Cyclic Humidity Changes Research Articles

Strain variation analysis of cross-laminated timber elements under cyclic moisture

Despite the growing use of cross-laminated timber (CLT) panels in the construction industry, their behaviours under weather conditions, especially cyclic moisture which generated additional strains, requires further investigation. To address this gap, an extensive experimental campaign was carried out to evaluate CLT elements, in which strains are measured, under cyclic moisture changes, using Digital Image Correlation (DIC). In this way, the effects of cyclic humidity changes on amplitude and distribution of strains on four different configurations of CLT panels, divided based on the free flow direction of moisture, are explained. Regarding RH cycle tests, CLT specimens were conditioned in climatic chambers for 324 days at constant temperature 20 (°C) with a humidity cycle varying every 21 days between 30% and 90%, getting constant at 65% after the first 142 days. Results show that, in the main face, drying periods present tensile strain while wetting periods present compressive strains in the longitudinal direction. But, in the tangential direction, restrained strains tend to reduce significantly from first to last drying cycles for all test configurations. Moreover, in the lateral face, drying periods present tensile strains while wetting periods present compressive strains in the tangential direction. Experimental results confirmed that the ratio between tangential and longitudinal directions tends to decrease with the number of RH cycles, and successive RH cycles do not cause cumulative strains on CLT panels.

Creep behavior of laminated veneer lumber from poplar under cyclic humidity changes

Many academic studies over the years have confirmed that mechano-sorptive (MS) creep is an inherent characteristic of wood. Unlike solid wood, bond lines are introduced into laminated veneer lumber (LVL), creating a laminated structure with different hygroscopicity. What are the effects of these differences on the MS creep of LVL? In this study, three groups of well-matched LVL samples were subjected to four-point bending loading within different relative humidity cycles. For each group, the applied load ranged from 15% to 35% of the short-term fracture load. The results showed that after the first hygroscopic process, LVL showed irreversible expansion (0.11 mm) and a relatively slow moisture adsorption rate. These made it difficult for LVL to show partial creep recovery during the first adsorption process no matter how low the load level was, while solid wood showed partial creep recovery when the load level was ≤ 25%. The following creep behavior of LVL was similar to that of solid wood: partial creep recovery started from the second adsorption stage when a moderate load level was applied.

Highly sensitive humidity sensors made from composites of HEC filled by carbon nanofillers

Resistive type humidity sensors made from the composite films of HEC (hydroxyethyl cellulose) and MWCNTs (multiwalled carbon nanotubes) were developed. Three kinds of HEC/MWCNT composite films were fabricated, which contained 7.0, 8.0 and 9.0 wt-% MWCNTs. Rapid response capability and high sensitivities of the composite sensors were confirmed. For the convenience of sensor’s calibration, the non-linear resistance change ratios can be linearised by performing natural logarithm operation on them. A new sensitivity factor was defined based on this linear relationship between the logarithm of the resistance change ratio and relative humidity. For comparison, a reference humidity sensor was also made from the composite film of HEC filled by 50.0 wt-% CBs (carbon blacks). Compared with the HEC/MWCNT sensors, the HEC/CB sensor was of much higher sensitivity, but lower repeatability and stability under cyclic humidity changes. Possible sensing mechanisms were discussed in detail.

Aging of Wood by Long-Term Heating at a Moderately High Temperature under Cyclic Humidity Changes

In order to attain the fruitful effects of wood properties by aging, we conducted the thermal treatment at a moderately high temperature (85°C), which does not cause drastic thermal degradation, under a cyclic humidity changes between 30 and 80% R.H. for 3 months. For wood and cellulose samples, the weight losses were significantly accelerated by the existence of moisture, however the weight loss of cellulose was 1 order lower than wood sample. Loss tangent of wood specimens decreased for a few percent by the heating for 3 months without humidity change, whereas it increased under cyclic humidity changes. By the heating accompanied with humidity change, the degradation may excessively occur even only 1 month and as low as 85°C. The decrease of holocellulose content and relative increase of Klason lignin content were observed, which were remarkable under the heating with humidity change. Because plural factors may participate to the changes of equilibrium moisture content and crystallinity, the changing behaviors were complicated. In conclusion, it was proved that the existence of moisture merely accelerated the thermal degradation and positive effect was not virtually founded.

Sol–gel BaTiO 3 thin film for humidity sensors

BaTiO 3 ceramic thin films were prepared by sol–gel processing and their humidity sensitive characteristics have been investigated. The films were formed on Al 2O 3 substrates with comb type Au electrodes by spin coating. It was found that the impedance of the thin films decreases with increasing relative humidity (RH). The humidity sensitivity of the sensor depends on the applied frequency of the electric signal and the activation energy for conduction is reduced with water adsorption. The BaTiO 3 thin films show a negligible hysteresis under cyclic humidity changes with a good long-term stability and fast response time. It appears that the sol–gel process produces films that can be used as efficient humidity sensors.

History-dependent dimensional stability of paper

History-dependent dimensional behavior of paper has been formulated within the framework of the general linear theory of viscoelasticity and the classical lamination theory. The effect of the drying history of the papermaking process was incorporated by introducing the “residual stress” in the reference configuration which was taken at the final drying stage in this study. This permits us to account for complex dimensional and form changes of paper in the converting and end use processes. In order to determine the prediction performance of the computer simulation model developed, in-plane and out-of-plane dimensional responses under cyclic humidity changes were predicted on the basis of hygroviscoelastic data of paper. The simulation examples demonstrated typical irreversible dimensional responses of paper both for the in-plane dimensional change and the curvature change (curl). The computer code can easily deal with the non-uniform distribution of 1) anisotropic viscoelastic properties, 2) hygrothermal properties, and 3) moisture and temperature through the thickness.