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Natural rubber — Its engineering characteristics
Natural R u b b e r its Engineering Characteristics Barbara Davies, Malaysian Rubber Producers' Research Association (MRPRA), Tun Abdul Razak Laboratoo,, Brickendonbuo,, Hertford SG13 8NL. Abstract
Natural rubber is an elastomer with excellent properties, which have been exploited in a wide range of applications. Despite the fact that it is a well-defined engineering material, with comprehensive documentation on both mechanical data and design principles, many engineers remain ignorant of natural rubber's potential. This article outlines the nature of the raw material and how it is compounded & shaped into useful products; the physical properties and engineering characteristics of natural rubber vulcanizates are also described. Environmental effects and their minimization by suitable compounding are discussed. Examples of several applications are given, including some showing the long service life of natural rubber engineering components. The aim is to introduce engineers to natural rubber and to show that information exists for the design of components with known mechanical properties and predictable in-service behaviour.
Introduction Natural rubber is the elastomer to consider in many engineering applications ranging from small bushes in mechanical engineering to building mounts and massive dock fenders in civil engineering. In many of these it functions as a spring, and from the rubbers available is chosen for its resilience, resistance to fatigue, low heat build-up, wide range of operating temperatures, good bonding to metals and ease of manufacture. Rubber springs also compare favourably with those of metal. It is possible to design a much simpler spring requiring no maintenance and having anisotropic stiffness. Rubber springs can accommodate some misalignment; (making installation easier). and the small amount of hysteresis inherent in the material will damp dangerous resonant vibrations in service. The above information suggests why rubber, particularly natural, should be the preferred material in spring applications. That this is not always the case owes much to engineers' greater familiarity with the conventional materials such as steel used routinely to design springs. Some engineers have taken on board new concepts encompassing the mechanical behaviour of rubber and a different design approach. They have discovered the advantages listed above, and have also proved that it is possible to design components in rubber which have known mechanical properties and will behave in service as predicted. A basic knowledge of these mech-
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anical properties is required before natural rubber can even be assessed as suitable or otherwise for a particular application. Once established as an appropriate material, preliminary designs may be produced and manufacturers approached. A standard component with the desired mechanical properties may be available, but often springs must be designed from scratch. Here the engineers may meet another problem: having learnt about the mechanical behaviour of rubber they still cannot communicate with the rubber technologists, who are familiar with the terminology of conventional physical properties. For this reason, engineers often regard rubber technology as a 'black art'. There are rubber manufacturers familiar with engineering requirements, who would find no problem in characterizing the material properties of a particular rubber compound. However, the number of complex measurements under different conditions that would be required would make this a costly exercise if more than just a few compounds were considered. The aim of this article is to show the common ground existing between the engineer and the rubber technologist. For natural rubber in particular, published mechanical data and design information are available: it is a welldefined engineering material. Before an outline of the engineering properties and design features of natural rubber, a brief description is given on the nature of rubber, how it is compounded and how this affects properties.
Raw Natural Rubber Natural rubber is a high molecular weight polymer, chemically cis-polyisoprene, occurring in nature as a colloidal suspension beneath the bark of certain trees. The commercial source is the species Hevea brasiliensis, which is cultivated in tropical regions mainly in Southeast Asia. The trees are tapped to obtain latex, and while about 15% of production is concentrated and used as such. most is coagulated to form dry rubber. A number of grades is available to suit different applications and technical specifications assure the quality of rubber produced. Malaysia, the world's largest producer, operates the Standard Malaysian Rubber (SMR) Scheme, within which there are a number of grades suitable for engineering applications. These are the top quality grades - SMR CV, LV, L and W F - produced from factory processed latex under tightly controlled conditions. In its raw state natural rubber consists of long, flexible polymer chains and its molecular weight ranges from 100 000 to over one million. In this state it has little practical use: it flows easily under load as the molecules slide over each other and fails to recover when the load is removed. It crystallizes at temperatures around 0°C, and becomes soft and sticky in hot weather. To convert rubber to a useful material, it must first be masticated to break down the long polymer chains and allow the introduction of other compounds.
MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986
C o m p o u n d i n g and Vulcanization To confer elastic properties over a wide range of temperatures, chemical crosslinks are introduced between the individual polymer chains, broken down by mastication into shorter lengths. Other ingredients of the compound will include fillers, protective agents and processing aids. Sulphur systems are most commonly used to vulcanize rubber, although urethane and peroxides are sometimes used. Stiffness of the rubber can be raised by increasing the number of crosslinks, but too high a crosslink density can lead to a deterioration in strength. A crosslink density to give high strength is normally chosen: stiffness can be increased or decreased by the addition of reinforcing fillers or oil. The filler used for high quality engineering applications is usually carbon black. Apart from stiffness, the filler may also influence hysteresis, strength properties and fatigue life. The raw rubber, vulcanizing system and fillers determine the initial properties of the compound. In service, it is necessary to protect a component so that these do not markedly deteriorate. Protective chemicals are therefore added to the mix to retard the changes brought about by oxidation, ozone attack, high service temperatures etc. The final mix may also contain materials to aid manufacturing processes. After thorough blending of all ingredients, the mix is heated and shaped by moulding or extrusion to give the final vulcanized product.
Physical Properties Physical constants for gum (unfilled) & filled vulcanizates, steel and water are compared in Table I. A rubber compound is frequently both specified and quality controlled, by hardness & tensile stress-strain properties. However, these are inadequate for engineering design purposes: those properties required are discussed later.Hardness is a measure of reversible elastic deformation under a specified load and is therefore related to Young's modulus. It is measured in International Rubber Hardness Degrees (IRHD), which are approximately the same as British Standard Hardness Degrees and readings on the Shore Durometer A Scale. The hardness of natural rubber vulcanizates used in engineering applications may vary from 20 to over 80 IRHD. Frequently-used tensile stress-strain properties include tensile strength, elongation at break and stress a t 1 0 0 % and 300% strain. The stiffness of natural rubber increases at high strains: this is due to chain extension between crosslinks approaching the limiting value and to strain-induced crystallization. The latter is responsible for natural rubber's high tensile strength, long fatigue life and excellent tear resistance.
three orders of magnitude greater than its shear modulus; Young's modulus is three to four times the low strain shear modulus. The load deflection curves for rubber in tension and compression are linear for strains of a few per cent. Values of Young's and shear modulus can be obtained from the linear region. Characteristic curves for three modes of deformation - sheer, compression and buckling - are shown in Fig. I. Behaviour in shear is more linear than that in compression. The relatively high bulk modulus means that rubber hardly changes volume under high compressive loads: a rubber block will bulge at the sides as constant volume is maintained. The thinner the rubber layer for a given cross-sectional area, the greater the
dellecl~
L o a d - D e f o r m a t i o n Characteristics Rubber is relatively incompressible: it has a Poisson's ratio close to 0.5. It can be seen from the table that rubber's high bulk modulus (ca 2000 MPa) is some
Fig. 1
Force deflection curves for rubber units under different m o d e s of deformation.
Table ! Physical Constants of Vulcanized Natural R u b b e r C o m p a r i s o n of a soft gum rubber, a harder black-filled rubber, mild steel and water. ( F r o m ref. 1)
Hardness* Tensile strength (TS) Elongation at break (EB) Young's modulus (Eo) Shear modulus (G) Bulk Modulus (Eo~,) Poisson's ratio Resilience Velocity of sound transmission Specific gravity Specific heat Thermal conductivity relative to water Coefficient of cubical expansion/deg C Electrical resistivity t Dielectric constant Power factor * t
IRHD MN/m 2 % MN/m 2 MN/m" MN/m 2 % m/s
ohms/cm cube
G u m rubber
Filled rubber
45 28 680 1.9 0.54 2000 0.4997 8O 37 0.93 0.45 0.25 67 x 10 .5 1.7 x 1016 3 0.0O2
65 21 420 5.9 1.37 2400 0.4997 60 37 1.16 0.41 0.31 56 x 10 .5 3 x 10 I° 15 0.1
Mild steel
Water
0 420 40 210 000 81 000 176 000 0.29 100 5000 7.7 0.116 73 3.5 x 10 .5
2100
1430 1 1 I 21 x 10 .5 9 x 10 .6 80
hardness scale range is 0 to 100. ( I R H D = International Rubber Hardness Degrees) but specially-compounded conductive rubbers 1-107 ohms/cm cube
MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986
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restriction to bulging and the stiffer the component will be. This feature is used in the design of compression springs: the compression modulus depends on the shape factor, defined as the ratio of one loaded surface to the total forcefree area. By incorporation of steel plates in a rubber spring unit, the spring can be designed to have changed stiffness in different directions, accommodating loads of varying magnitudes. Combined compression and shear is commonly used. Examples here include helicopter rotor bearings-', which are stiff in compression to accommodate the high centrifugal force (up to 75 000kgf), while relatively soft in shear to allow pitch, flap and lag; another example is the chevron spring commonly used in the suspensions of London Underground rolling stock 3. In both areas the use of steel-laminated natural rubber bearings has resulted in a considerable reduction in the number of parts used, elimination of maintenance, increased reliability and longer spring life. (Figs. 2&3). The smooth rising rate load-deflection relationship for rubber units under compression is used in simple compression springs - hollow rubber cylinders bonded at either end to metal plates. Such springs have been used for some time in the suspensions of commercial vehiclesL as the primary suspension in Formula 1 Grand Prix racing 5, and more recently in the World Land Speed Record car, Thrust 26. Compared with metal springs, which exhibit linear characteristics, this type of spring becomes progressively stiffer as the load increases. The result is a fail-safe spring which provides improved stability over a wide range of speeds and loads. The buckling mode of deformation is used less frequently, but is effective in certain types of dock fender, where it enables large amounts of energy to be absorbed without causing high peak forces to be tyansmitted to the vessel or structure. (Fig. 4).
Dynamic Mechanical Properties Most rubber springs must accommodate dynamic movements superimposed on the static load. The dynamic modulus depends on temperature, frequency and amplitude of deformation (see Fig. 5), although it is substantially independent of frequency below lO00Hz and at ambient temperatures. It may be measured in either shear or compression. Under dynamic conditions, carbon black affects strain amplitude, the effect diminishing with decreasing levels of carbon black. This is why spring vulcanizates of
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Fig. 2. A helicopter rotor bearing sectioned to show its laminated structure and the rotor head assembly showing position of the bearings. The bearing is mounted such that the high blade centrifugal force results in compression, while pitch, lag and flap produce shear deflection.
Fig. 3
A variety of steel laminated natural rubber springs, including a chevron spring of the type used in London Underground rollingstock.
about 50 IRHD are sometimes preferred. Generally, at higher carbon black loadings, dynamic shear moduli will decrease with increasing amplitude. Another effect due to carbon black is the marked softening of rubbers within the first loading cycles, caused by some breakdown of the black structure. For this reason, rubber engineering components may frequently be preloaded to the maximum load a few times before testing. Hysteresis, the energy loss between loading and unloading cycles, occurs in
all rubbers. Generally natural rubber has low hysteresis, but values increase with increasing carbon black loading. Low hysteresis means low heat-buildup, important in dynamic applications. However the existence of some hysteresis in natural rubber prevents excessive vibration amplitude at resonance. For good vibration isolation, the natural frequency of the loaded system should be less than one-third of that imposed. (see Fig. 6.) Hysteresis is often characterized by the loss of phase angles and varies with temperature,
MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986
frequency and. at high filler loadings, amplitude. If the amount of hysteresis is increased then the amount of damping at resonance is also increased. This can be desirable in some applications and high damping natural rubber compounds have been developed.
Design Data The design engineer needs information
Fig. 4
Natural rubber dock fenders installed at Port Lincoln, Australia. Each measures 2.44m long and 1.83m in diameter and weighs 6.5 tons.
on static and dynamic moduli and loss angles measured over a range of strains, frequencies, amplitudes and temperatures. It is in this area that published data are rare, except for natural rubber, which is the best characterized elastomer available. The Malaysian Rubber Producers' Research Association has expended a great deal of effort for many years in convincing engineers that natural rubber is a well-defined engineering material. As pan of this work. the Engineering Data Sheets 8 provide a basis on which rubber technologists and engineers can converse. Over 40 vulcanizates, selected to be typical of those used in engineering applications, are characterized. Each sheet provides a large amount of precise data on a specific natural rubber vulcanizate, including the basic properties familiar to the rubber technologist and, for the engineer, static and dynamic mechanical properties over a range of different strains, temperatures and frequencies. The design data provided by each sheet includes bulk and shear moduli, the latter over a range of strains from 2-350% at ambient temperature, for initial deformation and after nine cycles to 100% shear. Static compression IdB
Longevity The conditions under which a rubber component is stored or used may affect its performance. However, with modem compounding techniques any deterioration can be minimized or virtually eliminated. There are plenty of examples which bear witness to the longevity of natural rubber in even the most demanding applications. Before the advent of large-scale production of synthetics in the 1940s, natural rubber was the only elastomer available. Two of the oldest examples are a sewer ring installed in a London sewer in the 1850's and discovered during reconstruction work in 19639, and an AusI
1
--+20 $ I
Dynamic shear m o d u l u s M N / m ~
moduli are given over strains from 5-20% at six shape factors of 0.5-8.0. A range of low frequency (0.1 Hz) dynamic tests over temperatures ranging from -40°C to 150°C gives information on both static and dynamic shear moduli: the moduli at this frequency are essentially the same. At higher frequencies (1 Hz at 1-100% strain amplitude and 15 Hz at 2-10% strain amplitude) dynamic shear moduli are given over the same temperature range. Loss angles are also given for all three frequencies.
I
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Fig. 6
Variation of dynamic shear modulus with temperature and strain amplitude at 0.1 and 10Hz for gum and blackfilled vulcanizates.
M A T E R I A L S & D E S I G N Vol. 7 No. 2 M A R C H / A P R I L
1986
Dependence of transmissibility on ratio of imposed frequency n to natural frequency n o for various levels of hysteresial damping (expressed as the tangent of phase angle o'). Transmissibility is defined as the ratio of vibration amplitude on the 'protected' side of the spring to that of the disturbing vibration. When the ratio of disturbing frequency to natural frequency is unity (A) resonance occurs.
71
~
Comp¢ossive Iogd (kN) 8OO
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Fig. 7
A bridge bearing from the M2 motorway was removed by the Department of Transport for inspection in 1982, after 20 years of service• The stiffness of the bearings falls within the initial range of values obtained for the bearings supplied• Tests have also shown that the rubber in the bearing still fulfils all the requirements laid down at the time of installation, as well as those specified in the current more stringent standard (BS 5400).
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o
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tralian bridge pad installed in 1889 and still in service '°. Both these components were found to be in excellent condition, even without the benefits of modem compounding. More recently, extensive studies of bearings used to isolate buildings from vibration have provided evidence that such bearings will give excellent performance for at least 100 years ~t. Bridge bearings removed for inspection after 20 years service were found to be performing within the original specification limits z.,. (Fig. 7).
LOW TEMPERATURE STIFFENING -25* C O acrylonltrlle-butacllene 1000-
/
Natural rubber engineering vulcanizates can be produced to resist the effects of temperatures ranging from over 60°C to 100°C, and intermittently even higher. The static modulus of gum vulcanizates is approximately proportional to absolute temperatures, over the range -20°C to 70°C. Fillers reduce this effect and under dynamic conditions may even reverse it. In the design of components for use over a wide temperature range, allowance should be made for the temperature dependence of stiffness, thermal expansion and contraction. At temperatures below -20°C stiffness increases over a period of time: at -40°C it is double that at +20°C. However, use of suitable compounds and vulcanization conditions can slow this process so that significant stiffening can be avoided for many months. Two phenomena are involved;
72
x •
/
rubber
polychloroprene natural rubber
P SOO-
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Temperature
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Fig. 8a & 8b Comparison of low temperature stiffening in acrylonitrile-butadiene, polychloroprene and natural rubber, at -25°C and +3°C. At -25°C, N B R shows immediate and considerable stiffening, while polychloroprene stiffens markedly more than natural rubber after a few hours. On an expanded scale, and at more moderate temperatures, the increased stiffening of polychloroprene compared with natural rubber can be seen. low temperature crystallization, most rapid at-25 °C. and glass hardening at -70°C, when there is a transition to a brittle, glassy state and the modulus increases a thousandfold. Both effects are reversible, the vulcanizate returning to its original state when warmed. Low temperature stiffening is not generally a serious problem in natural rubber, unlike comparable synthetics which tend to stiffen more rapidly or at more moderate temperatures '3 (Fig 8a & 8b).
At high temperatures, physical properties are temperature dependent, but these effects are reversible provided no chemical changes take place. At temperatures approaching 140°C, chemical degradation may occur, resulting in a rapid loss of mechanical properties and increased hardness. Conventional natural rubber vulcanizates can be used up to 70°C, and other compound types will provide satisfactory service at 100°C and intermittently much higher
MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986
temperatures. The behaviour of a thickwalled engineering component, even at IO0°C, is very much better than tests on thin-sheet standard test pieces would suggest.
dants giving excellent resistance to oxidative degradation, antiozonants and/or waxes to provide resistance to ozone cracking and carbon black, which, in addition to its other effects, protects against ultra-violet radiation. Rubber components used in engineering tend to be of thick section, and the bulk of the rubber acts as a reservoir for the protective agents, which migrate to the surface replacing those which have already reacted with oxygen and ozone.
Weathering Oxygen, ozone and sunlight can all degrade natural rubber if it is not suitably protected. Engineering components, which are often required to function effectively over many years in exposed conditions, contain antioxi-
tO years
4 yeats b
I year
--
4 monlhs
--
1 month
4 days
D
1 week
10
01
10
1O 0
solvenls
T0 0 0
10 000
engine o,ls
heavy oils
v l s c o s t t y Ot IlQutd, m N s J m .
Fig. 9
The time taken for a liquid to penetrate a given stance into rubber depends on its viscosity. This graph shows the til ~ taken to penetrate 5 mm; it would take four times as long to penetrate I cm. (1 m N s / m e -1 centipoise).
Creep (mm)
1.0 3-8
Recent M R P R A research suggests that a 'skin' impervious to oxygen is formed on the surface after relatively short periods in service. While antioxidants are extremely efficient, complete protection against ozone cracking is less readily achieved by protective agents. For rubber subjected to continuous dynamic deformation, ozone cracks grow at about 0.25ram per year. The most effective protection is the removal of tensile stresses: in compression the rate of ozone attack is about 100 times slower than that in tension. Ideally, components should be designed so that there are no tensile stresses in the deformed state. In most engineering components the modes of deformation result in the restriction to the surface of any tensile stresses that are present, so cracks will not propagate into the bulk of the rubber and performance is not affected. Swelling in Liquids Natural rubber exhibits very low levels of swelling in fresh & sea water and most inorganic liquids. It is also swollen very little by acetone, alcohol and vegetable oils. It is therefore used extensively in marine applications such as dock fenders and on offshore oil rigs ~4. Natural rubber is more susceptible to swelling by oils and organic solvents, and if a large volume is absorbed it becomes very weak. This is a diffusion-controlled process, and there is a limit to the amount of liquid which can be absorbed. Up to the equilibrium swelling ratio, depth of penetration is proportional to the square root of time. The penetration rate of hydrocarbon liquids into natural rubber decreases as viscosity increases, (Fig. 9). Bulky components are relatively unaffected by thick or heavy oil; natural rubber is widely used for engine mounts since performance over the expected service life is little affected by occasional splashes of motor oil.
3.6 ~)-4
D.2
Z N
I
I
I
I
I
l
I
[
1
2
3
4
5
6
7
8 Time (y)
Fig. 10 Actual m e a s u r e m e n t s of creep of rubber building mounts in service. The broken lines are the upper arid lower limits of creep as predicted by laboratory experiments.
MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986
Fatigue Fatigue failure in rubber is .caused by the growth of cuts or cracks through a component ~5. Correct design and compound formulation ensures that the fatigue life for rubber components is very high. Use of antiozonants ensures that ozone cracking is limited to about 0.1 q mm/annum, where q is the ozone concentration in parts per hundred million (generally 2-3 outdoors, less than 0.25 indoors). Other crack growth may arise from small imperfections in the rubber surface. Under static loading conditions crack growth is unlikely to occur, but
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small-scale rupture can occur during dynamic deformation if the strains around the crack tip are low enough for strain-induced crystallization to disappear. In most rubber springs the cyclic deformation is imposed on a larger static deformation so that the strain on the rubber does not relax during cycling. The absence of tensile stresses in the bulk of the rubber and design to minimize surface stress concentrations will prevent crack growth. In non-crystallizing synthetic rubbers. cuts may continue to grow under static conditions until failure occurs or the strain is removed. The straincrystallization of natural rubber is partially responsible for its superiority in spring applications. Considerable work has been carried out at M R P R A to enable the fatigue crack growth rates in engineering components to be predicted from a fracture mechanics approach~5. ~6
Creep and Stress Relaxation This should be taken into account when designing with rubber. In natural rubber, both relaxation phenomena can be minimized by suitable compounding and pre-working the component to the service deformation before installation. Rates can be accurately predicted in laboratory tests and allowed for in the design (Fig. 10). Creep is the increase in strain with time under the action of a constant stress, i.e. the rubber continues to deform under a given load. W h e n the imposed load is removed, all but a few per cent of the deformation is recovered immediately;, the amount of deformation not recovered is known as permanent set. Stress relaxation is the decrease in stress under the action of constant strain. Both types of relaxation are a combination of physical and chemical effects. Chemical relaxation is mainly oxidative and directly proportional to time and proceeds faster at higher temperatures. Large engineering components are protected by their bulk and
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the use of antioxidants can greatly reduce the effects. Physical relaxation is proportional to the logarithm of time: rates are usually expressed as per cent per decade of time. The overall effect of a combination of physical and chemical relaxation is for rates to reduce with time following the application of the deformation. F o r applications such as building mounts, designed to support a heavy load over a very long period, the extent of creep is obviously critical to their performance. Studies have shown that this should not exceed 20% of the initial deflection during the first few weeks, with a further 5-10% over a period of many years. In absolute terms, a correctly compounded and designed steel laminated natural rubber building mount would show a total creep of less than 6ram in 100 years. In applications such as pipe sealing rings, stress relaxation is the more important consideration. These seals must maintain an adequate sealing force, accommodating movements over very long periods. Stress relaxation rates are substantially independent of the type or amount of deformation, but vary considerably with the composition and type of rubber. As is the case with creep, rates of natural rubber vulcanizates are generally lower than those of other rubbers. An appropriate vulcanizing system will provide a compound with a stress relaxation rate as low as 1.5% per decade.
Conclusion In the use of rubber in engineering design, three basic characteristics are exploited: that it is soft. relatively incompressible and capable of undergoing large deformations from which it rapidly recovers. Natural rubber has proved itself as the elastomer to be preferred in engineering applications, possessing good resilience, fatigue resistance, low heat-build-up, wide range of operating temperatures, good bonding to metals and ease of manufacture. Moreover, it is the most extensively
characterized rubber available, both in terms of mechanical data and information on design principles.
ReFerences
I. Lindley. P.B. Engineering Design with Natural Rubber, NR Technical Bulletin. MRPRA, 1978. 2. Herbst. P.T. Natural rubber as a bearing material for rotary wing aircraft applications. Paper H, NRPRA 3rd Rubber in Engineering Conference, 1973. 3. Halter, G.H.. and Morphew, D.J. Maintenance aspects of London Transport rolling stock. Railway Engineer, 3, 165, 1982. 4. Trucks ride well on transverse rubber springs. Rubber Developments, 30. 32. 1977. 5. Derham. C.J., and Elliott. D.J. Grand Prix success for natural rubber. Rubber Developments, 28, 3, 1975. 6. Record speed on natural rubber suspension, Rubber Developments, 36, 94, 1983. 7. Lau, M.G., Leaver. A.D.W.. and Lindlcy. P.B. Impact protection with NR - dock fenders and automobile bumpers. Proc Rubber in Engineering Conference. Kuala Lumpur, 1974. 8. Engineering Data Sheets. MRPRA. (Set of 50 sheets in a ring binder, price £12). 9. Dunkley, W.E. The longevity of rubber in pipes and pipelines. Rubber Developments. 17, 98. 1964. 10. Stevenson, A. Longevity of natural rubber in structural bearings. Plastics and Rubber - Processing and Applications, 5,253-258, 1985. l l. Long term tests confirm laboratory predictions. Rubber Developments. 28, 7, 1975. 12. Ab-Malek. K., and Stevenson, A. The assessment of the long term performance of rubber bridge bearings for use in highway bridges - MRPRA report for the Department of Transport, ref BE 22/2/0127, January 1984. 13.Stevenson, A. Chapter 2 in Rubber in Offshore Engineering (ed A. Stevenson). Adam Hilger Ltd, 1984. 14. Barriers and bearings. Rubber Developments, 35, 92, 1982. 15. Lake, G.J. Aspects of fatigue and fracture of rubber. Progress of Rubber Technology, 45, 89-143, 1983. 16. Stevenson, A. A fracture mechanics study of the fatigue of rubber in compression. Int. J. Fracture, 23, 47-59. 1983.
MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986
Natural rubber is an elastomer with excellent properties, which have been exploited in a wide range of applications. Despite the fact that it is a well-defined engineering material, with comprehensive documentation on both mechanical data and design principles, many engineers remain ignorant of natural rubber's potential. This article outlines the nature of the raw material and how it is compounded & shaped into useful products; the physical properties and engineering characteristics of natural rubber vulcanizates are also described. Environmental effects and their minimization by suitable compounding are discussed. Examples of several applications are given, including some showing the long service life of natural rubber engineering components. The aim is to introduce engineers to natural rubber and to show that information exists for the design of components with known mechanical properties and predictable in-service behaviour.
Introduction Natural rubber is the elastomer to consider in many engineering applications ranging from small bushes in mechanical engineering to building mounts and massive dock fenders in civil engineering. In many of these it functions as a spring, and from the rubbers available is chosen for its resilience, resistance to fatigue, low heat build-up, wide range of operating temperatures, good bonding to metals and ease of manufacture. Rubber springs also compare favourably with those of metal. It is possible to design a much simpler spring requiring no maintenance and having anisotropic stiffness. Rubber springs can accommodate some misalignment; (making installation easier). and the small amount of hysteresis inherent in the material will damp dangerous resonant vibrations in service. The above information suggests why rubber, particularly natural, should be the preferred material in spring applications. That this is not always the case owes much to engineers' greater familiarity with the conventional materials such as steel used routinely to design springs. Some engineers have taken on board new concepts encompassing the mechanical behaviour of rubber and a different design approach. They have discovered the advantages listed above, and have also proved that it is possible to design components in rubber which have known mechanical properties and will behave in service as predicted. A basic knowledge of these mech-
68
anical properties is required before natural rubber can even be assessed as suitable or otherwise for a particular application. Once established as an appropriate material, preliminary designs may be produced and manufacturers approached. A standard component with the desired mechanical properties may be available, but often springs must be designed from scratch. Here the engineers may meet another problem: having learnt about the mechanical behaviour of rubber they still cannot communicate with the rubber technologists, who are familiar with the terminology of conventional physical properties. For this reason, engineers often regard rubber technology as a 'black art'. There are rubber manufacturers familiar with engineering requirements, who would find no problem in characterizing the material properties of a particular rubber compound. However, the number of complex measurements under different conditions that would be required would make this a costly exercise if more than just a few compounds were considered. The aim of this article is to show the common ground existing between the engineer and the rubber technologist. For natural rubber in particular, published mechanical data and design information are available: it is a welldefined engineering material. Before an outline of the engineering properties and design features of natural rubber, a brief description is given on the nature of rubber, how it is compounded and how this affects properties.
Raw Natural Rubber Natural rubber is a high molecular weight polymer, chemically cis-polyisoprene, occurring in nature as a colloidal suspension beneath the bark of certain trees. The commercial source is the species Hevea brasiliensis, which is cultivated in tropical regions mainly in Southeast Asia. The trees are tapped to obtain latex, and while about 15% of production is concentrated and used as such. most is coagulated to form dry rubber. A number of grades is available to suit different applications and technical specifications assure the quality of rubber produced. Malaysia, the world's largest producer, operates the Standard Malaysian Rubber (SMR) Scheme, within which there are a number of grades suitable for engineering applications. These are the top quality grades - SMR CV, LV, L and W F - produced from factory processed latex under tightly controlled conditions. In its raw state natural rubber consists of long, flexible polymer chains and its molecular weight ranges from 100 000 to over one million. In this state it has little practical use: it flows easily under load as the molecules slide over each other and fails to recover when the load is removed. It crystallizes at temperatures around 0°C, and becomes soft and sticky in hot weather. To convert rubber to a useful material, it must first be masticated to break down the long polymer chains and allow the introduction of other compounds.
MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986
C o m p o u n d i n g and Vulcanization To confer elastic properties over a wide range of temperatures, chemical crosslinks are introduced between the individual polymer chains, broken down by mastication into shorter lengths. Other ingredients of the compound will include fillers, protective agents and processing aids. Sulphur systems are most commonly used to vulcanize rubber, although urethane and peroxides are sometimes used. Stiffness of the rubber can be raised by increasing the number of crosslinks, but too high a crosslink density can lead to a deterioration in strength. A crosslink density to give high strength is normally chosen: stiffness can be increased or decreased by the addition of reinforcing fillers or oil. The filler used for high quality engineering applications is usually carbon black. Apart from stiffness, the filler may also influence hysteresis, strength properties and fatigue life. The raw rubber, vulcanizing system and fillers determine the initial properties of the compound. In service, it is necessary to protect a component so that these do not markedly deteriorate. Protective chemicals are therefore added to the mix to retard the changes brought about by oxidation, ozone attack, high service temperatures etc. The final mix may also contain materials to aid manufacturing processes. After thorough blending of all ingredients, the mix is heated and shaped by moulding or extrusion to give the final vulcanized product.
Physical Properties Physical constants for gum (unfilled) & filled vulcanizates, steel and water are compared in Table I. A rubber compound is frequently both specified and quality controlled, by hardness & tensile stress-strain properties. However, these are inadequate for engineering design purposes: those properties required are discussed later.Hardness is a measure of reversible elastic deformation under a specified load and is therefore related to Young's modulus. It is measured in International Rubber Hardness Degrees (IRHD), which are approximately the same as British Standard Hardness Degrees and readings on the Shore Durometer A Scale. The hardness of natural rubber vulcanizates used in engineering applications may vary from 20 to over 80 IRHD. Frequently-used tensile stress-strain properties include tensile strength, elongation at break and stress a t 1 0 0 % and 300% strain. The stiffness of natural rubber increases at high strains: this is due to chain extension between crosslinks approaching the limiting value and to strain-induced crystallization. The latter is responsible for natural rubber's high tensile strength, long fatigue life and excellent tear resistance.
three orders of magnitude greater than its shear modulus; Young's modulus is three to four times the low strain shear modulus. The load deflection curves for rubber in tension and compression are linear for strains of a few per cent. Values of Young's and shear modulus can be obtained from the linear region. Characteristic curves for three modes of deformation - sheer, compression and buckling - are shown in Fig. I. Behaviour in shear is more linear than that in compression. The relatively high bulk modulus means that rubber hardly changes volume under high compressive loads: a rubber block will bulge at the sides as constant volume is maintained. The thinner the rubber layer for a given cross-sectional area, the greater the
dellecl~
L o a d - D e f o r m a t i o n Characteristics Rubber is relatively incompressible: it has a Poisson's ratio close to 0.5. It can be seen from the table that rubber's high bulk modulus (ca 2000 MPa) is some
Fig. 1
Force deflection curves for rubber units under different m o d e s of deformation.
Table ! Physical Constants of Vulcanized Natural R u b b e r C o m p a r i s o n of a soft gum rubber, a harder black-filled rubber, mild steel and water. ( F r o m ref. 1)
Hardness* Tensile strength (TS) Elongation at break (EB) Young's modulus (Eo) Shear modulus (G) Bulk Modulus (Eo~,) Poisson's ratio Resilience Velocity of sound transmission Specific gravity Specific heat Thermal conductivity relative to water Coefficient of cubical expansion/deg C Electrical resistivity t Dielectric constant Power factor * t
IRHD MN/m 2 % MN/m 2 MN/m" MN/m 2 % m/s
ohms/cm cube
G u m rubber
Filled rubber
45 28 680 1.9 0.54 2000 0.4997 8O 37 0.93 0.45 0.25 67 x 10 .5 1.7 x 1016 3 0.0O2
65 21 420 5.9 1.37 2400 0.4997 60 37 1.16 0.41 0.31 56 x 10 .5 3 x 10 I° 15 0.1
Mild steel
Water
0 420 40 210 000 81 000 176 000 0.29 100 5000 7.7 0.116 73 3.5 x 10 .5
2100
1430 1 1 I 21 x 10 .5 9 x 10 .6 80
hardness scale range is 0 to 100. ( I R H D = International Rubber Hardness Degrees) but specially-compounded conductive rubbers 1-107 ohms/cm cube
MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986
69
restriction to bulging and the stiffer the component will be. This feature is used in the design of compression springs: the compression modulus depends on the shape factor, defined as the ratio of one loaded surface to the total forcefree area. By incorporation of steel plates in a rubber spring unit, the spring can be designed to have changed stiffness in different directions, accommodating loads of varying magnitudes. Combined compression and shear is commonly used. Examples here include helicopter rotor bearings-', which are stiff in compression to accommodate the high centrifugal force (up to 75 000kgf), while relatively soft in shear to allow pitch, flap and lag; another example is the chevron spring commonly used in the suspensions of London Underground rolling stock 3. In both areas the use of steel-laminated natural rubber bearings has resulted in a considerable reduction in the number of parts used, elimination of maintenance, increased reliability and longer spring life. (Figs. 2&3). The smooth rising rate load-deflection relationship for rubber units under compression is used in simple compression springs - hollow rubber cylinders bonded at either end to metal plates. Such springs have been used for some time in the suspensions of commercial vehiclesL as the primary suspension in Formula 1 Grand Prix racing 5, and more recently in the World Land Speed Record car, Thrust 26. Compared with metal springs, which exhibit linear characteristics, this type of spring becomes progressively stiffer as the load increases. The result is a fail-safe spring which provides improved stability over a wide range of speeds and loads. The buckling mode of deformation is used less frequently, but is effective in certain types of dock fender, where it enables large amounts of energy to be absorbed without causing high peak forces to be tyansmitted to the vessel or structure. (Fig. 4).
Dynamic Mechanical Properties Most rubber springs must accommodate dynamic movements superimposed on the static load. The dynamic modulus depends on temperature, frequency and amplitude of deformation (see Fig. 5), although it is substantially independent of frequency below lO00Hz and at ambient temperatures. It may be measured in either shear or compression. Under dynamic conditions, carbon black affects strain amplitude, the effect diminishing with decreasing levels of carbon black. This is why spring vulcanizates of
70
Fig. 2. A helicopter rotor bearing sectioned to show its laminated structure and the rotor head assembly showing position of the bearings. The bearing is mounted such that the high blade centrifugal force results in compression, while pitch, lag and flap produce shear deflection.
Fig. 3
A variety of steel laminated natural rubber springs, including a chevron spring of the type used in London Underground rollingstock.
about 50 IRHD are sometimes preferred. Generally, at higher carbon black loadings, dynamic shear moduli will decrease with increasing amplitude. Another effect due to carbon black is the marked softening of rubbers within the first loading cycles, caused by some breakdown of the black structure. For this reason, rubber engineering components may frequently be preloaded to the maximum load a few times before testing. Hysteresis, the energy loss between loading and unloading cycles, occurs in
all rubbers. Generally natural rubber has low hysteresis, but values increase with increasing carbon black loading. Low hysteresis means low heat-buildup, important in dynamic applications. However the existence of some hysteresis in natural rubber prevents excessive vibration amplitude at resonance. For good vibration isolation, the natural frequency of the loaded system should be less than one-third of that imposed. (see Fig. 6.) Hysteresis is often characterized by the loss of phase angles and varies with temperature,
MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986
frequency and. at high filler loadings, amplitude. If the amount of hysteresis is increased then the amount of damping at resonance is also increased. This can be desirable in some applications and high damping natural rubber compounds have been developed.
Design Data The design engineer needs information
Fig. 4
Natural rubber dock fenders installed at Port Lincoln, Australia. Each measures 2.44m long and 1.83m in diameter and weighs 6.5 tons.
on static and dynamic moduli and loss angles measured over a range of strains, frequencies, amplitudes and temperatures. It is in this area that published data are rare, except for natural rubber, which is the best characterized elastomer available. The Malaysian Rubber Producers' Research Association has expended a great deal of effort for many years in convincing engineers that natural rubber is a well-defined engineering material. As pan of this work. the Engineering Data Sheets 8 provide a basis on which rubber technologists and engineers can converse. Over 40 vulcanizates, selected to be typical of those used in engineering applications, are characterized. Each sheet provides a large amount of precise data on a specific natural rubber vulcanizate, including the basic properties familiar to the rubber technologist and, for the engineer, static and dynamic mechanical properties over a range of different strains, temperatures and frequencies. The design data provided by each sheet includes bulk and shear moduli, the latter over a range of strains from 2-350% at ambient temperature, for initial deformation and after nine cycles to 100% shear. Static compression IdB
Longevity The conditions under which a rubber component is stored or used may affect its performance. However, with modem compounding techniques any deterioration can be minimized or virtually eliminated. There are plenty of examples which bear witness to the longevity of natural rubber in even the most demanding applications. Before the advent of large-scale production of synthetics in the 1940s, natural rubber was the only elastomer available. Two of the oldest examples are a sewer ring installed in a London sewer in the 1850's and discovered during reconstruction work in 19639, and an AusI
1
--+20 $ I
Dynamic shear m o d u l u s M N / m ~
moduli are given over strains from 5-20% at six shape factors of 0.5-8.0. A range of low frequency (0.1 Hz) dynamic tests over temperatures ranging from -40°C to 150°C gives information on both static and dynamic shear moduli: the moduli at this frequency are essentially the same. At higher frequencies (1 Hz at 1-100% strain amplitude and 15 Hz at 2-10% strain amplitude) dynamic shear moduli are given over the same temperature range. Loss angles are also given for all three frequencies.
I
I
3
4
A
+lOJ
-_---
Dynamic strain a m p l i t u d e
Dynamic shear modulus M N / m I
+2
.
\\
black
,<
--22~,~
oI~Ii
m~,.
.....
-20
. . . . . . . .
0
loll+
~/~
2 n/n o
1 . ~ o ~.
I
I
I
I
+50
0
50
100
Fig. 5
=
gum
:ix
o.iHt 0
-I0
I
150 T e m p e r a t u r e "C
Fig. 6
Variation of dynamic shear modulus with temperature and strain amplitude at 0.1 and 10Hz for gum and blackfilled vulcanizates.
M A T E R I A L S & D E S I G N Vol. 7 No. 2 M A R C H / A P R I L
1986
Dependence of transmissibility on ratio of imposed frequency n to natural frequency n o for various levels of hysteresial damping (expressed as the tangent of phase angle o'). Transmissibility is defined as the ratio of vibration amplitude on the 'protected' side of the spring to that of the disturbing vibration. When the ratio of disturbing frequency to natural frequency is unity (A) resonance occurs.
71
~
Comp¢ossive Iogd (kN) 8OO
II
.P.I
I '"g" ""l* I 1
:;,;::,:,,:;
soo
+ 3eC O polychloroprene n a t u r a l r u b b e r (21~pph S) x n a t u r a l r u b b e r (31/4 p p h S)
Incremental modulus 30 i
......
400 300
1 day I
TEM,ERATORE ST FFE""G I
1 week I
1 month I
I year
3 yeera /
I
PC 0
i j
co/if" 1
2
3
4
$
e
7
B
Compreollvo aeformlllon Imm)
Fig. 7
A bridge bearing from the M2 motorway was removed by the Department of Transport for inspection in 1982, after 20 years of service• The stiffness of the bearings falls within the initial range of values obtained for the bearings supplied• Tests have also shown that the rubber in the bearing still fulfils all the requirements laid down at the time of installation, as well as those specified in the current more stringent standard (BS 5400).
20
;/
0/'
oJ
10
NR-1. •o ~ • °
..Z.2--1---'-----, O 0.O1
011
• • "
"
;
.
--
•~°
~ ' ~
NR-2
,
,
10
100
1
1 O00
,
10000 Uma(hrs)
Incremental modulus MPa
o
15000
tralian bridge pad installed in 1889 and still in service '°. Both these components were found to be in excellent condition, even without the benefits of modem compounding. More recently, extensive studies of bearings used to isolate buildings from vibration have provided evidence that such bearings will give excellent performance for at least 100 years ~t. Bridge bearings removed for inspection after 20 years service were found to be performing within the original specification limits z.,. (Fig. 7).
LOW TEMPERATURE STIFFENING -25* C O acrylonltrlle-butacllene 1000-
/
Natural rubber engineering vulcanizates can be produced to resist the effects of temperatures ranging from over 60°C to 100°C, and intermittently even higher. The static modulus of gum vulcanizates is approximately proportional to absolute temperatures, over the range -20°C to 70°C. Fillers reduce this effect and under dynamic conditions may even reverse it. In the design of components for use over a wide temperature range, allowance should be made for the temperature dependence of stiffness, thermal expansion and contraction. At temperatures below -20°C stiffness increases over a period of time: at -40°C it is double that at +20°C. However, use of suitable compounds and vulcanization conditions can slow this process so that significant stiffening can be avoided for many months. Two phenomena are involved;
72
x •
/
rubber
polychloroprene natural rubber
P SOO-
/
/o
x ._.a0----x--x
,_:::~_, ~
Environmental Effects
Temperature
0
0.01
0.1
--I-1
~
:
I 10
.........
7-100
x
x
~
PC •
"~--~N.I 1OOO
1 1OOOO
time(his)
Fig. 8a & 8b Comparison of low temperature stiffening in acrylonitrile-butadiene, polychloroprene and natural rubber, at -25°C and +3°C. At -25°C, N B R shows immediate and considerable stiffening, while polychloroprene stiffens markedly more than natural rubber after a few hours. On an expanded scale, and at more moderate temperatures, the increased stiffening of polychloroprene compared with natural rubber can be seen. low temperature crystallization, most rapid at-25 °C. and glass hardening at -70°C, when there is a transition to a brittle, glassy state and the modulus increases a thousandfold. Both effects are reversible, the vulcanizate returning to its original state when warmed. Low temperature stiffening is not generally a serious problem in natural rubber, unlike comparable synthetics which tend to stiffen more rapidly or at more moderate temperatures '3 (Fig 8a & 8b).
At high temperatures, physical properties are temperature dependent, but these effects are reversible provided no chemical changes take place. At temperatures approaching 140°C, chemical degradation may occur, resulting in a rapid loss of mechanical properties and increased hardness. Conventional natural rubber vulcanizates can be used up to 70°C, and other compound types will provide satisfactory service at 100°C and intermittently much higher
MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986
temperatures. The behaviour of a thickwalled engineering component, even at IO0°C, is very much better than tests on thin-sheet standard test pieces would suggest.
dants giving excellent resistance to oxidative degradation, antiozonants and/or waxes to provide resistance to ozone cracking and carbon black, which, in addition to its other effects, protects against ultra-violet radiation. Rubber components used in engineering tend to be of thick section, and the bulk of the rubber acts as a reservoir for the protective agents, which migrate to the surface replacing those which have already reacted with oxygen and ozone.
Weathering Oxygen, ozone and sunlight can all degrade natural rubber if it is not suitably protected. Engineering components, which are often required to function effectively over many years in exposed conditions, contain antioxi-
tO years
4 yeats b
I year
--
4 monlhs
--
1 month
4 days
D
1 week
10
01
10
1O 0
solvenls
T0 0 0
10 000
engine o,ls
heavy oils
v l s c o s t t y Ot IlQutd, m N s J m .
Fig. 9
The time taken for a liquid to penetrate a given stance into rubber depends on its viscosity. This graph shows the til ~ taken to penetrate 5 mm; it would take four times as long to penetrate I cm. (1 m N s / m e -1 centipoise).
Creep (mm)
1.0 3-8
Recent M R P R A research suggests that a 'skin' impervious to oxygen is formed on the surface after relatively short periods in service. While antioxidants are extremely efficient, complete protection against ozone cracking is less readily achieved by protective agents. For rubber subjected to continuous dynamic deformation, ozone cracks grow at about 0.25ram per year. The most effective protection is the removal of tensile stresses: in compression the rate of ozone attack is about 100 times slower than that in tension. Ideally, components should be designed so that there are no tensile stresses in the deformed state. In most engineering components the modes of deformation result in the restriction to the surface of any tensile stresses that are present, so cracks will not propagate into the bulk of the rubber and performance is not affected. Swelling in Liquids Natural rubber exhibits very low levels of swelling in fresh & sea water and most inorganic liquids. It is also swollen very little by acetone, alcohol and vegetable oils. It is therefore used extensively in marine applications such as dock fenders and on offshore oil rigs ~4. Natural rubber is more susceptible to swelling by oils and organic solvents, and if a large volume is absorbed it becomes very weak. This is a diffusion-controlled process, and there is a limit to the amount of liquid which can be absorbed. Up to the equilibrium swelling ratio, depth of penetration is proportional to the square root of time. The penetration rate of hydrocarbon liquids into natural rubber decreases as viscosity increases, (Fig. 9). Bulky components are relatively unaffected by thick or heavy oil; natural rubber is widely used for engine mounts since performance over the expected service life is little affected by occasional splashes of motor oil.
3.6 ~)-4
D.2
Z N
I
I
I
I
I
l
I
[
1
2
3
4
5
6
7
8 Time (y)
Fig. 10 Actual m e a s u r e m e n t s of creep of rubber building mounts in service. The broken lines are the upper arid lower limits of creep as predicted by laboratory experiments.
MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986
Fatigue Fatigue failure in rubber is .caused by the growth of cuts or cracks through a component ~5. Correct design and compound formulation ensures that the fatigue life for rubber components is very high. Use of antiozonants ensures that ozone cracking is limited to about 0.1 q mm/annum, where q is the ozone concentration in parts per hundred million (generally 2-3 outdoors, less than 0.25 indoors). Other crack growth may arise from small imperfections in the rubber surface. Under static loading conditions crack growth is unlikely to occur, but
73
small-scale rupture can occur during dynamic deformation if the strains around the crack tip are low enough for strain-induced crystallization to disappear. In most rubber springs the cyclic deformation is imposed on a larger static deformation so that the strain on the rubber does not relax during cycling. The absence of tensile stresses in the bulk of the rubber and design to minimize surface stress concentrations will prevent crack growth. In non-crystallizing synthetic rubbers. cuts may continue to grow under static conditions until failure occurs or the strain is removed. The straincrystallization of natural rubber is partially responsible for its superiority in spring applications. Considerable work has been carried out at M R P R A to enable the fatigue crack growth rates in engineering components to be predicted from a fracture mechanics approach~5. ~6
Creep and Stress Relaxation This should be taken into account when designing with rubber. In natural rubber, both relaxation phenomena can be minimized by suitable compounding and pre-working the component to the service deformation before installation. Rates can be accurately predicted in laboratory tests and allowed for in the design (Fig. 10). Creep is the increase in strain with time under the action of a constant stress, i.e. the rubber continues to deform under a given load. W h e n the imposed load is removed, all but a few per cent of the deformation is recovered immediately;, the amount of deformation not recovered is known as permanent set. Stress relaxation is the decrease in stress under the action of constant strain. Both types of relaxation are a combination of physical and chemical effects. Chemical relaxation is mainly oxidative and directly proportional to time and proceeds faster at higher temperatures. Large engineering components are protected by their bulk and
74
the use of antioxidants can greatly reduce the effects. Physical relaxation is proportional to the logarithm of time: rates are usually expressed as per cent per decade of time. The overall effect of a combination of physical and chemical relaxation is for rates to reduce with time following the application of the deformation. F o r applications such as building mounts, designed to support a heavy load over a very long period, the extent of creep is obviously critical to their performance. Studies have shown that this should not exceed 20% of the initial deflection during the first few weeks, with a further 5-10% over a period of many years. In absolute terms, a correctly compounded and designed steel laminated natural rubber building mount would show a total creep of less than 6ram in 100 years. In applications such as pipe sealing rings, stress relaxation is the more important consideration. These seals must maintain an adequate sealing force, accommodating movements over very long periods. Stress relaxation rates are substantially independent of the type or amount of deformation, but vary considerably with the composition and type of rubber. As is the case with creep, rates of natural rubber vulcanizates are generally lower than those of other rubbers. An appropriate vulcanizing system will provide a compound with a stress relaxation rate as low as 1.5% per decade.
Conclusion In the use of rubber in engineering design, three basic characteristics are exploited: that it is soft. relatively incompressible and capable of undergoing large deformations from which it rapidly recovers. Natural rubber has proved itself as the elastomer to be preferred in engineering applications, possessing good resilience, fatigue resistance, low heat-build-up, wide range of operating temperatures, good bonding to metals and ease of manufacture. Moreover, it is the most extensively
characterized rubber available, both in terms of mechanical data and information on design principles.
ReFerences
I. Lindley. P.B. Engineering Design with Natural Rubber, NR Technical Bulletin. MRPRA, 1978. 2. Herbst. P.T. Natural rubber as a bearing material for rotary wing aircraft applications. Paper H, NRPRA 3rd Rubber in Engineering Conference, 1973. 3. Halter, G.H.. and Morphew, D.J. Maintenance aspects of London Transport rolling stock. Railway Engineer, 3, 165, 1982. 4. Trucks ride well on transverse rubber springs. Rubber Developments, 30. 32. 1977. 5. Derham. C.J., and Elliott. D.J. Grand Prix success for natural rubber. Rubber Developments, 28, 3, 1975. 6. Record speed on natural rubber suspension, Rubber Developments, 36, 94, 1983. 7. Lau, M.G., Leaver. A.D.W.. and Lindlcy. P.B. Impact protection with NR - dock fenders and automobile bumpers. Proc Rubber in Engineering Conference. Kuala Lumpur, 1974. 8. Engineering Data Sheets. MRPRA. (Set of 50 sheets in a ring binder, price £12). 9. Dunkley, W.E. The longevity of rubber in pipes and pipelines. Rubber Developments. 17, 98. 1964. 10. Stevenson, A. Longevity of natural rubber in structural bearings. Plastics and Rubber - Processing and Applications, 5,253-258, 1985. l l. Long term tests confirm laboratory predictions. Rubber Developments. 28, 7, 1975. 12. Ab-Malek. K., and Stevenson, A. The assessment of the long term performance of rubber bridge bearings for use in highway bridges - MRPRA report for the Department of Transport, ref BE 22/2/0127, January 1984. 13.Stevenson, A. Chapter 2 in Rubber in Offshore Engineering (ed A. Stevenson). Adam Hilger Ltd, 1984. 14. Barriers and bearings. Rubber Developments, 35, 92, 1982. 15. Lake, G.J. Aspects of fatigue and fracture of rubber. Progress of Rubber Technology, 45, 89-143, 1983. 16. Stevenson, A. A fracture mechanics study of the fatigue of rubber in compression. Int. J. Fracture, 23, 47-59. 1983.
MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986