Aged Tire Durability
Specific directions to the National Highway Traffic Safety Administration to upgrade tire safety standards came in November 2000, when the U.S. Congress passed the Transportation Recall Enhancement, Accountability and Documentation (TREAD) Act.
As tire wear life has increased over the years, interest in the aging of a tire’s internal components has increased. In the events leading up to the TREAD Act, Congress explicitly stated that there is a need for some type of aged tire durability test on light vehicle tires (under 10,000 lbs (4,536 kg) in gross vehicle weight). Currently, there is neither an industry nor a government standard practice for accelerating the aging of tires.
The goal of ASTM Subcommittee F09.30 and its aged tire durability task group is to develop a scientifically valid protocol for laboratory aging, and then test the aged tire such that certain of its properties and behavior correlate to field aged tires. However, before this standards development activity can move forward, research is necessary in order to determine appropriate laboratory aging and roadwheel endurance test conditions relating to field aged tire performance.
Formed in 2002, the aged tire durability task group has developed a two-phase approach (described below and illustrated in Figure 1) to conduct this research. Successful completion of Phase 1 hinged upon the support of the project sponsors and the effort of task group members. Task group participation has now grown to 33 members representing 20 organizations, including suppliers and consultants, testing organizations, tire manufacturers and retreaders, trade associations, vehicle manufacturers and government observers. To date, participants have contributed more than three man-years of volunteer time in nearly 60 full task group and data analysis sub-team meetings.
Overview of the Tire Aging Project
The first phase of the tire aging project employed a design of experiments (DOE) approach to determine the optimal conditions for accelerated aging for a selected single tire type. This exploratory evaluation included both static (in an oven) and dynamic (on a roadwheel) accelerated aging designs. Accelerated aging conditions for both approaches were examined by statistically and empirically analyzing property changes of both field and laboratory aged tires relative to the corresponding properties of a new tire. Phase 1 encompassed the evaluation of test data from field tires that were directly compared and analyzed against the data generated from tires that were laboratory aged on an accelerated basis. Phase 1 made use of field tire test data collected and provided in the public domain by NHTSA, saving both time and expense through the non-duplication of effort.
Phase 1 of the ASTM aging research focused on the objective of matching key physical and chemical properties between both field and laboratory aged tires. This objective assumed that if the changes in properties between field and laboratory aged tires were similar, then laboratory aging would mimic the aging mechanisms of tires in the field. Chemical (elemental analysis and solvent swelling), physical (tensile, indentation modulus and peel strength) and dynamic (viscoelastic and two-ply fatigue) measurements were made in Phase 1. The tire components/regions examined in this study are shown as a cutaway tire in Figure 2.
Static Laboratory Oven Aging of Tires
The first method selected to age tires in the laboratory was static, in temperature controlled ovens. Bearing in mind the requirement to most accurately reproduce the aged state of field-aged tires, a range of oven aging parameters was selected based on the collective prior experience of the task group members, with the intent of choosing final parameters for the method based on the test results.
As a result, static laboratory tire aging was conducted for durations of a number of weeks. Data collected from a new tire corresponding to zero weeks aging completed the statistical design. Other DOE parameters included oven temperature and tire inflation medium and pressure. The filling gas was either standard shop air or a mixture of oxygen and nitrogen known as a 50/50 mix. The shop air had an oxygen content of approximately 21 percent. The 50/50 mix produced an oxygen content of approximately 42 percent in the inflated tire, due to the fact it was mounted in an air environment, and the existing gas was not purged from the tire. A total of four variables are included in the static DOE.
Dynamic Laboratory Roadwheel Aging of Tires
The second method selected to age tires to known field states was dynamic roadwheel aging. Roadwheel testing is a method that has been used in the tire industry for many years to gauge the mechanical durability of a tire construction. As with the oven aging, the committee selected a range of parameters to exercise the tires, with the intent of selecting the most appropriate of these parameters based on the collected data.
The second design of experiments was conducted by cycling additional new tires on a 67.23 inch (1.707 m) diameter roadwheel. The dynamic DOE included a total of five test variables. Tires were run over a range of time, loads, speeds, and inflation pressures. The filling gas was again either standard shop air or a mixture of oxygen and nitrogen known as a 50/50 mix, which produced an oxygen content of approximately 42 percent in the inflated tire.
The ASTM F09.30 Aged Tire Durability Task Group decided not to retain dynamic aging as the laboratory aging protocol. Two reasons factored into this decision. First, the data indicate that the most severe aging conditions are needed to cover the targeted field aged range (two to six years of field aging). However, if these most severe conditions are applied, the result is an uneven aging in the tire, that is, the same aged state is not achieved throughout the tire. Second, when the lower severity dynamic aging conditions are applied, insufficient aging occurs within the limits of the DOE to cover the targeted field range.
The study has shown for static aging that temperatures above 65 °C should be avoided as a standard aging condition to avoid significant over-aging of tire components relative to field tire behavior. However, the data did not allow the committee to conclusively select a specific temperature, so a lower temperature range will be retained for further study in Phase 2.
Applying maximum (as calculated and as determined from test data) inflation pressure and maximum initial oxygen concentration yielded results most similar to field tires. However, without periodic refilling during the oven duration, oxygen depletion in the tire cavity will continuously occur via permeation into and reaction within the tire. This may result in oxygen partial pressures that are too low to effectively, oxidatively age the tires as intended. The aging response of a tire with pressure and oxygen concentration maintained during the course of static oven aging is currently being studied, the results of which will determine a definite reinflation protocol.
The amount of time this tire must spend in the oven has been found to be dependent upon both the oven temperature and the desired field aged state of the specific tire components studied.
This Phase 2 follow-up study is now under way and includes three tire types in order to confirm the laboratory static (oven) aging conditions developed in Phase 1. A smaller selection of physical, chemical, and dynamic properties will again be evaluated for these tires by statistical and comparative methods. Durability-type roadwheel testing will also be conducted on both new tires and on laboratory aged tires.
Phase 2 of the project includes the development of a roadwheel test plan. Eight experiments will make up this module. These experiments include a steady-state DOE, stepped-up load roadwheel testing, material property evaluations of aged tires, and five other experiments to provide information for the framing and tuning of the test conditions.
This Phase 2 roadwheel module shows the broad participation of the task group participants, with 12 organizations volunteering resources, as well as stakeholder commitment to assure industry consensus on the final test method(s) and standard(s).
A Phase 2 validation module will follow the roadwheel study. This module will evaluate a broader range of tire types using the recommended aging protocol developed from the final results of Phase 1 followed by a roadwheel durability evaluation. The testing information generated from Phase 1 and the two modules in Phase 2 will allow the task group to develop a laboratory aged tire durability test standard(s). The group anticipates drafting, balloting, and approval of the tire aging standard(s) in 2007. //