Full Field Stress Measurement for in Situ Structural Health Monitoring of Airframe Components and Repairs

Abstract : The fatigue usage monitoring systems installed on various military aircraft rely primarily on strain gauges for sensory information, and for good reason. Strain gauges have a well established certification framework, a relatively good track record of reliability and they directly target the parameter that drives fatigue. Extending the role of strain gauges to structural health monitoring however is problematic. The reasons are manifold but a key one is that strain gradients at ''fatigue hot spots" are often of a length scale shorter than the gauge. Stress imaging techniques have no such restriction but few have realistic prospects for application to in situ structural health monitoring. Recent developments in miniature low-cost microbolometer technology could make thermoelastic stress analysis an exception. The present paper examines the potential of this class of infrared detector to furnish a basis for in situ structural health monitoring. It first covers some preliminaries starting with a brief comparison of thermoelastic stress analysis to other full-field strain measurement techniques, followed by a discussion of several aircraft related applications illustrating the key properties of the methodology. Finally, it describes a case study in structural health monitoring involving the centre barrel assembly of an F/A-18 A/B fighter aircraft.