PU adhesive joint between timber and DVW

Project information
Location: Eindhoven, The Netherlands
Project type: Academic Research
Institution: Eindhoven University of Technology (TU/e)
Field: Timber Engineering

Project overview
This academic research project focused on assessing the accuracy of numerical models of polyurethane (PU) adhesive connections between spruce timber and densified veneer wood (DVW). The numerical models were evaluated and validated using experimental test results.

Connections are often unavoidable in timber structures due to limitations in the available dimensions and lengths of structural timber elements. When a connection governs the structural design, larger member cross-sections may be required locally to accommodate fasteners and prevent splitting. For practical and economic reasons, these increased cross-sections are often continued over a larger part, or even the entire length, of the structural member, resulting in inefficient use of material.

One of the principal considerations in timber connection design is the risk of timber splitting. Design standards therefore prescribe minimum edge, end and spacing distances for mechanical fasteners. These requirements can result in connection zones that are considerably larger than would otherwise be required based solely on the internal forces within the structural member.

The research investigated the use of densified veneer wood as a local reinforcement material for timber connections. Due to its high compressive strength and stiffness, densified veneer wood is capable of resisting high concentrated forces. By bonding the densified veneer wood to the structural timber at the connection zone, concentrated forces from the mechanical fasteners can be transferred into the densified veneer wood and distributed over a larger bonded surface before being transferred through the adhesive layer into the timber member.

This approach can reduce the risk of splitting while allowing smaller fastener edge and end distances. The increased local resistance may also enable the use of fewer but larger fasteners, potentially reducing assembly time and allowing smaller structural cross-sections. As a result, the connection can achieve a more efficient use of both material and labour.

A combination of experimental testing and numerical modelling was used to investigate the material properties and structural behaviour of the connection. Material tests were first performed to determine the mechanical properties required for the numerical models. The behaviour and capacity of the complete PU adhesive connection between spruce and densified veneer wood were subsequently investigated through shear testing.

The experimental results were then used to assess the ability of the numerical models to reproduce the observed structural behaviour of the adhesive connection.

Research Scope
The research focused on the mechanical properties and numerical modelling of PU adhesive connections between spruce timber and densified veneer wood, including:

• Mechanical properties of densified veneer wood
• Mechanical properties of the PU adhesive
• Shear behaviour of the bonded connection
• Influence of adhesive bond length
• Influence of adhesive layer thickness
• Load-bearing capacity of the bonded connection
• Numerical modelling of the adhesive connection
• Evaluation of numerical models against experimental results

Research Methods
• Experimental material testing
• Compression testing of densified veneer wood
• Tensile testing of PU adhesive
• Double-shear testing of bonded spruce–DVW connections
• Testing of different adhesive bond lengths
• Testing of different adhesive layer thicknesses
• Determination of material properties for numerical modelling
• Numerical modelling
• Comparison and validation of numerical models using experimental results