Sumitomo Rubber Industries has announced the results of a joint research project with the Leibniz Institute for Polymer Research Dresden, Germany, into the mechanisms behind rubber failure.
This groundbreaking research has shed light on the heretofore unknown mechanism that causes microscopic voids within the rubber (i.e. the presumed origin of rubber failure), leading to the formation and propagation of cracks.
The results of this recent joint research initiative will be used to develop performance sustaining technology, which will be incorporated into SRI’s Smart Tyre Concept.
While it has long been suspected that rubber failure (one of the major factors behind the phenomenon of tire wear) is due to the formation and growth of cracks in the rubber resulting from the fracturing of rubber molecules and the formation of voids within rubber at the microscopic scale, previous research had failed to fully explicate this theory. Thus, SRI set out to observe the formation of voids in synthetic rubber.
In 2015, the tire maker succeeded in shedding light on the formation of voids thanks to detailed molecular-level structural simulations utilizing SRI’s advanced 4D Nano Design technology to create new materials development technology, enabling the company to invent technologies to suppress void formation.
With this latest research, the two have succeeded in directly observing internal structural changes in combination with the mechanical behavior of molecules within actual synthetic rubber specimens through two different types of experiments. The findings of this research will open up possibilities for the development of rubber materials with extremely high durability by giving greater control over the viscoelastic properties of the rubber.
Using computed tomography (CT) to analyze changes in force, strain and volume in a thin rubber disk
A thin rubber disk specimen was placed between and affixed to two metal disk plates and stretched perpendicular to the contact surface (i.e. so that the plates were moving in opposing directions) in order to observe the relationship between applied stress and changes in volume as a function of apparent strain. At the same time, CT was utilized to directly observe the formation of voids within the rubber.
Analysis of applied force and volumetric change
Due to the inherent properties of rubber, when a thin disk-shaped specimen is stretched in this way, its natural inclination is to contract radially (i.e. perpendicular to the direction of tensile stress). However, because the specimen was affixed to the metal plates, the rubber directly attached to the plates could not contract radially as long as it remained solidly affixed to the plates. As a result, the rubber was forced to expand, allowing researchers to directly observe the formation of voids through CT observation of the rubber interior.
From this experiment, researchers also learned that the circumstances of void formation can vary. In particular, it was found that rubber failure in synthetic rubber containing filler material (such as silica or carbon black) occurred due to the formation of voids between agglomerates of filler material, while rubber failure in synthetic rubber containing no filler material occurred due to the formation of voids resulting from the sliding of rubber molecules.
Studying the relationship between tensile stress, apparent strain and apparent volumetric strain in specimens of both filled and unfilled synthetic rubber, it was found that apparent volumetric strain increased as apparent strain increased in both types of rubber. These results indicate that voids form within rubber due to constraint strain.
In filled synthetic rubber, voids formed between filler agglomerates and these voids then connected and combined to form cracks. The voids remain small due to the reinforcing effects of the filler material.
In unfilled synthetic rubber, voids mainly formed due to the sliding of rubber molecules. The initial voids then grow by merging of small voids due to crack propagation between these voids until the final formation of cracks.
Elucidation of synthetic rubber failure behavior through analysis using small-angle x-ray scattering (SAXS)
A notched sheet specimen of synthetic rubber was stretched laterally (i.e. in the planar direction) while small-angle x-ray scattering was utilized to observe the formation and growth of voids within the rubber at the notch point.
Using small-angle x-ray scattering to measure internal rubber density at the point of the notch in the sheet specimen of synthetic rubber, it was found that the density of the rubber nearer to the notch was lower than the rubber in areas farther away from the notch, suggesting that many voids were forming within the rubber near the notch point. These results indicate that, when sheet rubber is stretched laterally (i.e. in the planar direction), voids exist at the point where the rubber tears. Thus, researchers discovered that voids are directly involved in rubber failure, as had long been suspected.
More on the research being conducted by SRI and the Leibniz Institute for Polymer Research Dresden, Germany in the 2018 Tire Technology Annual Review.