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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

Effect of Crystal Orientation on Fatigue-Crack Initiation in Polycrystalline Aluminum Alloys

Description: Report presenting testing demonstrating that fatigue cracks initiate in preexisting slip bands on planes parallel to planes from tests on large-grained specimens of 1100 and 5052 aluminum alloys. The resolved shear stress on the planes in crystal where fatigue cracks had developed were compared to uncracked crystals. Results regarding the location fatigue cracks, number of stress cycles observed, and differences of the crystals observed in the two types of aluminum alloys are provided.
Date: August 1957
Creator: Weinberg, J. G. & Bennett, J. A.
Partner: UNT Libraries Government Documents Department

The Cause of Welding Cracks in Aircraft Steels

Description: The discussion in this article refers to gas welding of thin-walled parts of up to about 3 mm thickness. It was proven that by restricting the sulphur, carbon, and phosphorous content, and by electric-furnace production of the steel, it was possible in a short time to remove this defect. Weld hardness - i.e., martensite formation and hardness of the overheated zone - has no connection with the tendency to weld-crack development. Si, Cr, Mo, or V content has no appreciable effect, while increased manganese content tends to reduce the crack susceptibility.
Date: October 1940
Creator: Müller, J.
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

QUARTERLY REPORT OF THE SOLUTION MATERIALS SECTION FOR THE PERIOD ENDING OCTOBER 31, 1958

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

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

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

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

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

"SHARP"--A ONE-DIMENSIONAL HYDRODYNAMICS CODE FOR THE IBM 704

Description: The SHARP code designed to handle calculations of hydrodynamic motion in one-dimensional form, including provision for the disruption of the sample by spallation, is described. The code was prepared by means of the IBM logical coding scheme, FORTRAN. Generation of SHARP requires a set of initial conditions which define an established pressure pulse in the front portion of the sample. The code was developed to investigate the motion of the pressure pulse and the corresponding effects. The input for the code consists of the location and velocity of zone boundaries, values of the internal energy and relative volumes of active zones, the density of the material, and the constants required to establish the equation of state. (M.C.G.)
Date: April 1, 1958
Creator: Nance, O.; Wilkins, M.; Blandford, I. & Smith, H.
Partner: UNT Libraries Government Documents Department

Shock induced multi-mode damage in depleted uranium

Description: Recent dynamic damage studies on depleted uranium samples have revealed mixed mode failure mechanisms leading to incipient cracking as well as ductile failure processes. Results show that delamination of inclusions upon compression may provide nucleation sites for damage initiation in the form of crack tip production. However, under tension the material propagates cracks in a mixed shear localization and mode-I ductile tearing and cracking. Cracks tips appear to link up through regions of severe, shear dominated plastic flow. Shock recovery experiments were conducted on a 50 mm single stage light gas gun. Serial metallographic sectioning was conducted on the recovered samples to characterize the bulk response of the sample. Experiments show delaminated inclusions due to uniaxial compression without damage propagation. Further results show the propagation of the damage through tensile loading to the incipient state, illustrating ductile processes coupled with mixed mode-I tensile ductile tearing, shear localization, and mode-I cracking in depleted uranium.
Date: January 1, 2009
Creator: Koller, Darcie D; Cerreta, Ellen K & Gray, Ill, George T
Partner: UNT Libraries Government Documents Department

Predicting runaway reaction in a solid explosive containing a single crack

Description: Mechanically damaged high explosive (HE) undergoing defiagration has recently been shown capable of generating combustion pressures and flame speeds dramatically in excess of those observed in undamaged HE. Flame penetration of HE cracks large enough to support the reaction zone serves to increase the burning surface area and the rate of gas production. Cracks confine the product gas, elevating the local pressure and reducing the reaction zone thickness such that the flame can enter smaller-width cracks. As the reaction zone decreases sufficiently to enter the smallest cracks, the flame surface area will grow appreciably, rapidly pressurizing the cracks. This runaway of pressure and burning area, termed combustion bootstrapping, can dramatically accelerate the combustion mode and in the most extreme cases may result in deflagration-to-detonation transition [3, 4]. The current study is intended to help predict the conditions required for the onset of reaction runaway in a narrow slot in HE. We review experiments [5] where flames were observed to propagate though a narrow slot (intended to simulate a well-formed crack) in high explosive at velocities up to 10 km/s, reaching pressures in excess of 1 kbar. Pressurization of the slot due to gas-dynamic choking is then used to predict the onset of runaway reaction. This model agrees with experimental pressure measurements of observed reaction runaway in slots.
Date: January 1, 2009
Creator: Jackson, Scott I & Hill, Larry G
Partner: UNT Libraries Government Documents Department

EXAMINATION OF CORROSION SPECIMENS FROM SLURRY BLANKET MOCKUP RUN SM-4

Description: Generally low attack rates were displayed by specimens of type 347 stainless steel, titanium-75A, and Zircaloy-2 which were exposed for 2396 hr in slurry blanket mock-up Run SM-4. The leading coupon of the array, of type 347 stainless steel, suffered more severe attack are to erosive action by the slurry. Coupons of SA-212-B carbon steel were consumed during the run. No stress corrosion cracking was noted on samples, placed in the pressurizer, of types 347 and AM 350 stainless steels, titanium-6A14V alloy, or Zircaloy-2. (auth)
Date: June 1, 1958
Creator: Reed, S.A. & Compere, E.L.
Partner: UNT Libraries Government Documents Department

Predicting fracture in micron-scale polycrystalline silicon MEMS structures.

Description: Designing reliable MEMS structures presents numerous challenges. Polycrystalline silicon fractures in a brittle manner with considerable variability in measured strength. Furthermore, it is not clear how to use a measured tensile strength distribution to predict the strength of a complex MEMS structure. To address such issues, two recently developed high throughput MEMS tensile test techniques have been used to measure strength distribution tails. The measured tensile strength distributions enable the definition of a threshold strength as well as an inferred maximum flaw size. The nature of strength-controlling flaws has been identified and sources of the observed variation in strength investigated. A double edge-notched specimen geometry was also tested to study the effect of a severe, micron-scale stress concentration on the measured strength distribution. Strength-based, Weibull-based, and fracture mechanics-based failure analyses were performed and compared with the experimental results.
Date: September 1, 2010
Creator: Hazra, Siddharth S. (Carnegie Mellon University, Pittsburgh, PA); de Boer, Maarten Pieter (Carnegie Mellon University, Pittsburgh, PA); Boyce, Brad Lee; Ohlhausen, James Anthony; Foulk, James W., III & Reedy, Earl David, Jr.
Partner: UNT Libraries Government Documents Department