The average density of core plywood varies significantly based on the wood species used, but generally falls within 500–700 kg/m³ for most commercial hardwood-based plywood. This density directly correlates with the panel's bending strength (modulus of rupture, MOR) and resistance to deformation (modulus of elasticity, MOE). Below is a detailed analysis of this relationship:
1. Core Density and Bending Strength (MOR)
Higher density cores (e.g., 650–700 kg/m³ in birch/mixed hardwood) provide greater MOR (typically 70–100 MPa) due to:
Increased fiber density: Enhances load distribution across veneers.
Improved glue bond penetration: Dense cores allow phenolic resin adhesives (e.g., WBP glue) to form stronger bonds, reducing interlaminar shear.
Lower density cores (e.g., 450–550 kg/m³ in softwood) reduce MOR by 20–40%, as porous structures facilitate stress concentrations under bending loads.
2. Core Density and Deformation Resistance (MOE)
Density directly impacts MOE, which governs stiffness. For example:
At 600 kg/m³, MOE averages 10,000–12,000 MPa.
A 10% density increase (e.g., 500 → 550 kg/m³) typically raises MOE by 15–20%, reducing deflection under uniform loads.
Mechanism: Dense cores resist fiber compression and buckling, delaying elastic deformation. This is critical in applications like flooring or roof decking, where deflection limits are stringent.
3. Trade-offs and Limitations
Excessive density risks brittleness: Ultra-dense cores (e.g., >750 kg/m³) may reduce ductility, causing sudden failure under impact loads.
Anisotropic effects: Cross-grain lamination in plywood mitigates warping but complicates density-strength correlations. For instance, low-density cores with balanced veneers can outperform solid wood in stability.
Empirical validation: Testing (e.g., static bending per NF B51-016) shows MOR/MOE scaling with density via power-law relationships.
4. Practical Implications for Structural Design
Subfloors/Roofing: Specify cores ≥600 kg/m³ (MOR ≥80 MPa) for high live loads (e.g., >2.5 kN/m²).
Cost optimization: Use mixed hardwood cores (550–600 kg/m³) for balanced strength-weight efficiency in wall sheathing.
Standards compliance: Density must align with international codes (e.g., EN 13986) for MOR/MOE certification.
Key Parameters by Core Type
| Core Material | Density (kg/m³) | Typical MOR (MPa) | Typical MOE (MPa) |
|---|---|---|---|
| Birch/Hardwood | 650–700 | 90–100 | 11,000–12,500 |
| Mixed Hardwood | 580–650 | 70–85 | 9,500–11,000 |
| Softwood (Pine) | 450–550 | 50–65 | 7,000–9,000 |
| Eucalyptus | 600–680 | 75–95 | 10,000–12,000 |
Conclusion
Core density is a primary determinant of plywood's bending performance. While higher density enhances strength and stiffness, optimal selection requires balancing cost, weight, and ductility. For critical applications (e.g., seismic zones), pair dense cores with composite reinforcements (e.g., FRP skins) to suppress sudden failure. Always verify test certificates for density-MOR/MOE compliance per project specifications.
