Mechanical behaviour improvements of wood veneer laminates by stitching

Wood shows a wide range of strengths. Under tensile loads along the fibre, a hardwood such as birch has a strength of up to 140MPa. However, when subjected to shear loads transverse to the fibre, known as rolling shear, it has a strength of only about 4MPa.
In the case of plywoods, rolling shear failure is a common failure mode.
In civil engineering, the tensile failure of concrete structures or the transverse tensile failure of wooden structures is countered by inserting tension rods (reinforcements) or screws. Such tension rods could also prevent roll shear failure and delamination in veneer laminates.
The "Stitch!" project is investigating whether such tension rods can be introduced through stitching.
By stitching veneers, shear failure in the core of veneer laminates as well as delamination on the outer layers of veneer laminates can be avoided. This significantly increases the flexural strength and the energy absorption during flexural impact loads.

To evaluate the potential of reinforcing wood laminates by stitching, a wide number of parameters are being studied. These include, amongst others, the following: Wood moisture, bonding condition, thread materials and sizes, needle shapes and sizes. The influence of production-induced damage as a function of machine and substrate is also investigated.

In order to be able to investigate the very large parameter space as comprehensively as possible, Stitch! relies on a combination of numerical and experimental studies. With an advanced and well-validated model, many of the parameters and sensitivities can be studied while excluding variances and external influences.