Structural Integrity Assessment of Corroded Landing Gear Drag

Abstract

The reliability of a corroded helicopter landing gear drag beam has been investigated. The drag beam made of high strength steel has experienced a failure caused by corrosion initiated crack. The probability of fracture has been calculated from the intercept of a strength and a load distribution. The strength distribution of a drag beam has been obtained using thickness measurement data estimated by radiography. A model for thickness measurement has been developed using linear attenuation coefficient for both a base metal and a corrosion product. By comparing the photon intensities of corroded region with sound region of known thickness, the reduction of corrosion has been estimated. Due to uncertainties in model parameters which are the thickness of sound region, photon intensity with or without a corrosion product, and corrosion rate, Monte Carlo simulation has been conducted. A load distribution has been obtained using flight load data acquired from strain gages attached to the drag beams. The drag beam mostly carried compressive load and much smaller torsional load. The result showed that currently operated drag beams have enough margin of safety. With the consideration of uncertainties, the inspection of drag beam using radiography has sufficient capability for the reliability evaluation. Based on the risk of failure, part replacement schedule has been decided.