Richard E. Martin

NDE Methodologies for Composite Flywheels Certification

Manufacturing readiness of composite rotors and certification of flywheels depend in part on the maturity of nondestructive evaluation (NDE) technology for process optimization and quality assurance, respectively. Capabilities and limitations of x-ray-computed tomography and radiography, as well as advanced ultrasonics were established on NDE ring and rotor standards with EDM notches and drilled holes. Also, intentionally seeded delamination, tow break, and insert of bagging material were introduced in hydroburst-rings to study the NDE detection capabilities of such anomalies and their effect on the damage tolerance and safe life margins of subscale rings and rotors. Examples of possible occurring flaws or anomalies in composite rings as detected by NDE and validated by destructive metallography are shown. The general NDE approach to ensure quality of composite rotors and to help in the certification of flywheels is briefly outlined.

Impedance-based Structural Health Monitoring of a Ceramic Matrix Composite

This article addresses the use of electro-mechanical impedance-based structural health monitoring for assessing damage in a woven ceramic matrix composite consisting of a silicon carbide fibers and silicon carbide matrix. Controlled levels of damage were induced by conducting quasi-static, load/unload/reload tensile tests. The specimens localized mechanical impedance was assessed using piezoelectric-ceramic patches that act as both actuator and sensor. The impedance results were correlated to damage and compared to acoustic emissions data, stress—strain response, and data from previous studies concerning the material system. The impedance data was shown to correlate well with the progressive damage mechanisms within the composite material system.

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.

Thermal imaging technique to detect delaminations in CFRP-plated concrete

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.

Durability Testing of Docking System Seals for Space Applications

NASA is developing a new docking system to support future space exploration missions to low-Earth orbit, the Moon, and other destinations. This system will be used to dock future vehicles with the International Space Station or with each other. A key component of this system is the seal at the main docking interface which inhibits the loss of cabin air once docking is complete. Depending on the mission, the seal must be able to dock in either a sealon-flange or seal-on-seal configuration. Seal-on-flange mating occurs when a docking system equipped with a seal docks to a system with a flat metal flange. This would occur when a vehicle docks to a node on the International Space Station. Seal-on-seal mating occurs when two docking systems equipped with seals dock to each other. During and after the docking process, the seals will be subjected to scrubbing, or sliding, at the docking interface due to mechanical and thermal movements. To evaluate the effects of scrubbing on seal performance, durability tests were performed on subscale docking system seals manufactured from two candidate silicone elastomer compounds. These tests were performed under anticipated worst-case conditions of seal-on-flange mating at warm temperatures while under high compression levels. Seal performance was evaluated via leak and adhesion tests before and after the durability tests. Leak testing revealed no statistical differences in seal leak rates before and after durability testing. However, adhesion testing of the seals before and after the durability tests indicated that seal scrubbing during a mission could lead to increased adhesion during undocking.

Pulsed thermography of ceramic matrix composites

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.

Thermoelastic stress analysis of a pultruded composite double lap joint

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.

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.

Development and testing of an automated acousto-ultrasonic scan system

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.