Detecting Cracks in Helicopter Frame Station Structure using Unified Change Detection

Abstract

A unified change detection approach was proposed recently for detecting and identifying changes caused by various types of faults in rotating machinery. This novel, but unsophisticated method was validated using vibration data from rotating machines with several different types of faults, e.g. gear, bearing, pump, shaft and planet carrier faults. This paper further explores the application of unified change detection to the structural health monitoring field. We will present results of applying the method to detecting a crack in a Black Hawk helicopter frame structure using the in-flight strain gauge data, which were collected during a flight trial. During the trial, an excessive level of stress was detected in the 53rd test flight hour at a strain gauge located on a forward frame structure and by follow-up inspection a structural crack was found in close proximity to the gauge location. A previous study showed that, with the development of the structural crack, the dominant frequency components around the 4th shaft order of the main rotor were shifted from their nominal position by approximately 10 percent of the main rotor shaft order and were observed to vary with time (i.e. a crack-induced nonlinear effect). For this paper, the strain data acquired at various locations around the crack location have been analyzed using the unified change detection approach. Analysis results show that the changes in structural loading due to the crack can be detected and trends indicating a gradual change in behaviour are observed, even under the most common and uniform maneuvers such as the in-ground effect (IGE) hover that was experienced in every flight of the trial. The crack-induced change can be detected by most strain gauges around the crack location.