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'''Viscoelastic substances''', often referred to as viscoelastic materials, are substances that exhibit both viscous (fluid-like) and elastic (solid-like) properties. These materials display a time-dependent response to stress, meaning their behaviour changes over time when subjected to a constant load or force. Examples of viscoelastic materials include polymers, biological tissues, and some fluids. This article provides an overview of viscoelastic substances, including their characteristics, behaviour, and applications.
	'''Viscoelastic substances''' are materials that show both viscous and elastic behaviour when they are deformed. A purely elastic solid stores mechanical energy and returns quickly to its original shape. A purely viscous fluid flows and dissipates energy. A viscoelastic material sits between those ideal cases, so its response depends on time, loading rate, temperature, and material structure.

==Characteristics==
Viscoelastic substances exhibit several key characteristics:
	Common examples include rubber, many polymers, asphalt, gels, foams, biological tissue, cartilage, tendons, skin, some food products, and damping materials used in engineering.

'''Time-Dependent Response''': Viscoelastic materials respond to stress in a time-dependent manner. This means their behaviour changes over time when subjected to a constant load or force. For example, when a viscoelastic material is stretched, its stress-strain curve will exhibit different characteristics depending on the rate of deformation.
	== Core Behaviour ==
	The defining point is that stress and strain are time-dependent. A viscoelastic material may respond like a stiff solid during a quick impact but flow or relax when the same load is applied for longer.

'''Hysteresis''': Viscoelastic materials exhibit [[hysteresis]], which means there is a lag between the application and removal of stress. This results in energy dissipation, where some of the applied energy is lost as heat. The area enclosed by the stress-strain curve during loading and unloading represents the energy dissipated as heat.
	Important behaviours include:

'''Creep and Stress Relaxation''': Viscoelastic materials display creep, which is the gradual deformation under a constant load, and stress relaxation, which is the decrease in stress over time under a constant strain.
	* '''Creep''', where strain increases over time under a constant load.
	* '''Stress relaxation''', where stress falls over time when a fixed strain is held.
	* '''Hysteresis''', where loading and unloading do not follow the same path and some energy is lost as heat.
	* '''Rate-dependent stiffness''', where the material appears stiffer or softer depending on how quickly it is loaded.
	* '''Damping''', where vibration energy is dissipated.

==Behaviour==
The behaviour of viscoelastic materials is described by their stress-strain curves, which illustrate the relationship between stress (force per unit area) and strain (deformation). The stress-strain curve for a viscoelastic material typically exhibits three regions:
	These behaviours are why a rubber sole, a memory foam cushion, and a tendon do not behave like simple springs.

'''Elastic Region''': In the elastic region, the material behaves like a solid, and stress is directly proportional to strain (Hooke's law).
	== Creep and Relaxation ==
	Creep is seen when a constant stress is applied and the material continues to deform. A polymer under a hanging weight may stretch quickly at first and then more slowly as time passes.

'''Viscous Region''': In the viscous region, the material behaves like a fluid, and stress is not proportional to strain. Instead, strain increases with time even under a constant stress, as seen in creep tests.
	Stress relaxation is the opposite testing idea. The material is stretched to a fixed strain and held there. In a viscoelastic material, the measured stress can fall with time because internal molecular arrangements adjust.

'''Viscoelastic Region''': In the viscoelastic region, the material exhibits both elastic and viscous behaviour. The slope of the stress-strain curve in this region represents the viscoelastic modulus, which characterizes the material's resistance to deformation.
	Both effects are important in design. A seal, gasket, joint, or polymer support may work at first but gradually lose shape or force if creep and relaxation are ignored.

==Applications==
Viscoelastic materials have a wide range of applications across various industries:
	== Hysteresis and Energy Loss ==
	When a viscoelastic material is loaded and then unloaded, the unloading path is often different from the loading path. The loop between those paths represents energy that has been dissipated, usually as heat.

'''Polymeric Materials''': Many polymers, such as polyurethane foam and rubber, are viscoelastic. They are used in applications such as cushioning materials, sealants, and insulation.
	This effect is useful in vibration damping, tyres, shoe soles, protective padding, and impact-absorbing equipment. It can also be a weakness if too much energy loss causes heating, wear, or poor efficiency.

'''Biological Tissues''': Biological tissues, including skin, tendons, and cartilage, are viscoelastic. Understanding their viscoelastic properties is crucial for medical applications such as tissue engineering and prosthetics.
	== Material Examples ==
	=== Polymers and Rubber ===
	Polymers are often viscoelastic because their long molecular chains can rearrange under load. Rubber can recover after deformation but still loses energy during repeated loading.

'''Food Industry''': Many food products, such as bread and cheese, exhibit viscoelastic behaviour. This property influences their texture, shelf life, and processing methods.
	=== Biological Tissue ===
	Many tissues are viscoelastic. Tendons, cartilage, skin, and blood vessels all respond differently depending on speed and duration of loading. This matters in biomechanics, injury modelling, prosthetics, and tissue engineering.

'''Construction''': Viscoelastic materials are used in construction for seismic isolation, damping, and noise reduction in buildings and infrastructure.
	=== Foods and Gels ===
	Bread dough, cheese, jelly, and many processed foods have viscoelastic properties. Texture, mouthfeel, cutting behaviour, and shelf stability can depend on how these materials respond over time.

'''Aerospace''': Viscoelastic materials are used in aerospace applications for damping vibrations and reducing noise in aircraft and spacecraft.
	=== Engineering Materials ===
	Asphalt, foams, sealants, adhesives, damping pads, and vibration isolators may all be designed around viscoelastic response. Their performance can change with temperature and loading frequency.

==Conclusion==
	== Measurement ==
	Viscoelastic behaviour can be measured with tests such as creep testing, stress relaxation testing, dynamic mechanical analysis, indentation, rheometry, and cyclic loading. These tests help describe how a material behaves over different time scales and temperatures.

Viscoelastic substances are materials that exhibit both viscous and elastic behaviour. Their unique properties make them valuable in various industries, from polymers and biological tissues to food and construction. Understanding the viscoelastic behaviour of materials is essential for designing products, developing new materials, and optimizing processes across a range of applications.
	Simple elastic constants are often not enough. Engineers and researchers may use time-dependent models such as Maxwell, Kelvin-Voigt, standard linear solid, or Prony-series descriptions to represent the material response.

==References==
	== Applications ==
	Viscoelastic materials are used in:

"Viscoelastic Materials." University of California, Irvine
	* Vibration damping and acoustic control.
	* Shock absorption and protective padding.
	* Tyres, seals, gaskets, and flexible joints.
	* Medical implants, prosthetics, and tissue models.
	* Food processing and texture control.
	* Construction materials such as asphalt and damping layers.

"Polymer Viscoelasticity." Polymer Science Learning Center
	The same behaviour that makes a material useful can also create design problems. Long-term creep, heat build-up, ageing, and temperature sensitivity must be considered when the material is used in a safety-critical or load-bearing role.

	== References ==
	* [https://www.britannica.com/science/viscoelasticity Britannica: Viscoelasticity]
	* [https://courses.ems.psu.edu/matse202/node/717 Penn State MATSE 202: Polymers as viscoelastic materials]
	* [https://lakeslab.ep.wisc.edu/VEnotes2.html University of Wisconsin: Viscoelasticity notes]
	* [https://www.biolinscientific.com/blog/what-is-a-viscoelastic-material Biolin Scientific: What is a viscoelastic material?]

	[[Category:Materials science]]
	[[Category:Physics]]