Zähigkeit

Zähigkeit is a German noun derived from zäh (tough, tenacious) and denotes the quality of being resistant to fracture, deformation or separation under stress. In materials science and engineering it refers to the capacity of a material to absorb energy and plastically deform before fracturing. Quantitatively, bulk toughness is the area under a stress–strain curve and is expressed as energy per unit volume; practical measures include impact toughness tests (Charpy, Izod) and fracture toughness parameters (e.g., K_IC) used in fracture mechanics.

Zähigkeit is distinct from hardness (resistance to surface indentation) and ductility (extent of plastic deformation) though the properties are related: a highly brittle material may be hard but have low toughness, while many metals combine moderate hardness with high toughness. Toughness depends on microstructure, temperature, strain rate and the presence of flaws; for example, many steels exhibit reduced toughness at low temperatures, whereas polymers and composites show varied toughness depending on molecular structure and toughening mechanisms.

Outside technical contexts, zähigkeit is used in everyday German to describe persistence, perseverance or a chewy texture. In casual speech it can also connote viscosity when applied to fluids (e.g., zähflüssig), but the precise scientific term for fluid resistance to flow is Viskosität. The concept is central to material selection, safety assessments and design against fracture and impact.