Engineering topics

Smith, Stephen W.

Publications and source records attributed to Smith, Stephen W..

29 records · Page 2

NASA NTRS2007

Application of Terahertz Radiation to the Detection of Corrosion under the Shuttle's Thermal Protection System

There is currently no method for detecting corrosion under Shuttle tiles except for the expensive process of tile removal and replacement; hence NASA is investigating new NDE methods for detecting hidden corrosion. Time domain terahertz radiation has been applied to corrosion detection under tiles in samples ranging from small lab samples to a Shuttle with positive results. Terahertz imaging methods have been able to detect corrosion at thicknesses of 5 mils or greater under 1" thick Shuttle tiles and 7-12 mils or greater under 2" thick Shuttle tiles.

Madaras, Eric I.↗

NASA NTRS2007

Replica-based Crack Inspection

A surface replica-based crack inspection method has recently been developed for use in Space Shuttle main engine (SSME) hydrogen feedline flowliners. These flowliners exist to ensure favorable flow of liquid hydrogen over gimble joint bellows, and consist of two rings each containing 38 elongated slots. In the summer of 2002, multiple cracks ranging from 0.1 inches to 0.6 inches long were discovered; each orbiter contained at least one cracked flowliner. These long cracks were repaired and eddy current inspections ensured that no cracks longer than 0.075 inches were present. However, subsequent fracture-mechanics review of flight rationale required detection of smaller cracks, and was the driving force for development of higher-resolution inspection method. Acetate tape surface replicas have been used for decades to detect and monitor small cracks. However, acetate tape replicas have primarily been limited to laboratory specimens because complexities involved in making these replicas - requiring acetate tape to be dissolved with acetone - are not well suited for a crack inspection tool. More recently developed silicon-based replicas are better suited for use as a crack detection tool. A commercially available silicon-based replica product has been determined to be acceptable for use in SSME hydrogen feedlines. A method has been developed using this product and a scanning electron microscope for analysis, which can find cracks as small as 0.005 inches and other features (e.g., pits, scratches, tool marks, etc.) as small as 0.001 inches. The resolution of this method has been validated with dozens of cracks generated in a laboratory setting and this method has been used to locate 55 cracks (ranging in size from 0.040 inches to 0.004 inches) on space flight hardware. These cracks were removed by polishing away the cracked material and a second round of replicas confirmed the repair.

Newman, John A.↗

NASA NTRS2006

Flow Liner Slot Edge Replication Feasibility Study

Surface replication has been proposed as a method for crack detection in space shuttle main engine flowliner slots. The results of a feasibility study show that examination of surface replicas with a scanning electron microscope can result in the detection of cracks as small as 0.005 inch, and surface flaws as small as 0.001 inch, for the flowliner material.

Newman, John A.↗

NASA NTRS2005

Reaction Control System Thruster Cracking Consultation: NASA Engineering and Safety Center (NESC) Materials Super Problem Resolution Team (SPRT) Findings

The shuttle orbiter s reaction control system (RCS) primary thruster serial number 120 was found to contain cracks in the counter bores and relief radius after a chamber repair and rejuvenation was performed in April 2004. Relief radius cracking had been observed in the 1970s and 1980s in seven thrusters prior to flight; however, counter bore cracking had never been seen previously in RCS thrusters. Members of the Materials Super Problem Resolution Team (SPRT) of the NASA Engineering and Safety Center (NESC) conducted a detailed review of the relevant literature and of the documentation from the previous RCS thruster failure analyses. It was concluded that the previous failure analyses lacked sufficient documentation to support the conclusions that stress corrosion cracking or hot-salt cracking was the root cause of the thruster cracking and lacked reliable inspection controls to prevent cracked thrusters from entering the fleet. The NESC team identified and performed new materials characterization and mechanical tests. It was determined that the thruster intergranular cracking was due to hydrogen embrittlement and that the cracking was produced during manufacturing as a result of processing the thrusters with fluoride-containing acids. Testing and characterization demonstrated that appreciable environmental crack propagation does not occur after manufacturing.

MacKay, Rebecca A.↗

NASA NTRS2004

A Comparison of Weld-Repaired and Base Metal for Inconel 718 and CRES 321 at Cryogenic and Room Temperatures

Fatigue crack growth tests were conducted to characterize the performance of Inconel 718 and CRES 321 welds, weld heat-affect-zone and parent metal at room temperature laboratory air and liquid nitrogen (-196oC) environments. The results of this study were required to predict the damage tolerance behavior of proposed orbiter main engine hydrogen fuel liner weld repairs. Experimental results show that the room and cryogenic temperature fatigue crack growth characteristics of both alloys are not significantly degraded by the weld repair process. However, both Inconel 718 and CRES 321 exhibited lower apparent toughness within the weld repair region compared to the parent metal.

Newman, John A.↗

NASA NTRS2003

Characterization of Cracking and Crack Growth Properties of the C5A Aircraft Tie-Box Forging

Detailed destructive examinations were conducted to characterize the integrity and material properties of two aluminum alloy (7075-T6) horizontal stabilizer tie box forgings removed.from US. Air Force C5A and C5B transport aircraft. The C5B tie box forging was,found to contain no evidence of cracking. Thirteen cracks were found in the CSA,forging. All but one of the cracks observed in the C5A component were located along the top cap region (one crack was located in the bottom cap region). The cracks in the C5A component initiated at fastener holes and propagated along a highly tunneled intergranular crack path. The tunneled crack growth configuration is a likelv result of surface compressive stress produced during peening of the .forging suijace. The tie box forging ,fatigue crack growth, fracture and stress corrosion cracking (SCC) properties were characterized. Reported herein are the results of laboratory air ,fatigue crack growth tests and 95% relative humidity SCC tests conducted using specimens machined from the C5A ,forging. SCC test results revealed that the C5A ,forging material was susceptible to intergranular environmental assisted cracking: the C5A forging material exhibited a SCC crack-tip stress-intensity factor threshold of less than 6 MPadn. Fracture toughness tests revealed that the C5A forging material exhibited a fracture toughness that was 25% less than the C5B forging. The C5A forging exhibited rapid laboratory air fatigue crack growth rates having a threshold crack-tip stress-intensity factor range of less than 0.8 MPa sup m. Detailed fractographic examinations revealed that the ,fatigue crack intergranular growth crack path was similar to the cracking observed in the C5A tie box forging. Because both fatigue crack propagation and SCC exhibit similar intergranular crack path behavior, the damage mechanism resulting in multi-site cracking of tie box forgings cannot be determined unless local cyclic stresses can be quantified.

Piascik, Robert S.↗

NASA NTRS2003

Simulation of Fatigue Crack Initiation at Corrosion Pits With EDM Notches

Uniaxial fatigue tests were conducted to compare the fatigue life of laboratory produced corrosion pits, similar to those observed in the shuttle main landing gear wheel bolt-hole, and an electro-discharged-machined (EDM) flaw. EDM Jaws are used to simulate corrosion pits during shuttle wheel (dynamometer) testing. The aluminum alloy, (AA 7050) laboratory fatigue tests were conducted to simulate the local stress level contained in the wheel bolt-hole. Under this high local stress condition, the EDM notch produced a fatigue life similar to test specimens containing corrosion pits of similar size. Based on the laboratory fatigue test results, the EDM Jaw (semi-circular disc shaped) produces a local stress state similar to corrosion pits and can be used to simulate a corrosion pit during the shuttle wheel dynamometer tests.

Smith, Stephen W.↗

NASA NTRS2002

Orbiter Cold Plate Intergranular Corrosion: Development of NDE Standards and Assessment of NDE Methods

During pre-servicing of a space shuttle (orbiter vehicle, OV-102), helium leak detection of an avionics cold plate identified a leak located in the face sheet oriented towards the support shelf. Subsequent destructive examination of the leaking cold plate revealed that intergranular corrosion had penetrated the 0.017-inch thick aluminum (AA6061) face sheet. The intergranular attack (IGA) was likely caused by an aggressive crevice environment created by condensation of water vapor between the cold plate and support shelf. Face sheet susceptibility to IGA is a result of the brazing process used in the fabrication of the cold plates. Cold plate components were brazed at 1000 F followed by a slow cooling process to avoid distortion of the bonded cold plate. The slow cool process caused excessive grain boundary precipitation resulting in a material that is susceptible to IGA. The objectives of this work are as follows: (1) Develop first-of-a-kind nondestructive evaluation (NDE) standards that contain IGA identical to that found in the orbiter cold plates; and (2) Assess advanced NDE techniques for corrosion detection and recommend methods for cold plate examination. This report documents the results of work performed at Langley Research Center to fulfill these objectives.

Smith, Stephen W.↗

NASA NTRS2002

Detection of Intergranular Corrosion in Cold Plate Face Sheets

Cold plates are critical for cooling electronic systems in the shuttle. As a result of the environmental conditions in which they operate, water can condense between them and a support shelf. In some cases, this water results in intergranular corrosion in the face sheet. If the intergranular corrosion sufficiently penetrates the face sheet, a coolant leak could occur and jeopardize cold plate operation. This paper examines techniques for detecting and characterizing the intergranular corrosion, to enable recertification of cold plates that have been in operation for 15 plus years. Intergranular corrosion was artificially induced in the face sheets of a series of cold plate specimens using an electrochemical process. Some of the cold plate specimens were separated for destructive characterization of the extent of corrosion produced by the electrochemical process and to insure the induced corrosion was intergranular. The rest of the specimens were characterized nondestructively using several techniques. X-ray tomography and ultrasonic techniques provided the best indication of corrosion in these specimens and will be the focus of this paper. An x-ray tomography technique was shown to be the most effective technique for characterizing depth of the intergranular corrosion. From these measurements, corrosion profile maps were developed that were consistent with subsequent destructive evaluations of the specimens. This enabled the assessment of NDE (ondestructive evaluation) standards to evaluate the viability of other NDE techniques. Due to system constraints, a different technique must be used to inspect an entire cold plate. An ultrasonic technique was shown to be very reliable for detection of corrosion in the unbacked regions of the face sheet. The ultrasonic technique was performed in an alcohol bath to avoid additional corrosion during the NDE evaluation. A pulse echo technique that focuses on the RMS value of the signal is shown to be very sensitive to the presence of intergranular corrosion.

Winfree, William P.↗

NASA NTRS2001

The Effect of O2, H2O, and N2 on the Fatigue Crack Growth Behavior of an Alpha + Beta Titanium Alloy at 24 C and 177 C

To study the effects of atmospheric species on the fatigue crack growth behavior of an a+B titanium alloy (Ti 6-2-2-2-2) at room temperature and 177 C, fatigue tests were performed in laboratory air, ultrahigh vacuum, and high purity water vapor, oxygen, nitrogen and helium at various partial pressures. Accelerated fatigue crack growth rates in laboratory air compared to ultrahigh vacuum are linked to the damaging effects of both water vapor and oxygen. Observations of the fatigue crack growth behavior in ultrahigh purity environments, along with surface film analysis using X-ray photoelectron spectroscopy (XPS), suggest that multiple crack-tip processes govern the damaging effects of air. Three possible mechanisms are proposed: 1) at low pressure (less than 10(exp -1) Pa), accelerated da/dN is likely due to monolayer adsorption on crack-tip surfaces presumably resulting in decreased bond strengths at the fatigue crack tip, 2) for pressures greater than 10(exp -1) Pa, accelerated da/dN in oxygen may result from oxidation at the crack tip limiting reversible slip, and 3) in water vapor, absorption of atomic hydrogen at the reactive crack tip resulting in process zone embrittlement.

Smith, Stephen W.↗

NASA NTRS2001

Fatigue Crack Growth Characteristics of Thin Sheet Titanium Alloy Ti 6-2-2-2-2

Fatigue crack growth rates of Ti 6-2-2-2-2 as a function of stress ratio, temperature (24 or 177 C), tensile orientation and environment (laboratory air or ultrahigh vacuum) are presented. Fatigue crack growth rates of Ti 6-2-2-2-2 are also compared with two more widely used titanium alloys (Timetal 21S and Ti 6Al-4V). The fatigue crack growth rate (da/dN) of Ti 6-2-2-2-2 in laboratory air is dependent upon stress ratio (R), particularly in the near-threshold and lower-Paris regimes. For low R (less than approximately 0.5), da/dN is influenced by crack closure behavior. At higher R (> 0.5), a maximum stress-intensity factor (K(sub max)) dependence is observed. Fatigue crack growth behavior is affected by test temperature between 24 and 177 C. For moderate to high applied cyclic-stress-intensity factors (delta-K), the slope of the log da/dN versus log delta-K curve is lower in 177 C laboratory air than 24 C laboratory air. The difference in slope results in lower values of da/dN for exposure to 177 C laboratory air compared to room temperature laboratory air. The onset of this temperature effect is dependent upon the applied R. This temperature effect has not been observed in ultrahigh vacuum. Specimen orientation has been shown to affect the slope of the log da/dN versus log delta-K curve in the Paris regime.

Smith, Stephen W.↗