Bridging Layers: A Discrete Model for Interface and Fracture in Laminated Timber

Introduction
Laminated timber systems, such as glued laminated timber (glulam) and cross-laminated timber (CLT), are increasingly used in structural applications due to their favourable strength-to-weight ratio and sustainability. Their response is governed by the interaction of orthotropic timber laminae and adhesive interfaces, where complex mechanisms such as tensile cracking, compressive crushing, shear-induced splitting, and interfacial delamination occur.
Current modelling approaches are predominantly based on continuum formulations with smeared damage or cohesive interfaces. While suitable for global response prediction, they often lack a physically consistent description of fracture processes and do not provide a unified energetic framework linking bulk material behaviour and interface failure.
The key knowledge gap is the absence of a discrete modelling approach in which orthotropic fracture, irreversible deformation, and interfacial debonding are consistently described within a energy-based formulation.
 
Materials and methods
The research will develop a discrete modelling framework inspired by the Lattice Discrete Particle Model (LDPM), adapted to laminated timber systems.
Each lamina will be discretised into interacting particles, with facet-based kinematics defined through displacement jumps. The constitutive response at each facet will be formulated in terms of work-conjugate traction–separation laws derived from a Helmholtz free energy potential. The facet-level constitutive behaviour will include:
• Elastic response reflecting orthotropic stiffness associated with grain orientation
• Damage-driven fracture in tension and shear, governed by fracture energy
• Plastic (irreversible) mechanisms in compression and shear, capturing crushing, frictional sliding, and permanent deformations
Adhesive interfaces between laminae will be represented by dedicated interface facets sharing the same kinematic description but with distinct constitutive parameters. Their behaviour will combine adhesion, damage evolution, and frictional sliding, enabling the simulation of progressive debonding. A central feature of the formulation is that both plastic dissipation and fracture energy dissipation are embedded within a unified thermodynamic framework. The evolution of damage and plasticity is driven by energy release and dissipation potentials, ensuring consistency between bulk and interface responses and allowing a natural transition between intra-laminar cracking and interfacial failure. 
The model will be calibrated and validated using:
• Three-point bending tests on glulam and CLT panels
• Uniaxial tension and compression tests with different grain orientations
• Compact tension tests for fracture characterisation
• (Optional) Delamination or shear tests for interface behaviour
Advanced measurement techniques (e.g. Digital Image Correlation) may be used to capture strain localisation and crack evolution. 
 
Expected results
The expected outcome is a discrete, energy-based modelling framework capable of capturing the coupled mechanisms governing the behaviour of laminated timber. 
The model will:
• Provide an energetically consistent description of orthotropic fracture and irreversible deformation within  timber laminae
• Capture interfacial damage, frictional sliding, and delamination using the same kinematic and energetic framework
• Reproduce the interaction between plasticity and fracture processes under complex loading conditions
• Enable predictive simulations across different laminate configurations and loading scenarios
In addition, the research will deliver:
       • Constitutive laws combining fracture energy and plastic dissipation for timber and adhesive interfaces
       • A validated numerical tool for analysing laminated timber structures
       • Improved understanding of competing failure mechanisms (cracking, crushing, delamination)
       • A basis for the development of more physically grounded modelling and design approaches
Overall, the proposed framework advances current modelling strategies by introducing a unified thermodynamic formulation that consistently couples fracture and plasticity at the meso-scale, providing improved predictive capability for laminated timber systems.