This contribution presents a novel strategy to achieve real-time prediction of non-penetrating impact-induced
damage, especially delamination between plies of composite laminated structures. The proposed strategy aims to create
an optimum-sized database of simulated damage information on a given laminated structure using numerical failure
models and pattern recognition technique. A multi-stage data clustering algorithm is implemented to identify regions in
the structural Finite Element (FE) model that have similar damage signatures. The generated database is linked to the
real-time estimate of impact location and load-time history obtained from piezoelectric transducers based passive impact
monitoring system. A composite stiffener panel is selected as a model problem and it is shown that the proposed strategy
based on pattern recognition will result in large savings in computational cost for the database generation besides
providing real-time damage diagnostic capabilities for in-service adverse impact events within certain confidence
bounds.
KEYWORDS: Sensors, Composites, Passive remote sensing, Data modeling, Failure analysis, System integration, Inverse problems, Calibration, Structural health monitoring, Systems modeling
Next generation technology of integrated health management systems for air-transportation structures will utilize
SHM methods in combination with simulation techniques for the prediction of structural degradation induced
by adverse events such as impacts. The contribution focuses on the development of an advanced real-time
monitoring system for impact loads using passive sensing networks. Starting from the fundamental approach of
real-time monitoring based on system identification models, problems of model order, signal conditioning and
efficient model training will be addressed. Finally, the load monitoring system is interactively linked to a damage
prediction module based on numerical failure analysis employing composite failure criteria. The utilization of appropriate database techniques allows a real-time prediction of impact induced damage after detection of any adverse impact event making information available on developing degradation at the earliest possible state.
Transient ultrasonic waves in an elastic half-space generated by an ultrasonic transducer of finite size are modeled by the
Distributed Point Source Method (DPSM). DPSM which is a Green's function based semi-analytical mesh-free
technique is modified to incorporate the transient loading from a finite size acoustic transducer. Fast Fourier transform
(FFT) of the transient loading is computed and then DPSM is used to compute the ultrasonic field at different
frequencies and then inverse fast Fourier transform (IFFT) is taken to get the transient response of an elastic half-space
excited by a bounded acoustic beam. Numerical results are generated for elastic half-space excited with normal incidence
of acoustic beam. Then the transient Rayleigh wave in the solid half-space is generated. The modeling is then extended
to the transient response of an elastic half-space containing a crack, struck by a bounded acoustic beam. It is discussed in
the paper what type of useful information that is hidden in the steady state solution can be obtained from the transient
results.
In order to take full advantages of composites and enable future composite structures to operate at their physical limits
rather than limits predetermined from computational design assumptions and safety factors, there is a need to develop an
embeddable sensing system to allow a structure to "feel" and "think" its structural state. In this paper, the concept of
multi-modal sensing capabilities using a network of multifunctional sensors integrated with a structure has been
developed. Utilizing this revolutionary concept, future structures can be designed and manufactured to provide multiple
modes of information that when synthesized together can provide capabilities for intelligent sensing, environmental
adaptation and multi-functionality. To demonstrate the feasibility of multi-modal sensing capabilities with built-in sensor
network, one single type of piezoelectric sensor was selected to perform the measurements of dynamic strain,
temperature, damage detection and impact monitoring. The uniqueness of the sensing system includes (1) Flexible,
multifunctional sensor networks for integration with any type of composite structural component, (2) Scalable sensor
network for monitoring of a large composite structure, (3) Reduced number of connecting wires for sensors, (4) Hybrid
diagnostics with multiple sensing capabilities, (5) Sensor network self-diagnostics and self-repair for damaged sensor
system.
In NDT (nondestructive testing) often a side-drilled hole (circular cavity) is used for calibration. In this paper scattering
of ultrasonic waves by a circular hole is studied. The ultrasonic wave is generated by a transducer of finite dimension. A
newly developed semi-analytical technique called Distributed point source method (DPSM) has been adopted to solve
this problem. Even though this is an old problem the complete field of the scattered waves in presence of a hole in a half
space near its boundary has not been shown in the literature yet. The scattered ultrasonic field (stress and displacement)
is computed using DPSM and presented in the paper. Solution of this problem will also help us to understand the
distortion of the ultrasonic field in the half-space due to the presence of a circular anomaly (cavity or inclusion) which
plays an important role in structural health monitoring.
Triangulation technique for impact point location works very well when the acoustic emission sensors are placed at a
relatively large distance from the point of impact. In this situation the time of arrival measurement is not affected
significantly by the small error that might arise from not being able to pinpoint the exact time of arrival of the acoustic
signal. The conventional technique also requires that the wave speed in the medium is well-known and non-dispersive in
the frequency range of interest. If the receiving wave is a P-wave or S-wave or a non-dispersive Rayleigh wave then the
conventional triangulation technique is reliable. In this paper it is shown that the conventional triangulation technique is
not very reliable for locating the impact point in a plate when the sensors are placed close to the striking point for two
reasons - first, it is difficult to pin point the exact time of arrival of the signal and secondly the Lamb modes in a plate
are dispersive. Dispersive signals attenuate differently at various frequencies and propagate with different speeds
causing distortions in the received signals and thus introduce more error in the time of flight measurement. In this paper
an alternative approach is proposed to locate the impact point more accurately. Experiments are carried out with an
aluminum plate. The impact points predicted by the conventional triangulation technique and the proposed modified
method are compared.
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