PSI - Issue 77

Eva Graf et al. / Procedia Structural Integrity 77 (2026) 331–338 Graf et al. / Structural Integrity Procedia 00 (2026) 000–000

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1. Introduction The mobility sector is a major contributor to global greenhouse gas (GHG) emissions (Filonchyk et al., 2024). Thus, automobile manufacturers are under increasing pressure to replace conventional metals with more sustainable alternative materials. In this regard, lightweight materials such as fiber reinforced plastics (FRP) are particularly attractive, because reducing the weight directly improves the fuel efficiency and reduces emissions (Helms and Lambrecht, 2007). However, not only decreasing the vehicle weight, but also replacing GHG-intensive materials with more sustainable alternative materials is required (Kawajiri et al., 2020). Wood fulfils these requirements, as wood is a renewable, naturally grown material. If loaded parallel to its fiber orientation, wood offers high specific strength comparable to steel, aluminum alloys, and even FRP (Kohl et al., 2016). Consequently, the interest in using wood or wood-based materials continuously rises even for applications beyond the building sector (Baumann et al., 2019) (Heyner et al., 2022) (Wurm et al., 2025). The achieved mechanical performance strongly depends on the wood species and their density (Kallakas et al., 2020). Hardwoods such as birch and beech demonstrate favorable compromises between strength and weight for structural use in automobiles (Baumann et al., 2019). However, using simple solid wood panels or laminates for structural applications in automobiles is associated with several challenges (Kohl et al., 2016). In order to prevent catastrophic failures in crash events, materials for crash-relevant components must provide high impact toughness, which includes a favorable combination of strength and ductility. However, despite providing numerous advantageous properties, wood is inherently anisotropic, demonstrates brittle fracture behavior under tensile loading, and is prone to splintering during impact. Furthermore, the mechanical properties may distinctly vary due to natural defects (Green et al., 1999), and the properties depend on the moisture content and temperature (Graf et al., 2024, 2025). Reinforcement of wood using fibers or metals has shown promising potential to overcome these drawbacks (Ungerer et al., 2025) (Graf et al., 2025). For instance, Ungerer et al. (2025) investigated the impact strength of birch wood laminates reinforced with cellulose filaments using emulsion polymer isocyanate- and epoxy-based adhesive systems to improve the impact energy absorption and the fracture toughness compared to simple wood laminates. Graf et al. (2024, 2025) investigated the quasi-static bending behavior of laminated aluminum-wood composites, which exhibited higher bending strength and greater formability as simple wood by preventing premature brittle fracture. However, varying properties and bonding issues between aluminum and wood were major challenges that must be solved for future applications of aluminum-wood hybrids. An approach for improving the homogeneity is the use of veneers that are alternately stacked with different fiber orientations. For instance, plywood with several veneers in cross-wise stacking order possesses a more homogeneous strength and stiffness than solid wood (Stark et al., 2010). The thinner the veneers, the greater the number of layers with different fiber orientations for a given overall thickness, which reduces the influence of local wood defects and anisotropy by homogenizing the mechanical properties. Additionally, decreasing the veneer thickness reduces the depth of lathe checks and unfavorable bending during manufacturing (Pramreiter et al., 2021) (Rohumaa et al., 2013). The present study investigates the bending behavior of aluminum-wood composites consisting of 0.5 mm-thick birch veneers reinforced with a single 1 mm-thick EN AW-6016-T4 aluminum alloy sheet under quasi-static and impact loadings. The suitability of three different adhesives was investigated for bonding the aluminum alloy sheet to the plywood plate. 2. Materials and methods 2.1. Sample preparation Peeled 0.5 mm-thick birch veneers were produced using a Raute 3HV66 veneer peeling lathe. The veneers were cut to dimensions of 420 mm × 420 mm using a veneer guillotine and subsequently dried for 45 s in a Raute veneer dryer with 170 °C steam. After drying, the veneers were conditioned in a climate chamber at 25 °C and 20 % relative humidity. To manufacture the plywood, nine layers of veneers were alternately stacked with cross-wise fiber orientation, considering either a (i) perpendicular (P) or (ii) a longitudinal (L) stacking order with either perpendicular (90°) or parallel (0°) wood fiber orientation of the outer veneers, as schematically shown in Figure 1. Every second veneer was coated with about 100 g/m 2 of conventional thermosetting phenol formaldehyde (PF) adhesive, prepared