22_FFG_Stitch - Stitching of veneers for prevention of rolling shear and delamination failure

Wood shows a wide range of strengths. Under longitudinal tensile loads, hardwood - such as birch - has a strength of up to 140MPa. However, under shear loads (‘rolling shear’), it only exhibits a strength of around 4MPa. Especially with materials made of rotary cut veneers, this failure is provoked by production-induced damage (so-called "lathe checks"). In the case of plywoods or laminated veneer lumber, rolling shear failure therefore frequently observed - in particular when the plywood features high-strength face layers, e.g. of GRP or CFRP. In civil engineering, the tensile failure of concrete structures or the transverse tensile failure of wooden structures is tackled by inserting tension rods (reinforcements) or bolts. Though tension rods could prevent rolling shear failure and delamination in veneer laminates, a similar approach was so far not adopted here. The "Stitch!" project is investigating whether tension rods can be inserted through sewing-threads. The research hypotheses of the project "Stitch!" are: can be avoided or delayed. This significantly increases the bending strength and also the energy absorption in the case of bending impact loads. The sewing of veneers is already used in furniture design as a joining technique or for aesthetic reasons. In "Stitch!", the targeted strengthening of materials through sewing of veneers is investigated.