The effect of flaws on fatigue life in bushings.

Rubber-metal composites are widely used engineering applications because they offer a unique combination of properties and are very durable. Often natural rubber (NR) is the rubber of choice in these applications. Its effectiveness is influenced by a number of factors, for example crosslinking (ref 1).

Other important factors are (ref. 2):

* occurrence or absence of complete relaxation during each compression cycle;

* presence or absence of ozone;

* shape effects;

* boundary conditions.

This article focuses on Silentbloc bushings as an example of a widely used rubber-metal composite. These have a long history of successful use in automotive suspension systems where good fatigue life is essential. Figure I shows a rubber insert before and after assembly (ref 3).

Before assembly, the outside diameter (O.D.) of the rubber insert is larger than the inside diameter (I.D.) of the outer metal; the insert I.D. is smaller than the O.D. of the inner metal sleeve. Bushings are assembled by squeezing a rubber insert between inner and outer metal sleeves using a special lubricant.

In use, bushings experience radial and axial loads and they rotate through a torsion angle as shown in figure 2.

Earlier work (ref. 4) showed that the addition of 0.5 parts per hundred hydrocarbon (pphr) of sand (30/100 mesh) halved the fatigue life of a natural rubber vulcanizate. This result emphasizes the need for clean rubber compounds if satisfactory fatigue life is to be realized.

Gent describes the effect of flaws on breaking strength and fatigue fife (ref. 5). He suggests that a variety of fracture processes begin from a natural flaw of the order of 40 micrometers in size. Flaws like this occur in raw elastomers as well as mixed compounds.

The present study extends this earlier work by examining a range of silica particle diameters at the 0.5 pphr level on fatigue life in NR bushings. Table I shows the experimental compounds, their mesh classifications and associated particle diameters.

Table 1 - effect of silica with different particle
diameters on bushing fatigue life

Compound                 A     B        C        D        E

Silica particles

Mesh size*
   Through               -     200/     100/      60/      20/
   Retained on                 325       200      100       60

Diameter
   Micrometers           -      74/     149/     250/     840/
                                 44       74      149      250
   Inches                -   0.003/   0.006/   0.010/   0.033/
                              0.002    0.003    0.006    0.010
Fatigue life (kc)**242          199      194      182      150

Compound                    F

Silica particles

Mesh size*
   Through                  10/
   Retained on               20

Diameter
   Micrometers            2,000/
                             840
   Inches                  0.079
                           0.033
Fatigue life (kc)**242       139

* Standard sieves, ASTM E 11 ** Average value for three bushings tested

Compounds B-F all contain added particles that are larger than the naturally occurring flaws described above. These compounds all show lower fatigue lives than compound A, die control. Fatigue life generally decreases with increasing particle size, with compound F having the lowest fatigue life. It should be noted that this occurred even though the number of added particles in compound F is least, because particles were added on a constant weight basis.

This work demonstrates the need for cleanliness to obtain good fatigue life in rubber products.

References

[1.] J. G. Sommer, "Effect of crosslinking systems on fatigue life," to be published in Rubber World, December 1997.

[2.] P.B. Lindley and A. Stevenson, Rubber Chem. Technol., 55, 337(1982).

[3.] D.A. Meyer and J.A. Welch, Rubber Chem. Technol., 50, 145 (1977).

[4.] G.J. Lake and P.B. Lindley, Rubber Journal, 146 (11) 34 (1964).

[5.] A.N. Gent, Chapter 10, in "Science and technology of rubber," Academic Press, New York, E Eirich, Ed., p. 419.