to avoid multi-layer composition in specimens induced by annual rings which can produce discontinuity of strain profile with the sollitation. The average density have been determined ρ re f = 0.48 (at T = 24˚C, RH = 58 %). Preliminary measurements have been performed by X-ray diffraction. The mean microfibril angle has been measured before creep test as 16˚and the grain angle as 1˚.

Creep equipment and measurements
Four-point bending devices with 40 mm inner span and 80 mm outer span were used. Serial arrangement by pair of specimens has been done to increase repeatability in same environment conditions. Two sets of 4 specimens (i-index) have been placed in controlled environment produced by a climatic chamber. Two specimens were kept unloaded for free swelling and shrinkage determination ( j-index). On each of these specimens strain ε + was measured on tensile face and ε − on compression face using electric strain gages. Three specimens were weighted occasionally for moisture content estimation w.

Data analysis
In those bending tests the strain evolution is quantified by calculating mean strain < ε > and differential strain ∆ε for each sample.
The mean strain reveals hygromechanical processes occurring during sorption (as swelling and shrinkage). The differential strain reveals sample deflection. A preliminary acclimatizing phase (t=0-360 h) without specimen loading have been performed to clean out stress history of specimens and improve compliance calculations. The compliance have been calculated for each loaded specimen (i-index) using the strain of corresponding unloaded specimen (eq. 2). A correction has been applied to eliminate small differences in density between specimens.
A linear function have been established during acclimatizing phase with graphical analysis of ∆ε i = f i ∆ε j to correct heterogeneities of specimen against sollicitation. The stress σ i was applied with lead mass and fixed below the elastic limit (around 8 MPa).
14th International Conference on Experimental Mechanics

Results and discussion
The complete environment history of the experiment and specimen response is describe in figure 3. All these results have been made on longitudinal measurement. In this document only the creep part will be detailed.

Moisture content influence on viscoelastic creep
Constant environment periods at low and high relative humidity level have been used to quantify the influence of moisture content on viscoelastic creep.  In figure 4 the set (A) maintained in dry state (w ≃ 5%) presents as expected a lower elastic compliance than set (B) at wet state (w ≃ 27%).

Mecanosorptive influence on viscoelastic response
In the previous section the viscoelastic response have been analyzed near fiber saturated point (w ≃ 27%). A drying and a subsequent wetting of those specimens creates an strain increase. This phenomenon is called mechanosorptive creep.   This changes in slope indicates that viscoelastic creep depends on mecanosorptive creep accumulation indicating an interaction between both phenomenons. This result confirm Hanhijarvi's results on the influence between the viscoelastic response and mechanosorption [3]. It also seems to exhaust the viscoelastic response of material.

Influence of moisture content at loading on mechanical response
By loading the specimens at different moment of environment history we have studied the interaction between viscoelastic creep and mechanosorptive response.  The specimens (A) reach the deformation level of the specimens (B) suggesting that the "++" effect mostly results from the increase of compliance due to a higher moisture content. This observation supports the assumption that the mechano-sorptive effect does not occur significantly during adsorption at high humidity. In subsequent sorption cycles both sets (A) and (B) exhibit mechanosorption. In set (B), neither one week of wet creep nor subsequent sorption cycles could exhaust mechano-sorption. This results suggests that mechanosorption and pure creep are two separate phenomena.  However the effect of these dry-wet cycles is much less pronounced with set (A), which reached an apparent stable-state creep limit whatever the amplitude of dry-wet cycles. This indicates that the mechanosorptive response interacts in a complex manner with pure creep and that it cannot be simply described by an additive strain component. These results evidence the influence of moisture content history during loading and give useful indications on the interaction between mechanosorptive effect and viscoelasticity.