0.3

0.2

Extension

0.1

0

0.1

0.2

Compression

0.3

0.4

0.8

0.9

1.0

1.1

1.2

λ

the ratio of the nominal stress to the apparent

10%

20%

Young's modulus can be written as

σ

λ2 - λ

=

,

(4)

3

where σ is the nominal stress acting on the seal

and λ is the extension ratio of the current joint

width to the original joint width. The relation is

depicted in Figure 9. Gent and Lindley illustrated

with experimental results that the approximation

reasonably represents the compression of vulca-

(0.275 in.

6.99 mm)

nized natural rubber blocks with *d*/*w *ratios from

(0.300 in.

0.36 to 1.7, and with lengths equal to *w*, to approxi-

7.62 mm)

mately 25% compression. These results, by the in-

fluence of the apparent modulus on the large defor-

173

mation response, provide further elucidation of the

influence of the shape factor on the stress of a seal.

It should be noted, however, that the use of eq 4

implies the applicability of a strain energy func-

tion with the form of the first term in eq 1. Al-

though this term provides a good representation

of rubber constitutive behavior, it does not repre-

sent rubber behavior in general (Treloar 1974).

Cook (1965a, 1965b) later considered the homo-

geneous rubber elasticity relation of eq 4 for seal-

(0.300 in.

7.62 mm)

ant materials, but inappropriately used the equa-

tion directly for the inhomogeneous deformation

of butt joint seal specimens without distinguish-

ing between the material Young's modulus and

the apparent Young's modulus, as shown to be nec-

essary by Gent and Lindley (1959).

13

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