Traditionally damage has been studied based on a Stored Energy approach, including an internal variable or independently to predict failure life epitomized by Miner's rule. The fundamental assumption of continuum damage mechanics is that the coalescence of voids accrues until complete rupture. This assumption is given a mathematical form by the definition of effective stress obtained by augmenting the strain energy potential with a damage variable. A tempting hypothesis based on effective stress is that the stiffness of rubber degrades until total rupture. However, experimental evidence is yet to prove the relation between the property changes in rubber with fatigue life remains an open question.
Hence, a systematic experimental procedure was implemented to study the fatigue-induced property variation in natural rubber compounds. The variation of moduli at different strains, hyper-elasticity, viscoelasticity, and failure properties viz., tensile strength, Elongation at break, and strain energy density at break were considered. The experiments aimed to understand damage-induced mechanical property variations and explore whether they can be used for damage evolution law. As per the evolution law used in continuum damage mechanics, the constitutive equation includes an internal damage variable, altering the stress-strain relationship. Therefore, the experimental results were analyzed to ascertain whether: (i) a significant and monotonic degradation of stiffness which can potentially quantify the fatigue-induced damage is observed; (ii) the variation in any of the properties mentioned above is monotonic and a relatable indicator for fatigue damage so that a sound damage evolution law can be developed.
The experimental results do not indicate a monotonous stiffness decay with fatigue life. Partial recovery of stiffness was observed at intermediate stages of fatigue life- a phenomenon that CDM cannot capture. When a failure-based damage law is used to depict hyperelastic behavior through finite element analysis, the results do not agree with the experimental results. On the other hand, the variation in Elongation at break and strain energy density at break demonstrated a unique correlation to the cyclic strain and fraction of life consumed.
The interplay of fatigue damage and inelasticity of elastomers is an open problem. The parameters of a viscoelasticity constitutive equation were used to trace the evolution of elastomer inelasticity with fatigue damage. Contrary to the Continuum-damage-mechanics-based hypothesis, the decay of stiffness of the equilibrium network was not monotonic with fatigue life. The variation of the inelastic network parameters suggests interplay between damage and inelasticity. However, a trend suggestive of the effect of damage on viscoelasticity was not found. High-temperature fatigue demonstrated no permanent impact on stress-strain response. Similar to hyperelasticity, the evolution of inelasticity with fatigue damage also confirms that the changes of the viscoelastic properties are not an indicator of fatigue-induced damage.
Unlike the conventional perception of damage strain energy to arise from stored energy available for deformation, it is asserted that the damage accumulates at the cost of ultimate properties.