Structural adhesives are designed for high strength, creep resistance, and resistance to harsh service environments such as high temperatures and chemicals. As a result, the base resins used in structural adhesive formulations tend to be highly crosslinked, relatively brittle materials with high modulus and poor resistance to crack initiation and growth. They are characterized by a lack of toughness and generally by low impact, peel, and fatigue strength, especially at high strain rates.
Toughness is adhesive's ability to absorb energy. It is directly related to the area under the stress-strain curve when the adhesive joint is tested. Tough adhesives generally have both a high degree of elongation as well as a high ultimate strength. Toughness is a characteristic that depends on the viscoelastic or time-dependent nature of the adhesive material.
Properties that are dependent on the adhesive's viscoelastic nature are considered to be dynamic properties. These include those that are dependent on time or rate of loading such as impact, peel, and fatigue.