Seth S. Kessler

Damage detection in composite materials using lamb wave methods

Cost-effective and reliable damage detection is critical for the utilization of composite materials. This paper presents part of an experimental and analytical survey of candidate methods for in situ damage detection of composite materials. Experimental results are presented for the application of Lamb wave techniques to quasi-isotropic graphite/epoxy test specimens containing representative damage modes, including delamination, transverse ply cracks and through-holes. Linear wave scans were performed on narrow laminated specimens and sandwich beams with various cores by monitoring the transmitted waves with piezoceramic sensors. Optimal actuator and sensor configurations were devised through experimentation, and various types of driving signal were explored. These experiments provided a procedure capable of easily and accurately determining the time of flight of a Lamb wave pulse between an actuator and sensor. Lamb wave techniques provide more information about damage presence and severity than previously tested methods (frequency response techniques), and provide the possibility of determining damage location due to their local response nature. These methods may prove suitable for structural health monitoring applications since they travel long distances and can be applied with conformable piezoelectric actuators and sensors that require little power.

Damage detection in composite materials using frequency response methods

Cost-effective and reliable damage detection is critical for the utilization of composite materials. This paper presents part of an experimental and analytical survey of candidate methods for the in situ detection of damage in composite materials. The experimental results are presented for the application of modal analysis techniques applied to graphite/epoxy specimens containing representative damage modes. Changes in natural frequencies and modes were found using a laser vibrometer, and 2-D finite element models were created for comparison with the experimental results. The models accurately predicted the response of the specimens at low frequencies, but coalescence of higher frequency modes makes mode-dependant damage detection difficult for structural applications. The frequency response method was found to be reliable for detecting even small amounts of damage in a simple composite structure, however the potentially important information about damage type, size, location and orientation were lost using this method since several combinations of these variables can yield identical response signatures.

Structural health monitoring in composite materials using frequency response methods

Cost effective and reliable damage detection is critical for the utilization of composite materials in structural applications. Non-destructive evaluation techniques (e.g. ultrasound, radiography, infra-red imaging) are available for use during standard repair and maintenance cycles, however by comparison to the techniques used for metals these are relatively expensive and time consuming. This paper presents part of an experimental and analytical survey of candidate methods for the detection of damage in composite materials. The experimental results are presented for the application of modal analysis techniques applied to rectangular laminated graphite/epoxy specimens containing representative damage modes, including delamination, transverse ply cracks and through-holes. Changes in natural frequencies and modes were then found using a scanning laser vibrometer, and 2-D finite element models were created for comparison with the experimental results. The models accurately predicted the response of the specimems at low frequencies, but the local excitation and coalescence of higher frequency modes make mode-dependent damage detection difficult and most likely impractical for structural applications. The frequency response method was found to be reliable for detecting even small amounts of damage in a simple composite structure, however the potentially important information about damage type, size, location and orientation were lost using this method since several combinations of these variables can yield identical response signatures.

Impact of carbon nanotube length on electron transport in aligned carbon nanotube networks

Here, we quantify the electron transport properties of aligned carbon nanotube (CNT) networks as a function of the CNT length, where the electrical conductivities may be tuned by up to 10× with anisotropies exceeding 40%. Testing at elevated temperatures demonstrates that the aligned CNT networks have a negative temperature coefficient of resistance, and application of the fluctuation induced tunneling model leads to an activation energy of ≈14 meV for electron tunneling at the CNT-CNT junctions. Since the tunneling activation energy is shown to be independent of both CNT length and orientation, the variation in electron transport is attributed to the number of CNT-CNT junctions an electron must tunnel through during its percolated path, which is proportional to the morphology of the aligned CNT network.

Design of a piezoelectric-based structural health monitoring system for damage detection in composite materials

Cost-effective and reliable damage detection is critical for the utilization of composite materials. This paper presents the conclusions of an experimental and analytical survey of candidate methods for in-situ damage detection in composite structures. Experimental results are presented for the application of modal analysis and Lamb wave techniques to quasi-isotropic graphite/epoxy test specimens containing representative damage. Piezoelectric patches were used as actuators and sensors for both sets of experiments. Modal analysis methods were reliable for detecting small amounts of global damage in a simple composite structure. By comparison, Lamb wave methods were sensitive to all types of local damage present between the sensor and actuator, provided useful information about damage presence and severity, and present the possibility of estimating damage type and location. Analogous experiments were also performed for more complex built-up structures. These techniques are suitable for structural health monitoring applications since they can be applied with low power conformable sensors and can provide useful information about the state of a structure during operation. Piezoelectric patches could also be used as multipurpose sensors to detect damage by a variety of methods such as modal analysis, Lamb wave, acoustic emission and strain based methods simultaneously, by altering driving frequencies and sampling rates. This paper present guidelines and recommendations drawn from this research to assist in the design of a structural health monitoring system for a vehicle. These systems will be an important component in future designs of air and spacecraft to increase the feasibility of their missions.

Durability Assessment of Lamb Wave-Based Structural Health Monitoring Nodes

Structural health monitoring (SHM) is an emerging technology leading to the development of systems capable of continuously monitoring structures for damage to improve safety and reduce life-cycle costs. SHM involves integration of one or more nondestructive test methods into a vehicle in order to facilitate quick and accurate damage detection with minimal human intervention. Aerospace structures have one of the highest payoffs for SHM systems since damage can lead to catastrophic and expensive failures, and the vehicles involved undergo regular costly inspections. Current work in SHM has focused on damage detection methods and sensor optimization, however, the topics of durability, reliability, and longevity of these systems has not been sufficiently addressed. Experimental results from durability testing of piezoelectric Lamb-wave nodes (transceivers) are presented and a framework for developing SHM test standards is offered. Existing standards for the durability, reliability, and longevity of commercial and military aircraft components are identified, and the relation of their standards to SHM systems is discussed. These standards include susceptibility to environmental testing, mechanical durability, and electro-magnetic interference (EMI), as well as a host of other extreme aircraft conditions (shock, vibration, fluids, etc.). Using these existing standards, a test matrix to assess the durability of the SHM sensors is developed, as well as criteria to establish whether a sensor/structural system has been affected by the various environments. Lamb-wave sensors have been tested in a variety of environments— including high temperature and large strain—so that their operational envelop can be characterized. Future environmental testing will include low temperature, high humidity, fluid susceptibility, low-velocity impact, and high altitude (low pressure). While the aircraft component industry is in general well regulated, it is evident that there is a need for a supplemental standard geared specifically towards smart structure technologies. This would incorporate SHM and other embedded or surface mounted smart structure components and systems, including interactions between the smart/active component and the structure. The field of SHM has progressed significantly in recent years, and it will become critical to address these topics explicitly before SHM systems can be successfully utilized in prognostic applications.

Optimization of Lamb wave actuating and sensing materials for health monitoring of composite structures

In a continuing effort to examine the effectiveness of Lamb wave methods for the health monitoring of composite structures, this paper presents the conclusions of an analytical and experimental study optimizing piezoelectric patches to detect damage within composite laminates. Previous research has demonstrated the ability of Lamb waves to provided useful information about the presence of damage in simple narrow coupons, and they have yielded the possibility of estimating severity and location of damage as well. During the course of this NRO funded research program, several types of piezoelectric materials in various configurations were analyzed in order to produce the highest force actuator and best resolution sensor at the lowest power level. Consideration was also placed towards directionality of wave propagation, and durability, reliability and reproducibility of the sensing patch itself. Experiments were then carried out on narrow coupon laminates to qualify and tune these actuating/sensing patches. New algorithms were used to filter and decompose the resulting signals to more efficiently detect the presence of damage for automated use, and gather information relating to the damage type, severity and location. SHM technologies will enable condition-based maintenance for efficient structural design, will reduced overall life-cycle costs, and eliminate scheduled inspections.

Electrothermal Icing Protection of Aerosurfaces Using Conductive Polymer Nanocomposites

United States. Dept. of the Navy. Small Business Innovation Research (Contract N68335-11-C-0424)

Selection of materials and sensors for health monitoring of composite structures

Embedded structural health monitoring systems are envisioned to be an important component of future transportation systems. One of the key challenges in designing an SHM system is the choice of sensors, and a sensor layout, which can detect unambiguously relevant structural damage. This paper focuses on the relationship between sensors, the materials of which they are made, and their ability to detect structural damage. Sensor selection maps have been produced which plot the capabilities of the full range of available sensor types vs. the key performance metrics (power consumption, resolution, range, sensor size, coverage). This exercise resulted in the identification of piezoceramic Lamb wave transducers as the sensor of choice. Experimental results are presented for the detailed selection of piezoceramic materials to be used as Lamb wave transducers.

Tomographic Electrical Resistance-based Damage Sensing in Nano-Engineered Composite Structures

Abstract : Aligned carbon nanotubes (CNTs) are being investigated as a means for enhancing structural performance of composite structures. Inherent in introducing CNTs into existing polymer-matrix composites are new multifunctional attributes such as significantly enhanced electrical conductivity and piezoresistivity that may be used for damage sensing and inspection. Here, fiber-reinforced polymer-matrix laminates with aligned CNTs grown in-situ are coupled with a non-invasive sensing scheme utilizing the enhanced electrical conductivity of the laminates to infer damage based on resistance changes. The laminates contain long (~10 micron) aligned CNTs throughout the woven plies of the laminate, including at the ply interfaces. Electrodes are written onto the laminate surfaces using a direct-write process, and 3D damage inspection (in-plane and through-thickness) is demonstrated for impacted composite plates.