Viscoelastic properties of thermo-hydro-mechanically treated beech ( Fagus sylvatica L.) determined using dynamic mechan
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ORIGINAL ARTICLE
Viscoelastic properties of thermo‑hydro‑mechanically treated beech (Fagus sylvatica L.) determined using dynamic mechanical analysis Andreja Kutnar1,2 · Jane O’Dell3 · Christopher Hunt3 · Charles Frihart3 · Frederick Kamke4 · Matthew Schwarzkopf1,2 Received: 11 March 2020 / Accepted: 7 November 2020 © The Author(s) 2020
Abstract Thermo-hydro-mechanical (THM) processing can improve the intrinsic properties of wood, produce new materials, and give desired form and function to new applications. THM treatments change the mechanical properties of wood and may change its viscoelastic properties as well. Therefore, the objective of this study was to assess the viscoelastic properties of THM-treated wood at several humidity and load levels. To explore these changes, this study applied a THM treatment to beech (Fagus sylvatica L.) wood with steam (620 kPa) and heat (170 °C), followed by densification and increased temperature (200 °C) in a hot-press, which was then cooled while under pressure. Two initial specimen thicknesses before THM treatment were used to study the difference between density ratios. Specimens were tested in a humidity-controlled dynamic mechanical analyser (DMA) to apply creep stress with different loading levels (20% and 30% of expected modulus of rupture) and relative humidity levels (30%, 50%, and 65% RH). The creep compliance/recovery response was monitored, and dynamic moduli were measured before and after the application of creep stress. The loss modulus measured was highest for specimens tested at 65% RH and lowest in specimens tested at 30% RH, which is a direct result of the viscous response of the material. Increased damping of the specimens was also observed at higher RH, which is typical for wood products due to added moisture in the cell wall acting as a plasticizer to cell wall polymers. Like previous studies, THM treatment lowered the equilibrium moisture content (EMC) of densified wood specimens, which affected their mechanical performance. THM treatment yields higher strength and lower EMC, suggesting that this product could be suitable for structural applications where their service life is in an indoor environment. THM treatment also resulted in decreased creep compliance and recovery compliance as compared to control specimens.
1 Introduction Forest-based industries are continually developing advanced processing, materials, and wood modification solutions to meet evolving demands and increase competitiveness. One challenging aspect to these solutions is the viscoelastic behaviour of wood (Wolcott et al. 1994). Owing to the viscoelastic nature of wood, its mechanical properties depend on time, temperature, and moisture. At relatively short * Matthew Schwarzkopf [email protected] 1
InnoRenew CoE, Livade 6, Izola 6310, Slovenia
2
University ofPrimorska, Andrej Marušič Institute, Muzejski trg 2, Koper 6000, Slovenia
3
Forest Products Laboratory, Madison, WI, USA
4
Oregon State University, Oregon, USA
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