KEYWORDS: Transducers, 3D modeling, Waveguides, Structural health monitoring, Photoelasticity, Wave propagation, Finite element methods, Signal attenuation, Aerospace engineering, Wavefronts
Structural health monitoring (SHM) technologies, which use integrated sensing for damage detection, are expected to
improve system reliability, availability, and operational cost. Guided waves can propagate great distances while
experiencing low attenuation. They have been successfully used for damage detection in structures of relatively low
geometric complexity such as plates and cylindrical pipes. The use of guided waves for this purpose becomes
increasingly difficult as the geometric complexity of the structure increases. Aerospace structural components such as
fuel tanks, wings, etc. often are comprised of substructures that consist of plates with integral stiffeners. This work
reports on finite element simulations of guided waves in integrally stiffened plate structures. In these studies, the guided
waves are generated by PZT wafer-type transducers mounted on the structure. Transient dynamic finite element
simulations using PZFlex, in 2D and in 3D, were used to model both the structure and transducers. The interaction of the
guided waves with cracks, simulated by notches of varying dimensions, is also modeled. This allows appraisal of the
sensitivity of various modes for crack detection by providing insight into mode conversion and scattering resulting from
the guided wave and crack interaction.
Durable integrated sensor systems are needed for long-term health monitoring evaluations of aerospace systems. For
legacy aircraft the primary means of implementing a sensor system will be through surface mounting or bonding of the
sensors to the structure. Previous work has shown that the performance of surface-bonded piezo sensors can degrade
due to environmental effects such as vibrations, temperature fluctuations, and substrate flexure motions. This
performance degradation included sensor cracking, disbonding, and general loss of efficiency over time. In this research
effort, the bonding state of a piezo sensor system was systematically studied to understand and improve the long-term
durability and survivability of the sensor system. Analytic and computational models were developed and used to
understand elastic wave generation and reception performance for various states of sensor disbond. Experimental studies
were also conducted using scanning laser vibrometry, pitch-catch ultrasound, and pulse-echo ultrasound methods to
understand elastic wave propagation effects in thin plate materials. Significant performance loss was observed for
increasing levels of sensor disbond as well as characteristic frequency signatures which may be useful in understanding
sensor performance levels for future structural health monitoring systems.
Surface bonded sensors have significant potential for detecting and characterizing damage in legacy aircraft structures as
part of a Structural Health Monitoring system. In this effort, research has been undertaken to understand the impact of
adhesive viscoelastic properties on the generation of elastic wave energy by surface-bonded sensors in thin plates.
Previous work has shown that bonded sensors can degrade and fail due to exposure to weather, vibration, temperature,
and mechanical loading. In that work, experimental and analytical studies were performed to characterize the transfer of
static load from a structure into a surface-bonded sensor. The results indicated that the sensor should be decoupled from
the nearly static loading induced from the structure to improve its durability. In this effort, we build on that original work
to determine what effect the adhesive has on elastic wave generation and reception in the host structure. The results
indicate that strong coupling with the structure is required for effective generation and reception of elastic waves, where
the elastic material properties of the sensor, bond, and host structure were considered. Although the two goals appear to
be contradictory (sensor durability and elastic wave generation), the use of a strongly viscoelastic adhesive is viewed as
potential solution for both by allowing weak coupling at low frequencies and strong coupling at high frequencies.
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