The ability to expand greatly and to return quickly is what distinguishes a rubber from a plastic. This property not only makes possible the catapult but also allows designers to use rubbers to supply constant forces, either in tension or compression.
High quality rubber compounds will remain elastic for their full design lives, virtually irrespective of the movement cycles they undergo. However, all rubbers will relax to some extent under constant deformation and this should be specified if significant.
Where rubber is to be used continuously in tension, consideration should be given to the effects of failure and trials carried out as required.
All rubber types are elastic. Natural rubbers are tough and strong but may have limited life if exposed to ozone or sunlight. Thermoplastic rubbers generally have lower elasticity and the softer grades relax when deformed, giving rise to permanent set.