What type of materials are most susceptible to crack growth starting from tiny imperfections?

Brittle materials are indeed the type most susceptible to crack growth originating from tiny imperfections. The reason for this lies in their inherent structural properties. Brittle materials, such as glass and certain ceramics, do not undergo significant plastic deformation when subjected to stress. Instead, they tend to fracture suddenly when the applied stress exceeds a certain threshold. Because of this sudden failure characteristic, even the smallest flaws or imperfections in their microstructure can serve as stress concentrators, leading to rapid crack propagation.

In contrast, ductile materials possess the ability to deform plastically before fracturing, allowing them to absorb energy and distribute stress more effectively. This characteristic can help to mitigate the effects of small imperfections, as they can undergo some deformation that prevents immediate crack growth.

Composite materials, which are made from two or more constituent materials, can exhibit a range of behavior depending on their matrix and reinforcement components. While some composites may be susceptible to issues related to existing flaws, their behavior is usually more complex due to their layered structure and the diversity of material properties.

Metallic alloys can, of course, display a wide range of mechanical properties. Many metallic alloys have been engineered for ductility and toughness, making them less susceptible to sudden fracture from tiny imperfections compared to brittle materials.