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Early detection of cracks resulting from fatigue stressing

Description: Report discusses an apparatus that may be used for indicating the formation of fatigue cracks or for following the progress of cracks. Details of the experiment and discussion of the way in which the deflection-crack area relationship may be used to monitor fatigue cracks are included.
Date: September 1944
Creator: Bennett, John A.
Partner: UNT Libraries Government Documents Department

Status Report on Studies of Recovery Boiler Composite Floor Tube Cracking

Description: Cracking of the stainless steel layer of co-extruded 304L stainless steel/SA210 Gd A 1 carbon steel black liquor recovery boiler floor tubes has been identified as one of the most serious material problems in the pulp and paper industry. A DOE-funded study was initiated in 1995 with the goal of determining the cause of and possible solutions to this cracking problem. These studies have characterized tube cracking as well as the chemical and thermal environment and stress state of floor tubes. Investigations of possible cracking mechanisms indicate that stress corrosion cracking rather than thermal fatigue is a more likely cause of crack initiation. The cracking mechanism appears to require the presence of hydrated sodium sulfide and is most likely active during shut-downs and/or start-ups. Based on these results and operating experience, certain alloys appear to be more resistant than others to cracking in the floor environment, and certain operating practices appear to significantly lessen the likelihood of cracking. This report is the latest in a series of progress reports presented on this project.
Date: September 12, 1999
Creator: Eng, P.; Frederick, L.A.; Hoffmann, C.M.; Keiser, J.R.; Mahmood, J.; Maziasz, P.J. et al.
Partner: UNT Libraries Government Documents Department

Random polycrystals of grains containing cracks: Model ofquasistatic elastic behavior for fractured systems

Description: A model study on fractured systems was performed using aconcept that treats isotropic cracked systems as ensembles of crackedgrains by analogy to isotropic polycrystalline elastic media. Theapproach has two advantages: (a) Averaging performed is ensembleaveraging, thus avoiding the criticism legitimately leveled at mosteffective medium theories of quasistatic elastic behavior for crackedmedia based on volume concentrations of inclusions. Since crack effectsare largely independent of the volume they occupy in the composite, sucha non-volume-based method offers an appealingly simple modelingalternative. (b) The second advantage is that both polycrystals andfractured media are stiffer than might otherwise be expected, due tonatural bridging effects of the strong components. These same effectshave also often been interpreted as crack-crack screening inhigh-crack-density fractured media, but there is no inherent conflictbetween these two interpretations of this phenomenon. Results of thestudy are somewhat mixed. The spread in elastic constants observed in aset of numerical experiments is found to be very comparable to the spreadin values contained between the Reuss and Voigt bounds for thepolycrystal model. However, computed Hashin-Shtrikman bounds are much tootight to be in agreement with the numerical data, showing thatpolycrystals of cracked grains tend to violate some implicit assumptionsof the Hashin-Shtrikman bounding approach. However, the self-consistentestimates obtained for the random polycrystal model are nevertheless verygood estimators of the observed average behavior.
Date: July 8, 2006
Creator: Berryman, James G. & Grechka, Vladimir
Partner: UNT Libraries Government Documents Department

Erosion of a geopolymer.

Description: Solid-particle erosion studies were conducted on a representative geopolymer. The test conditions were normal impact of 390-{micro}m angular Al{sub 2}O{sub 3} erodent particles moving at 50, 70, or 100 m/s. Steady-state erosion rates were obtained and the material-loss mechanism was studied by scanning electron microscopy. The geopolymer responded as a classic brittle material. Elastic-plastic indentation events led to formation of brittle cleavage cracks that resulted in spallation of material. The erosion rate was proportional to erodent velocity to the 2.3 power. The erosion rate and mechanism for the geopolymer were nearly identical to what has been observed for erosion of Si single crystals.
Date: July 2, 2002
Creator: Goretta, K. C.; Chen, N.; Routbort, J. L.; Lukey, G. C. & van Deventer, J. S. J.
Partner: UNT Libraries Government Documents Department

De-alloying and stress corrosion cracking. Final report, July 1, 1990--June 30, 1993

Description: Results of work on fracture properties of porous dealloyed gold structures indicates that this material undergoes a brittle-ductile transition as the size scale of the porosity increases. Aspects of the work reported on and proposed address fundamental issues related corrosion in alloy systems. De-alloyed film induce brittle fracture experiments are being performed on Ag-Au and Cu-Au alloy thin sheets. An indirect potential drop technique is being developed to measure dynamic crack motion. Preliminary work is being performed to determine optimum conditions for film thickness-crack penetration experiments.
Date: April 1, 1996
Creator: Sieradzki, K.
Partner: UNT Libraries Government Documents Department

Reformulation of Elasticity Theory for Discontinuities and Long-Range Forces

Description: Some materials may naturally form discontinuities such as cracks as a result of deformation. As an aid to the modeling of such materials, a new framework for the basic equations of continuum mechanics, called the "peridynamic" formulation, is proposed. The propagation of linear stress waves in the new theory is discussed, and wave dispersion relations are derived. Material stability and its connection with wave propagation is investigated. It is demonstrated by an example that the reformulated approach permits the solution of fracture problems using the same equations either on or off the crack surface or crack tip. This is an advantage for modeling problems in which the location of a crack is not known in advance.
Date: October 1, 1998
Creator: Silling, S.A.
Partner: UNT Libraries Government Documents Department


Description: A simulated HRT fuel solution was unstable when passed through a heated Zircaloy-2 bypass section installed on a Ti loop. Losses of uranyl, cupric, and nickel sulfates were observed when the temperature of the solution emerging from the bypass was as low as 290 deg C. The 0.04 m UO/sub 2/SO/sub 4/ solutions containing Please delete abstract number 7731
Date: October 31, 1959
Creator: Griess, J.C.; Savage, H.C.; Greeley, R.S.; English, J.L.; Bolt, S.E.; Hess, D.N. et al.
Partner: UNT Libraries Government Documents Department

Stress intensity factor solutions for cracks in threaded fasteners

Description: Nondimensional stress intensity factor (K) solutions for continuous circumferential cracks in threaded fasteners were calculated using finite element methods that determined the energy release rate during virtual crack extension. Assumed loading conditions included both remote tension and nut loading, whereby the effects of applying the load to the thread flank were considered. In addition, K solutions were developed for axisymmetric surface cracks in notched and smooth round bars. Results showed that the stress concentration of a thread causes a considerable increase in K for shallow cracks, but has much less effect for longer cracks. In the latter case, values of K can be accurately estimated from K solutions for axisymmetric cracks in smooth round bars. Nut loading increased K by about 50% for shallow cracks, but this effect became negligible at crack depth-to-minor diameter ratios (a/d) greater than 0.2. An evaluation of thread root acuity effects showed that root radius has no effect on K when the crack depth exceeds 2% of the minor diameter. Closed-form K solutions were developed for both remote-loading and nut-loading conditions and for a wide range of thread root radii. The K solutions obtained in this study were compared with available literature solutions for threaded fasteners as well as notched and smooth round bars.
Date: February 1, 1999
Creator: Oster, D.M. & Mills, W.J.
Partner: UNT Libraries Government Documents Department

How Tough is Human Cortical Bone? In-Situ Measurements on Realistically Short Cracks

Description: Bone is more difficult to break than to split. Although this is well known, and many studies exist on the behavior of long cracks in bone, there is a need for data on the orientation-dependent crack-growth resistance behavior of human cortical bone which accurately assesses its toughness at appropriate size-scales. Here we use in-situ mechanical testing in the scanning electron microscope and x-ray computed tomography to examine how physiologically-pertinent short (<600 mu m) cracks propagate in both the transverse and longitudinal orientations in cortical bone, using both crack-deflection/twist mechanics and nonlinear-elastic fracture mechanics to determine crack-resistance curves. We find that after only 500 mu m of cracking, the driving force for crack propagation was more than five times higher in the transverse (breaking) direction than in the longitudinal (splitting) direction due to major crack deflections/twists principally at cement sheathes. Indeed, our results show that the true transverse toughness of cortical bone is far higher than previously reported. However, the toughness in the longitudinal orientation, where cracks tend to follow the cement lines, is quite low at these small crack sizes; it is only when cracks become several millimeters in length that bridging mechanisms can develop leading to the (larger-crack) toughnesses generally quoted for bone.
Date: May 10, 2008
Creator: Ritchie, Robert O.; Koester, K. J.; Ager III, J. W. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Behavior of structural clay tile infilled frames

Description: Steel frames infilled with structural clay tile have been used in commercial and industrial buildings for most of this century. Often these buildings are located in moderate to high seismic zones and are likely to experience earthquake forces. Little prior research has been conducted to investigate the behavior of clay tile infills under lateral loading. Twenty-one large-scale clay tile infilled frames were tested to determine their behavior and correlate the results with other available experimental data. The infills greatly increased the in-plane stiffness and strength of the otherwise flexible framing. Two in-plane failure mechanisms were observed, diagonal cracking and comer crushing. Under uniform out-of-plane load, the infills cracked along the mortar joints and developed membrane forces. Tremendous out-of-plane capacity was observed as the panels arched vertically and then horizontally, remaining stable after ultimate capacity was reached. Under sequential and combined bidirectional loadings, the panels remained stable with little interaction of the in-plane and out-of-plane behavior, particularly in the frame member forces. Analytical comparisons of measured versus predicted stiffness, ultimate capacity, and frame member forces were performed. A numerical model based on a piecewise linear equivalent strut was developed. Recommendations for evaluation of clay tile infills subjected to seismic loads were proposed.
Date: December 18, 1994
Creator: Flanagan, R.D.
Partner: UNT Libraries Government Documents Department

Techniques for in situ HVEM mechanical deformation of nanostructural materials

Description: We have developed two in-situ HVEM techniques which allow us to begin fundamental investigations into the mechanisms of deformation and fracture in nonstructured materials. A procedure for the observation of tensile deformation and failure in multilayers materials in cross-section is given and also the development of an in-situ HVEM nanoindentor of surfaces and films on surfaces in cross-section.
Date: August 7, 1995
Creator: Wall, M.A.; Barbee, T.W. Jr. & Dahmen, U.
Partner: UNT Libraries Government Documents Department

Dislocation emission from a three-dimensional crack -- A large-scale molecular dynamics study

Description: A series of massively parallel molecular dynamics simulations with up to 35 million atoms is performed to investigate dislocation emission from a three-dimensional crack. The authors observe dislocation loops emitted from the crack front--the first time this has been seen in computer simulations. The sequence of dislocation emission in the process of crack blunting process strongly depends on the crystallographic orientation of the crack front and differs strikingly from anything previously conjectured. This finding is essential to establish a precise dislocation emission criterion (i.e., intrinsic ductility criterion). They also find that boundary conditions and interatomic force laws have a significant effect on jogging or blunting dislocation emission modes.
Date: December 31, 1996
Creator: Zhou, S.J.; Beazley, D.M.; Lomdahl, P.S.; Voter, A.F. & Holian, B.L.
Partner: UNT Libraries Government Documents Department

R-curve behavior of whisker-reinforced ceramic composites

Description: Ceramics have attracted great interest for advanced heat engines and industrial applications as a result of their stiffness, light weight, corrosion resistance, and superior performance at high temperatures. However, their utilization in engineering applications is severely limited by their brittleness. One approach toward substantially reducing this brittleness is by incorporating strong whiskers into the ceramic matrix. Studies of whisker-reinforced ceramic composites have shown both rising fracture resistance with crack extension (i.e. R-curve behavior) and a three- to four-fold increase in steady-state toughness. The primary toughening mechanism is bridging of the crack surface by whiskers. This paper discusses the bridging mechanism in detail and presents R-curves for SiC whisker-reinforced Al{sub 2}O{sub 3} composites.
Date: June 1, 1995
Creator: Hsueh, Chun-Hway & Becher, P.F.
Partner: UNT Libraries Government Documents Department

Acceptance criteria for ultrasonic flaw indications in the inner liner of double-shell waste storage tanks

Description: Radioactive defense waste, resulting from the chemical processing of spent nuclear fuel, has been stored in double-shell tanks (DSTS) at the Hanford Site since 1970. As part of the program to assure that the DSTs maintain their structural integrity, an inspection plan is being developed and implemented. This report provides recommendations and technical bases for acceptance criteria for flaw indications detected during ultrasonic inspection of inner liners of the DSTS. The types of indications addressed are crack-like flaws, wall thinning, and pitting. In establishing acceptable flaw sizes, the evaluations have taken into consideration the potential for crack growth by the mechanism of stress corrosion cracking. Consideration was given to technical approaches used in ASME Codes, for reactor tanks at the Department of Energy Savannah River facilities, and in recommendations by the Tank Structural Integrity Panel. The goal was to ensure that indications discovered during inspections are not large enough to ever cause a leak or rupture of the tank inner liner. The acceptance criteria are intended to be simple to apply using a set of tables giving acceptable flaw sizes. These tables are sufficiently conservative to be applicable to all double-shell tanks. In those cases that a flaw exceeds the size permitted by the tables, it is proposed that additional criteria permit more detailed and less conservative evaluations to address specific conditions of stress levels, operating temperature, flaw location, and material properties.
Date: July 1, 1995
Creator: Simonen, F.A.; Graves, R.E. & Johnson, K.I.
Partner: UNT Libraries Government Documents Department

Fatigue crack nucleation in metallic materials

Description: The process of fatigue crack nucleation in metallic materials is reviewed placing emphasis in results derived for pure FCC metals with wavy slip behavior. The relationship between Persistent Slip Bands (PSB`s) and crack initiation will be examined for both single crystals and polycrystals, including the conditions for inter- and transgranular crack nucleation and their connection to type of loading, crystallography and slip geometry. The latter has been found to be an important parameter in the nucleation of intergranular cracks in polycrystals subjected to high strain fatigue, whereby primary slip bands with long slip lengths impinging on a grain boundary produce intergranular crack nucleation under the right conditions. Recent results related to intergranular crack nucleation in copper bicrystals and crack nucleation in Cu/Sapphire interfaces indicate that this mechanism controls crack nucleation in those simpler systems as well. Furthermore, it is found that under multiple slip conditions the crack nucleation location is controlled by the presence of local single slip conditions and long slip lengths for a particular Burgers vector that does not have to be in the primary slip system.
Date: April 1, 1999
Creator: Peralta, P.; Laird, C.; Ramamurty, U.; Suresh, S.; Campbell, G.H.; King, W.E. et al.
Partner: UNT Libraries Government Documents Department

Linear and nonlinear methods for detecting cracks in beams

Description: This paper presents experimental results from the vibration of a polycarbonate beam containing a crack that opens and closes during vibration. Several techniques were employed to detect and locate the crack making use of the nonlinearity. ``Harmonic mode shapes`` proved to be more sensitive to damage than conventional mode shapes. Instantaneous frequency and time-frequency methods also showed clear signatures for the crack. The results indicate that nonlinearities may provide increased capabilities for structural damage detection and location.
Date: December 31, 1995
Creator: Prime, M.B. & Shevitz, D.W.
Partner: UNT Libraries Government Documents Department

Compressive strength, plastic flow properties, and surface frictional effects of 1100, 3003 and 6061 aluminum alloys

Description: The purpose of this study is to find aluminum alloys that are effective for use as wire vacuum seals in the 800MeV particle accelerator located at the Louis Anderson Meson Physics Facility (LAMPF) in Los Alamos, NM. Three alloys, Al 1100, Al 3003, and Al 6061, are investigated under uniaxial compression to determine stresses for a given height reduction from 0 to 70 percent, and to find plastic flow and surface interaction effects. Right-circular cylindrical specimens are compressed on-end (cylindrically) and radially (for modeling as compressed wire). Aluminum 1100 and 3003 alloys are compared for length to diameter ratios of 1 and 2 for both compression types, and are then compared to results of radial compression of annealed small diameter Al 1100 wire currently used at LAMPE. The specimens are also compressed between three different platen surfaces, polished steel, etched steel, and aluminum 6061-T6, to determine effects of friction. The Al 3003 alloy exhibits 20 to 25% lower stresses at all height reductions than Al 1100 for both cylindrical and radial compression.
Date: December 31, 1993
Creator: Pinkerton, G.W.
Partner: UNT Libraries Government Documents Department

Stress measurement with non-indentation

Description: The feasibility of using nano-indentation to measure residual stress in glasses was studied. Indents were placed on the side of flexure specimens at four different distances from the neutral axis while the specimens were under load in four-point-bending. Three different glasses (soda-lime, boro-silicate, and fused silica) were indented with a cube-corner indenter using 2 to 30 mN indentation loads. A high resolution scanning electron microscope was used to measure the length of the cracks emanating from the corners of the indents while the specimen remained under load. The measured crack lengths were correlated to the local stress using indentation theory. For the correlation, elastic beam theory was used to calculate the magnitude of the local stress at the indentation sites. Results derived from crack lengths were in good agreement with local stress within experimental scatter. However, this scatter was found to be rather large as a result of the stochastic nature of crack formation. It can be concluded from this study that nano-indentation can be used to measure residual surface stresses with high spatial resolution provided that a sufficient number of indents are used to assure good statistical accuracy.
Date: November 1998
Creator: Jakus, K.; Evans, N.D. & Hay, J.C.
Partner: UNT Libraries Government Documents Department

Modeling complex phenomena: Multiple length and time scales in extended dynamical systems

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). Using nonlinear techniques and large-scale simulations, we have systematically studied meso-scale pattern formation and dynamics in nonlinear, nonequilibrium systems exhibiting topological excitations (dislocations, vortices, vortex lines, domain walls); dislocation generation from crack fronts in ductile materials; the smoothing of rough surfaces in solid-on-solid models; ordering and melting of moving flux lattices in three-dimensional Josephson junction arrays with external magnetic field, current, and disorder; filamentary and plastic vortex flow in disordered thin films superconductors; magnetic vortices in Heisenberg spin layers; and hierarchical twinning and tweed texture in elastic models.
Date: December 31, 1998
Creator: Lomdahl, P.; Bishop, A. & Jensen, N.G.
Partner: UNT Libraries Government Documents Department

Crack detection on HC-130H aircraft using low frequency eddy current

Description: An eddy current inspection method was developed at the Federal Aviation Administration`s Airworthiness Assurance NDI Validation Center (AANC) to easily and rapidly detect subsurface fatigue cracks in the wheel well fairing on the US Coast Guard (USCG) HC-130H aircraft caused by fatigue. The inspection procedure locates cracks as small as 10.2 millimeters in length at 2.54 mm below the skin surface at raised fastener sites. The test procedure developed baseline three USCG aircraft. Inspection results on the three aircraft reveals good correlation with results made during subsequent structural disassembly.
Date: February 1, 1998
Creator: Moore, D.G.; Mihelic, J.E. & Barnes, J.D.
Partner: UNT Libraries Government Documents Department