Don J. Roth

Thermographic Imaging for High-Temperature Composite Materials—A Defect Detection Study

Abstract. The ability of a thermographic imaging technique for detecting flat-bottom hole defects of various diameters and depths was evaluated in four composite systems (two types of ceramic-matrix composites, one metal-matrix composite, and one polymer-matrix composite) of interest as high-temperature structural materials. The holes ranged from 1 to 13 mm in diameter and 0.1 to 2.5 mm in depth in samples approximately 2—3 mm thick. The thermographic imaging system utilized a scanning mirror optical system and infrared (IR) focusing lens in conjunction with a mercury—cadmium—telluride infrared detector element to obtain high-resolution infrared images. High-intensity flash lamps located on the same side as the infrared camera were used to heat the samples. After heating, up to 30 images were sequentially acquired at 70—150 ms intervals. Limits of detectability based on depth and diameter of the flat-botton holes were defined for each composite material. Ultrasonic and radiographic images of the samples were obtained and compared with the thermographic images. This study was done under a nonreimbursable Space Act Agreement between NASA—Lewis Research Center and Bales Scientific, Inc., to allow several heating configurations to be evaluated in a cost-effective and timely fashion.

Signal Processing Approaches for Terahertz Data Obtained from Inspection of the Shuttle External Tank Thermal Protection System Foam

Foam shedding from the shuttle external tank remains a critical problem regarding Shuttle orbiter safety. Flaws present in the foam can result in initiation sites for foam loss, and NASA is continuing to look at improving existing NDE methods for foam inspection as well as developing new methods. Terahertz NDE, greatly enhanced over the last several years with respect to its use for external tank foam inspection, has been a focus for continued improvement through signal and image processing improvements. In this study, results from various signal processing approaches to improve terahertz image flaw resolution for external tank foam are described.

Protocol and Assessment of Signal Processing and Feature Extraction Methods for Terahertz NDE for Spray‐On Foam Insulation

A quantitative assessment protocol was developed to evaluate potential signal processing methods for THz NDE of spray‐on foam insulation. Automated feature extraction methods were developed to replicate the important visual classification characteristics used by inspectors and a new merit assessment approach was developed to identify the most promising combination of signal processing algorithms. Through application of the protocol to test data, several signal processing methods were found to improve the discrimination of defects.

Microstructural and Defect Characterization in Ceramic Composites Using an Ultrasonic Guided Wave Scan System

In this study, an ultrasonic guided wave scan system was used to characterize various microstructural and flaw conditions in two types of ceramic matrix composites, SiC/SiC and C/SiC. Rather than attempting to isolate specific lamb wave modes to use for characterization (as is desired for many types of guided wave inspection problems), the guided wave scan system utilizes the total (multi‐mode) ultrasonic response in its inspection analysis. Several time‐ and frequency‐domain parameters are calculated from the ultrasonic guided wave signal at each scan location to form images. Microstructural and defect conditions examined include delamination, density variation, cracking, and pre/post‐infiltration. Results are compared with thermographic imaging methods. Although the guided wave technique is commonly used so scanning can be eliminated, applying the technique in the scanning mode allows a more precise characterization of defect conditions.

Development of a High Performance Acousto‐Ultrasonic Scan System

Acousto‐ultrasonic (AU) interrogation is a single‐sided nondestructive evaluation technique employing separated sending and receiving transducers. It is used for assessing the microstructural condition/distributed damage state of the material between the transducers. AU is complementary to more traditional NDE methods such as ultrasonic c‐scan, x‐ray radiography, and themographic inspection that tend to be used primarily for discrete flaw detection. Through its history, AU has been used to inspect polymer matrix composite, metal matrix composite, ceramic matrix composite, and even monolithic metallic materials. The development of a high‐performance automated AU scan system for characterizing within‐sample microstructural and property homogeneity is currently in a prototype stage at NASA. In this paper, a review of essential AU technology is given. Additionally, the basic hardware and software configuration, and preliminary results with the system, are described.

Approaches for non-uniformity correction and dynamic range extension for acoustography

Acoustography is a full-field ultrasonic imaging process where a high resolution 2D acousto-optic sensor based on liquid crystal technology is employed to directly convert the ultrasound into a visual image in near real time. Unprocessed acoustography images typically suffer from non-uniformity due to spatial variations in the optical brightness response of the acousto-optic sensor field to ultrasonic intensity. Additionally, dynamic range of the acousto-optic sensor is limited to approximately 20 to 30 db. The nonuniformity and dynamic range limitation can result in difficulty in acoustography image interpretation, impracticality for large field application, and difficulty for use on samples having a wide range of attenuation. The approach of this ongoing study is to apply various methodologies that address these limitations in hopes of extending the usefulness and applications of acoustoography for nondestructive testing. This article shows initial results of methodologies developed to correct for image non-uniformity and explains the proposed approach to extend the dynamic range of acoustography images.

Characterization of C/Enhanced SiC Composite During Creep-Rupture Tests Using an Ultrasonic Guided Wave Scan System

Summary An ultrasonic guided wave scan system was used to nondestructively monitor damage over time and position in a C/enhanced SiC sample that was creep tested to failure at 1200 o C in air at a stress of 69 MPa (10 ksi). The use of the guided wave scan system for mapping evolving oxidation profiles (via porosity gradients resulting from oxidation) along the sample length and predicting failure location was explored. The creep-rupture tests were interrupted for ultrasonic evaluation every two hours until failure at ~17.5 cumulative hours. Introduction Ceramic matrix composites (CMC) are being developed for advanced aerospace propulsion applications in order to save weight, to improve reuse capability, and to increase performance. C/SiC materials have shown some promise in these applications but are extremely susceptible to oxidation damage. C/Enhanced SiC (the “enhancement” is in boron carbide, which is put in the composite matrix in order to protect the carbon fibers from oxidation) have shown increased lifetimes over the conventional C/SiC materials.

3-D surface profiling using focused air-coupled ultrasound

Surface topography is an important variable in the performance of many industrial components and is normally measured with diamond-tip profilometry over a small area or using optical scattering methods for larger area measurement. This article shows quantitative surface topography profiles as obtained using only high-frequency focused air-coupled ultrasonic pulses. The method is simple and reproducible because it relies mainly on knowledge and constancy of the sound velocity through the air. The air transducer is scanned across the surface and sends pulses to the sample surface where they are reflected back from the surface along the same path as the incident wave. Time-of-flight images of the sample surface are acquired (over large depths if required) and converted to depth/surface profile images using the simple relation (d=V*t/2) between distance (d), time-of-flight (t), and the velocity of sound in air (V). The system has the ability to resolve surface depression variations as small as 25 μm, is useab...

ULTRASONIC PHASED ARRAY SIMULATIONS OF WELDED COMPONENTS AT NASA

Comprehensive and accurate inspections of welded components have become of increasing importance as NASA develops new hardware such as Ares rocket segments for future exploration missions. Simulation and modeling will play an increasing role in the future for nondestructive evaluation in order to better understand the physics of the inspection process, to prove or disprove the feasibility for an inspection method or inspection scenario, for inspection optimization, for better understanding of experimental results, and for assessment of probability of detection. This study presents simulation and experimental results for an ultrasonic phased array inspection of a critical welded structure important for NASA future exploration vehicles.

Oxidation of Carbon/Carbon through Coating Cracks

Reinforced carbon/carbon (RCC) is used to protect the wing leading edge and nose cap of the Space Shuttle Orbiter on re-entry. It is composed of a lay-up of carbon/carbon fabric protected by a SiC conversion coating. Due to the thermal expansion mismatch of the carbon/carbon and the SiC, the SiC cracks on cool-down from the processing temperature. The cracks act as pathways for oxidation of the carbon/carbon. A model for the diffusion controlled oxidation of carbon/carbon through machined slots and cracks is developed and compared to laboratory experiments. A symmetric cylindrical oxidation cavity develops under the slots, confirming diffusion control. Comparison of cross sectional dimensions as a function of oxidation time shows good agreement with the model. A second set of oxidation experiments was done with samples with only the natural craze cracks, using weight loss as an index of oxidation. The agreement of these rates with the model is quite reasonab