Holzforschung 2021; 75(9): 847–856 Original article Maximilian Pramreiter*, Sabine C. Bodner, Jozef Keckes, Alexander Stadlmann, Florian Feist, Georg Baumann, Emad Maawad and Ulrich Müller Predicting strength of Finnish birch veneers based on three different failure criteria https://doi.org/10.1515/hf-2020-0209 plane fibre orientation. It was shown that the failure Received September 30, 2020; accepted February 1, 2021; criteria commonly used for manmade fibre reinforced published online March 15, 2021 composites are also applicable for thin birch veneers. Abstract: The use of wood in high-performance compos- Keywords: failure criteria; fibre orientation; non-destruc- ites based on laminated veneer products, plywood or wood tive testing; strength prediction; wide angle X-ray hybrid elements thereof requires accurate prediction of scattering. strength of each individual ply. Previous research has shown that one dominating factor influencing the strength of birch veneers is the fibre orientation. The present study 1 Introduction investigates the validity of the failure criteria after Tsai- Hill, Hoffmann and Kollmann for thin birch veneers under On a macroscopic scale manmade fibre reinforced com- tensile loading. The fibre orientation in- and out-of-plane posites are a combination of fibres such as glass or carbon was measured by means of wide-angle X-ray scattering. embedded in an amorphous matrix e.g. epoxy (Nielsen and Tensile strength and threshold values were determined in Landel 1994). The orientation of the fibres within the laboratory experiments. Pearson correlation between the composite can be arranged freely to optimize strength and predicted strength and actual strength ranged from 0.836 fit the demand of the finished product (Flemming and Roth r up to 0.883. Best correlation ( = 0.883) was achieved for 2003). Wood is a natural material structured at multiple Kollmann using a combined angle between in- and out-of- hierarchical levels (Fratzl and Weinkamer 2007). On the cell-wall level it can be understood as a natural fibre *Corresponding author: Maximilian Pramreiter, Department of reinforced composite made up of fibrous cellulose Material Science and Process Engineering, Institute of Wood embedded in amorphous lignin and hemicellulose (Fratzl Technology and Renewable Materials, University of Natural Resources fi and Life Sciences Vienna, Austria (BOKU), Konrad Lorenz Strasse 24, 2007). On the tree ring level bre orientation is the pre- 3430, Tulln a.d. Donau, Austria, dominating factor influencing strength and stiffness E-mail: [email protected]. https://orcid.org/0000- (Kollmann and Cote 1968). The fibre orientation itself is 0003-0283-5936 influenced by a multitude of growth and environmental Sabine C. Bodner and Jozef Keckes, Department of Materials Science, factors (Richter 2015). Therefore, wood can be seen as a University Leoben, Jahnstrasse 12, 8700, Leoben, Austria, biological fibre reinforced composite with somewhat arbi- E-mail: [email protected] (S.C. Bodner), fi [email protected] (J. Keckes) trary bre orientation. It is usually considered in applica- Alexander Stadlmann and Ulrich Müller, Department of Material tions with static loads that require high elastic stability e.g. Science and Process Engineering, Institute of Wood Technology and beams or columns for building constructions (Bodig and Renewable Materials, University of Natural Resources and Life Goodman 1973; Panshin and Zeeuw 1964). Due to its vari- Sciences Vienna, Austria (BOKU), Konrad Lorenz Strasse 24, 3430, ability within the same species as well as within a single Tulln a.d. Donau, Austria, E-mail: [email protected] (A. Stadlmann), [email protected] (U. Müller) tree (Marra 1979) predictability of different properties of Florian Feist and Georg Baumann, Vehicle Safety Institute (VSI), Graz individual wood elements (i.e. lumber, beams, veneers, University of Technology, Inffeldgasse 23/I, 8010, Graz, Austria, etc.) for load-bearing structures is an important issue E-mail: fl[email protected] (F. Feist), [email protected] (Bucur 2003; Ross 2015). (G. Baumann) Besides established fields of application for wood and Emad Maawad, Institute of Materials Physics, Helmholtz-Zentrum wood products e.g. glue-laminated-timber, laminated- Geesthacht, Max-Planck-Str.1, D-21502 Geesthacht, Germany, E-mail: [email protected]. https://orcid.org/0000-0003-4500- veneer-lumber, plywood, furniture, pulp and paper, cur- 2301 rent research efforts also investigate the capabilities of Open Access. © 2021 Maximilian Pramreiter et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. 848 M. Pramreiter et al.: Predicting strength of birch veneers wood for high performance based composites such as The threshold values for tension or compression are still automotive parts (Baumann et al. 2019; Berthold 2016; needed. Additionally, the threshold in-plane shear Müller et al. 2019b). To reduce costs and time the product strength of the material is needed. This constitutes the development cycle within the automotive industry is biggest difference to Kollmann (1951) and is true for failure strongly supported by computer aided engineering (CAE) criteria after Tsai-Hill (1950), Tsai and Wu (1971) and e.g. finite element modelling (FEM) (Gu 2017). Predicting Hoffmann (1967). the behaviour of wood using FEM is a key challenge when The present study aims to investigate the accuracy of introducing wood in such high-performance applications. different failure criteria for fibre reinforced composites for A constitutive model describes the material response under the bio-composite wood. Eberhardsteiner (2002) discussed mechanical (or multi-physical) loading. A material card is the application of failure criteria according to Tsai-Hill an implementation of a constitutive model in a FE code, (1950), Tsai and Wu (1971) and Hoffmann (1967) for spruce where characteristic values describing e.g. elasticity, wood. The failure criterion Tsai and Wu (1971) and its ac- plasticity, damage and failure are entered (Klein 2012). curacy when applied to wood have been investigated in Needed material values include the modulus of elasticity, detail (Eberhardsteiner 2002; Mascia and Nicolas 2012) and the shear modulus, the Poisson’s ratio, the density and the was therefore not included in this work. The Tsai and Wu strength. Additionally, the viscoelastic behaviour (Navi (1971) criterion exhibits satisfactory capabilities to predict and Stanzl-Tschegg 2009) and strain rate dependency strength based on its increased number of terms in the (Polocoșer et al. 2017) also need to be considered when curve fitting equation. However, the determination of the utilizing wood for structural applications. To support reli- interaction coefficient (F12) requires an expensive biaxial able simulation results the representing material values test without distinctly increasing the accuracy (Jones 1999). need to be precise and reliable (Müller et al. 2019a). Therefore, the focus of the present work was laid on criteria Based on these requirements it is clear that an accu- after Tsai-Hill (1950) as well as Hoffmann (1967). Further- rate, reliable, reproducible and non-destructive method to more, the accuracy of the empirical failure criteria predict relevant material parameters of wood, including Kollmann (1951) should be evaluated for thin hardwood strength, is needed. Current research in the area of strength veneers. Thin birch veneers with different fibre orientation prediction is dealing with either standing trees or logs were used for the tests. The fibre in-plane and out-of-plane (Mahler and Hauffe 2000; Turpening 2011), sawn lumber orientations were determined by means of wide-angle (Diebold et al. 2000; Rohanova et al. 2011; Sandoz et al. X-ray scattering (WAXS). The tensile strength was deter- 2000; Stapel and Van De Kuilen 2014) and engineered mined according to the standard DIN 789 (2005) using an wood products (EWPs) (Gilbert et al. 2017a, 2017b; Hunt universal testing machine. et al. 1989; Mansfield et al. 2016; Meder et al. 2002). Most of It was hypothesized that a sound prediction of strength these methods include the measurement of strength can be achieved using different failure criteria according to influencing factors such as density, dynamic modulus of Kollmann (1951), Tsai-Hill (1950) or Hoffmann (1967). elasticity or fibre orientation. One dominating factor Additionally, it was hypothesized that Tsai-Hill (1950) and influencing strength of wood is the fibre deviation within Hoffmann (1967) show higher correlation with strength the material (Keckes et al. 2003; Pramreiter et al. 2020a). than Kollmann (1951), because they are not empirical Within the wood industry the most common failure criteria models but are solely based on fundamental strength to describe strength in relation to fibre orientation was first concepts. Furthermore, it was assumed that the combina- described by Hankinson (1921). The model is based on tion of in- and out-of-plane fibre orientation yields a higher empirical results for compression under different fibre correlation than maximum in in-plane fibre orientation orientations. Kollmann (1934) later adopted the model in alone. order to increase the applicability. Based on empirical re- sults the model can be used for strength as well as elastic properties and can be established for different wood spe- 2 Materials and methods cies. To build the curve threshold values