Effective maximum tensile stress and strain for industrial fatigue design of rubber products made from filled NR and SBR

Design engineers in the industry are facing considerable challenges in achieving the fatigue requirement of products within a tight time frame. An adequate, simple, practical and reasonably accurate criterion would be desirable. Since R ratios (the ratio between the minimum stress/strain and the maximum stress/strain) play an essential role in fatigue life prediction, the author has developed two criteria, i.e., effective maximum tensile stress and strain, to evaluate the fatigue life of industrial products in a stage of design by taking R ratios into account. The developed criteria can simulate the response of rubber specimens under fatigue loads with different R ratios without any fitted parameters, which is a novelty of the proposed approach. To verify the criteria, three standard types of samples, i.e., CD (cylindrical dumbbell) specimens made from filled NR (Natural Rubber), AE42 specimens and AE2 specimens made from filled SBR (Styrene-Butadiene Rubber), were utilised. The published experimental data, 30 cases (R <0, R = 0 and R >0) for CD under uniaxial loads, 40 cases (R = 0, R >0) for AE42 under tension, torsion, and tension-torsion loads, and 128 cases (R = 0, R >0) for AE2 under tension, torsion, and tension-torsion loads, were collected for validation. The results have demonstrated the effectiveness of the new criteria. The observed crack locations in all three types of samples were consistently associated with the maximum values of the developed criteria. The scatter factor of prediction against the experimental data was achieved within ±0.25 with R2 = 0.86 for 30 cases using the CD specimens and within ±0.35 with R2 = 0.91 for 168 cases using the AE42 and the AE2 samples. This successful validation provides design engineers with a reliable tool they can use at no additional cost. The S-N curves obtained are also a valuable resource for industrial applications.