Mechanical behavior of walnut ( Juglans regia L .) and cherry ( Prunus avium L .) wood in tension and compression in all anatomical directions. Revisiting the tensile / compressive stiffness ratios of wood.

The mechanical properties of walnut (Juglans regia L.) and cherry (Prunus avium L.) woods, as frequent raw materials in cultural heritage objects, have been investigated as a function of the anatomical directions and the moisture content (MC). The strength data are decreasing with increasing MC, whereas the tensile strength in the longitudinal direction is higher by factors of 1.5–2 compared to the compression strength. Moreover, the inequality of tensile and compressive stiffness is discussed, which is a matter of debate since a long time. This so-called bimodular behavior is difficult to describe in a generalized mode due to the high data variability if tension and compression properties are analyzed on different samples. If tensile and compressive stiffness tests are performed on the same samples of walnut and cherry wood, the ratio between these properties is significantly higher than 1. Keywords: cherry wood (Prunus avium L.), stiffness ratios, strength ratios, tension and compression strengths, walnut wood (Juglans regia L.

Significant influence of lignin on axial elastic modulus of poplar wood at low microfibril angles under wet conditions

Wood is extensively used as construction material. Despite accumulating knowledge on its mechanical properties, the contribution of the cell wall matrix polymers to wood mechanics is still not well understood. Previous studies have shown that axial stiffness correlates with lignin content only for cellulose microfibril angles larger than around 20°, while no influence was found for smaller angles. Here, by analyzing the wood of poplar plants with reduced lignin content due to down-regulation of CAFFEOYL SHIKIMATE ESTERASE, we showed that lignin content influences axial stiffness also at smaller angles. Micro-tensile tests of the xylem revealed that axial stiffness was strongly reduced in the low-lignin transgenic lines. Strikingly, microfibril angles were around 15° for both wild type and transgenic poplars, suggesting that cellulose orientation is not responsible for the observed changes in mechanical behavior. Multiple linear regression analysis showed that the decrease in stiffness was almost completely related to the variation in both density and lignin content. We suggest that the influence of lignin content on axial stiffness may gradually increase as a function of the microfibril angle. The obtained results may help setting up comprehensive models of the cell wall for unraveling the individual role of the cell wall matrix polymers

Mechanical factors contributing to the Venus flytrap's rate-dependent response to stimuli.

Funder: schweizerischer nationalfonds zur förderung der wissenschaftlichen forschung; doi: http://dx.doi.org/10.13039/501100001711Funder: ETH ZurichThe sensory hairs of the Venus flytrap (Dionaea muscipula Ellis) detect mechanical stimuli imparted by their prey and fire bursts of electrical signals called action potentials (APs). APs are elicited when the hairs are sufficiently stimulated and two consecutive APs can trigger closure of the trap. Earlier experiments have identified thresholds for the relevant stimulus parameters, namely the angular displacement [Formula: see text] and angular velocity [Formula: see text]. However, these experiments could not trace the deformation of the trigger hair's sensory cells, which are known to transduce the mechanical stimulus. To understand the kinematics at the cellular level, we investigate the role of two relevant mechanical phenomena: viscoelasticity and intercellular fluid transport using a multi-scale numerical model of the sensory hair. We hypothesize that the combined influence of these two phenomena and [Formula: see text] contribute to the flytrap's rate-dependent response to stimuli. In this study, we firstly perform sustained deflection tests on the hair to estimate the viscoelastic material properties of the tissue. Thereafter, through simulations of hair deflection tests at different loading rates, we were able to establish a multi-scale kinematic link between [Formula: see text] and the cell wall stretch [Formula: see text]. Furthermore, we find that the rate at which [Formula: see text] evolves during a stimulus is also proportional to [Formula: see text]. This suggests that mechanosensitive ion channels, expected to be stretch-activated and localized in the plasma membrane of the sensory cells, could be additionally sensitive to the rate at which stretch is applied

Bio-Inspired Wooden Actuators for Large Scale Applications

<div><p>Implementing programmable actuation into materials and structures is a major topic in the field of smart materials. In particular the bilayer principle has been employed to develop actuators that respond to various kinds of stimuli. A multitude of small scale applications down to micrometer size have been developed, but up-scaling remains challenging due to either limitations in mechanical stiffness of the material or in the manufacturing processes. Here, we demonstrate the actuation of wooden bilayers in response to changes in relative humidity, making use of the high material stiffness and a good machinability to reach large scale actuation and application. Amplitude and response time of the actuation were measured and can be predicted and controlled by adapting the geometry and the constitution of the bilayers. Field tests in full weathering conditions revealed long-term stability of the actuation. The potential of the concept is shown by a first demonstrator. With the sensor and actuator intrinsically incorporated in the wooden bilayers, the daily change in relative humidity is exploited for an autonomous and solar powered movement of a tracker for solar modules.</p></div

Mechanical loading of a bilayer in the field experiment.

<p>Colour code: black: unloaded bilayer (3.8mm beech, 2mm spruce), red: bilayer (4mm beech, 1.6mm spruce) loaded with a point load of 100g at the free end of the bilayer. Arrows indicate application and removal of the weight.</p

Actuation of bilayers after step-wise change of relative humidity from 85% to 35%.

<p>Color code: red: 0.2mm spruce layer (thickness ratio m = 0.05); blue: 0.8mm (0.2), green: 1mm (0.25), orange: 2mm (0.5), purple: 3mm (0.75), brown: 4mm (1). a) Wood moisture content, b) curvature, c) curvature of bilayers with different m (note that the overall thickness h also changes), d) curvature of a bilayer 1,2,4,8, and 24 hours after transfer. The actuation is also made visible in a movie (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0120718#pone.0120718.s004" target="_blank">S1 Movie</a>). e) Comparison of experimentally derived and calculated specific curvature k 24 hours after transfer. Calculations are shown for values of n = E1/E2 of 10 (dotted line), 20 (solid line), and 30 (patched line). f) Relative amplitude of k as a function of n and m.</p

Configuration of beech strips, spruce strips and bilayers and their response to drying.

<p>a) Cutting direction of single layers of beech and spruce and their dimensional changes after a decrease in wood moisture content of 10%. L = longitudinal direction, R = radial direction, T = tangential direction; WMC = wood moisture content; rH = relative humidity; green arrows indicate fibre direction and cellulose microfibril orientation (as the microfibrils are oriented almost parallel to the fibre axis). The relative dimensional changes along the long axis of the strips are given in red (specific values for the strips shown). b) Bending of a corresponding bilayer following the change in wood moisture content.</p

Long-term actuation of bilayers during field test in full weathering conditions.

<p>a) Daily change of curvature (lines), and mean (sliding average over 11days) curvature (dots), b) daily change of relative humidity (lines), mean relative humidity (dots), and temperature, c) ratio of curvature change and change in relative humidity (small squares) and average ratio (sliding average with a window of 11 data points) (dots).</p

Wooden bilayers in the long-term field experiment.

<p>a) Down-bending of the bilayers in the morning on Sept. 13th 2013, b) Up-bending of the bilayers in the afternoon. Two bilayers are mechanically loaded. The actuation of the bilayers on that day is shown in the <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0120718#pone.0120718.s005" target="_blank">S2 Movie</a>.</p