Advanced composites are being used increasingly in state-of-the-art aircraft and aerospace structures. In spite of
their many advantages, composite materials are highly susceptible to hidden flaws that may occur at any time during
the life cycle of a structure, and if undetected, may cause sudden and catastrophic failure of the entire structure. This
paper is concerned with the detection and characterization of hidden defects in composite structures before they
grow to a critical size. A methodology for automatic damage identification and localization is developed using a
combination of vibration and wave propagation data. The structure is assumed to be instrumented with an array of
actuators and sensors to excite and record its dynamic response, including vibration and wave propagation effects. A
damage index, calculated from the measured dynamical response of the structure in a previous (reference) state and
the current state, is introduced as a determinant of structural damage. The indices are used to identify low velocity
impact damages in increasingly complex composite structural components. The potential application of the approach
in developing health monitoring systems in defects-critical structures is indicated.
This work aims at developing a compact and wireless structural health monitoring system (WSHM). The system samples
ultrasonic wave propagation data, analyzes the collected data using a statistical damage index (SDI) approach and
transmits the results to a remote location. The analysis provides an insight into the state of health of the structure under
test as a function of time. The approach is designed to overcome the complexity and variability of the signals in the
presence of damage as well as the geometric complexity of the structure, requiring minimal operator intervention. The
approach establishes a baseline drawn from measurements done on an undamaged or partially damaged structure. This
baseline is used to monitor for changes in the health of the structure. Damage indices are evaluated "instantly" by
comparisons between the frequency response of the monitored structure and an unknown damage under the same
ambient conditions. The approach is applied to identify several types of structural defects in steel girders and stiffened
composite panels for different arrangements of the ultrasonic source and the ultrasonic receivers. The objectives are to
deliver an early indication of the risk associated with the defect and to develop inspection and mitigation strategies to
manage the risk using detailed, local, nondestructive evaluation of the areas identified with possible defects. The
wireless data acquisition system and the automated data analysis tool developed under this work should improve the
reliability of the defects detection capability and aid in the development of near real-time health monitoring systems for
Advanced composites are being used increasingly in state-of-the-art aircraft and aerospace structures. In spite of their
many advantages composite materials are highly susceptible to hidden flaws that may occur at any time during the life
cycle of a structure and if undetected, may cause sudden and catastrophic failure of the entire structure. An example of
such a defects critical structural component is the "honeycomb composite" in which thin composite skins are bonded
with adhesives to the two faces of extremely lightweight and relatively thick metallic honeycombs. These components
are often used in aircraft and aerospace structures due to their high strength to weight ratio. Unfortunately, the bond
between the honeycomb and the skin may degrade with age and service loads leading to separation of the load-bearing
skin from the honeycomb (called "disbonds") and compromising the safety of the structure. This paper is concerned with
the noninvasive detection of disbonds using ultrasonic guided waves. Laboratory experiments are carried out on a
composite honeycomb specimen containing localized disbonded regions. Ultrasonic waves are launched into the
specimen using a broadband PZT transducer and are detected by a distributed array of identical transducers located on
the surface of the specimen. The guided wave components of the signals are shown to be very strongly influenced by the
presence of a disbond. The experimentally observed results are being used to develop an autonomous scheme to locate
the disbonds and to estimate their size.
This paper is concerned with the detection and characterization of impact damage in stiffened composite structures
using high frequency Lamb waves and low frequency modal vibrations. The geometric and material complexities of
the structure present practical difficulties in the direct analysis of both wave propagation and modal vibration data
using theoretical constructs. An improved test setup, consisting of high fidelity sensor arrays, laser scanning
vibrometer, data acquisition boards, signal conditioning and dedicated software has been implemented. The
conceptual structural health monitoring (SHM) system presented here involves a low level computational effort, has
high reliability, and is able to treat the acquired data in real-time to identify the presence of existing as well as
emerging damage in the structure. A statistical damage index algorithm is developed and automated by utilizing a
diagnostic imaging tool to identify a defect right from its appearance, with high degree of confidence. The main
advantage of the method is that it is relatively insensitive to environmental noise and structural complexities as it is
based on the comparison between two adjacent dynamical states of the structure and the baseline for comparison is
continuously updated to the previous state. The feasibility of developing a practical Intelligent Structural Health
Monitoring (ISHM) System, based on the concept of "a structure requesting service when needed," is discussed.
A high energy Nd:YAG laser source is used to determine the intrinsic adhesion strength of thin films deposited on substrates. The specimen is designed to convert the thermal energy of the short duration laser pulse to a strong compressive stress on the back face of the substrate. The compressive stress propagates through the layered structure, and upon reflection from the free surface of the film, generates a tensile wave which produces tensile failure of the interface. The stress associated with the interface failure is calculated from a theoretical model of wave propagation through the layered medium. The compressive stress produced by the laser source is determined from a second experiment involving the homogeneous substrate by removing the film. Examples of the applications of the technique in cell biology are presented.
The paper presents a unified computer assisted automatic damage identification technique based on a damage
index, associated with changes in the vibrational and wave propagation characteristics in damaged structures.
An improved ultrasonic and vibration test setup consisting of distributed, high fidelity, intelligent, surface
mounted sensor arrays is used to examine the change in the dynamical properties of realistic composite
structural components with the appearance of damage. The sensors are assumed to provide both the low
frequency global response (i.e., modal frequencies, mode shapes) of the structure to external loads and the
(local) high frequency signals due to wave propagation effects in either passive or active mode of the
ultrasonic array. Using the initial measurements performed on an undamaged structure as baseline, the
damage indices are evaluated from the comparison of the frequency response of the monitored structure with
an unknown damage. The technique is applied to identify impact damage in a woven stiffened composite
plate that presents practical difficulties in transmitting waves across it due to scattering and other energy
dissipation effects present in the material and the geometry of the structure. Moreover, a sensitivity analysis
has been carried out in order to estimate a threshold value of the index below which no reliable information
about the state of health of the structure can be achieved. The feasibility of developing a practical Intelligent
Structural Health Monitoring (ISHM) System, based on the concept of "a structure requesting service when
needed," is discussed.
This paper is concerned with the detection of low velocity impact and the associated internal damage in composite structures using Lamb waves. Impact tests are carried out on a cross ply graphite epoxy plate using an instrumented impact testing system. The contact force and the surface motion caused by the impact load are recorded at several points on the plate surface away from the impact location and are analyzed based on theoretical simulations. The Lamb waves generated by the impact load and internal damage to the plate caused by it are shown to be highly effective tools for damage detection in laboratory specimens. Ultrasonic and impact tests are also conducted on a stiffened, woven composite panel in an effort to examine the propagation characteristics of ultrasonic waves in realistic composite structural components. Preliminary analysis of the recorded waveforms indicates that Lamb waves can be used to interrogate relatively large composite structures.
In this paper, an efficient and accurate semi-analytical method based on the wavenumber integral representation of the elastodynamic field is described to calculate the surface responses produced by localized dynamic loads in a relatively thick composite plate. Two types of loads are considered: a pencil lead break source located on the surface and a localized shear delamination within the interior of the plate. In the case of the pencil lead break source, the calculated results for the surface motion are compared with those obtained in laboratory experiments on a 4.4 mm thick 32 layered cross-ply graphite/epoxy using high-fidelity broad band transducers. The waveforms consist of both flexural and extensional modes; the amplitude variations of these modes are found to be strongly dependent on their propagation direction. For the delamination source at the mid plane, the results from the exact calculation are compared with those from an approximate laminate theory with shear correction factor and “moment tensor” representation of the source. The results obtained by the two methods are shown to have excellent agreement in the low frequency ranges. Although, the motion due to the delamination is dominated by flexural waves of lower frequency in both thin and thick plates, the presence of extensional waves are observed in thicker laminates. The acoustic emission waveforms from the initiation of a shear delamination source at various interfaces are also calculated. It is found that the amplitude of the flexural modes decreases and that of the extensional modes increases as the source moves farther away from the mid plane.
This paper is concerned with the detection and characterization of hidden defects in advanced structures before they grow to a critical size. A methodology for automatic damage identification and localization is developed using a combination of vibration and wave propagation data. The structure is assumed to be instrumented with an array of actuators and sensors to excite and record its dynamic response, including vibration and wave propagation effects. A damage index, calculated from the measured dynamical response of the structure in a reference state and the current state, is introduced as a determinant of structural damage. The indices are used to identify low velocity impact damages in composite as well as aluminium plates for different arrangements of the source and the receivers. The potential applications of the approach in developing health monitoring systems in defects-critical structures is discussed.
Development of efficient tools to successfully localize and characterize hidden damage in critical structural components is an important task in the design and construction of structural health monitoring systems in aging as well as new structures. In this paper two methodologies for damage identification and localization will be presented. The first is an automatic numerical scheme using a state space system identification approach and the second is based on certain damage correlation indices associated with changes in the frequency response of the structure in presence of flaws. In each case, the structure is to be instrumented with an array of sensors to record its dynamic response including vibration and wave propagation effects. To determine the type and location of an unknown defect, the sensor data detected is used to identify a new system, which then is compared to a database of state-space models to find the nearest match. The second method deals with the definition of a set of damage correlation indices obtained from the frequency response analysis of the structure. Two types of indices have been considered. The first uses the correlation between the responses of the defect free and damaged structure at the same point, and the second uses correlation at two different points. The potential application of the general approach in developing health monitoring systems in defects-critical structures is discussed.
The behavior of the wave field produced in a thin unidirectional graphite/epoxy composite plate by a dynamic point load is studied using an approximate shear deformation plate theory (S.D.P.T) and a finite element analysis (F.E.A). Comparisons are made for propagation at 0°, 45°, and 90° directions relative to the fibers showing excellent agreement between the two model approaches. The approximate method is then used to calculate the response of a composite plate as well as of an aluminum plate to a uniform dynamic surface load distributed in a circular region. A periodic reversal in the phase of the signal with propagation distance is observed. It is found that this is caused by the strong dispersion of the first antisymmetric waves at low frequencies. For clarification, the steepest descent method is applied to obtain a closed form analytical expression for the far field response in the aluminum plate for a Dirac delta source. It is shown that the waveform carries a singularity that reverses its phase at regular intervals. The present work should be helpful in understanding the nature of waveform signals produced by impact loads and in the detection and characterization of impact damage in composite structures.
Hidden damage caused by foreign object impact in a composite structure, if left undetected, can grow and lead to a catastrophic failure of the structure. Detection of impact events and characterization of the degree of damage caused by them, preferably in real time, would be extremely helpful in safe continued operation of composite structures. In this paper, low velocity impact experiments are carried out on AS4/3501-6 [0/90]8S cross-ply graphite epoxy composite plates. An instrumented impact testing system is used to record the contact force and the surface motion at locations away from the impact point. The response of the plate due to localized sources is calculated using a modified laminate theory providing detailed information on the relationship between the impact load and the signals generated by the load. For thin plates, the far-field response is dominated by plate guided Lamb waves. It is shown that the occurrence of an impact loading can be easily detected from the recorded signals. Delamination damage, if any, can also be determined through careful analysis of the recorded waveforms. Practical applications of the technique in structural health monitoring will require careful investigation and elimination of environmental noise.
This paper is concerned with the nondestructive materials and defects characterization of foams using ultrasonics. Propagation of ultrasonic waves in a thick plate made out of highly porous, open cell, carbon foam is studied in an effort to understand their relationship with the elastic properties of the material. The foam is assumed to have a tetrakaidecahedral structure and its geometric and overall elastic properties are determined from microscopic data and wave propagation experiments. A simple, one-dimensional model, based on a periodic spring-mass system is proposed for propagation in the through-thickness direction. Due to the difficulty in performing experiments under dry coupling conditions, ultrasonic experiments were carried out with the foam immersed in fluid in a test bed. The wave speed in the fluid filled foam is calculated after obtaining the average elastic moduli of the fluid saturated medium. The calculated wave speed is found to be in good agreement with that measured in the experiment. Significant discontinuities within the plate are also detected through ultrasonic and radiographic experiments. The present models can be employed effectively to characterize any open-cell foam.