In most materials, linear stress-strain behavior to high strain — even if it is elastic — is the exception, not the rule.

For a rubber, divide the engineering stress by the stretch ratio (1 + strain). This is the so-called “true stress", which accounts for the reduction in cross-sectional area. This plot shows that rubbers are fairly linear to 40–60% strain, much higher than any other material.

Rubbers are Elastomers and their Behaviour is Governed by their “Bulk Modulus” which is Greatly Influenced by a Very High Poissons Ratio whereas Metals have Very Low Poissons Ratios so their behaviour is Very Nearly Linear Rubbers are Decidedly Non-Linear !!!!!