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At least 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Quality Factor and Microslipping of Fatigue Cracks in Thin Plates at Resonant Vibration

Resonant vibrations have been stimulated in thin metal plates using a non-contacting electromagnetic driver. A sinusoidal force was applied in a swept frequency fashion and the resulting surface displacements were monitored through the use of an acoustic microphone. It has been found that the presence of a fatigue crack in the sample causes a broadening of the second resonance peak. The Q factors of the resonance curves were determined and are directly correlated with the presence of fatigue cracks in the samples. The broadening of the curves is explained in terms of a microslipping at the crack face walls which reduces the amplitude of the resonant vibration by increasing the damping of the system. A comparison is made between the resonance characteristics of fatigue damaged and notched samples, where the stiffness of the two systems is nearly constant while the interaction between crack face walls is eliminated in the latter.

Wincheski, B.↗
NASA NTRS2002

Power Law Versus Exponential Form of Slow Crack Growth of Advanced Structural Ceramics: Dynamic Fatigue

The life prediction analysis based on an exponential crack velocity formulation was examined using a variety of experimental data on glass and advanced structural ceramics in constant stress-rate ("dynamic fatigue") and preload testing at ambient and elevated temperatures. The data fit to the strength versus In (stress rate) relation was found to be very reasonable for most of the materials. It was also found that preloading technique was equally applicable for the case of slow crack growth (SCG) parameter n > 30. The major limitation in the exponential crack velocity formulation, however, was that an inert strength of a material must be known priori to evaluate the important SCG parameter n, a significant drawback as compared to the conventional power-law crack velocity formulation.

Choi, Sung R.↗
NASA NTRS2004

Applicability of a Crack-Detection System for Use in Rotor Disk Spin Test Experiments Being Evaluated

Engine makers and aviation safety government institutions continue to have a strong interest in monitoring the health of rotating components in aircraft engines to improve safety and to lower maintenance costs. To prevent catastrophic failure (burst) of the engine, they use nondestructive evaluation (NDE) and major overhauls for periodic inspections to discover any cracks that might have formed. The lowest cost fluorescent penetrant inspection NDE technique can fail to disclose cracks that are tightly closed during rest or that are below the surface. The NDE eddy current system is more effective at detecting both crack types, but it requires careful setup and operation and only a small portion of the disk can be practically inspected. So that sensor systems can sustain normal function in a severe environment, health-monitoring systems require the sensor system to transmit a signal if a crack detected in the component is above a predetermined length (but below the length that would lead to failure) and lastly to act neutrally upon the overall performance of the engine system and not interfere with engine maintenance operations. Therefore, more reliable diagnostic tools and high-level techniques for detecting damage and monitoring the health of rotating components are very essential in maintaining engine safety and reliability and in assessing life.

Abdul-Aziz, Ali↗
NASA NTRS2005

RSRM TP-H1148 Main Grain Propellant Crack Initiation Evaluation

Pressurized TP-HI 148 propellant fracture toughness testing was performed to assess the potential for initiation of visually undetectable cracks in the RSRM forward segment transition region during motor ignition. Two separate test specimens were used in this evaluation. Testing was performed in cold-gas and hot-fire environments, and under both static and dynamic pressurization conditions. Analysis of test results demonstrates safety factors against initiation of visually undetectable cracks in excess of 8.0. The Reusable Solid Rocket Motor (RSRM) forward segment is cast with PBAN propellant (TP-HI 148) to form T an 1 1-point star configuration that transitions to a tapered center perforated bore (see Figure 1). The geometry of the transition region between the fin valleys and the bore causes a localized area of high strain during horizontal storage. Updated analyses using worst-case mechanical properties at 40 F and improved modeling techniques indicated a slight reduction in safety margins over previous predictions. Although there is no history of strain induced cracks or flaws in the transition region propellant, a proactive test effort was initiated to better understand the implications of the new analysis, primarily the resistance of TP-H1148 propellant to crack initiation' during RSRM ignition.

Earnest, Todd E.↗
NASA NTRS2006

Crack Initiation and Growth in Rigid Polymeric Closed-Cell Foam Cryogenic Applications

Cryogenic vessels, such as the Space Shuttle External Tank, are often insulated with closed-cell foam because of its low thermal conductivity. The coefficient of thermal expansion mismatch between the foam and metallic substrate places the foam under a biaxial tension gradient through the foam thickness. The total foam thickness affects the slope of the stress gradient and is considered a significant contributor to the initiation of subsurface cracks. Rigid polymeric foams are brittle in nature and any subsurface cracks tend to propagate a finite distance toward the surface. This presentation investigates the relationship between foam thickness and crack initiation and subsequent crack growth, using linear elastic fracture mechanics, in a rigid polymeric closed-cell foam through analysis and comparison with experimental results.

Sayyah, Tarek↗
NASA NTRS2006

Development of Eddy Current Technique for the Detection of Stress Corrosion Cracking in Space Shuttle Primary Reaction Control Thrusters

A recent identification of stress corrosion cracking in the Space Shuttle Primary Reaction Control System (PRCS) thrusters triggered an extensive nondestructive evaluation effort to develop techniques capable of identifying such damage on installed shuttle hardware. As a part of this effort, specially designed eddy current probes inserted into the acoustic cavity were explored for the detection of such flaws and for evaluation of the remaining material between the crack tip and acoustic cavity. The technique utilizes two orthogonal eddy current probes which are scanned under stepper motor control in the acoustic cavity to identify cracks hidden with as much as 0.060 remaining wall thickness to the cavity. As crack growth rates in this area have been determined to be very slow, such an inspection provides a large safety margin for continued operation of the critical shuttle hardware. Testing has been performed on thruster components with both actual and fabricated defects. This paper will review the design and performance of the developed eddy current inspection system. Detection of flaws as a function of remaining wall thickness will be presented along with the proposed system configuration for depot level or on-vehicle inspection capabilities.

Wincheski, Buzz↗
NASA NTRS2006

High Resolution Millimeter Wave Detection of Vertical Cracks in the Space Shuttle External Tank Spray-On-Foam Insulation (SOFI)

Space Shuttle Columbia s catastrophic failure, the separation of a piece of spray-on-foam insulation (SOFI) from the external tank (ET) in the Space Shuttle Discovery s flight in 2005 and crack detected in its ET foam prior to its successful launch in 2006 emphasize the need for effective nondestructive methods for inspecting the shuttle ET SOFI. Millimeter wave nondestructive testing methods have been considered as potential and effective inspection tools for evaluating the integrity of the SOFI. This paper presents recent results of an investigation for the purpose of detecting vertical cracks in SOFI panels using a focused millimeter wave (150 GHz) reflectometer. The presented images of the SOFI panels show the capability of this reflectometer for detecting tight vertical cracks (also as a function of crack opening dimension) in exposed SOFI panels and while covered by a piece of SOFI ramp simulating a more realistic and challenging situation.

Kharkovsky, S.↗
NASA NTRS2007

Effects of Different R ratios on Fatigue Crack Growth in Laser Peened Friction Stir Welds

The influence of laser peening on the fatigue crack growth behavior of friction stir welded (FSW) Aluminum Alloy (AA) 7075-T7351 sheets was investigated. The surface modification resulting from the peening process on the fatigue crack growth of FSW was assessed for two different R ratios. The investigation indicated a significant decrease in fatigue crack growth rates resulting from using laser shock peening compared with unpeened, welded and unwelded specimens. The slower fatigue crack growth rate was attributed to the compressive residual stresses induced by the peening.

Hatamleh, Omar↗
NASA NTRS2007

In-Plane Cracking Behavior and Ultimate Strength for 2D Woven and Braided Melt-Infiltrated SiC/SiC Composites Tensile Loaded in Off-Axis Fiber Directions

The tensile mechanical properties of ceramic matrix composites (CMC) in directions off the primary axes of the reinforcing fibers are important for architectural design of CMC components that are subjected to multi-axial stress states. In this study, 2D-woven melt-infiltrated (MI) SiC/SiC composite panels with balanced fiber content in the 0 degree and 90 degree directions were tensile loaded in-plane in the 0 degree direction and at 45 degree to this direction. In addition, a 2D triaxially-braided MI composite panel with balanced fiber content in the plus or minus 67 degree bias directions and reduced fiber content in the axial direction was tensile loaded perpendicular to the axial direction tows (i.e., 23 degrees from the bias fibers). Stress-strain behavior, acoustic emission, and optical microscopy were used to quantify stress-dependent matrix cracking and ultimate strength in the panels. It was observed that both off-axis loaded panels displayed higher composite onset stresses for through-thickness matrix cracking than the 2D-woven 0/90 panels loaded in the primary 0 degree direction. These improvements for off-axis cracking strength can in part be attributed to higher effective fiber fractions in the loading direction, which in turn reduces internal stresses on critical matrix flaws for a given composite stress. Also for the 0/90 panel loaded in the 45 degree direction, an improved distribution of matrix flaws existed due to the absence of fiber tows perpendicular to the loading direction. In addition, for the +67/0/-67 braided panel, the axial tows perpendicular to the loading direction were not only low in volume fraction, but were also were well separated from one another. Both off-axis oriented panels also showed relatively good ultimate tensile strength when compared to other off-axis oriented composites in the literature, both on an absolute strength basis as well as when normalized by the average fiber strength within the composites. Initial implications are discussed for constituent and architecture design to improve the directional cracking of SiC/SiC CMC components with MI matrices.

Morscher, Gregory N.↗
NASA NTRS2007

Development of Eddy Current Techniques for the Detection of Cracking in Space Shuttle Primary Reaction Control Thrusters

A recent identification of cracking in the Space Shuttle Primary Reaction Control System (PRCS) thrusters triggered an extensive nondestructive evaluation effort to develop techniques capable of identifying such damage on installed shuttle hardware. As a part of this effort, specially designed eddy current probes inserted into the acoustic cavity were explored for the detection of such flaws and for evaluation of the remaining material between the crack tip and acoustic cavity. The technique utilizes two orthogonal eddy current probes which are scanned under stepper motor control in the acoustic cavity to identify cracks hidden with as much as 0.060 remaining wall thickness to the cavity. As crack growth rates in this area have been determined to be very slow, such an inspection provides a large safety margin for continued operation of the critical shuttle hardware. Testing has been performed on thruster components with both actual and fabricated defects. This paper will review the design and performance of the developed eddy current inspection system. Detection of flaws as a function of remaining wall thickness will be presented along with the proposed system configuration for depot level or on-vehicle inspection capabilities.

Wincheski, Buzz A.↗
NASA NTRS2007

A Semi-Analytical Method for Determining the Energy Release Rate of Cracks in Adhesively-Bonded Single-Lap Composite Joints

A semi-analytical method for determining the strain energy release rate due to a prescribed interface crack in an adhesively-bonded, single-lap composite joint subjected to axial tension is presented. The field equations in terms of displacements within the joint are formulated by using first-order shear deformable, laminated plate theory together with kinematic relations and force equilibrium conditions. The stress distributions for the adherends and adhesive are determined after the appropriate boundary and loading conditions are applied and the equations for the field displacements are solved. Based on the adhesive stress distributions, the forces at the crack tip are obtained and the strain energy release rate of the crack is determined by using the virtual crack closure technique (VCCT). Additionally, the test specimen geometry from both the ASTM D3165 and D1002 test standards are utilized during the derivation of the field equations in order to correlate analytical models with future test results. The system of second-order differential field equations is solved to provide the adherend and adhesive stress response using the symbolic computation tool, Maple 9. Finite element analyses using J-integral as well as VCCT were performed to verify the developed analytical model. The finite element analyses were conducted using the commercial finite element analysis software ABAQUS. The results determined using the analytical method correlated well with the results from the finite element analyses.

Yang, Charles↗
NASA NTRS2008

Space Shuttle Orbiter Atlantis Liquid Oxygen Pre-Valve Detent Roller Cracking Investigation

During routine inspections of the Space Shuttle s Main Propulsion System Liquid Oxygen (LO2) pre-valve, the mechanism provided to maintain the valve in the open position was found cracked. The mechanism is a Vespel roller held against the valve visor by a stack of Belleville springs. The roller has been found cracked 3 times. All three instances were in the same valve in the same location. There are 6 pre-valves on each orbiter, and only one has exhibited this problem. Every-flight inspections were instituted and the rollers were found to be cracked after only one flight. Engineers at Marshall Space Flight Center, Johnson Space Center, and Kennedy Space Center worked together to determine a solution. There were several possible contributors to the failure: a misaligned visor, an out-of-specification edge with a sharp radius, an out-of-specification tolerance stack up of a Belleville spring stack that caused un-predicted loads on the Vespel SP-21 roller, and a dimple machined into the side of the roller to indicate LO2 compatibility that created a stress riser. The detent assembly was removed and replaced with parts that were on the low side of the tolerance stack up to eliminate the potential for high loads on the detent roller. After one flight, the roller was inspected and showed fewer signs of wear and no cracks.

Holleman, Elizabeth↗
NASA NTRS2008

Space Shuttle Orbiter Atlantis Liquid Oxygen Prevalve Detent Roller Cracking Investigation

During routine inspections of the Space Shuttle's Main Propulsion System (MPS) Liquid Oxygen (LO2) pre-valve, the mechanism provided to maintain the valve in the open position was found cracked. The mechanism is a Vespel roller held against the valve visor by a stack of Belleville springs. The roller has been found cracked 3 times. All three instances were in the same valve in the same location. There are 6 pre-valves on each orbiter, and only one has exhibited this problem. Every-flight inspections were instituted and the rollers were found to be cracked after only one flight. Engineers at Marshall Space Flight Center, Johnson Space Center and Kennedy Space Center worked together to determine a solution. There were several possible contributors to the failure: a mis-aligned visor, an out of specification edge with a sharp radius, an out of specification tolerance stack up of a Belleville spring stack that caused un-predicted loads on the Vespel SP-21 roller, and a dimple machined into the side of the roller to indicate LO2 compatibility that created a stress riser. The detent assembly was removed and replaced with parts that were on the low-side of the tolerance stack up to eliminate the potential for high loads on the detent roller. After one flight, the roller was inspected and showed fewer signs of wear and no cracks.

Holleman, Elizabeth↗
NASA NTRS2008

Oxidation of Carbon/Carbon through Coating Cracks

Reinforced carbon/carbon (RCC) is used to protect the wing leading edge and nose cap of the Space Shuttle Orbiter on re-entry. It is composed of a lay-up of carbon/carbon fabric protected by a SiC conversion coating. Due to the thermal expansion mismatch of the carbon/carbon and the SiC, the SiC cracks on cool-down from the processing temperature. The cracks act as pathways for oxidation of the carbon/carbon. A model for the diffusion controlled oxidation of carbon/carbon through machined slots and cracks is developed and compared to laboratory experiments. A symmetric cylindrical oxidation cavity develops under the slots, confirming diffusion control. Comparison of cross sectional dimensions as a function of oxidation time shows good agreement with the model. A second set of oxidation experiments was done with samples with only the natural craze cracks, using weight loss as an index of oxidation. The agreement of these rates with the model is quite reasonab

Jacobson, N. S.↗
NASA NTRS2009

Calculation of Stress Intensity Factors for Interfacial Cracks in Fiber Metal Laminates

Stress intensity factors for interfacial cracks in Fiber Metal Laminates (FML) are computed by using the displacement ratio method recently developed by Sun and Qian (1997, Int. J. Solids. Struct. 34, 2595-2609). Various FML configurations with single and multiple delaminations subjected to different loading conditions are investigated. The displacement ratio method requires the total energy release rate, bimaterial parameters, and relative crack surface displacements as input. Details of generating the energy release rates, defining bimaterial parameters with anisotropic elasticity, and selecting proper crack surface locations for obtaining relative crack surface displacements are discussed in the paper. Even though the individual energy release rates are nonconvergent, mesh-size-independent stress intensity factors can be obtained. This study also finds that the selection of reference length can affect the magnitudes and the mode mixity angles of the stress intensity factors; thus, it is important to report the reference length used with the calculated stress intensity factors.

Wang, John T.↗
NASA NTRS2009

A Relationship Between Constraint and the Critical Crack Tip Opening Angle

Of the various approaches used to model and predict fracture, the Crack Tip Opening Angle (CTOA) fracture criterion has been successfully used for a wide range of two-dimensional thin-sheet and thin plate applications. As thicker structure is considered, modeling the full three-dimensional fracture process will become essential. This paper investigates relationships between the local CTOA evaluated along a three-dimensional crack front and the corresponding local constraint. Previously reported tunneling crack front shapes were measured during fracture by pausing each test and fatigue cycling the specimens to mark the crack surface. Finite element analyses were run to model the tunneling shape during fracture, with the analysis loading conditions duplicating those tests. The results show an inverse relationship between the critical fracture value and constraint which is valid both before maximum load and after maximum load.

Johnston, William M.↗
NASA NTRS2010

Back-Face Strain for Monitoring Stable Crack Extension in Precracked Flexure Specimens

Calibrations relating back-face strain to crack length in precracked flexure specimens were developed for different strain gage sizes. The functions were verified via experimental compliance measurements of notched and precracked ceramic beams. Good agreement between the functions and experiments occurred, and fracture toughness was calculated via several operational methods: maximum test load and optically measured precrack length; load at 2 percent crack extension and optical precrack length; maximum load and back-face strain crack length. All the methods gave vary comparable results. The initiation toughness, K(sub Ii) , was also estimated from the initial compliance and load.The results demonstrate that stability of precracked ceramics specimens tested in four-point flexure is a common occurrence, and that methods such as remotely-monitored load-point displacement are only adequate for detecting stable extension of relatively deep cracks.

Salem, Jonathan A.↗
NASA NTRS2010

Ultrasonic Monitoring of Ply Crack and Delamination Formation in Composite Tube Under Torsion Load

As a simple model of a rotor spar, a circular graphite-epoxy composite laminate cylinder was subjected to cyclic torsional load. The test section of the cylindrical specimen varied from four to six plies of plus or minus 45 degree fibers, due to intentional ply overlaps and gaps. A layer of 13-micrometer Teflon film was inserted between plies at three locations to serve as delamination initiators. A commercial X-Y scanner was mounted to the load frame to enable ultrasonic inspection without removing the specimen. A focused immersion probe was mounted in a captive water column with a rugged Nitrile membrane tip, which was coupled to the cylinder using a mist of soapy water. The transducer was aligned normal to the cylinder surface using the X-axis. Scanning was performed along the length of the specimen with the Y-axis and the specimen was incrementally rotated by the torsion head of the load frame. After 350k cycles of torsion, several linear 45 degree diagonal indications appeared as 5-40% attenuation of the back wall echo, with no apparent echoes from the interior of the composite, suggesting through-ply cracks in the innermost ply. Crack indications grew and new cracks appeared as torsion cycling continued. Internal reflections from delaminations associated with the growing ply cracks appeared after 500k cycles. Three areas of extensive multi-layer delaminations appeared after 1150k cycles. Failure of the specimen occurred at 1600k cycles. The observed progressive damage was not associated with the Teflon inclusions. Concurrent thermographic measurements provided lower resolution confirmation of the damage observed.

Johnston, P. H.↗