Relating MOE decrease and mass loss due to fungal decay in plywood and MDF using resonalyser and X-ray CT scanning Wanzhao Li a, * , Jan Van den Bulcke a , Imke De Windt a , Nele Defoirdt a , Jelle Dhaene b , Manuel Dierick b , Hugo Sol c , Joris Van Acker a a UGCTeWoodlab-UGent, Laboratory of Wood Technology, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000, Gent, Belgium b UGCT, Department of Physics and Astronomy, University Ghent Centre for X-ray Tomography, Proeftuinstraat 86, 9000, Ghent, Belgium c Department of Mechanics of Materials and Constructions, Vrije Universiteit Brussel, Pleinlaas 2, Brussels, Belgium article info Article history: Received 18 November 2015 Accepted 13 March 2016 Keywords: Fungal decay Plywood MDF Resonalyser X-ray CT scanning abstract Both plywood and medium density fiberboard (MDF) are often intended to be applied in conditions with fungal decay risk, which can lead to a decrease in physical and mechanical properties. It is thus essential to understand the detailed performance of the panels under fungal decay over time. In the presented research, the specimens were exposed to wet vermiculite inoculated with Pleurotus ostreatus for 14 weeks. A non-destructive flexural vibration method (resonalyser) was used to measure the Young's modulus (MOE) of the specimens, whereas the internal structure and mass loss distribution were monitored with X-ray CT scanning. The significant MOE decrease of MDF for interior use is a result of internal cracking. MDF produced with acetylated fibers showed only 1.5% mass loss and 10% MOE decrease. Plywood specimens exhibit a MOE decrease of approximately 12% at incipient fungal decay partially resulting from water dynamics and structural changes. There is a linear relationship between MOE decrease and mass loss, yet different for different types of plywood. Most mass loss occurs in the surface layers causing significant MOE decrease of entire plywood specimens, whereas the mass loss of the inner layers is influenced by structural properties of the veneers such as grain direction. © 2016 Published by Elsevier Ltd. 1. Introduction Wood based products are intensively used in the building in- dustry as a construction material. They are flexible in use, have a high strength to weight ratio and are sustainable materials, which are clear advantages compared to alternative materials such as plastics, metals and concrete (George, 1991; Issa and Kmeid, 2005). Wood based products, however, are prone to water uptake and biological degradation in service, which can decrease their me- chanical strength (Schirp and Wolcott, 2005). Understanding the impact of biological degradation on the mechanical strength of wood based products is important from a construction point of view. At the incipient stage, fungal decay causes changes in the chemical composition of wood and induces a measurable strength decrease before measurable mass loss (Winandy and Morrell, 1993). After the incipient stage, there is a direct relationship between strength loss and mass loss of wood (Curling et al., 2002). Plywood is an important representative of the wood-based panels (WBP) group and widely used in class 3 situations (Van Acker and De Smet, 2007) and the only WBP considered suitable for load bearing application. Currently, several medium density fiberboard (MDF) products produced with treated fibers are also considered to have a good durability in service (Suttie et al., 2015). The me- chanical strength of plywood and MDF during several years of outdoor exposure was studied by other researchers by periodically measuring the modulus of rupture (MOR) and internal bond strength (IB) (Kojima et al., 2011; River, 1994). The use of acceler- ated aging to evaluate mechanical strength decrease of wood- based panels was also investigated to compare with outdoor exposure (Kojima and Suzuki, 2011). Fungal decay of plywood in outdoor exposure was further studied by Van den Bulcke et al. (2011, 2009) reporting on the importance of the glue type on the biological durability of plywood. Although substantial research on the overall biological degradation of plywood and MDF has been performed, there is a need for non-destructive measurements of strength and structure in function of decay rate. Investigating the * Corresponding author. E-mail address: Wanzhao.Li@UGent.be (W. Li). Contents lists available at ScienceDirect International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod http://dx.doi.org/10.1016/j.ibiod.2016.03.012 0964-8305/© 2016 Published by Elsevier Ltd. International Biodeterioration & Biodegradation 110 (2016) 113e120