Structural materials suffer fatigue damage by repeated loadings and unloadings – more so if subjected to stress reversals of tension and compression. We use this phenomenon ourselves to break wire by bending it back and forth several times. Well-documented instances of spectacular fatigue failures made headlines in the mid-20th century thanks to the mass-produced American Liberty ships of the 1940s and the pioneering British de Havilland Comet jetliners of the 1950s. The ships were the first to feature an all-welded steel design, and a significant number failed catastrophically in the cold waters of the North Atlantic during World War II. Fatigue cracks nucleated at the corners of square hatches and propagated rapidly by brittle fracture. In earlier ships, riveted plates acted as unintended crack arresters. The Comet was the world’s first commercial jet. Within a year of introduction, three of the airliners had broken up in flight. Repeated repressurization of the stressed alloy fuselage during take-offs and landings led to fatigue cracks emanating and propagating from the corners of the cabin windows, producing the fatal flight failures. Less spectacular, but fatigue related nonetheless, are the spider-like networks of surface cracks often seen on asphalt highways due to the repeated loading and unloading of traffic.
Tires are also subjected to fluctuating stresses during highway service. These stresses vary in magnitude and direction. Like all materials, the physical properties of rubber, including fatigue life, deteriorate when subjected to a sizeable number of dynamic deformations. Failures can develop from progressive fracturing (crack growth) initiating at points of high stress; cracks generally form perpendicular to the direction of maximum tension. Consider that a vehicle tire experiences about 30 million load-unload cycles during 40,000 miles of operation, while the ‘million-mile casing’ of a long-haul radial truck tire undergoes 500 million cycles with three retreadings. Resulting fatigue damage can occur unseen in cord or rubber components and can sometimes be seen as microcracks on exterior tire surfaces. Such damage after long service is a consequence of interacting mechanical, chemical and/or thermal processes.
When radial tires first penetrated the US auto market, the industry experienced two new forms of fatigue damage not seen in bias ply constructions: belt-edge separations and cord ‘socketing’ (also known as ‘walking wire’). These breakdowns produced looseness at the belt edge and rendered many end-of-life PCR tires unsuitable for retreading, partially contributing to the demise of that market. The worst-case scenario: tread separations or belt detachments occurring during highway service and causing an accident. On the other hand, superficial cracks sometimes seen at the base of tread grooves at tire wear-out mileages are usually cosmetic unless cord reinforcements are visible.
Polymer selection and compounding ingredients have a major influence on the physical properties of a formulation. For example, for fatigue abatement, PCR and TBR sidewalls are often based on blends of natural rubber and polybutadiene. But we have a compounder’s dilemma: improving one property of a rubber recipe usually has an unwanted effect on another. Consider that carbon black particles embedded in rubber promote stiffness and strength while tending to blunt fatigue crack growth – all at the expense of increased hysteresis. Similarly, cobalt in steel belt skim stocks promotes cord-rubber adhesion but adversely influences crack growth. And the eight or more rubber compounds in a tire usually require varying states of cure to achieve the desired properties, including fatigue resistance – a balance of vulcanizate trade-offs.
Many types of laboratory fatigue testers have been developed to assess crack initiation, crack growth, or life-to-failure characteristics of rubber test specimens subjected to tensile, compressive and/or shearing stresses. Sample configurations vary – rings, dumb-bells, flat strips and so on. Not surprisingly, correlation between the results of lab tests, whether aged or unaged, and tire tests is not clear cut due to the wide scatter in test data and the varied nature of tire service conditions. In the laboratory, fatigue life can be doubled or halved by a temperature change of 10°C (18°F) in strain amplitude-controlled testing. However, one result is unequivocal: higher temperatures accelerate thermo-oxidative degradation and reduce the fatigue life of both test specimens and tires.
Might it be possible to develop a self-healing rubber that could be applied to critical tire components during assembly (for example, at cut steel cord endings) that would inhibit the nucleation and growth of microcracks, averting subsequent fatigue damage during service?