John T. Welter

Focusing of longitudinal ultrasonic waves in air with an aperiodic flat lens.

Modeling and experimental results of an ultrasonic aperiodic flat lens for use in air are presented. Predictive modeling of the lens is performed using a hybrid genetic-greedy algorithm constrained to a linear structure. The optimized design parameters are used to fabricate a lens. A method combining a fiber-disk arrangement and scanning laser vibrometer measurements is developed to characterize the acoustic field distribution generated by the lens. The focal spot size is determined to be 0.88 of the incident wavelength of 80-90 kHz at a distance of 2.5 mm from the lens. Theoretically computed field distributions, optimized frequency of operation, and spatial resolution focal length are compared with experimental measurements. The differences between experimental measurements and the theoretical computations are analyzed. The theoretical calculation of the focal spot diameter is 1.7 mm which is 48% of the experimental measurement at a frequency of 80-90 kHz. This work illustrates the capabilities of a hybrid algorithm approach to design of flat acoustic lenses to operate in air with a resolution of greater than the incident wavelength and the challenges of characterizing acoustic field distribution in air.

Corrosion damage detection with piezoelectric wafer active sensors

Since todays aging fleet is intended to far exceed their proposed design life, monitoring the structural integrity of those aircraft has become a priority issue for todays Air Force. One of the most critical structural problems is corrosion. In fact the KC-135 now costs

Volumetric characterization of delamination fields via angle longitudinal wave ultrasound

1.2 billion a year to repair corrosion. In this paper, we plan to show the use of Lamb waves to detect material loss in thin plates representative of aircraft skins. To do this we will use embedded transducers called Piezoelectric Wafer Active Sensor (PWAS) in a pitch-catch configuration. The sensors were placed on a grid pattern. Material loss through corrosion was simulated by removing the material mechanically with an abrasive tool. Thus, simulated corrosion pits of various depths and area coverage were made. Three-count tone burst wave packets were used. The Lamb wave packets were sent in a pitch-catch mode from one transmitter PWAS to the other PWAS in the grid acting as receivers. The Lamb wave mode used in these experiments was A1, since this was found to be more sensitive to changes due to material loss. At the frequencies considered in our experiments, the A1 waves are highly dispersive. It was found that, as the Lamb wave travels through simulated corrosion damage, the signal changes. The observed changes were in the signal wavelength (due to change in the dispersive properties of the medium) and in signal amplitude (due to redistribution of energy in the wave packet). This change in signal can be correlated to the magnitude of damage. To achieve this, we have used several approaches: (a) direct correlation between the sent and the received signals; (b) wavelet transform of the signal followed by correlation of the wavelet coefficients time-frequency maps; (c) Hilbert transform of the signal to produce the signal envelope and comparison of the resulting envelope signals (d) neural network correlation between the sent and received signals. It was found that these methods work well together in a complementary way.

Development of nondestructive non-contact acousto-thermal evaluation technique for damage detection in materials

The volumetric characterization of delaminations necessarily precedes rigorous composite damage progression modeling. Yet, inspection of composite structures for subsurface damage remains largely focused on detection, resulting in a capability gap. In response to this need, angle longitudinal wave ultrasound was employed to characterize a composite surrogate containing a simulated three-dimensional delamination field with distinct regions of occluded features (shadow regions). Simple analytical models of the specimen were developed to guide subsequent experimentation through identification of optimal scanning parameters. The ensuing experiments provided visual evidence of the complete delamination field, including indications of features within the shadow regions. The results of this study demonstrate proof-of-principle for the use of angle longitudinal wave ultrasonic inspection for volumetric characterization of three-dimensional delamination fields. Furthermore, the techniques developed herein form the...

Progress on the development of automated data analysis algorithms and software for ultrasonic inspection of composites

This paper presents the development of a new non-contact acousto-thermal signature (NCATS) nondestructive evaluation technique. The physical basis of the method is the measurement of the efficiency of the material to convert acoustic energy into heat, and a theoretical model has been used to evaluate this. The increase in temperature due to conversion of acoustic energy injected into the material without direct contact was found to depend on the thermal and elastic properties of the material. In addition, it depends on the experimental parameters of the acoustic source power, the distance between sample and acoustic source, and the period of acoustic excitation. Systematic experimental approaches to optimize each of the experimental variables to maximize the observed temperature changes are described. The potential of the NCATS technique to detect microstructural-level changes in materials is demonstrated by evaluating accumulated damage due to plasticity in Ti-6Al-4V and low level thermal damage in polymer matrix composites. The ability of the technique for macroscopic applications in nondestructive evaluation is demonstrated by imaging a crack in an aluminum test sample.

Fundamentals of angled-beam ultrasonic NDE for potential characterization of hidden regions of impact damage in composites

Progress is presented on the development and implementation of automated data analysis (ADA) software to address the burden in interpreting ultrasonic inspection data for large composite structures. The automated data analysis algorithm is presented in detail, which follows standard procedures for analyzing signals for time-of-flight indications and backwall amplitude dropout. ADA processing results are presented for test specimens that include inserted materials and discontinuities produced under poor manufacturing conditions.

Model-driven optimization of oblique angle ultrasonic inspection parameters for delamination characterization

In this study, the use of angled-beam ultrasonic NDE was explored for the potential characterization of the hidden regions of impact damage in composites. Simulated studies using CIVA FIDEL 2D were used to explore this inspection problem. Quasi-shear (qS) modes can be generated over a wide range of angles and used to reflect off the backwall and interrogate under the top delaminations of impact damage. Secondary probe signals that do propagate normal to the surface were found to be significant under certain probe conditions, and can potentially interfere with weakly scattered signals from within the composite panel. Simulations were used to evaluate the source of the multiple paths of reflections from the edge of a delamination; time-of-flight and amplitude will depend on the depth of the delamination and location of neighboring delaminations. For angled-beam inspections, noise from both the top surface roughness and internal features was found to potentially mask the detection of signals from the edge of delaminations. Lastly, the study explored the potential of generating “guided” waves along the backwall using an angled-beam source and subsequently measuring scattered signals from a far surface crack hidden under a delamination.In this study, the use of angled-beam ultrasonic NDE was explored for the potential characterization of the hidden regions of impact damage in composites. Simulated studies using CIVA FIDEL 2D were used to explore this inspection problem. Quasi-shear (qS) modes can be generated over a wide range of angles and used to reflect off the backwall and interrogate under the top delaminations of impact damage. Secondary probe signals that do propagate normal to the surface were found to be significant under certain probe conditions, and can potentially interfere with weakly scattered signals from within the composite panel. Simulations were used to evaluate the source of the multiple paths of reflections from the edge of a delamination; time-of-flight and amplitude will depend on the depth of the delamination and location of neighboring delaminations. For angled-beam inspections, noise from both the top surface roughness and internal features was found to potentially mask the detection of signals from the edge of...

Progress on automated data analysis algorithms for ultrasonic inspection of composites

Characterization of delamination fields in 3D for impact damaged composites is necessary to achieve the USAF objective of damage tolerance for polymer matrix composites [1, 2]. In order to characterize the hidden region of delamination fields, oblique angle ultrasound inspection is being investigated [3]. Benchmark studies with side-drilled hole (SDH) specimens are presented for model verification. The results from CIVA FIDEL 2D are compared to results from PZFlex, and experimental data. CIVA FIDEL 2D offers a computationally faster model using a semi-analytical beam model to reduce the finite difference time domain (FDTD) size and thus reduce the simulation run time. This modeling approach contrasts with the expense of the accuracy that a full finite element model (FEM) such as PZFlex provides. It is shown that a large number of lower fidelity models can be run and evaluated in an expedient manner with CIVA FIDEL to efficiently reduce the possible design space. Key cases can then be simulated with more computationally expensive, but highly accurate models, followed by experiments for verification. Model results showing the possibility of oblique angle ultrasonic detection of shadowed delaminations is presented.Characterization of delamination fields in 3D for impact damaged composites is necessary to achieve the USAF objective of damage tolerance for polymer matrix composites [1, 2]. In order to characterize the hidden region of delamination fields, oblique angle ultrasound inspection is being investigated [3]. Benchmark studies with side-drilled hole (SDH) specimens are presented for model verification. The results from CIVA FIDEL 2D are compared to results from PZFlex, and experimental data. CIVA FIDEL 2D offers a computationally faster model using a semi-analytical beam model to reduce the finite difference time domain (FDTD) size and thus reduce the simulation run time. This modeling approach contrasts with the expense of the accuracy that a full finite element model (FEM) such as PZFlex provides. It is shown that a large number of lower fidelity models can be run and evaluated in an expedient manner with CIVA FIDEL to efficiently reduce the possible design space. Key cases can then be simulated with more c...

Non-contact acousto-thermal signatures in as received and fatigue damaged Ti-6Al-4V

Progress is presented on the development and demonstration of automated data analysis (ADA) software to address the burden in interpreting ultrasonic inspection data for large composite structures. The automated data analysis algorithm is presented in detail, which follows standard procedures for analyzing signals for time-of-flight indications and backwall amplitude dropout. New algorithms have been implemented to reliably identify indications in time-of-flight images near the front and back walls of composite panels. Adaptive call criteria have also been applied to address sensitivity to variation in backwall signal level, panel thickness variation, and internal signal noise. ADA processing results are presented for a variety of test specimens that include inserted materials and discontinuities produced under poor manufacturing conditions. Software tools have been developed to support both ADA algorithm design and certification, producing a statistical evaluation of indication results and false calls us...

Broadband aperiodic air coupled ultrasonic lens

Abstract : Interaction of high amplitude acoustic waves with materials produces a small increase in the temperature that can be detected and measured using an IR camera. The changes in temperature as a function of time, due to interaction of high amplitude 20 kHz acoustics, with as received and fatigue damaged polycrystalline Ti-6Al-4V samples are compared. The maximum temperature reached by the sample has been found to increase with increasing fatigue cycles. The role of multiple physical mechanisms, responsible for conversion acoustic energy to heat, like the sample geometry (finite dimension), the microstructure (grain size), and dislocation density are examined. The theoretically evaluated temperature changes are observed to be in reasonable agreement with experimental measurements. The significance of the details of microstructure and dislocation properties needed in theoretical evaluation of temperature changes are used to explain the observed differences between experimental measurements and theoretical calculations.