Influence of layer architecture on fracture toughness and specimen stiffness in polymer multilayer composites

Abstract

The objective of this contribution was to increase the fracture toughness of talcum reinforced polypropylene (PP) while preserving specimen stiffness. This was accomplished by introducing soft interlayers (ILs) made of standard PP (PP-St) or very compliant PP (PP-Soft) and utilizing the so-called material inhomogeneity effect. Architectures with one or two ILs of either 0.3 or 0.9 mm thickness were tested in single edge notched bending experiments. Layers of PP-Soft always arrested growing cracks due to their low Young’s modulus, E, and yield stress, σy, which is called an (E-σy)-inhomogeneity. However, the increase in fracture toughness came at the cost of specimen stiffness. For ILs made of PP-St, E was still lower compared to the matrix material, but σy was similar (pure E-inhomogeneity). Specimen stiffness remained high for these composites, but crack arrest could not be achieved in most cases, which could be explained by plastic deformation of the soft layers. Plastic deformation could be contained within the ILs in one of the architectures, where two large ILs were used. Crack arrest could be achieved in this adapted IL design, leading to excellent fracture toughness in combination with high stiffness.

Keywords

• Polypropylene
• Multilayer
• Fracture mechanics
• Biomimetic design
• Material inhomogeneity effect