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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

Control Design Strategies to Enhance Long-Term Aircraft Structural Integrity

Over the operational lifetime of both military and civil aircraft, structural components are exposed to hundreds of thousands of low-stress repetitive load cycles and less frequent but higher-stress transient loads originating from maneuvering flight and atmospheric gusts. Micro-material imperfections in the structure, such as cracks and debonded laminates, expand and grow in this environment, reducing the structural integrity and shortening the life of the airframe. Extreme costs associated with refurbishment of critical load-bearing structural components in a large fleet, or altogether reinventoring the fleet with newer models, indicate alternative solutions for life extension of the airframe structure are highly desirable. Increased levels of operational safety and reliability are also important factors influencing the desirability of such solutions. One area having significant potential for impacting crack growth/fatigue damage reduction and structural life extension is flight control. To modify the airframe response dynamics arising from command inputs and gust disturbances, feedback loops are routinely applied to vehicles. A dexterous flight control system architecture senses key vehicle motions and generates critical forces/moments at multiple points distributed throughout the airframe to elicit the desired motion characteristics. In principle, these same control loops can be utilized to influence the level of exposure to harmful loads during flight on structural components. Project objectives are to investigate and/or assess the leverage control has on reducing fatigue damage and enhancing long-term structural integrity, without degrading attitude control and trajectory guidance performance levels. In particular, efforts have focused on the effects inner loop control parameters and architectures have on fatigue damage rate. To complete this research, an actively controlled flexible aircraft model and a new state space modeling procedure for crack growth have been utilized. Analysis of the analytical state space model for crack growth revealed the critical mathematical factors, and hence the physical mechanism they represent, that influenced high rates of airframe crack growth. The crack model was then exercised with simple load inputs to uncover and expose key crack growth behavior. To characterize crack growth behavior, both "short-term" laboratory specimen test type inputs and "long-term" operational flight type inputs were considered. Harmonic loading with a single overload revealed typical exponential crack growth behavior until the overload application, after which time the crack growth was retarded for a period of time depending on the overload strength. An optimum overload strength was identified which leads to maximum retardation of crack growth. Harmonic loading with a repeated overload of varying strength and frequency again revealed an optimum overload trait for maximizing growth retardation. The optimum overload strength ratio lies near the range of 2 to 3 with dependency on frequency. Experimental data was found to correlate well with the analytical predictions.

Newman, Brett A.↗
NASA NTRS2002

Micro-Mechanical Modeling of Ductile Fracture in Welded Aluminum-Lithium Alloys

This study describes a 3-D computational framework to model stable extension of a macroscopic crack under mode I conditions in ductile metals. Material separation occurs through a local fracture mechanism based on the growth and coalescence of micro voids dispersed in the material. The Gurson-Tvergaard plasticity model for void materials describes the eventual loss of material stress capacity under sustained loading. Fixed-sized computational cell elements defined on a thin layer adjacent to the crack plane provide an implicit length scale for continuum damage model. Outside this layer, the material remains undamaged by void growth, consistent with metallurgical observations. The key micro-mechanics parameters are D, the thickness of cell layer, and f 0 , the initial cell porosity.

Ahmed Ibrahim↗
NASA NTRS1986

Grain boundary oxidation and its effects on high temperature fatigue life

Fatigue lives at elevated temperatures are often shortened by creep and/or oxidation. Creep causes grain boundary void nucleation and grain boundary cavitation. Grain boundary voids and cavities will accelerate fatigue crack nucleation and propagation, and thereby shorten fatigue life. The functional relationships between the damage rate of fatigue crack nucleation and propagation and the kinetic process of oxygen diffusion depend on the detailed physical processes. The kinetics of grain boundary oxidation penetration was investigated. The statistical distribution of grain boundary penetration depth was analyzed. Its effect on high temperature fatigue life are discussed. A model of intermittent micro-ruptures of grain boundary oxide was proposed for high temperature fatigue crack growth. The details of these studies are reported.

Liu, H. W.↗
NASA NTRS2000

Lack of Penetration in Friction Stir Welds: Effects on Mechanical Properties and NDE Feasibility

This presentation reviews the issue of lack of penetration (LOP) in Friction Stir Welding and the feasibility of using non-destructive tests to detect . Friction Stir Welding takes place in the solid phase below the melting point of the materials to be joined. It thus gives the ability to join materials which are difficult to fusion weld, for example 2000 and 7000 aluminium alloys. This process though can result in a lack of penetration, due to an incomplete penetration of the DXZ. This is frequently referred to as a "kissing bond", which requires micro examination to detect. The presentation then discusses the surface crack tension tests. It then reviews the simulated service test and results. It then discusses the feasibility of using non-destructive examination to detect LOP, the forms of test which can be used, and the results the tests.

Kinchen, David G.↗
NASA NTRS2018

Effect of Control Mode and Load Rate on Fracture Toughness of Brittle Ceramics

The effects of control mode and rate on the fracture toughness of ceramics were measured by using chevron-notched flexure specimen in accordance with ASTM C1421. The use of stroke control gave consistent results with about 2% variation in measured fracture toughness for a very wide range of rates (0.005 to 0.5 mm/min). Use of strain or CMOD control gave an approximately 5% variation over a very wide range of rates, with the measurements being a function of rate. However, the effect was eliminated by use of dry nitrogen, implying a stress corrosion effect rather than a stability effect. With the use of nitrogen for strain control, fracture toughness values were within about 1% over a wide range of rates (1 to 80 micro-epsilon/sec.). CMOD or strain control did allow stable crack extension well past maximum load, and thus is preferred for energy calculations. The effort is being used to confirm recommendations in ASTM International standard C1421 on fracture toughness measurement.

control mode↗
NASA NTRS1990

Micro-mechanical analysis of damage growth and fracture in discontinuous fiber reinforced metal matrix composites

The near-crack-tip stresses in any planar coupon of arbitrary geometry subjected to mode 1 loading may be equated to those in an infinite center-cracked panel subjected to the appropriate equivalent remote biaxial stresses (ERBS). Since this process can be done for all such mode 1 coupons, attention may be focused on the behavior of the equivalent infinite cracked panel. To calculate the ERBS, the constant term in the series expansion of the crack-tip stress must be retained. It is proposed that the ERBS may be used quantitatively to explain different fracture phenomena such as crack branching.

Goree, James G.↗
NASA NTRS2015

Evaluation of MEMS-Based Wireless Accelerometer Sensors in Detecting Gear Tooth Faults in Helicopter Transmissions

The diagnostics capability of micro-electro-mechanical systems (MEMS) based rotating accelerometer sensors in detecting gear tooth crack failures in helicopter main-rotor transmissions was evaluated. MEMS sensors were installed on a pre-notched OH-58C spiral-bevel pinion gear. Endurance tests were performed and the gear was run to tooth fracture failure. Results from the MEMS sensor were compared to conventional accelerometers mounted on the transmission housing. Most of the four stationary accelerometers mounted on the gear box housing and most of the CI's used gave indications of failure at the end of the test. The MEMS system performed well and lasted the entire test. All MEMS accelerometers gave an indication of failure at the end of the test. The MEMS systems performed as well, if not better, than the stationary accelerometers mounted on the gear box housing with regards to gear tooth fault detection. For both the MEMS sensors and stationary sensors, the fault detection time was not much sooner than the actual tooth fracture time. The MEMS sensor spectrum data showed large first order shaft frequency sidebands due to the measurement rotating frame of reference. The method of constructing a pseudo tach signal from periodic characteristics of the vibration data was successful in deriving a TSA signal without an actual tach and proved as an effective way to improve fault detection for the MEMS.

gears↗
NASA NTRS2005

Micro-Flying Robotics in Space Missions

The Columbia Accident Investigation Board issued a major recommendation to NASA. Prior to return to flight, NASA should develop and implement a comprehensive inspection plan to determine the structural integrity of all Reinforced Carbon-Carbon (RCC) system components. This inspection plan should take advantage of advanced non-destructive inspection technology. This paper describes a non-intrusive technology with a micro-flying robot to continuously monitor inside a space vehicle for any stress related fissures, cracks and foreign material embedded in walls, tubes etc.

Bardina, Jorge↗
NASA NTRS2010

Nondestructive Evaluation (NDE) for Characterizing Oxidation Damage in Cracked Reinforced Carbon-Carbon

In this study, coated reinforced carbon-carbon (RCC) samples of similar structure and composition as that from the NASA space shuttle orbiter's thermal protection system were fabricated with slots in their coating simulating craze cracks. These specimens were used to study oxidation damage detection and characterization using nondestructive evaluation (NDE) methods. These specimens were heat treated in air at 1143 C and 1200 C to create cavities in the carbon substrate underneath the coating as oxygen reacted with the carbon and resulted in its consumption. The cavities varied in diameter from approximately 1 to 3mm. Single-sided NDE methods were used because they might be practical for on-wing inspection, while X-ray micro-computed tomography (CT) was used to measure cavity sizes in order to validate oxidation models under development for carbon-carbon materials. An RCC sample having a naturally cracked coating and subsequent oxidation damage was also studied with X-ray micro-CT. This effort is a follow-on study to one that characterized NDE methods for assessing oxidation damage in an RCC sample with drilled holes in the coating.

Roth, Don J.↗
NASA NTRS2009

NDE for Characterizing Oxidation Damage in Reinforced Carbon-Carbon

In this study, coated reinforced carbon-carbon (RCC) samples of similar structure and composition as that from the NASA space shuttle orbiter s thermal protection system were fabricated with slots in their coating simulating craze cracks. These specimens were used to study oxidation damage detection and characterization using NDE methods. These specimens were heat treated in air at 1143 and 1200 C to create cavities in the carbon substrate underneath the coating as oxygen reacted with the carbon and resulted in its consumption. The cavities varied in diameter from approximately 1 to 3 mm. Single-sided NDE methods were used since they might be practical for on-wing inspection, while x-ray micro-computed tomography (CT) was used to measure cavity sizes in order to validate oxidation models under development for carbon-carbon materials. An RCC sample having a naturally-cracked coating and subsequent oxidation damage was also studied with x-ray micro-CT. This effort is a follow-on study to one that characterized NDE methods for assessing oxidation damage in an RCC sample with drilled holes in the coating. The results of that study are briefly reviewed in this article as well. Additionally, a short discussion on the future role of simulation to aid in these studies is provided.

Roth, Don J.↗
NASA NTRS2009

Nondestructive Evaluation (NDE) for Characterizing Oxidation Damage in Cracked Reinforced Carbon-Carbon (RCC)

In this study, coated reinforced carbon-carbon (RCC) samples of similar structure and composition as that from the NASA space shuttle orbiter's thermal protection system were fabricated with slots in their coating simulating craze cracks. These specimens were used to study oxidation damage detection and characterization using nondestructive evaluation (NDE) methods. These specimens were heat treated in air at 1143 and 1200 C to create cavities in the carbon substrate underneath the coating as oxygen reacted with the carbon and resulted in its consumption. The cavities varied in diameter from approximately 1 to 3 mm. Single-sided NDE methods were used since they might be practical for on-wing inspection, while x-ray micro-computed tomography (CT) was used to measure cavity sizes in order to validate oxidation models under development for carbon-carbon materials. An RCC sample having a naturally-cracked coating and subsequent oxidation damage was also studied with x-ray micro-CT. This effort is a follow-on study to one that characterized NDE methods for assessing oxidation damage in an RCC sample with drilled holes in the coating.

Roth, Don J.↗
NASA NTRS1984

Acousto-ultrasonic measurements to monitor damage during fatigue of composites

An acousto-ultrasonic nondestructive testing method used to monitor damage during static and fatigue loading of thin graphite epoxy laminates is described. The experimental procedure, the signal analysis by the Fast Fourier Transform (FFT) algorithm, and the results of this analysis are discussed. Quasi-static tension tests showed a sharp decrease in the quantitative parameters when transverse cracks developed in the 90 degrees plies of a (0, 90/2/)s laminate. When internal micro-delaminations unite to form macro-delaminations, a sharp decrease in the parameters is also observed. The parameters are found to correlate well with other indications of damage development such as stiffness and degradation. The root mean square value of the moment is found to be more sensitive to damage than stiffness. Various signals and spectrums of graphite epoxy systems are presented.

Govada, A.↗
NASA NTRS1989

Fractography of composite delamination

Delamination is a major failure mode of carbon fiber organic matrix composites. It can occur under a variety of loading conditions. Efforts to develop predictive models of the delamination of carbon fiber composites are hampered by a lack of information about the micromechanics of impact damage and delamination growth. Crack formation and propagation in these materials cannot be observed in sufficient detail to determine micro-damage using currently available nondestructive methods such as acoustic backscattering or x ray imaging. Consequently, destructive methods are required. Delamination of composites in Mode I, Mode II and after low energy impact loads were investigated using metallographic techniques of potting the failed specimens, sectioning and examining the cut sections for damage modes.

Bascom, W. D.↗
NASA NTRS2009

Assessment of Composite Delamination Self-Healing Under Cyclic Loading

Recently, the promise of self-healing materials for enhanced autonomous durability has been introduced using a micro-encapsulation technique where a polymer based healing agent is encapsulated in thin walled spheres and embedded into a base polymer along with a catalyst phase. For this study, composite skin-stiffener flange debonding specimens were manufactured from composite prepreg containing interleaf layers with a polymer based healing agent encapsulated in thin-walled spheres. Constant amplitude fatigue tests in three-point bending showed the effect of self-healing on the fatigue response of the skin-stiffener flange coupons. After the cycling that created debonding, fatigue tests were held at the mean load for 24 hours. For roughly half the specimens tested, when the cyclic loading was resumed a decrease in compliance (increase in stiffness) was observed, indicating that some healing had occurred. However, with continued cycling, the specimen compliance eventually increased to the original level before the hold, indicating that the damage had returned to its original state. As was noted in a prevoius study conducted with specimens tested under monotonically increasing loads to failure, healing achieved via the micro-encapsulation technique may be limited to the volume of healing agent available relative to the crack volume.

O'Brien, T. Kevin↗
NASA NTRS2017

Validation Testing of a Peridynamic Impact Damage Model Using NASA's Micro-Particle Gun

Through a collaborative effort between the Virginia Commonwealth University and Raytheon, a peridynamic model for sand impact damage has been developed1-3. Model development has focused on simulating impacts of sand particles on ZnS traveling at velocities consistent with aircraft take-off and landing speeds. The model reproduces common features of impact damage including pit and radial cracks, and, under some conditions, lateral cracks. This study focuses on a preliminary validation exercise in which simulation results from the peridynamic model are compared to a limited experimental data set generated by NASA's recently developed micro-particle gun (MPG). The MPG facility measures the dimensions and incoming and rebound velocities of the impact particles. It also links each particle to a specific impact site and its associated damage. In this validation exercise parameters of the peridynamic model are adjusted to fit the experimentally observed pit diameter, average length of radial cracks and rebound velocities for 4 impacts of 300 μm glass beads on ZnS. Results indicate that a reasonable fit of these impact characteristics can be obtained by suitable adjustment of the peridynamic input parameters, demonstrating that the MPG can be used effectively as a validation tool for impact modeling and that the peridynamic sand impact model described herein possesses not only a qualitative but also a quantitative ability to simulate sand impact events.

Baber, Forrest E.↗
NASA NTRS1980

JANTX1N5614 switching siode

Diode manufactured by Micro Semiconductor and Semtech were tested. Main failure mode was surface inversions caused by leakage of contaminants through cracks in glass. Most failures in groups 2 and 3 were visual.

Source record↗
NASA NTRS1973

A potential means of using acoustic emission for crack detection under cyclic-load conditions

A preliminary investigation was conducted to assess the feasibility of monitoring acoustic emission signals from fatigue cracks during cyclic bend tests. Plate specimens of 6A1-4V titanium, 2219-T87 aluminum, and 18-Ni maraging steel were tested with and without crack starter notches. It was found that significant acoustic emission signals could be detected in the frequency range from 100 kHz to 400 kHz. Cracks emanating from starter notches were monitored by the ultrasonic pulse-echo technique and periodically measured by micro-optical examination. Methods used to reduce the effects of extraneous noises (i.e., machine noises, fretting) are described. A frequency spectrum analyzer was used to characterize the emissions and to evaluate methods used to acquire the signals (i.e., transducer location, bandwidth selection). The investigation indicated that it was possible to extract meaningful acoustic emission signals in a cyclic bend machine environment.

Vary, A.↗