Polyurethane (PU) core sandwich panels are widely used in construction, transportation, and other industries due to their excellent mechanical properties, insulation, and lightweight nature. The mechanical behavior of these panels largely depends on the characteristics of the PU core, the materials and thickness of the face sheets, and the overall structure. Here’s an overview of the key mechanical behaviors:
1. Compressive Strength
- PU core sandwich panels have good compressive strength, with the PU core providing the main support in the panel's thickness direction. The core absorbs compressive loads, while the face sheets bear tensile and shear forces, creating a balanced load distribution. Higher-density PU cores offer increased compressive strength but add weight.
2. Shear Strength
- Shear strength in sandwich panels is critical, as these forces often act between the core and face sheets. PU cores typically offer moderate shear strength. To enhance this property, sandwich panels may incorporate additional bonding agents or adhesives between layers. This property is essential for applications under torsional or bending stresses, such as in flooring systems.
3. Flexural Strength
- The combination of a lightweight core and rigid face sheets offers high flexural rigidity, with the face sheets providing most of the resistance to bending. The core resists shear and prevents the face sheets from buckling, enabling the panel to withstand higher flexural stresses and deformations with minimal weight increase.
4. Impact Resistance
- PU core sandwich panels generally exhibit moderate impact resistance. They are not as resistant to localized impacts as metal or composite cores, but the flexibility of PU helps absorb some impact forces, preventing immediate catastrophic failure. The addition of fiberglass or metal skins can improve impact resistance.
5. Creep and Long-term Deformation
- Under sustained loads, PU core materials can exhibit creep, or gradual deformation, over time, especially at elevated temperatures. However, panels with closed-cell PU foams tend to perform better due to the foam structure, which distributes stress and slows creep.
6. Thermal Expansion and Contraction
- PU cores have low thermal expansion, which helps maintain structural stability under temperature changes. However, when the panel is subjected to extreme temperature fluctuations, differential expansion between the core and face sheets may lead to delamination or cracking over time.
7. Fire Performance
- Pure PU cores can be flammable, but fire-retardant additives or coatings are often added to enhance fire performance. Panels made with modified PU cores can exhibit improved fire resistance and maintain mechanical integrity longer in high-temperature conditions.
8. Moisture Resistance and Durability
- PU is a closed-cell foam with good moisture resistance, which prevents water ingress that could degrade the core and impact mechanical strength. The panel faces, typically made of aluminum or fiberglass, are also resistant to corrosion and further protect the core.
For applications, understanding these behaviors helps in selecting the right sandwich panel configurations, such as PU density and face sheet material, to meet specific mechanical and environmental demands.
