Rivlin-Thomas 0-10%, in more rare cases a maximum of 15% strain is considered. The yield strength is the amount of stress that a material can undergo before transitioning from reversible elastic to irreversible plastic deformation, which is also known as yielding. The ultimate tensile strength (UTS) is a measure of the maximum amount of stress that a material can withstand, while being stretched or pulled, before it starts failing, eventually leading to material fracture. It is usually the highest point of the stress-strain curve obtained from the tensile test and a measure of the hydrogel's strength and resistance to break under tension. Hydrogels and more generally soft elastic materials often do not experience plastic yielding and therefore, do not exhibit a true yield strength. Instead of the UTS, the fracture stress at break is measured where ultimate tensile failure occurs (σ b ). Finally, the toughness W measures a material's ability to absorb energy before breaking and therefore equals to the work of extension until failure. Sometimes referred to as strain-energy density, the toughness W can be obtained through integration of the area under the stress-strain (or stress-stretch) curve using the equation below (Figure ):