Additive manufacturing is a rapidly growing field where 3-dimensional parts can be produced layer by layer. NASA’s electron beam freeform fabrication (EBF3) technology is being evaluated to manufacture metallic parts in a space environment. The benefits of EBF3 technology are weight savings to support space missions, rapid prototyping in a zero gravity environment, and improved vehicle readiness. The EBF3 system is composed of 3 main components: electron beam gun, multi-axis position system, and metallic wire feeder. The electron beam is used to melt the wire and the multi-axis positioning system is used to build the part layer by layer. To insure a quality deposit, a near infrared (NIR) camera is used to image the melt pool and solidification areas. This paper describes the calibration and application of a NIR camera for temperature measurement. In addition, image processing techniques are presented for deposit assessment metrics.
Critical components of propulsion systems frequently operate at high stress levels for long periods of time. The integrity of these parts must be proven by non-destructive evaluation (NDE) during various manufacturing steps and also during systematic overhaul inspections. Conventional NDE methods, however, have unacceptable limits. Some of these techniques are time-consuming and inconvenient for service aircraft testing. Impedance-based structural-health-monitoring (SHM) uses piezoelectric (PZT) patches that are bonded onto or embedded in a structure; each individual patch both actuates the surrounding structural area and senses the resulting structural response. The size of the excited area varies with the geometry and material composition of the structure. A series of experiments on simple geometry specimens (thin-gage aluminum square plates) was conducted for assessing the potential of E/M impedance method for structural damage detection. Based on the results of this preliminary study, further testing was conducted on a subscale disk specimen. Based on the results it can be concluded that the E/M impedance method has the potential to be used for damage detection of structures. The experimental method, signal processing, and damage detection algorithm should be tuned to the specific method used for structural interrogation.
This paper presents the results of a comparison study of three ultrasonic nondestructive evaluation (NDE) methods applied to polymer matrix composite (PMC) specimens subjected to impact damage. Samples mainly consisted of various thicknesses of graphite/epoxy coupon panels impacted with various energy levels. Traditional pulse-echo and through transmission ultrasonic c-scan techniques were applied to impacted samples and served as the basis for comparison. Specimens were then inspected using acoustography, a large field ultrasonic inspection technique that is analogous to real-time X-ray imaging. Acoustography utilizes a unique, wide area two-dimensional (2-D) detector, called an acousto-optic (AO) sensor, to directly convert ultrasound into visual images; much like an image intensifier in real-time radiography. Finally, a newly developed guided wave scanning system was utilized to inspect the same set of samples. This system uses two transducers in a pitch catch configuration to examine 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. Results are presented for all of the methods demonstrating each technique's detection capabilities and highlighting their advantages and disadvantages.
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.
The replacement of chemical batteries with composite flywheels offers many potential advantages in space applications. Before such flywheels can be successfully employed, it is imperative to ensure their integrity using NDE techniques. Previously, the use of traditional C-scan for the NDE of flywheels was compared to Scanning Ultrasonic Spectroscopy. However, both C-scan and scanning ultrasonic spectroscopy are point-by-point inspection techniques, and are thus inherently limited in inspection speed. In this paper, the application of Acoustography for the NDE of flywheels is reported. Acoustography provides an efficient and economical alternative to point-by-point ultrasonic scanning; in this approach a novel, wide area (AO) sensor is employed to provide full-field, real time ultrasonic images similar to x-ray imaging, significantly decreasing the required inspection time. Side by side images generated using Acoustography and traditional C-scan techniques for Plexiglas cylinder and a composite ring standard (both with known defects) are presented.
Acousto-ultrasonics (AU) is a 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 used to indirectly quantify the attenuation are the ultrasonic decay rate as well as various moments of the frequency power spectrum. For the most part, AU is used to gage the damage state of materials subjected to various mechanical or environmental loads. The AU technique has been applied to polymer matrix composites, ceramic matrix composites, metal matrix composites as well as metallic alloys. Historically, AU has been a point by point, manual technique with waveforms collected at discrete locations and post-processed. Data collection and analysis of this type limits the amount of detail that can be obtained. Also, the manual movement of the sensors is prone to user error and is time consuming. This paper discusses an automated AU scanning system recently developed and assembled at NASA Glenn Research Center. The paper will include a description of the hardware and software systems as well as the techniques for data reduction and presentation. In order to demonstrate the system capabilities, AU scan results for a SiC/SiC composite panel are presented.
Pulsed thermography is a powerful method for the nondestructive evaluation (NDE) of composite materials since it provides a large field means for detecting subsurface material defects and material variations. The main difficulty in the use of this technique, however, is the processing and interpretation of the acquired thermal image data. This paper examines three methods for processing pulsed thermography results concerning a composite plate with material inserts that simulate delamination type defects. Using the same temporal data set, a specimen is analyzed utilizing peak contrast, peak slope and a newly developed thermal image reconstruction technique. Comparisons are made on a composite panel with differing defect sizes and depths. In addition, results are compared to images gathered using through transmission ultrasonics and microfocus radiography. With subjective manipulation, pulsed thermography was able to show the defects more clearly than either the ultrasonic or radiographic techniques. A discussion of this subjectivity and future directions for automation are provided in an effort to better understand the potential of the 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.
KEYWORDS: Ultrasonics, Spectroscopy, Composites, Nondestructive evaluation, Transducers, Data acquisition, Inspection, Standards development, Acoustics, Control systems
Composite flywheels are being considered as a replacement for chemical batteries aboard the International Space Station (ISS). Due to the serious consequences of a failure in a space environment, extensive testing of the flywheel systems must be conducted prior to flight certification. In addition to standard material testing and characterization, non destructive evaluation (NDE) of the rotors must be preformed to identify processing flaws and to understand the damage progression in a rotor under load.
This paper describes the development of a scanning ultrasonic spectroscopy system for the non destructive evaluation of composite flywheels. Emphasis is placed on the novel resonance spectroscopy method that was investigated and its integration into a scanning system. Results of a scan of a composite ring standard and Plexiglas cylinder with known defects are presented and compared to ultrasonic pulse-echo c-scans.
The use of composite materials, in particular glass/epoxy systems for structural applications has seen widespread growth. Recent examples include a bridge in Butler County, Ohio and a covered pedestrian bridge that is scheduled to be installed in Akron, Ohio. Both of these structures employ pultruded composites for the main structural members due to their high strength, light weight and the ease of manufacture into common structural shapes such as wide flanges, I-beams and box sections. The use of these shapes gives the designer the ability to use many of the same types of structural details that are common to steel design. This paper will examine the most common method of joining structural members, bolted connections. The analysis of bolted connections in composite materials has been widely reported in the literature. Analysis methods have ranged from two and three dimensional finite element analysis to more empirical methods of calculating the stress concentration factors based on experimental data. This paper will focus on the use of the thermoelastic stress analysis method to determine the stress concentration around a steel pin loaded in double shear by a pultruded glass fiber composite. Further studies were conducted to determine the time dependent material behavior on the thermoelastic stress analysis signal output. The following is a description of the theory, experimental setup, and a summary of results.
A thermographic technique for detecting disbonds and delaminations in concrete beams reinforced with bonded sheets of unidirectional carbon fiber reinforced plastic is described. Concrete slab test specimens are fabricated with seeded flaws of various thickness and depth from the surface. The specimens are briefly heated with a projector heat source and infrared images are captured. Digital image processing techniques are applied in order to improve image quality. Flaws are identified. Advantages and limitations of this method are discussed.
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