PSI - Issue 13

Jacopo Schieppati et al. / Procedia Structural Integrity 13 (2018) 642–647 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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component is frequency independent. Therefore, the static component at small frequencies is more relevant and the overall crack growth rate results higher. This theory is not related to the energy dissipation and therefore seems to represent a proper explanation to our results. Analogous trend of increasing crack growth rate with lower frequencies were also reported by Busfield et al. (2002) by testing a styrene-butadiene copolymer. 3.2. Thermal conductivity Rubbers are poor thermal conductors and this characteristic associated with considerable internal heat generation, leads to consistent increase in temperature in thick rubber components. Therefore, thermal conductivity was measured in the range of possible utilization of the rubber components with two different techniques. Fig. 4 summarizes the thermal conductivity as a function of temperature for both measurement techniques (guarded heat and laser flash). Both methods reveal results in very good agreement along the investigated temperature range. Moreover, it is worth notice that the values remain almost constant along the tested range of temperature. The average values across all the range of temperature for the two methods are reported in Table 1. Compared to the guarded heated method the results for the laser flash method are more scattered.

Table 1. Average values of thermal conductivity obtained with two techniques. Measurement method L (W/m ∙ K) Error (W/m ∙ K) Guarded heat 0,372 ± 0,004 Laser flash 0,380 ± 0,061

4. Conclusion

Measurements to determine the fatigue crack growth rate and thermal conductivity were implemented in order to study the impact of temperature on the fatigue properties of rubbers. Through the monitoring of the surface temperature during crack growth experiments a further increase of the temperature at the crack tip was observed, which was related to higher local strain in the region. The temperature increase during cyclic loading dependents on the testing frequency, resulting in higher temperatures with higher frequencies. These differences can be explained with higher energy dissipations at high frequencies. Moreover, by varying the loading frequency variations of the crack growth rates were found as well. The differences were not connected to higher energy dissipations during cyclic loading, but to the viscoelastic contribution due to the static growth component of the overall crack growth rate, Lake and Lindley (1964). Furthermore, the thermal conductivity was measured and seems constant along the considered temperature range. The reported work represents a first step towards a better comprehension of the impact of temperature on the fatigue properties of rubbers. These basic concepts will be taken into account for further characterizations at different environmental

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Fig. 4. Values of thermal conductivity for the material between 30 and 130 °C measured with two different techniques: in black the data obtained with the guarded heat method and in red the one obtained with the laser flash method.