Patrick H. Johnston

Applicability of ultrasonic tissue characterization for longitudinal assessment and differentiation of calcification and fibrosis in cardiomyopathy

Progress in tissue characterization of myocardium with ultrasound suggests that quantitative recognition of ischemic or scarred tissue will be achieved. Despite the increasing recognition and importance of cardiomyopathy, its diagnosis generally requires invasive procedures such as cardiac catheterization and biopsy. To investigate methods that permit the characterization of longitudinal cardiomyopathic changes that might ultimately be extended for noninvasive studies in patients, quantitative ultrasonic methods were utilized for in vitro tissue characterization of hearts from Syrian hamsters of selected age of either 2 to 3 or 5 to 7 months. Normal hamsters were used as controls. Myocardial sites (n = 600) from the young Syrian hamsters exhibited values (+/- standard error) of integrated ultrasonic backscatter averaging -53.87 +/- 0.26 dB, which were significantly different from values (n = 500) in age-matched control hamsters (-58.07 +/- 0.08 dB; p less than 0.001). Cardiomyopathic hearts from older animals exhibited backscatter values (n = 500 sites) averaging -50.87 +/- 0.22 dB, again significantly different from values (n = 300 sites) in age-matched control hamsters (-55.91 +/- 0.11 dB; p less than 0.001). In addition, ultrasonic attenuation was significantly different for hearts from the control and cardiomyopathic hamsters of both age ranges. The results correlated with sequential calcification and fibrosis characteristics assessed histopathologically. This study indicates that quantitative characterization of myocardium with ultrasound may permit longitudinal assessment of cardiomyopathic changes in diverse disease entities and their response to therapy.

Ultrasonic monitoring of ply crack and delamination formation in composite tube under torsion load

As a simple model of a rotor spar, a circular graphite-epoxy composite laminate cylinder was subjected to cyclic torsional load. The test section of the cylindrical specimen varied from four to six plies of ±45° fibers, due to intentional ply overlaps and gaps. A layer of 13-μm Teflon film was inserted between plies at three locations to serve as delamination initiators. A commercial X-Y scanner was mounted to the load frame to enable ultrasonic inspection without removing the specimen. A focused immersion probe was mounted in a captive water column with a rugged Nitrile membrane tip, which was coupled to the cylinder using a mist of soapy water. The transducer was aligned normal to the cylinder surface using the X-axis. Scanning was performed along the length of the specimen with the Y-axis and the specimen was incrementally rotated by the torsion head of the load frame. After 350k cycles of torsion, several linear 45° diagonal indications appeared as 5–40% attenuation of the back wall echo, with no apparent echoes from the interior of the composite, suggesting through-ply cracks in the innermost ply. Crack indications grew and new cracks appeared as torsion cycling continued. Internal reflections from delaminations associated with the growing ply cracks appeared after 500k cycles. Three areas of extensive multi-layer delaminations appeared after 1150k cycles. Failure of the specimen occurred at 1600k cycles. The observed progressive damage was not associated with the Teflon inclusions. Concurrent thermographic measurements provided lower-resolution confirmation of the damage observed.

Thermography inspection for detection and tracking of composite cylinder damage during load testing

Two thermography techniques, passive and active, are used to detect damage initiation and progression in a cyclically loaded composite cylinder. The passive thermography tracks damage progression in real time during cyclic loading. Active flash thermography, using a flash tube enclosed within the cylinder, images delaminations. A differential thermography processing technique eliminates normal material variations and improves sensitivity to and sizing of delaminations. The thermography results were compared to non‐immersion ultrasonic results.

Modeling the pulse-echo response of a two-dimensional phase-insensitive array for NDE of layered media

The use of a pulse-echo phase-insensitive array as the approach for the NDE (nondestructive evaluation) of uneven bondlines in the Space Shuttle Solid Rocket Motor is discussed. Experimental results with a scanned-element pseudoarray indicate that the sensitivity of detecting total disbonds at a rough interface between two rubberlike materials can be improved by more than a factor of 2 by using phase-insensitive detection. Computer simulations are being used to evaluate the pulse-echo characteristics of various arrangements of pulse-echo arrays with coplanar interspersed transmitting and receiving elements. This approach allows flexibility for exploring various array configurations, and as well as independent selection of materials for optimizing transmit and receive characteristics.<<ETX>>

Tomographic imaging of an ultrasonic field in a plane by use of a linear array: theory and experiment

Quantitative ultrasonic characterization of inhomogeneous and anisotropic materials is often difficult due to undesired phenomena such as beam steering and phase aberration of the insonifying field. We introduce a method based on tomographic reconstruction techniques for the visualization of an ultrasonic field using a linear array rotated in a plane. Tomographic reconstruction of the ultrasonic field is made possible through the phase-sensitive nature of the tall, narrow piezoelectric elements of a linear array that act as parallel line integrators of the pressure field. We validate the proposed imaging method through numerical simulations of propagated ultrasonic fields based upon the angular spectrum decomposition technique. We then demonstrate the technique with experimental measurements of two textile composites and a reference water path. We reconstruct images of the real and imaginary parts of a transmitted 2 MHz ultrasonic field that are then combined to reconstruct images of the power and unwrapped phase. We also construct images of the attenuation and phase shift for several regions of the composites. Our results demonstrate that tomographic imaging of an ultrasonic field in a plane using a rotated linear array can potentially improve ultrasonic characterization of complex materials.

Ultrasonic Studies of Composites Undergoing Thermal and Fatigue Loading

New composite materials possess attractive properties for use in advanced aircraft. A necessary requirement for their introduction into aeronautic use is an accurate understanding of their long term aging processes so that proper design criteria can be established. In order to understand those properties, these composites must be exposed to thermal and load cycles that are characteristic of flight conditions. Additionally, airline companies will require nondestructive evaluation (NDE) methods that can be used in the field to assess the condition of these new materials as they age.

Effect of Rivet Rows on Propagation of Lamb Waves in Mechanically Fastened Two-Layer Aluminum Plates

Feasibility of using Lamb waves for disbonds and corrosion detections in aircraft fuselage structures was investigated in recent years. Measurement performed on various laboratory-fabricated specimens as well as on panels removed from aircrafts has shown consistent results and demonstrated its potential applications for large area structural integrity evaluation [1–3]. It has been observed that structural flaws existing on the path of Lamb waves not only changed amplitude of waves but also affected their velocities as well. Amplitude change caused by a disbond of size less than 0.5 in. × 0.5 in. was significant and has been measured. Variation in phase velocity was used to quantify the corrosion-induced thickness reduction in aluminum sheets. An area of size 1 in. × 1 in. with 8% thickness loss in subsurface of an 1 mm thick aluminum plate was detected by monitoring the phase velocity increase of SO mode. While these tests made major progresses toward developing a practical and low cost flaw assessment system, effects due to the presence of certain structural elements, such as coatings and fasteners, on the propagation of Lamb waves are becoming important issues, and need to be analyzed. These effects are themselves interesting physical phenomenon and worth investigation, however, it is hoped that propagation variations of waves induced by structural defects can be separated from these effects and be quantitatively correlated to the physical properties of defects.

Mode conversions of Lamb waves for inspection of disbonds

Disbond detection with Lamb waves is tested on lap splice joints and on laboratory specimens consisting of bonded aluminium sheets. Significant amplitude variation is observed between propagation over bond and disbond. Measurements conducted on laboratory specimens with different geometry of disbonds and on aircraft skins show results consistent with those obtained by standard C-scan ultrasonic test and other techniques.<<ETX>>

Comparison of ultrasonic signals in aircraft skin obtained through experiment and simulation

An ultrasonic pulse-echo technique is used to detect disbonds in adhesive bonds in lap joints and those between aircraft skin and reinforcing doublers. Experimental signals are acquired from fabricated as well as commercial aircraft lap joints. It is observed that the ultrasonic signals from unpainted surfaces contain clearly distinguishable echoes from successive layers of the sample. A better understanding of the signals is obtained from sound propagation in multilayered structures. Good correlation between simulation and experimental signals is obtained for both bonded and disbonded locations in the test region. Infrared thermometry is also used to image the regions that are tested ultrasonically. The ultrasonic results are found to be in good agreement with results using thermography.<<ETX>>

PULSE‐ECHO PHASED ARRAY ULTRASONIC INSPECTION OF PULTRUDED ROD STITCHED EFFICIENT UNITIZED STRUCTURE (PRSEUS)

A PRSEUS test article was subjected to controlled impact on the skin face followed by static and cyclic axial compressions. Phased array ultrasonic inspection was conducted before impact, and after each of the test conditions. A linear phased array probe with a manual X‐Y scanner was used for interrogation. Ultrasound showed a delamination between the skin and stringer flange adjacent to the impact. As designed, the stitching in the flange arrested the lateral flaw formation. Subsequent ultrasonic data showed no delamination growth due to continued loading.