Before non-destructive testing came on the scene some years ago, the best and most dedicated inspectors in a retread plant could only inspect tires visually. While they might have had the brightest lights and the sharpest probe tools, they weren't Clark Kent and didn't have the x-ray vision necessary to see through the tires.
Today, however, retreaders have a number of non-destructive methods of tire inspection available, each of which can aid in determining whether incoming tires, or casings, are good candidates for retreading. Let's examine a few.
One of the most popular non-destructive methods of tire inspection, shearography can be a very effective tool in locating certain types of casing separations or anomalies. The output from the shearography scan, which is displayed visually, offers a two-dimensional graphic representation of the change in elevation of a three-dimensional surface.
Technicians make subjective judgments of the condition of a particular casing based on their interpretation of the visual display. But since there's a gray area between the extremes of a perfectly sound casing and one that's clearly unacceptable, interpretation can vary from technician to technician, and even from one equipment manufacturer to another.
Although a shearography scan can enable a technician to identify anomalies known as separations, and most separations are considered detrimental to the durability of the casing, some of the smaller anomalies are inconsequential and don't affect the tire performance.
However, shearography is not effective in finding separations associated with open penetrations, cuts, snags, potential sidewall ruptures (also known as zipper failures) and similar in-service damage.
The most popular machine using electrical impulses is the NTD Tire Tester, developed by Hawkinson and used by retread plants worldwide. By subjecting the casing to electrical impulses, technicians can detect nails, nail holes, cuts, tears and bad repairs that often are not visible to the naked eye. This unit is able to catch tire failures before the equipment gets back on the highway, making it another critical element in the non-destructive testing toolbox.
Giving inspectors yet another view of the casing, x-rays can show broken steel cords and other defects that are invisible to the naked eye. X-rays literally see inside the casing. The units found in retread plants use low doses of radiation well within government-established guidelines.
Ultrasonic technology sends sound waves through a casing to a receiver. The strength of the sound waves is measured as they pass through the casing, which alerts inspectors to potential trouble spots. Problem areas are outlined with patterns of colored dots. The operator can then examine the dot pattern to determine the shape, size and location of the irregularity and make a decision as to whether the casing should continue through the retread process.
Your retreader should be using one or more of the above non-destructive testing methods to ensure that your tires are receiving the best and most thorough inspection available before they enter the retread process. By insisting on this as part of your criteria, you will ensure that your tire failures are left at the retread plant — not on some highway in the middle of nowhere at three o'clock in the morning.
Harvey Brodsky is managing director of the Tire Retread Information Bureau.