Tensile, Compression and Shear stresses can best be visualized by comparing an unloaded tire section to a loaded one as shown above. The following points can be made:

1. An aircraft tire is designed so that in the unloaded condition the internal tensile forces acting on each layer of fabric are uniform.

2. Due to the high deflection of the tire section under the load, the tensile forces on the outer plies will be higher than those on the inner plies.

3. Due to the force gradient from outer to inner plies, shear forces are developed between the various layers of fabric.

4. Underinflating or overloading a tire will increase these shear forces, thus rapidly decreasing the life of an aircraft tire.

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Carcass Fatigue Due to Underinflation

To demonstrate how rapid carcass fatigue can occur due to underinflation, the chart above shows the average of three different tire sizes run at the following conditions: 1. One tire of each size was run on successive taxi cycles consisting of 35,000 ft. each at 40 mph. This was repeated until tire failure occurred. Since this tire was properly inflated, the test result was recorded as 100% durability performance.

2. A second tire of each group was run to the same test, but was 5% underinflated.

3. A third tire of each group was also run to the same test, but at 10% underinflation.

Obviously, one would expect the tire durability to decrease with underinflation. What's impressive, however, is the magnitude of reduction.

To further study the effect of underinflation on tire failure, additional tests were run on the dynamometer. Several tires, at various degrees of underinflation, were run to failure. Some tires were run to take-off cycles and others to 10,000 ft. taxi cycles. As would be expected, the cycles to failure decrease as the percent of underinflation increases.

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Cycles to Failure vs. Underinflation

A couple interesting findings in this study were that all the taxi cycle failures were blowouts in the lower sidewall, while the take-off cycle failures were thrown treads. From the shape of the curves, we see that take-off cycles were more sensitive to underinflation than were taxi cycles.

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Cycles to Failure vs. Overload

To determine if overloading has the same detrimental affect on tire life as underinflation, the same tests were run on several tires with increasing overloads. As also would be expected, the more a tire is overloaded the quicker it fails.

A couple interesting findings in this study were that all the taxi cycle failures were still lower sidewall blowouts, and only thrown treads occurred during the take-off cycles. This test shows that taxi cycles are more sensitive to tire overloading.

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Tensile, compression, and shear forces in aircraft tires are extremely high. When the tires are not properly maintained, these forces go even higher until the compound and/or fabric start rapid deterioration. When this happens the following problems can occur:

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Shoulder Separation

Shoulder separation is most likely to occur between outer plies where the shear forces are highest.

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Lower Sidewall Compression Break

This is the start of the type of failure caused by underinflation or overloading. This photo shows carcass cords above the bead area that are starting to fail due to flex fatigue.

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These photos show how underinflation or overloading can cause lower sidewall compression flex breaks.

Sidewall Crack - The first signs of compression flex break in the lower sidewall can appear on the outside sidewall or the inside liner. This photo shows a crack developing in lower sidewall.

Liner Crack - The first signs of a compression flex break can also appear on the inside liner. This condition will also be apparent by a tire pressure loss. This pressure loss then magnifies the problem, resulting in sidewall blowout.

Massive Separation - During the creation of a sidewall or liner crack, the carcass plies on the inside become severely deteriorated, along with massive separations. This results in possible sidewall blowout.

These three photographs show the stages of progression. Never mistake these conditions for simply a sidewall or liner crack, as a blowout is imminent.

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Courtesy of The Goodyear Tire & Rubber Company