For future structural health monitoring (SHM) systems, the knowledge of past and present operational loads in the form
of forces/moments at critical system interfaces will be invaluable for performing accurate prognostics and augmenting
SHM capabilities. However, this information is not a direct product of traditional operational loads monitoring (OLM)
techniques employed on current fleet aircraft and is not easily achieved using existing force measurement devices. In
recognition of this limitation, this paper addresses the development of an accurate in-situ multiaxis force measurement
system for directly monitoring dynamic operational loads at critical mechanical interfaces without altering the existing
The proposed methodology utilizes a strain gage-based measurement technique in which a series of sensors is calibrated
with a set of known loading configurations. The sensitivity matrix relating the measured strains to the loads forms the
core of the system. The feasibility of the proposed technique was demonstrated both analytically and experimentally on a
representative aircraft weapon store/rail interface exhibiting nonlinearity in the system. The results are conclusive in that
the outlined trained network approach is able to accurately predict all six force/moment interface loads with less than 8
percent total error under various loading conditions.
An approach based upon the employment of piezoelectric transducer rosettes is proposed for passive damage or impact
location in anisotropic or geometrically-complex structures. The rosettes are comprised of rectangular Macro-Fiber
Composite (MFC) transducers which exhibit a highly directive response to ultrasonic guided waves. The MFC response
to flexural (A0) motion is decomposed into axial and transverse sensitivity factors, which allow extraction of the
direction of an incoming wave using rosette principles. The wave source location in a plane is then simply determined by
intersecting the wave directions detected by two rosettes.
The rosette approach is applicable to anisotropic or geometrically-complex structures where conventional time-of-flight
source location is challenging due to the direction-dependent wave velocity. The performance of the rosettes for source
location is validated through pencil-lead breaks performed on an aluminum plate, an anisotropic CFRP laminate, and a
complex CFRP-honeycomb sandwich panel.
The monitoring of adhesively-bonded joints through the use of ultrasonic guided waves is the general topic of this paper. Specifically, composite-to-composite joints representative of the wing skin-to-spar bonds of Unmanned Aerial Vehicles (UAVs) are examined. This research is the first step towards the development of an on-board structural health monitoring system for UAV wings based on integrated ultrasonic sensors. The study investigates two different lay-ups for the wing skin and two different types of bond defects, namely poorly-cured adhesive and disbonded interfaces. The guided wave propagation problem is studied numerically by a semi-analytical finite element method that accounts for viscoelastic damping, and experimentally by utilizing macro fiber composite (MFC) transducers which are inexpensive, flexible, highly robust, and viable candidates for application in on-board monitoring systems. Based upon change in energy transmission, the presence of damage is successfully identified through features extracted in both the time domain and discrete wavelet transform domain. A unique "passive" version of the diagnostic system is also demonstrated experimentally, whereby MFC sensors are utilized for detecting and locating simulated active damage in an aluminum plate. By exploiting the directivity behavior of MFC sensors, a damage location algorithm which is independent of wave speed is developed. Application of this approach in CFRP components may alleviate difficulties associated with damage location in highly anisotropic systems.
In this paper the Semi-Analytical Finite Element (SAFE) method for modeling guided wave propagation is extended to
account for linear viscoelastic material damping. Linear viscoelasticity is introduced by allowing for complex stiffness
constitutive matrices for the material. Dispersive characteristics of viscoelastic waveguides, such as phase velocity,
attenuation, energy velocity and cross-sectional wavestructures are extracted. Knowledge of the above-mentioned
dispersive properties is important in any structural health monitoring attempt that uses ultrasonic guided waves for long
range inspection. The proposed damped formulation is applied to several waveguides with different mechanical and
geometric properties. In particular, a viscoelastic isotropic plate, a railroad track and a pipe are studied.
Unmanned Aerial Vehicles (UAVs) are being increasingly used in military as well as civil applications. A critical part of the structure is the adhesive bond between the wing skin and the supporting spar. If not detected early, bond defects originating during manufacturing or in service flight can lead to inefficient flight performance and eventual global failure. This paper will present results from a bond inspection system based on attached piezoelectric disks probing the skin-to-spar bondline with ultrasonic guided waves in the hundreds of kilohertz range. The test components were CFRP composite panels of two different fiber layups bonded to a CFRP composite tube using epoxy adhesive. Three types of bond conditions were simulated, namely regions of poor cohesive strength, regions with localized disbonds and well bonded regions. The root mean square and variance of the received time-domain signals and their discrete wavelet decompositions were computed for the dominant modes propagating through the various bond regions in two different inspection configurations. Semi-analytical finite element analysis of the bonded multilayer joint was also carried out to identify and predict the sensitivity of the predominant carrier modes to the different bond defects. Emphasis of this research is based upon designing a built-in system for monitoring the structural integrity of bonded joints in UAVs and other aerospace structures.