This lecture explores the toughness and fracture mechanisms of composite materials, focusing on how cracks propagate and the energy consumed during the process. The instructor discusses the importance of fiber orientation, interface strength, and the bipartition mechanism in enhancing the toughness of composites. The lecture also covers the concept of porosity in materials, its effects on mechanical properties, and the deformation behavior under compression. Examples of porous materials like wood are provided to illustrate these concepts.