Structural sandwich materials composed of triaxially braided polymer matrix composite material face sheets sandwiching a foam core are being utilized for applications including aerospace components and recreational equipment. Since full scale components are being made from these sandwich materials, it is necessary to develop proper inspection practices for their manufacture and in field use. Specifically, nondestructive evaluation (NDE) techniques need to be investigated for analysis of components made from these materials. Hockey blades made from sandwich materials were examined with multiple NDE techniques including thermographic, radiographic, and laser based methods to investigate the manufactured condition of blades and damage induced from play. Hockey blades in an as received condition and damaged blades used in play were investigated with each technique. NDE images from the blades were presented and discussed. Structural elements within each blade were observed with radiographic imaging. Damaged regions and some structural elements of the hockey blades were identified with thermographic imaging. With shearography, structural elements, damaged regions, and other material variations were detected in the hockey blades. Each technique's advantages and disadvantages were considered in making recommendations for inspection of components made from these types of materials.
SiC/SiC composite materials targeted as turbine components for next generation aero-engines are being investigated at NASA Glenn Research Center. In order to examine damage mechanisms in these materials, SiC/SiC coupons were impacted with 1.59 mm diameter steel spheres at increasing velocities from 115 m/s to 400 m/s. Pulsed thermography, a nondestructive evaluation technique that monitors the thermal response of a sample over time, was utilized to characterize the impact damage. A thermal standard of similar material was fabricated to aid in the interpretation of the thermographic data and to provide information regarding thermography system detection capabilities in 2.4 mm thick SiC/SiC composite materials. Flat bottom holes at various depths with aspect ratios greater than 2.5 were detectable in the thermal images. In addition, the edges of holes at depths of 1.93 mm into the sample were not as resolvable as flat bottom holes closer to the surface. Finally, cooling behavior was characterized in SiC/SiC materials and used to determine impact damage depth within an 8.5% error of a known depth.
A guided wave scanning system was developed and is being refined at NASA Glenn Research Center. Instead of isolating a single Lamb wave mode, this guided wave scan system utilizes a multi-mode ultrasonic response consisting of multiple, overlapping wave modes. Various time and frequency related parameters are calculated from the time domain waveform at each scan location to create images. In order to optimize the performance of the guided wave scanning device, many experimental conditions need to be considered. In this study, the effects of the transducer contact force, dry couplant pad configuration, and scan step size on the repeatability of the guided wave parameters and the intensity and quality of the ultrasonic waveform were investigated. Based on the results, an optimal couplant configuration was recommended for future use with the scanning device.
Strong, lightweight, temperature-resistant ceramic matrix composite (CMC) materials such as carbon fiber reinforced silicon carbide (C/SiC) are being developed for use in reusable launch vehicles. C/SiC coupons were developed to investigate damage behavior due to tensile and fatigue testing. In order to describe the nature of damage in this material a nondestructive evaluation technique that can detect damage progression is necessary. This study determines acousto-ultrasonics’ (AU) capabilities and limitations for the detection of damage in these composites. AU parameters were evaluated for two sets of C/SiC coupons prior to interrupted fatigue testing. In addition, a single coupon was tested with two different loading configurations. The statistical significance of several AU parameters is determined for characterizing this composite material. Ten AU waveforms were collected along the gauge length of the C/SiC coupons prior to tensile and fatigue testing. Three operators collected the waveforms from each set of coupons to check repeatability. These waveforms were processed with an analysis routine that calculates AU parameters such as ultrasonic decay rate, the first moment of the power spectrum (M0), and the centroid of the power spectrum (fc). The results will recommend the most repeatable AU parameters and loading configuration for future evaluation of C/SiC components.
Acousto-ultrasonics (AU) is a nondestructive evaluation (NDE) technique that utilizes two ultrasonic transducers to interrogate the condition of a test specimen. The sending transducer introduces an ultrasonic pulse at a point on the surface of the specimen while a receiving transducer detects the signal after it has passed through the material. The aim of the method is to correlate certain parameters of the detected waveform to characteristics of the material between the two transducers. The waveform parameter of interest is the attenuation due to internal damping for which information is being garnered from the frequency domain. The parameters utilized to indirectly quantify the attenuation are the ultrasonic decay rate as well as various moments of the frequency power spectrum. Here, the sensitivity for each of the parameters was assessed in respect to changing boundary conditions during the experiments. Three conditions were controlled during the ultrasonic characterization of the specimens. First, issues concerning the contact force of the transducer were studied. Second, the support structure of the specimen was addressed. Lastly, the damage state of the material itself was considered. After analyzing the various AU parameters, the overall sensitivity of the AU technique to material change or damage were quantified and compared to changes in the AU values resulting from the experimental boundary conditions. This investigation showed that certain AU parameters could be utilized to gauge damage in composites, although, the experimental boundary conditions may slightly influence the results. The results of this study are important due to the fact that at this point AU is an empirical method.
In a previous study by the authors, the ultrasonic spectroscopy technique identified possible disbonds or delaminations that went unsubstantiated by other NDE (nondestructive evaluation) methods. The specimens were polymer matrix composite (PMC) rings sectioned from flywheel rotors. For this study, polymer matrix composite (PMC) rings were further investigated to determine the sensitivity of the ultrasonic spectroscopy technique in detecting tight disbonds or delaminations. The ultrasonic system utilizes a continuous swept sine waveform as the input. After the swept sine wave traverses the material, the captured waveform is subjected to two fast Fourier transforms (FFT); i.e. an FFT operation is performed on the amplitude versus frequency plot obtained by the first FFT. The second FFT along with equalization of the frequency spectrum allows for the evaluation of the fundamental resonant frequency as a function of material properties and thickness. Here, a study of ultrasonic spectroscopy's sensitivity to delaminations was conducted. Data was collected while opening a controlled delamination. The delamination opening was monitored using optical methods. The full thickness resonance, the resonance corresponding to the location of the intentional disbond, and the frequency spectrum were examined in an effort to characterize the sensitivity of the NDE method concerning various delamination conditions.
Flywheel energy storage devices comprised of multilayered composite rotor systems are being studied extensively for utilization in the international space station. These composite material systems were investigated with a recently developed ultrasonic resonance spectroscopy technique. The system, UltraSpecTM, employs a swept frequency approach and performs a fast Fourier transform (FFT) on the frequency spectrum of the response signal. In addition, the system allows for equalization of the frequency spectrum, providing all frequencies with equal amounts of energy to excite higher order resonant harmonics. Interpretation of the second FFT, along with equalization of the frequency spectrum, offers greater assurance in acquiring and analyzing the fundamental frequency, or spectrum resonance spacing. The range of frequencies swept in a pitch-catch mode was varied up to 8 MHz depending on the material and geometry of the component. Single and multilayered material samples, with and without known defects, were evaluated to determine how the constituents of a composite material system affect the resonant frequency.
Amplitude and frequency changes in the spectrum and spectrum resonance spacing domains were examined from ultrasonic response of a flat composite coupon, thin composite rings, and thick composite rings. Also, the ultrasonic spectroscopy responses from areas with an intentional delamination and a foreign material insert, similar to defects that may occur during manufacturing malfunctions, were compared to those from defect free areas in thin composite rings.A thick composite ring with varying thickness was tested to investigate the full thickness resonant frequency and any possible bulk interfacial bond issues. Finally, the effect on the frequency response of naturally occuring single and clustered voids in a composote ring was established.