Noncontact Evaluation of Defects in Thin Plate With Multimodes Lamb’s Waves and Wavelet Transform

Multimode’s Lamb waves in aluminum plates with various defects were excited by a Q-switched Nd:YAG laser. The Lamb waves past through the defects were received a laser interferometer. The received signals of the Lamb waves are processed by the wavelet transformation. The wavelet transformation is generally shown on the time-frequency domain. By dividing a propagation distance by the time, the group velocities are identified. In this way, group velocity dispersion maps of multimode’s Lamb waves are constructed with the received temporal signals. By changing the shape of the mother wavelet, Gabor function, we can identify the dispersion curves of the higher mode Lamb waves. The group velocity dispersion maps of a intact specimen agree well on theoretical dispersion curves of S0, A0, S1, A1, S2, A2, and A3 modes. The difference between the dispersion maps of the intact specimen and that with defects clearly visualizes the existence of defects. This non-contact method is effective for inspecting various defects in thin plate structures.

A Disposable PVDF Sensor Applied for the Detection of Fatigue Crack Growth in Bonded Composite Joints

The application of polyvinylidene di-fluoride (PVDF) film as an acoustic emission (AE) sensor appears to be practical to detect the fatigue crack growth in composite materials and structures. A commercially available PVDF film sensor was employed to detect AE due to fatigue crack growth in the single-lap joints of graphite/epoxy laminates. Although the signal-to-noise ratio is not as high as those measured by PZT sensors, the result showed that the location of crack front could be predicted by the linear location of AE signals detected by the PVDF sensors. Since the composite materials usually produce very energetic AE signals, the extremely cost-effective form of PVDF sensors can be permanently mounted on composite structures for structural health monitoring as disposable ones. Piezoelectric polymer sensors are expected to be eventually embedded in composite structures provided the current limit of use temperature being increased by introducing co-polymerization with some heat resistant constituents.

Basic NDE of Some Nano Composites

The influence of the nano-sized particulate or slender structures admixed into the material of regular composite structures of various architectures is being enthusiastically studied in many places across the world, but the study is yet in its infancy because there are so many aspects to be investigated. This work is basically on foam-cored structural sandwich composites, and even here there are many variables involved, for example the nano-enhancer can be introduced into the reinforcement, the matrix or the foam. The focus of the work is on some possible effects of the presence of the nano-materials on the NDE process in testing these composites. Acoustic emission is emphasized in these studies, as it appears to hold promise for non-destructively testing materials of this nature, and basic and standard mechanical test methods are employed.

Manufacturing and Analysis of 3-D Woven Sandwich Composite Under Compressive Loading With Acoustic Emission Monitoring

Composite sandwich panel with 3-d woven fabric preform were manufactured through a vacuum assisted resin transfer molding (VARTM) process. The 3-d sandwich fabric preform known as Parabeam, consist of two bi-directional woven E-glass fabric surfaces, which are inter connected mechanically with several vertical woven piles. Parabeam provides a new type of sandwich composite that possess several multifunctional benefits in addition to light weight, bending stiffness and core skin debonding resistance advantages. After consolidation of the panels, polyurethane foam was injected into the core. Conventional sandwich panels were also fabricated with precast polyurethane foam and E-glass face sheet for comparison with the Parabeam sandwich. Edge wise compression tests were conducted and the corresponding acoustic emission (AE) responses were monitored by an advance AE system. Piezoelectric sensor was used to detect the AE signals. On the basis AE signals, the damage source of the specimen were identified, and classified. Furthermore, cracking size were estimated and used to determine the delamination events among the AE signals. Consequently, the AE signals were correlated to the load-displacement curve in the time domain in order to elucidate the types of damage in the specimen. Details of the manufacturing process, the compression tests, and the analysis of the AE signals are presented in this paper.

Nondestructive Inspection of Corrosion and Delamination at the Concrete-Steel Reinforcement Interface

This paper explores the feasibility of detecting and quantifying corrosion and delamination at the interface between reinforcing steel bar and concrete using ultrasonic guided waves. The problem of corrosion and delamination of the reinforcing steel in the aging infrastructure has increased significantly in the last three decades and is likely to keep on increasing. Ultrasonic cylindrical guided waves that can propagate a long distance along the reinforcing steel bar are found to be sensitive to the interface conditions between steel bar and concrete. Ultrasonic transducers are used to launch and detect cylindrical guided waves along the steel bar. The traditional ultrasonic testing methods, for instance the pulse-echo method, where reflection, transmission, and scattering of longitudinal waves are used for detecting large voids in concrete, are not very efficient for detecting corrosion and delamination at the interface between concrete and steel bar. For this study four sets of specimens were prepared. They are rebars and plain steel bars with corrosion and physical separation. Transducers used during the experiment are the Electromagnetic Acoustic Transducer (EMAT) and the Piezoelectric Transducer (PZT). The experiment suggests that the guided wave inspection technique is feasible for the health monitoring of reinforced concrete structures. It also reveals that the ultrasonic guided waves are sensitive to the type of steel used and to the rib patterns on the reformed steel bars.

The Development and the Use of Fiber Optic Sensors for the Structural Health Monitoring of Composite (GFRP) Structures

The development of a fiber optic sensor system for the long-term monitoring of composite glass fiber-reinforced polymer (GFRP) poles will be presented. There is a growing interest in developing techniques for evaluating and monitoring their structural integrity. In the past few years, fiber optic sensor technology has emerged as a promising alternative to conventional methods of monitoring such as strain gauges. Fiber optic sensors can be attached to or embedded in civil structures such as bridges, dams and buildings. They operate by responding to changes in temperature and strain on the structure. The long gauge fiber optic sensor measures the average strain between two points on a structure. Long gauges are interferometric sensors that involve phase matching two optical paths to create a maximum interference pattern at the photodetector. The strain is calculated using the distance in which the reference arm of the interferometer is moved in order to match the optical path lengths.

Study on Damage Progression Process for Plain-Woven Carbon Fabric Composites Under Cyclic Loading

In this study, the damage progression process for plain-woven carbon fabric composites (PW-CFCs) under cyclic loading was investigated. Under cyclic loading, the damage progression was estimated intermittently by using the thermo-elastic damage analysis (TDA) method. Crack density for each layer of the specimen was also estimated at several stages of fatigue. It was found from these results that the damage progression process was divided into three stages. In the first stage, especially, the damage accumulation occurred independently in the PW-CFC specimen and progressed easily along the transverse fiber bundles. All experimental results showed that the damage progression in the first fatigue stage was explained by considering damage-units (a unit area of damage progression and accumulation). Based on the percolation theory, the damage progression for PW-CFC under cyclic loading can be characterized by re-generating the distribution pattern of damage-units.

Non Destructive Diagnosis for Minimum Invasive Access Preparation in Endodontic Treatment

Root canal treatment of infected root canals represents a large percentage of business in general dental practice. It is an expensive process and often prone to failure. During root canal treatment, destructive access preparation by removing parts of tooth crown and dentin is usually needed even before a clinician’s inspection and diagnosis. This paper presents a non-destructive method for accessing the internal tooth geometry by building a 3-D tooth model from 2-D radiograph. The geometry of root canals is then formulated into a mathematical model. Based on this mathematical model, the treatment procedures utilizing the dental tools/instruments are planned by a computer aided prescription system, which yields the tool selection and tool path for the root canal preparation by an intelligent micro drilling machine with on-line monitoring. To minimize the removal of healthy tooth crown and dentin, thus protecting the strength of the patient’s infected tooth, an optimization algorithm is utilized for planning the access preparation in the root canal treatment. Although an opening of a tooth crown is still needed so that dental instruments can reach the root canal, the non-destructive 3-D modeling and the optimization of the access preparation in the new approach makes the root canal treatment minimally invasive compared to present techniques.

Non-Contact Laser Ultrasonic Inspection for Cryogenically Introduced Interlayer Defects in Epoxy-Based Composites

Traditional transducer-based techniques for Non-Destructive Evaluation (NDE) are limited by fixed frequency-bandwidth for generation and sensing, and thus provide unsatisfactory resolution for certain types of material defects. Thermo-Acousto-Photonic NDE (TAP-NDE) is a proven alternative that is non-invasive and non-contact, and suited for real-time applications. This paper focuses on employing TAP-NDE to examine the presence of microcracks and fissures in multi-layered composites. Tests were performed on layered composite panels of specific epoxyresin composition and constant thickness to identify localized delaminations formed by subjecting the specimen to cryogenic cycling. Interrogation of the undamaged specimen using laser-generated broadband surface waves revealed a standard reference knowledge base, as seen in the instantaneous frequency-time domain. Tests were repeated after each specimen was subjected to a set number of cycles of liquid nitrogen cycling, which caused damages at the micron scale in the bulk material. Analyses showed changes in the time of wave arrival and absence of prominent high frequency components. Wave velocity and dispersion characteristics of the cycled specimen were altered. Thus, the specimen, on cryogenic cycling, was found to undergo a decrease in stiffness, which is speculatively the result of micro-voids, fissures or delaminations between layers. Hence, when combining with the basic notion of instantaneous frequency, TAP-NDE acts as an effective broadband generation and sensing technique, demonstrating feasibility and greater versatility for inspecting layered composites as against contemporary narrowband techniques.

Nondestructive Methods for the Damage Assessment of Cylindrically Curved Composite Laminates Subjected to Low-Velocity Impact

Curved composite laminates appeared to be more vulnerable than flat ones to rapid transverse loading. Damage induced by low-velocity impact on the cylindrically curved composite laminates has been experimentally investigated. Graphite/epoxy shells with the radius of curvatures of 150 mm showed quite different impact response and damage behavior from that of flat laminate. Under the same impact energy level, the maximum contact force varied with the radius of curvatures, which is directly related to the impact damage. Delamination was distributed rather evenly at each interface along the thickness direction of curved laminates on the contrary to the case of flat laminates, where delamination is typically concentrated at the interfaces away from the impact point. Due to the presence of curvature, the acoustic microscopy could not be directly applied to the layer-by-layer assessment of delamination damage. As an alternative, the penetrant-enhanced X-radiography (PEXR) was introduced and the results from PEXR were compared with those from destructive examination of the cross-section by scanning electron microscopy.