Experimental Verification of Two Stage Algorithm for Health Monitoring of Aerospace Structures

Abstract

A system of continuous structural health monitoring would be desirable for early warning of distress in major engineering systems such as space structures and orbiting spacecrafts since they are susceptible to the impact of meteoroids and orbital debris. However, the complexity of some large space structures makes the use of the traditional non-destructive evaluation (NDE), such as visual inspection and instrumental evaluation methods, impractical. A recent development in structural health monitoring systems (SHM) is the use of vibration-based damage identification (VBDI) methods. These methods use limited instrumentation to detect the changes in the measured modal characteristics of the structure, that is, its frequencies and mode shapes. These characteristics change with the physical properties of the structure (stiffness, mass and damping matrices) and can be used to help find the location and extent of damage. Optimal matrix update method is one of the VBDI algorithms that depend on finite element modeling (FEM) of the structure. The FRF differences method is also one of the VBDI techniques that depends on the directly measured frequency response functions data and is therefore referred to as non model-based damage identification algorithm. A proposed two stage algorithm combining the aforementioned model-based and non-model based methods introduced to improve the reliability of damage detection. The algorithm presented a simple robust sequential scheme of VBDI techniques and has proven an acceptable level of success when tested through numerical simulation in the presence of simulated random errors. The present paper focuses on the experimental verifications through the implementation of the algorithm to evaluate its efficiency in identifying damage in real large-scale space structures. The experimental verifications are highlighted through modal tests designed to provide estimates of damage in a 3D eight-bay freefree frame representing part of the International Space Station ISS. Details of tests on a healthy structure as well as on a damaged structure in which predetermined damage has been introduced are presented. This combination allowed identifying different levels of damage for real monitoring of structures using minimum modal data base even when the structure is somewhat complex.