Anthony J. Croxford

Remote inspection system for impact damage in large composite structure

A significant opportunity for reducing the weight of composite aircraft is through the development of an economically efficient method to detect barely visible or invisible impact damage sustained in service. In this paper, a structurally integrated, inert, wireless system for rapid, large-area impact damage detection in composite is demonstrated. Large-area inspection from single sensors using ultrasonic-guided waves is achieved with a baseline-subtraction technique. The wireless interface uses electromagnetic coupling between coils in the embedded sensor and inspection wand. Compact encapsulated sensor units are built and successfully embedded into composite panels at manufacture. Chirp-based excitation is used to enable single-shot measurements with high signal-to-random-noise ratio to be obtained. Signal processing to compensate for variability in inspection wand alignment is developed and shown to be necessary to obtain adequate baseline subtraction performance for damage detection. Results from sensors embedded in both glass fibre and carbon fibre-reinforced composite panels are presented. Successful detection of a 10u2009J impact damage in the former is demonstrated at a range of 125u2009mm. Quantitative extrapolation of this result suggests that the same level of impact damage would be detectable at a range of up to 1000u2009mm with an inspection wand alignment tolerance of 4u2009mm.

Investigation of inductively coupled ultrasonic transducer system for NDE

Inductive coupling offers a simple solution to wirelessly probe ultrasonic transducers. This paper investigates the theory and feasibility of such an inductively coupled transducer system in the context of nondestructive evaluation (NDE) applications. The noncontact interface is based on electromagnetic coupling between three coils; one of the coils is physically connected to the transducer, the other two are in a separate probing unit, where they are connected to the transmit and receive channels of the instrumentation. The complete system is modeled as a three-port network with the measured impedance of a bonded piezoelectric ceramic disc representing a sensor attached to an arbitrary solid substrate. The developed transmission line model is a function of the physical parameters of the electromagnetic system, such as the number of turns and diameter of each coil, and their separation. This model provides immediate predictions of electrical input impedance and pulse¿echo response. The model has been validated experimentally and a sensitivity analysis of the input parameters performed. This has enabled optimization of the various parameters. Inductively coupled transducer systems have been built for both bulk and guided wave examples. By using chirped excitation and baseline subtraction, inspection distance of up to 700 mm is achieved in single-shot, guidedwave pulse¿echo mode measurements with a 5 mm separation between the probing coils and transducer coil on an aluminum plate structure. In the bulk wave example, a delamination in an 8.9-mm-thick carbon fiber composite specimen is successfully identified from the changes in the arrival time of a reflected pulse.

Design of an embedded sensor, for improved structural performance

Low velocity impact damage to composite laminates can result in a complicated network of matrix cracks and delaminations beneath the laminates surface, which are extremely difficult to detect by visual inspection. Current non-destructive evaluation (NDE) techniques such as ultrasonic C-scan and x-ray imaging create significant downtime, which leads to costly inspection programmes. Embedded sensors offer the potential to increase the automation of inspection, and decrease the downtime when compared with current NDE practices. However, for such systems to be practical, sensors must be integrated within composite structures without producing unacceptable loss of structural performance. This paper identifies techniques for embedding slim sensors with comparatively large in-plane dimensions inside composite materials. Interlaminar shear strength tests were used to identify an encapsulating layer for the sensors. Flexural strength testing was carried out on laminates containing sensors embedded towards the compressive surface of flexural specimens. The experimental study was complemented with finite element analysis, which identified the load paths within different embedment configurations and aided with the interpretation of the experimental results. The results show that with careful selection of sensor materials, geometry, embedding location and embedment technique, sensors can be integrated within composite structures without producing any significant reduction of mechanical performance.

Diffuse field full matrix capture for near surface ultrasonic imaging

ABSTRACT This article reports a technique for near‐surface ultrasonic array imaging. Information equivalent to an undelayed full matrix of inter‐element responses is produced through cross‐correlation of a later time diffuse full matrix. This reconstructed full matrix lacks the nonlinear effects of early time saturation present in a directly acquired response. Consequently the near‐surface material information usually obscured by this effect is retrieved. Furthermore it is shown that a hybrid full matrix formed through a temporally weighted sum of coherent and reconstructed matrices allows for effective near‐surface and bulk material imaging from a single direct‐contact experimental realisation.

Sensitivity images for multi-view ultrasonic array inspection

The multi-view total focusing method (TFM) is an imaging technique for ultrasonic full matrix array data that typically exploits ray paths with zero, one or two internal reflections in the inspected object and for all combinations of longitudinal and transverse modes. The fusion of this vast quantity of views is expected to increase the reliability of ultrasonic inspection; however, it is not trivial to determine which views and which areas are the most suited for the detection of a given type and orientation of defect. This work introduces sensitivity images that give the expected response of a defect in any part of the inspected object and for any view. These images are based on a ray-based analytical forward model. They can be used to determine which views and which areas lead to the highest probability of detection of the defect. They can also be used for quantitatively analyzing the effects of the parameters of the inspection (probe angle and position, for example) on the overall probability of detection. Finally, they can be used to rescale TFM images so that the different views have comparable amplitudes. This methodology is applied to experimental data and discussed.The multi-view total focusing method (TFM) is an imaging technique for ultrasonic full matrix array data that typically exploits ray paths with zero, one or two internal reflections in the inspected object and for all combinations of longitudinal and transverse modes. The fusion of this vast quantity of views is expected to increase the reliability of ultrasonic inspection; however, it is not trivial to determine which views and which areas are the most suited for the detection of a given type and orientation of defect. This work introduces sensitivity images that give the expected response of a defect in any part of the inspected object and for any view. These images are based on a ray-based analytical forward model. They can be used to determine which views and which areas lead to the highest probability of detection of the defect. They can also be used for quantitatively analyzing the effects of the parameters of the inspection (probe angle and position, for example) on the overall probability of detec...

Design, application, and validation of embedded ultrasonic sensors within composite materials

The layer wise construction of laminated composites offers the potential to embed sensors within composite structures. One possible solution is the embedding of sensors that are inductively coupled to an external probe; which allows for the efficient contactless transfer of electrical signals to the sensor. Embedding sensors within structures is an attractive option, due to the physical protection offered to the sensor by the host structure. However, for embedding sensors to be viable, sensor integration must result in minimal degradation of the laminates mechanical performance. This work focuses on designing embedded inductively coupled sensors for structural performance. A suitable sensor coating for the sensor unit was identified using interlaminar shear strength testing. Sensors were then embedded into quasi-isotropic four-point bend flexural strength specimens, and different embedding strategies demonstrated. In addition to providing the sensor with physical protection, embedding sensors within a composite host offers the additional benefit of monitoring the curing process of the surrounding composite. A single inductively coupled sensor was embedded into a large glass fiber epoxy plate, and the measured guided wave pulse echo response used to monitor the curing process. This novel cure monitoring technique was then benchmarked against direct scanning calorimetry.

Near surface ultrasonic imaging

Ultrasonic phased arrays offer excellent performance for detecting and classifying defects; however when inspecting near the array, there is typically a deadzone where electrical cross talk saturates the response making it impossible to measure. In many situations this can be mitigated through the use of a physical standoff, however for permanently installed systems or in situ inspection of components in access restricted areas such as gas turbines such a solution is impossible. In addition, such a standoff typically reduces the amplitude of the received signals degrading the signal to noise ratio (SNR). This paper reports on an approach that allows ultrasonic measurements to be made of the near surface region. Specifically by measuring the diffuse response of the system it is possible to reconstruct the greens function between any pair of transducers. As this is created from data that is not saturated there is no deadzone in the resulting image. When combined with advanced sampling techniques using hadam...

Health monitoring of composite structures throughout the life cycle

This study demonstrates the capability of inductively coupled piezoelectric sensors to monitor the state of health throughout the lifetime of composite structures. A single sensor which generated guided elastic waves was embedded into the stacking sequence of a large glass fiber reinforced plastic plate. The progress of cure was monitored by measuring variations in the amplitude and velocity of the waveforms reflected from the plate’s edges. Baseline subtraction techniques were then implemented to detect barely visible impact damage (BVID) created by a 10 Joule impact, at a distance of 350 mm from the sensor embedded in the cured plate. To investigate the influence of mechanical loading on sensor performance, a single sensor was embedded within a glass fiber panel and subjected to tensile load. The panel was loaded up to a maximum strain of 1%, in increments of 0.1% strain. Guided wave measurements were recorded by the embedded sensor before testing, when the panel was under load, and after testing. The ultrasonic measurements showed a strong dependence on the applied load. Upon removal of the mechanical load the guided wave measurements returned to their original values recorded before testing. The results in this work show that embedded piezoelectric sensors can be used to monitor the state of health throughout the life-cycle of composite parts, even when subjected to relatively large strains. However the influence of load on guided wave measurements has implications for online monitoring using embedded piezoelectric transducers.

Investigation of capacitively coupled ultrasonic transducer system for nondestructive evaluation

Capacitive coupling offers a simple solution to wirelessly probe ultrasonic transducers. This paper investigates the theory, feasibility, and optimization of such a capacitively coupled transducer system (CCTS) in the context of nondestructive evaluation (NDE) applications. The noncontact interface relies on an electric field formed between four metal plates-two plates are physically connected to the electrodes of a transducer, the other two are in a separate probing unit connected to the transmit/receive channel of the instrumentation. The complete system is modeled as an electric network with the measured impedance of a bonded piezoelectric ceramic disc representing a transducer attached to an arbitrary solid substrate. A transmission line model is developed which is a function of the physical parameters of the capacitively coupled system, such as the permittivity of the material between the plates, the size of the metal plates, and their relative positions. This model provides immediate prediction of electric input impedance, pulse-echo response, and the effect of plate misalignment. The model has been validated experimentally and has enabled optimization of the various parameters. It is shown that placing a tuning inductor and series resistor on the transmitting side of the circuit can significantly improve the system performance in terms of the signal-to-crosstalk ratio. Practically, bulk-wave CCTSs have been built and demonstrated for underwater and through-composite testing. It has been found that electrical conduction in the media between the plates limits their applications.