Micromechanical properties of beech cell wall measured by micropillar compression test and nanoindentation mapping

Wood exhibits very different behavior and properties at different scales. One important scale is the cell wall (CW) that is commonly tested by nanoindentation. Common nanoindentation provides important insight into the material but has limitations because it does not apply uniaxial stress and provides data from single spots. Therefore, the aim was to examine beech CW using two stateof-the-art techniques: micropillar compression (MCo) and nanoindentation mapping (NIP). The mean strength of the beech CW was found to be about 276 MPa and the mean yield stress was 183 MPa. These values were higher than those in most cited literature, which was attributed to the fact that libriform fibers from beech late wood were measured. Mean E obtained from MCo was about 7.95 GPa, which was lower than the values obtained on a macrolevel and about 61% of the value obtained from NIP. NIP also showed that E of the CW around the middle lamella (ML) was about 64% of the value at the location attributed to the S2 layer. Lower E from MCo may be caused by sinking of the micropillar into the wood structure under the load. Failure of the micropillars showed gradual collapse into themselves, with debonding at the S3 layer or the MLs.