Characterization and understanding of mechanical variability of elastomer: application in automotive valve testing in low cycles fatigue

Abstract

Currently, rubber automotive valves are appropriate for passenger vehicles that operate at speeds lower than 210 km/h. However, beyond this threshold, the mechanical stress imposed on the elastomer is far more intense, increasing the risk of cracks caused by the cyclic accelerations and decelerations of the vehicle. This work delves into valve damage at high speed to gain insights into the factors contributing to failures. Fractographic analysis on valves has facilitated a thorough comprehension of valve damage by precisely pinpointing the location of crack initiation and its propagation within the volume of the elastomer. Nevertheless, the correlation between failures and valve durability is not straightforward, primarily due to variations in bench test equipment. Therefore, in order to eliminate the influence of bench test equipment-related variations, a fatigue campaign was conducted on laboratory specimens. This aimed to exclusively characterize the variability in rubber fatigue. Additionally, to achieve a higher level of representativeness of valve application, Hencky's invariants were employed to establish an equivalent kinematic mechanical valve state on these specimens. Experimental results attest an intrinsic variability of the rubber material. Subsequently, a fractography study on these specimens has provided a clearer insight into the primary material weaknesses, specifically focusing on the agglomeration of black carbon. A microstructural analysis using scanning electron microscopy (SEM) was performed to assess the batch dispersion state in correlation with specimen durability.

Keywords

• elastomer
• tire valve
• variability
• fatigue
• structure
• fractography
• hencky’s invariants