The Rheology Handbook. Thomas MezgerЧитать онлайн книгу.
following processes are taking place when a load is applied to a VE solid.
5.1.2.1.10Elastic behavior
When performing a deformation process, a more or less large proportion of the deformation energy applied will be stored by the strained material. When removing the load, only the stored proportion of the deformation energy will be completely available afterwards. Therefore, this proportion of the deformation energy corresponds to the elastic behavior of a sample being the driving force for the re-formation process finally (see Chapter 4.3.1b).
5.1.2.1.11Viscous behavior
In general, when a material is forced to flow, a certain relative motion occurs between its components (molecules, particles, superstructures). Therefore, a process is taking place which is always combined with internal friction, leading to the so-called viscous heating . The proportion of the deformation energy used up by the material during the shear process corresponds to the permanently remaining changes in the internal structure. Of course, this consumed portion is no longer available, e. g. for a complete reformation process after all. A part of the produced friction heat may heat up the sample itself, and another part may be conducted to the outside, being therefore lost (dissipated) for the sample, e. g. heating up the environment then (see also Chapter 2.3.1b).
5.1.2.1.12Examples from daily practice:
5.1.2.1.13a) Rubber buffers, and damping of mechanical vibrations
Viscoelastic behavior of a rubber buffer has to be balanced. If the viscous portion is too small, absorption properties are reduced, e. g. to damp the effects of mechanical vibrations. In this case, a buffer may be too rigid and inflexible, i. e., a large part of the deformation energy arising from mechanical oscillations cannot be used up in the energy-absorbing (viscous) material components in order to render the consequences of this energy harmless.
Example: An insulating mat for damping vibrations under a machine
5.1.2.1.14b) Noise protection, and damping of sound waves
Acoustic absorption properties are reduced if viscoelastic sound insulation materials show a too small viscous proportion. In this case, the material is unable to damp and absorb a sufficiently high portion of deformation energy which is occurring here in the form of the energy of motion produced by the molecules in the air. The goal is to transfer as much as possible of the mechanical energy of the sound waves into heat energy. Without sufficient absorption, this sound energy will be reflected by the material or it is directed through the material to the outside then. As a consequence, noise pollution arises. Specialists in acoustics are aiming to optimize the damping factor which is the crucial characteristic parameter to evaluate absorption of structure-borne sound of materials under dynamic, i. e., oscillating load. This material-specific factor depends on the occurring frequency and on the temperature [5.15]. See also Chapter 8.2.4a: damping factor, oscillatory tests.
Examples: Noise protection materials for vehicles and other machines, walls of sound absorbing rooms, sound insulating mats. See also Example 3 of Chapter 8.7.1c: Acoustic damping behavior of technical rubbers (using the WLF method for time/temperature shift).
5.1.2.1.15c) Car tires, deformation energy and viscous heating
On the other hand, if a damping material has a too large viscous proportion, this can lead to excessive viscous heating, and therefore, to an even destructive degree of deformation finally. In this case, too much deformation energy would be transformed into heat, more than can be simultaneously stored or transported through the material to the outside. As a consequence, further supply of deformation energy might lead to partial or even complete destruction, i. e., to softening or even to the break of the material.
Example: Car tires, and lost deformation energy
Tire air pressure being 0.2/0.4/0.6 bar too low reduces the working life of the tires by 10/30/45 %. Further negative effects are increased rolling resistance, changed driving characteristics of the vehicle for the worse, extended braking distance, and increased fuel consumption [5.16].
5.1.2.1.16d) Automotive bumpers
A bumper of a car should be able to absorb as much mechanical impact energy as possible, or alternatively, should lead it away to other components of the vehicle without any permanently remaining deformation or even destruction. For this purpose, material scientists, physicists and automotive engineers work closely together to develop design modifications and modern VE composite materials in order to optimize the spectrum of the mechanical and thermal properties. For example, integrated damping elements such as crushable bins or crash-boxes are used which are deforming or folding in a defined, desired way when it comes to an accident, therefore absorbing most of the impact energy [5.17]. Re-formable casings and facings are available which can resist collisions with a speed of up to 15 km/h without any damage.
5.1.2.1.17e) Shock absorbers, and the Kelvin/Voigt model
For a shock absorber consisting of a spring/dashpot combination, the spring should keep the deflection within a defined limit and the dashpot should absorb a part of the impact energy by the viscous dashpot fluid (e. g. oil). After the impact, the shock absorber should be pushed back to its initial position by the elastic behavior of the spring. Therefore here, it is aimed to perform a delayed but completely reversible process, comparable to the behavior of the Kelvin/Voigt model.
5.1.2.1.18f) Equipment for sportive activities, and viscoelastic properties
Nowadays, success in sports also depends crucially on the VE properties of the equipment used which should neither be too rigid nor too flexible. Some games are only possible due to the equipment’s VE behavior, and others are only attractive and tricky due to this. Sports goods, sports kit and sportswear etc. have become a paying playground for developers of VE composite materials.
Examples: The pole for pole vaulting; shoes when running and jumping (using “gel soles”); balls containing an air bubble or made of composite or solid material; the combination of the racket handle, racket strings, ball and net in tennis, squash, badminton, table tennis or ping-pong. See also Experiment 8.2 of Chapter 8.4.3a: Bouncing rubber balls (and tanδ).
5.1.2.1.19g) Biological materials, and synthetic “bio-materials”
Life itself could not have developed in all its diversity, were it not for the complex interplay of natural VE liquids and VE solids.
Examples: Blood (if the elastic portion of this VE liquid is too high, there is the risk of a stroke; haemo-rheology, i. e., blood-rheology), mucus, the synovial fluid (to lubricate the knee joint), the vitreous body of the eyes (the water content of this gel-like material is more than 98 %); soft, flexible, swellable, taut or firm tissues (such as skin, cornea, cartilages, body fat); arteries and veins (if they are becoming too inflexible, this may lead to arteriosclerosis, which means artery hardening), leaves, twigs, tendons, barks, bones, wood, tree trunks.
The spine is a complex natural composite material consisting of disks (fibers with a gelatinous core), vertebrae (spongy bone containing bone marrow), ligaments, joints and connecting tissue. Its task is to give stability to the body, to absorb impacts, to show elasticity and also to enable us to make bending, stretching and torsional motions [5.25] [5.26] [5.27].
These areas of medical technology/biotechnology are opening up new opportunities