Recent interest in bonded composite patch repair technology for aerospace systems is because this method can be carried out at a reduced cost and time and can easily be applied to complex geometric structures.
This paper details the development of a dual stiffness/energy sensor for monitoring the integrity of a composite patch used to repair an aluminum structural component. The smart sensor has the ability to predict the elastic field of a given host structure based on the strain state of two sub-sensors integrated into the structure. The present study shows the possibility of using the sensor to deduce the local instantaneous host stiffness. Damaged structures are characterized by a reduction in their elastic stiffness that evolve from microstructural defects. A local smart sensor can be developed to sense the local average properties on a host. In this paper, sensors are attached to a structure and a modified Eshelby's equivalent inclusion method is used to derive the elastic properties of the host. An analytical derivation and a sensitivity analysis for the quasistatic application is given in a papers by Majed, Dasgupta, Kelah and Pines. A summary of the derivation of the dynamic Eshelby tensor is presented. This is of importance because damage detection in structures undergoing vibratory and other motions present a greater challenge than those in quasistatic motion.
An in-situ health monitoring active sensor system for a real structure (an aluminum plate with an attached repair patch) under close-to real lifecycle loading conditions is developed. The detection of the onset of any damage to the structure as well as the repair patch and the subsequent monitoring of the growth of this damage constitute important goals of the system. Both experimental and finite element methods were applied. Experimental results are presented for tests of the aluminum plate with the repair patch under monotonic quasi-static and dynamic loading vibratory conditions. In summary, the study shows that smart bonded composite repair patches are very effective in the repair of thin aluminum structures since they are able to determine the integrity of the repair structure as well as the repair patch.
A new mechanical multifunctional dual-stiffness sensor for in-situ real-time stiffness and energy density measurements was developed at the University of Maryland. This sensor is composed of 2 sub-sensors - a stiff and compliant subsensor. The sensor has the ability to predict the elastic field of a given host structure based on the strain state of the two sub-sensors integrated into the structure. This study showed the possibility of using the sensor to deduce the local instantaneous host stiffness and the strain energy density. Because the sensors can be embedded in a structure that is subjected to a complex stress state, Eshelbys equivalent inclusion method was used to derive the elastic properties of the host. An analytical derivation and a sensitivity analysis is given in a paper by Majeed and Dasgupta. Majeed and Dasgupta showed how the stiffness sensor is used to estimate the hosts stiffness. The present study evaluates the use of a dual stiffness sensor using piezoelectric sub-sensors to diagnose damage and to determine the remaining life of structures. The objective of this research is to develop an in-situ health monitoring active sensor for a real structure under its actual lifecycle loading condition. The detection of the onset of any damage, the subsequent monitoring of its growth and the help in predicting the remaining life of the structure constitute the important goals of the system. A numerical verification using finite element analysis (FEA) is presented and the results indicate that the sensor can be used for diagnostics of structures. A distributed array of sensors is used to determine the location of damage. Experimental results are presented for uniaxial experimental tests under monotonic quasistatic loading conditions and indicate that the concept can easily be implemented.