Structural Health Monitoring of a Metallic Sandwich Panel by the Method of Virtual Forces

A structural health monitoring system for assessing thermal damage in a metallic sandwich panel was developed in this article. The method of virtual forces was utilized to detect, locate, and quantify damage. The method assumes that a damaged structural response is a summation of the undamaged response and the response from an effective forcing function, which represents the effects of the material damage. The effective forcing function, or virtual force, is an estimate of the forces that the damage mechanism exerts on the undamaged structure. Virtual forces were shown to detect changes in the mass, stiffness, and damping matrices in a lumped parameter dynamic model. A finite element model of a mechanically attached sandwich metallic panel was used to detect a stiffness loss at different locations between two sensor locations. These results were then compared with experimental results involving thermal debond between the face sheet and core of an Al—Al honeycomb panel. Virtual forces indicated there was damage associated with magnitude and frequency shifts in the frequency response function measurements made on the damaged panel. Damage was located relative to the sensor/actuator measurement locations. Damage that was not adjacent to measurement locations was less accurately located and quantified. Variability in producing localized thermal debond using an acetylene torch was also apparent in the virtual force estimates. The virtual force estimates also indicated that damage was spread over several measurement locations because highly localized thermal debond was difficult to produce.