Local Triads of Sensors to Mine Collocated Acceleration for Structural Health Monitoring of an Aluminum Beam With Bolted Joints

Several aerospace, space, and naval platforms process a core structural system assembled wholly or partially by aluminum alloy elementary structures, such as beams, plates, and cylinders, among others. Aluminum alloys are preferred due to light weight and corrosion resistance. The formed complex aluminum alloy structure is characterized by a number of joints where different structural elements are joined together. Areas with joints are prone to damage. Some joints are made by bolts for assembly and reassembly purposes. A loose bolt can be considered as a man-induced damage in the whole complex structure. This damage can result in a catastrophic failure. The vibration and elastic wave propagation-based monitoring of a critical area bearing bolt joints can offer a reliable monitoring. In this work, we present a basic experimental research effort. We consider an aluminum alloy beam composed of two sunbeams joined by means of two bolts. The bolts are healthy in the sense that they are tight to the level of design recommended force. We use three mono-axial sensors in three dimensional arrangement and mine vibration-wave data in the form of collocated signals. These signals are analyzed by means of the proper orthogonal decomposition transform. The local tri-axial arrangement of mono-axial sensors is used to collect collocated acceleration signals in two local areas. One of the local areas contains the bolted joints. The other local area contains the free end of the structure. This is not considered as a critical area but a boundary accessible area. The POD modal structures of collocated vibration signals are quite different. The behavior is compared to that of structure without bolted joints.