Stress Relaxation

Stress relaxation is the more relevant property for sealing products such as O-ring , rubber seals and rubber gaskets. When rubber is held at constant deformation, there is a decrease in stress as a function of time.This phenomenon can be of great impoertance in sealing application,where the material of the seal is required to maintain a specific level of sealing force to prevent leakage. Stress relaxation can be the dominant factor that limits the effect life of rubber seals and rubber gaskets.

Stress relaxation is usually defined as the loss in stress expressed as a percentage of the initial stress. Thus

stree relaxation = 100*(So – St)/So

So is initial stress

St is stress at time t

Stress relaxation and creep rate are related to one another if the shape of the force-deflection curve is known. According to this the relationship between the two parameter is determined by the incremental stiffness at the point on the force-deflection curve relevant to the stress relaxation or creep measurement .Thus

where C is the creep rate, S is the stress relaxation rate .Since it has been established that creep and stress relaxation can be related in this way,in the next topic i will be following discussion both are referred to as relaxation processes.

Pic. Stress relaxation phenomenon

**Ref. http://www.engin.umich.edu/class/bme332/ch10ligten/bme332ligamenttendon.htm

Rubber Properties : Hardness Test

Hardness test is the most widely used test on vulcanised rubber material , hardness is rubber material property which the engineer will often need to specify when defining his requirements. The term “hardness” as applied to valcanised rubber material mean resistance to deformation, or more precisely, indentation by rigid body The British Standard method of expressing hardness of rubber material has been so devised that the result, express in British Standard and International Rubber Hardness Degree, bears a known relation to the elastic modulus .However from the engineer’s point of view the hardness test serves essentially as an indirect measure of the elastic modulus, of greater or less accuracy according to the circumstances just mentioned. The basis of the British Standard Hardness Test is the following relationship between the elastic modulus of the rubber material and the depth of penetration by rigid ball:

F/E = 0.00017*(r^0.65)*(p^1.35)

where F = the indenting force (kg)

E = Elastic modulus (kg/cm^2)

r = radius of ball (cm)

p = depth of penetration (mm/100)

Creep

Creep is a time dependent increase in deformation under constant load . If an elastomer component is subjected to a static preload, then this load cause a progressive increase in deformation as a function of time.It can be important in a wide variety of application, such as suspension engine rubber mounts and building mounts etc. Creep is expressed as a percentage of the initial deflection . Thus

Creep at time t (%) = 100*(l – l0)/l0

where l0 is initial deflection and l is the deflection at time t .

The initial deflection must be measured at a define initial time to , which should be about 10 times longer than time taken to apply the deformation. Creep rate is express as creep divided by function of time. This function may be the logarithm of time if the relaxation mechanism is physical or time if it is chemical . Creep in rubber material consist of both physical (due to molecular chain slippage) and chemical(due to molecular chain breaking) . Physical creep rates (A) decrease in time and are usually expressed as a percentage of the original deflection per decade of time. Chemical creep rate (B) at a constant temperature are approximately linear with time, and thus the total creep is given by

creep(%) = A log10(t/t0) + B(t – t0)

**Engineering with Rubber (2001)

Pic1 Creep test

Silicone Rubber

Which have carbon-carbon backbones, silicone rubber contain very flexible siloxane backbones,and have very low glass transition temperature. The most common silicone rubber is polydimethyl siloxane with a Tg of –127 *C . Silicone rubbers have both excellent hight temperature resistance and low temperature flexibility. In addition, they process good biocompatibility and thus are use in implants and prosthese. Other uses include gasket , seal O-ring , Ice Tyre.

Rubber Tire : Basic function of a tire

1. Provide Load carrying capacity
2. Provide cushioning and damping
3. Transmit driving and breaking torque
4. Provide cornerring force
5. Provide dimensional stability
6. Resist abrasion
7. Generate steering response
8. Have low rolling resistance
9. Provide minimum noise and minimum vibration
10. Be durable throughout the expected life span

Prinary Purpose of the Tire

The primary purpose of the tire is to transfer the driver’s action such as accelerating,steering and breaking to the road surface.

The part of the tire that rolls on the road is know as the contact patch

The friction between the road surface and the contact patch is all that ensures the vehicle follows the driver’s commands.

Picture 1 Contact Patch

Three major performances are demanded

1. Safety
2. Economy
3. Comfort

Rubber Properties: Thermal Expansivity , Pressure

The coefficient of thermal expansion for elastomer is approximately 4.8 x 10E-04 /K similar to a hydrocarbon liquid. Addition of fillers reduces the value slightly. Comparing this expansivity to that of steel (3.5 x E-05 /K) ,a tenfold difference, one begins to understand the built-in interfacial strain in a bonded rubber-metal or composite structure.

The high coefficient of expansion couple with the high bulk modulus mean that a potential exists for subtantial thermal pressure .If the elastomer is confined and subsequently heated. This describes the mouldind enveronment, with thermal pressure a neccessity for replicating mould contours and surface finish. The moulding procedure of “bumping” vent excess rubber create by this thermal expansion. If not relieved, the thermal pressure could exceed the press clamping force. This can make the mould open at the parting line, creating a condition referred to as “Backrinding”.

Beerbower has calculate these pressure using the ratio of thermal expansion at constant pressure to compressibility at constant temperature .Using Beerbower's Equation. It is possible to predict the pressure developed .

= 1.13 for saturate backbone hydrocarbon that identify according to ASTM D 1418 designation "M" ; e.g., EPDM and 1.22 for unsaturated elastomer ASTM D1418 designation " R" ; e.g., NBR , SBR , NR

T is temperature (K)

Design Principles

Rubber material is an engineering material : To design adquately ,basic machanical properties must be appreciated. The three elementary type of strain for isotropic material are simple tension simple shear and uniform compression (hydrostatic) . The behavior for these cases is define by constant : Young’s modulus E (tension) ,rigidity modulus G (shear) ,bulk modulus K (compression) and Poisson’s ratio v . Poisson’s ratio is 0.5 for a totally incompressible solid. For elastomer v is 0.499 + (for steel v is approximately 0.3) .For an isotropic incompressible material , E = 3 G. The essential incompressibility of rubber material has many consequences in design , manufacturing and application.

Natural Rubber

Of the range of elastomers available to technologists natural rubber (NR) is among the most important , because it is the building block of most rubber compound used in product today. It is a material that is capable of rapid deformation and recovery , and it is insoluble in a range of solvents, though it will swell when immersed in organics solvents at elevated temperature. Its many attributes include abrasion resistance , good hysteretic properties, high tear strength, high tensile strength and high green strength. However , it may also display poor fatigue resistance. It is difficult to process in factories, and it can show poor tire performance in areas such as traction or wet skid compared to selected synthetic elastomer.