Active Detection of Structural Damage in Aluminum Alloy Using Magneto-elastic Active Sensors (meas)

Many structural damage detection methods utilize piezoelectric sensors. While these sensors are efficient in supporting many structural health monitoring (SHM) methodologies, there are a few key disadvantages limiting their use. The disadvantages include the brittle nature of piezoceramics and their dependence of diagnostic results on the quality of the adhesive used in bonding the sensors. One viable alternative is the utilization of Magneto-Elastic Active Sensors (MEAS). Instead of mechanically creating elastic waves, MEAS induce eddy currents in the host structure which, along with an applied magnetic field, generate mechanical waves via the Lorentz force interaction. Since elastic waves are generated electromagnetically, MEAS do not require direct bonding to the host structure and its elements are not as fragile as PWAS. This work exp lores the capability of MEAS to detect damage in aluminum alloy. In part icular, methodologies of detecting fatigue cracks in thin plates were exp lored. Specimens consisted of two identical aluminum p lates featuring a machined slot to create a stress riser for crack fo rmation. One specimen was subjected to cyclic fatigue load. MEAS were used to transmit elastic waves of d ifferent characteristics in order to explore several SHM methodologies. Experiments have shown that the introduction of fatigue cracks created measurable amplitude changes in the waves passing through the fatigued region of the aluminum plate. The phase indicated sensitivity to load conditions, but manifestation in the cracked region lacked stability. Nonlinear effects were studied using plate thickness resonance, which revealed b irefringence due to local stresses at the site of the fatigue crack. The resonance spectrum has also shown a frequency decrease apparently due to stiffness loss. Preliminary results suggest opportunities for fatigue damage detection using MEAS. Application of MEAS for the diagnosis of complex structures is currently being investigated. INTRODUCTION Modern structural health monitoring (SHM) increasingly relies on a diverse suite of sensing technologies. Piezoelectric transducers are widely using in SHM because of their h igh efficiency; however, limitations that can affect their application include the brittle nature of piezoelectric material as well as their dependence on the quality of an adhesive bond layer [1]. It is desirable to improve survivability of SHM sensors and eliminate, if possible, dependence of monitoring results on the quality of a bond layer between the sensors and the structure. Magneto-elastic active sensors (MEAS) [2] offer an alternative to piezoelectric transducers in applications where direct bonding to the structure is not desired, bond quality is a concern, or where a more mechanically robust sensor would be required. MEAS utilize an effect of electromagnetic generation and reception of ultrasonic waves in conductive materials [3] and hence operate on the same princip le as electromagnetic acoustic transducers (EMATs). However, in contrast to bulky and heavy EMATs, MEAS design is optimized and min iaturized for potentially embeddable applications and currently features sensors with diameters as small as 1⁄2 inch [4]. MEAS require two components; a wire co il and a stationary magnetic field. A time vary ing current is passed through the coil adjacent to a conductive host structure generating eddy currents. In nonferrous materials these eddy currents combined with a stationary magnetic field penetrating the material allows the generation of elastic waves through the Lorentz-force interaction [5]. MEAS also act as sensors where an elastic wave passing through a stationary magnetic field will induce current flow in the sensor coils. The primary advantages of MEAS are, first, the non-contact (or through paint) nature of their operation which allows performance independent of the integrity of the bond. Even though the bond layer does not impact sensor performance increasing liftoff results in decreased efficiency Proceedings of the ASME 2011 Conference on Smart Materials, Adaptive Structures and Intelligent Systems SMASIS2011 September 18-21, 2011, Scottsdale, Arizona, USA