The Fatigue Threshold of Rubber and Its Characterization Using the Cutting Method

Below a limiting value of tearing energy called the intrinsic strength or fatigue threshold (T₀), cracks will not grow in rubber due to fatigue; hence, this material characteristic is important to understand from both fundamental and practical perspectives. We summarize key aspects of the fatigue threshold, including the Lake-Thomas molecular interpretation of T₀ in terms of fracture of polymer network chains in crosslinked elastomers. The various testing approaches for quantifying T₀ are also discussed, with a focus on the classic Lake-Yeoh cutting method which was recently revived by the introduction of a commercial testing instrument that applies this procedure, the Intrinsic Strength Analyser (ISA). A validation of the cutting method is also given by demonstrating that a 2-h test on the ISA yields a value of T₀ that is essentially identical to the T₀ from near-threshold fatigue crack growth (FCG) measurements that require 7.5 months of continuous testing. Compound formulation effects – polymer type, crosslink density, type and amount of reinforcing fillers, and addition of oils/plasticizers – are examined based on the limited published research in this area and our new results. At the end, some insights are offered into using the fatigue threshold to develop highly durable rubber products.