Structural fatigue measurement

Detecting metal cracks before they can be seen by non-destructive testing instruments


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Non-destructive testing methods such as magnetic particle inspection, shown above, are difficult to use outside of the lab and fail to detect very small cracks.

In 2011, a Southwest Airlines Boeing 737 suffered a five-foot rupture to its fuselage at 34,000 feet resulting in an explosive decompression and emergency landing. Experts believe that this was caused by metal fatigue.

Every metal part, no matter how well it is manufactured, contains some flaws as a result of the manufacturing process. The flaws are randomly distributed, some are on the surface and some are located internally. Flaws close to a stress concentration point are subjected to increased strain and therefore have an increased probability of developing micro-cracks. These micro-cracks subsequently join with each other to form a bigger crack. When the part is subjected to in-service loads, the crack begins to grow. The rate at which a micro-crack grows depends upon several factors that include load levels and the number of load cycles to which the material is subjected.

Due to the need to tightly monitor metal fatigue in high-performance aircraft structures, the Navy has developed a method and apparatus to detect metal stress earlier than traditional non-destructive test instruments. The system calls for mounting a fatigue gauge made from the identical material in the component under test. An electrical power source repeatedly applies current to the gauge – a coupon approximately 6 inches in length – and resistance is measured over time. These coupons could be mounted on in-service aircraft, without the electrical source, removed at periodic intervals, and resistance-tested for fatigue. Such testing would provide insights into the effects of fatigue on aircraft flying particular missions and could inform maintenance schedules and inspections.

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