James R. Bodis

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.

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...

Turbine engine disk rotor health monitoring assessment using spin tests data

Detecting rotating engine component malfunctions and structural anomalies is increasingly becoming a crucial key feature that will help boost safety and lower maintenance cost. However, achievement of such technology, which can be referred to as a health monitoring remains somewhat challenging to implement. This is mostly due to presence of scattered loading conditions, crack sizes, component geometry and material properties that hinders the simplicity of imposing such application. Different approaches are being considered to assist in developing other means of health monitoring or nondestructive techniques to detect hidden flaws and mini cracks before any catastrophic events occur. These methods extend further to assess material discontinuities and other defects that have matured to the level where a failure is very likely. This paper is focused on presenting data obtained from spin test experiments of a turbine engine like rotor disk and their correlation to the development of a structural health monitoring and fault detection system. The data collected includes blade tip clearance, blade tip timing measurements and shaft displacements. The experimental results are collected at rotational speeds up to 10,000 Rpm and tests are conducted at the NASA Glenn Research Centers Rotordynamics Laboratory via a high precision spin system. Additionally, this study offers a closer glance at a selective online evaluation of a rotating disk using advanced capacitive, microwave and eddy current sensor technology.

Capability of Single-Sided Transient Thermographic Imaging Method for Detection of Flat Bottom Hole Defects in High-Temperature Composite Materials

A portion of the development effort for high temperature composite materials is dedicated to the assessment of nondestructive evaluation (NDE) technologies for detecting flaws in these materials [1,2]. To illustrate the importance of defect detection and characterization, figure l(a) shows the results of a delamination sensitivity analysis on a CMC material in consideration for use as a hot section material in advanced aircraft engines. The study indicates that as the size of delaminations increases from 3×3 mm to 25×25 mm, the hot surface temperature increases up to 50 percent making the material unusable for hot section application. Recent technological advancements in infrared camera technology and computer power have made thermographic imaging systems worth evaluating as a nondestructive evaluation tool for advanced composites. Thermography offers the advantages of real-time inspection, no contact with sample, non-ionizing radiation, complex-shape inspection capability, variable field of view size, and portability. The objective of this study was to evaluate the ability of a thermographic imaging technique for detecting flat-bottom hole defects of various diameters and depths in 4 composite systems of interest as high-temperature structural materials. The technique used in this study utilized high intensity flash lamps to heat the sample located on the same side of the detecting infrared camera. The composite systems were (fiber/matrix): silicon carbide/calcia-alumina-silica (SiC/ CAS) CMC, silicon carbide/silicon carbide (SiC/SiC) CMC, silicon carbide/titanium alloy (SiC/Ti) MMC, and graphite/polyimide PMC. The holes ranged from 1 to 13 mm in diameter and 0.1 to 2.5 mm in depth in samples approximately 2 to 3 mm thick. Ultrasonic and radiographie images of the samples were obtained and compared with the thermographic images.

A Novel Ultrasonic Method for Accurate Characterization of Microstructural Gradients in Monolithic and Composite Tubular Structures

Prior studies have shown that ultrasonic velocity/time-of-flight imaging that uses back surface echo reflections to gauge volumetric material quality is well suited (perhaps more so than is the commonlyused peak amplitude c-scanning) for quantitative characterization of microstructural gradients. Such gradients include those due to pore fraction, density, fiber fraction, and chemical composition variations [11–15]. Variations in these microstructural factors can affect the uniformity of physical performance (including mechanical [stiffness, strength], thermal [conductivity], and electrical [conductivity, superconducting transition temperature], etc. performance) of monolithic and composite [1,3,6,12]. A weakness of conventional ultrasonic velocity/time-of-flight imaging (as well as to a lesser extent ultrasonic peak amplitude c-scanning where back surface echoes are gated [17] is that the image shows the effects of thickness as well as microstructural variations unless the part is uniformly thick. This limits this type of imaging’s usefulness in practical applications. The effect of thickness is easily observed from the equation for pulse-echo waveform time-of-flight (2τ) between the first front surface echo (FS) and the first back surface echo (B1), or between two successive back surface echoes where:

A Novel Ultrasonic Method for Characterizing Microstructural Gradients in Tubular Structures

2\tau = {{\left( {2d} \right)} \over V}

Commercial implementation of NASA-developed ultrasonic imaging methods via technology transfer

(1) where d is the sample thickness and V is the velocity of ultrasound in the material. Interpretation of the time-of-flight image is difficult as thickness variation effects can mask or overemphasize the true microstructural variation portrayed in the image of a part containing thickness variations. Thickness effects on time-of-flight can also be interpreted by rearranging equation (1) to calculate velocity: