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Viscoelastic compounds show time-dependent elongation under a constant load, a characteristic known as creep. They act like liquids under this type of strain, but recover partially once the strain is removed.
These compounds became a subject of investigation when the need for power surged from the nineteenth century onwards. This raised the demand for compounds capable of resisting higher temperatures and pressures than ever before.
Metals were by then known to creep significantly at high temperatures. Creep in electrical bulbs, for instance, can lead to short-circuits as the filament sags and coils come into contact with each other. This underlay the development of creep-resistant alloys, or superalloys, formulated from nickel, chromium, aluminum, molybdenum and titanium, to name a few.
The same is true of synthetic polymers produced from plastics and many biomechanical materials. Thus, compounds with high viscoelasticity are those which can creep and recover, experience stress relaxation and absorb energy.
Viscoelasticity in Natural Materials
Some examples of creep in nature are:
Human Intervertebral Discs
These discs cushion the vertebra against friction while various spinal movements are taking place. They are made of fibrocartilage, which is both elastic and energy-absorbing. In normal circumstances, the discs gradually thin out and become wider, showing signs of creep as the body weight bears down on them. When a person lies down, weight is reduced and the discs slowly regain their original width and diameter (creep recovery). This means that after a few hours of rest in bed a person may gain a few centimeters in height. The best way to measure one’s true height is therefore in the morning, after a night’s sleep.
Joint cartilage is highly viscoelastic and absorbs shock to a high degree. When it wears down or breaks down, as in inflammatory arthritis or osteoarthritis, the bone ends grate against each other, making the joints extremely painful. Synthetic viscoelastic materials are sometimes suitable for intra-articular injection into such a joint, to cover the joint bones where cartilage is lacking. This provides shock-absorption.
The human skin tends to retain its folds momentarily when pinched up, and the longer the pinch is maintained, the longer the recovery period. This property reflects its viscoelasticity.
The sagging of wood beams in old houses reflects the tendency of the wood to creep under the weight of the roof and their own weight.
How Can Viscoelastic Creep be Beneficial?
Natural Creep in Newborn Skulls
A newborn’s head is often quite misshapen as it undergoes creep while pressing through the birth canal, formed of ligaments and bones. This helps it recover its normal shape after birth.
Creep in Foam Cushions
Another use of creep is in polymer foam cushions used for people in wheelchairs and in hospital beds, which allow the body to press the cushion to its shape, reducing pressure on the bony points of the body.
Absorption of Viscoelastic Energy Absorption
Absorbing Vibrational Energy
Human operators of vibrating tools like jackhammers suffer loss of efficiency and even long-term neurological sequelae from the noise and vibration. If these adverse effects are reduced the benefits would include a safer environment for machine operation, reduced downtime, longer machine life and reduced damage to the floor.
One way to do this is by using shock absorbers to isolate these machines from vibration sources, using a combination of mass, springs and dampers made of material with high viscoelasticity and density, and of the right thickness.
Materials with high viscoelasticity can absorb energy and are used to make dampers in tall buildings, for instance. Such buildings vibrate when they are subjected to wind or earthquakes. These dampers are made up of steel plates which are covered with viscoelastic polymer and are attached to certain diagonal bracing elements of the building structure. They take up the energy of the vibration and thus stabilize the building.
Other dampers are made from copper-manganese alloys, as in the propellers that drive naval ships. These, as well as zinc-aluminum alloys, are also used to make pneumatic rock crushers and to line gloves for people working with intensely vibrating tools like jackhammers and pneumatic drills.
Helicopter noise is generated from multiple sources, but especially the rotary turbine and the gears, amplified by the fuselage skin resonance. Thus, fiberglass-vinyl cloth sandwiches are used to make acoustic protective blankets. These are used to line the fuselage to prevent noise amplification by absorbing vibrations.
In situations which involve impacts and collisions, the impact can also be absorbed using viscoelastic materials like elastomers, reducing its force by half. Such materials are used to make automobile crash fenders, bumpers, foam padding in helmets, wrestling mats, and the insoles of shoes to shield the skeleton from impact.
Other applications of viscoelastic materials include reducing body impact in high-impact sports, designing ergonomic equipment, damping sounds and vibrations in electronic goods, and improving acoustic performance.
Delicate or important components could be damaged by vibration, impact or collision. Protecting them against these helps increase the operational life and enhance the efficiency of expensive machinery. Dampers with high viscoelasticity help protect such components or machines against friction, or when they need to be moved from one spot to another.
Can Viscoelastic Materials Prevent Vibrational Creep?
Vibrational creep occurs when a severely vibrating machine moves out of its designated place during its operation. This can be damped using high viscoelastic materials, which not only prevent friction and thus prevent creep, but also protect the machine from damage.
Machine connections often suffer from high vibrations, and materials with high viscoelasticity could be used to protect these connections.
Thus, materials with high viscoelasticity may be ideal for a number of applications, and these materials are finding new uses in workplaces, homes and buildings, making them safer and more durable.
Sources and Further Reading