PSI - Issue 77

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

335

5

3. Results and discussion 3.1. Quasi-static three-point bending

Figure 3 (a-d) shows the bending force-displacement curves, and Figure 3 (e) shows the energy uptake and bending strength determined for the aluminum-wood composites and the aluminum alloy sheets. It is evident that for L-stacked composites the curves were very consistent. PF- and PUR-bonded composites exhibited nearly identical bending behavior. The composite achieved high plastic deformation without brittle fracture of wood, as the aluminum alloy sheet absorbed the critical stress in the tension zone during bending, even in plywood with a relatively low moisture content of about 8 %. The coefficient of variation was calculated as CV (%) = 100 ( s/ x̄ ) from the standard deviation s and the mean value x̄ . The CV of the bending strength (i.e., of the maximum bending stress) was 3.1 %, 1.7 % and 0.4 % for PF-, PUR-, and EP-bonded L-stacked composites, respectively. For comparison, the typical CV reported for various mechanical properties of clear wood is in the range of 16-34 % (U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, 2010), and the CV of the bending strength of engineered wood products such as glued laminated timber (glulam) and laminated veneer lumber (LVL) is in the range of 10-20 % (Wang et al., 2024). The CV of the bending strength of aluminum-wood composites composed of four 1 mm-thick birch veneers in cross-wise stacking order and reinforced with an aluminum alloy sheet was 6.0 % (Graf et al., 2025). Although using thinner veneers may have contributed to the CV reduction, the consistency of the adhesive bond and the prevailing debonding likely played a more significant role in reducing variability.

Fig. 3. (a-d) Bending force-displacement curves and (e) energy absorption to max. bending force and bending strength of three-point bending tests for (a,b,c) aluminum-wood composites adhesive-bonded with (a) PF, (b) PUR, and (c) EP, and for (d) 2 mm- and 4 mm-thick aluminum alloy sheets. P and L refer to the perpendicular and longitudinal stacking order. All sample configurations were tested three times, except the P-stacked composites, which were only tested once. PF- and PUR-bonded P-stacked composites had about 10 % lower bending strength than L-stacked composites, which is attributed to the higher number of weaker veneers with perpendicular wood fiber orientation (U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, 2010). From a load-bearing and lightweight perspective, the aluminum-wood composites outperformed the 2 mm-thick aluminum alloy sheets of comparable mass, as composites sustained approximately twice the maximum load.