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New perspectives on the fracture of Nicalon fibers

Description: Experimental studies have been conducted to examine the strength and fracture behavior of monofiliment Nicalon{sup 3} SiC fibers ranging in diameter from 8 to 22 {mu}m. The effects of varying fiber diameter, flaw location and flaw population on the tensile behavior of individual fibers were investigated using fractography. Results indicate that variations in fiber diameter influence the apparent fiber fracture toughness K{sub 1c}, with higher K{sub 1c} values observed for decreasing fiber diameters. Observations also suggest that the location of the critical flaw may play a role in the fracture of Nicalon fibers. In addition to surface flaws, three distinct internal flaw populations are seen to cause fracture in Nicalon fibers.
Date: November 1, 1997
Creator: Taylor, S.T.; Lowe, T.C. & Butt, D.P.
Partner: UNT Libraries Government Documents Department

Metallographic and Fractographic Observations of Posttest Creep-Fatigue Specimens of Weld-Deposited Type 308 CRE Stainless Steel

Description: Type 308 CRE stainless steel weld specimens were subjected to metallographic and fractographic analysis after failure in elevated temperature (593 degrees C) creep-fatigue tests. The failure mode for specimens tested under continuous-cycle fatigue conditions were predominantly transgranular. When the test cycle was modified to include a hold time at the maximum tensile strain, the failure mode became predominantly inter-phase. Sigma phase was observed within the delta-ferrite regions of the weld. However, the presence of sigma phase did not appear to affect the failure mode.
Date: August 1978
Creator: Williams, M. W.
Partner: UNT Libraries Government Documents Department

On the in vitro fracture toughness of human dentin

Description: The in vitro fracture toughness of human dention has been reported to be of the order of 3 MPa sqrt m. This result, however is based on a single study for a single orientation, and furthermore involves notched, rather than fatigue precracked, test samples.
Date: February 5, 2002
Creator: Imbeni, V.; Nalla, R.K.; Bosi, C.; Kinney, J.H. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Mechanical Behavior and Fractography of 304 Stainless Steel with High Hydrogen Concentration

Description: Hydrogen embrittlement of 304 stainless steel with different hydrogen concentrations has been investigated. An electrochemical technique was used to effectively charge the high level of hydrogen into 304 stainless steel in a short period of time. At 25 ppm of hydrogen, 304 stainless steel loses 10 percent of its original mechanical strength and 20 percent plasticity. Although the ductile feature dominates the fractography, the brittle crown area near the outer surface shows the intergranular rupture effected by hydrogen. At 60 ppm of hydrogen, 304 stainless steel loses 23 percent of its strength and 38 percent plasticity, where the brittle mode dominates the fracture of the materials. Experimental results show that hydrogen damage to the performance of 304 stainless steel is significant even at very low levels. The fractograph analysis indicates the high penetration ability of hydrogen in 304 stainless steel. This work also demonstrates the advantages of the electrochemical charging technique in the study of hydrogen embrittlement.
Date: February 5, 2003
Creator: Au, M.
Partner: UNT Libraries Government Documents Department

Effect of Fe- and Si-induced flaws on fracture of Si{sub 3}N{sub 4}

Description: Fracture studies were performed to detect and assess the effect of flaws on the fracture behavior of hot-pressed Si{sub 3}N{sub 4} with Fe or Si inclusions. The addition of 5 and 0.5 wt.% Fe inclusions of 88--250 {micro}m size reduced the strength of Si{sub 3}N{sub 4} specimens by {approx} 40 and 15%, respectively. Similarly, addition of 1 and 0.5 wt.% Si inclusions of < 149 {micro}m size reduced the strength of Si{sub 3}N{sub 4} specimens by {approx} 50 and 39%, respectively. Fractography indicated that failure occurred primarily from internal flaws which included Fe- and Si-rich inclusions and/or regions of Si{sub 3}N{sub 4} matrix that were degraded as a result of reaction between Si{sub 3}N{sub 4} and molten Fe or Si. For inclusion-induced internal flaws, the critical flaw sizes calculated by fracture mechanics were always larger than the fractographically measured flaw sizes. This observation suggested local degradation in fracture toughness of the Si{sub 3}N{sub 4} matrix. A ratio, K, of {approx} 3.5--4.2 appeared to exist between the calculated and measured values of the critical internal flaw sizes of specimens that contained Fe inclusions. A similar ratio of 1.7--3.1 was observed for specimens that contained Si inclusions. The ratio K has important implications for strength predictions that are based on observed flaw size.
Date: October 1, 1997
Creator: Singh, J.P.
Partner: UNT Libraries Government Documents Department

Effect of heat treatment and heat-to-heat variations in the fatigue-crack growth response of Alloy 718. Part 2. Microscopic observation

Description: The microstructural aspects that influenced the room temperature and elevated temperature fatigue-crack propagation response of annealed, conventional, and modified heat-treated Alloy 718 were studied. Electron fractographic examination of Alloy 718 fatigue fracture surfaces revealed that operative crack growth mechanisms were dependent on heat treatment, heat-to-heat variations, temperature, and prevailing crack tip stress intensity level. In the low temperature regime (below 538{sup 0}C), all fracture surfaces exhibited a faceted appearance at low {Delta} levels, which is indicative of crystallographic fracture along intense inhomogeneous slip bands. The facets in the modified Alloy 718, however, were found to be rather poorly defined since the modified heat treatment tends to promote more homogeneous slip processes. Under progressively higher stress intensity levels, the room temperature and elevated temperature fatigue fracture surfaces exhibited striations, followed by a combination of striations and dimple rupture at the highest {Delta} values. Striation spacing measurements in all three heat-treated conditions were generally found to be in agreement with macroscopic growth rates at 24 and 538{sup 0}C. Under high temperature conditions (above 538{sup 0}C), evidence of intergranular fracture was also detected on the fatigue fracture surfaces, particularly at low stress intensity levels. This intergranular failure mechanism was found to be more extensive in the modified heat-treated Alloy 718. 17 figures.
Date: April 1, 1980
Creator: Mills, W.J. & James, L.A.
Partner: UNT Libraries Government Documents Department

Room and elevated temperature mechanical properties of PM TiAl alloy Ti-47Al-2Cr-2Nb

Description: A TiAl alloy powder with the composition Ti-47Al-2Cr-2Nb (at. %) was prepared by rotary atomization, followed by hot-extrusion and subsequent heat treatments to produce refined lamellar structures and fine duplex structures. The mechanical properties of the TiM alloy were determined at temperatures to 1000C in air, and the microstructures were characterized by TEM, SEM, and electron microprobe analyses. The alloy with the refined lamellar structure showed excellent mechanical properties at both room and elevated temperatures. It exhibited a plastic strain of 1.4% and a yield strength of 971 MPa (140.9 ksi) at room temperature. The yield strength remained approximately constant up to 800C and decreased to 577 MPa (83.7 ksi) at 1000C. The transverse fracture toughness, estimated by three-point bend testing of chevron-notched specimens at room temperature, was 22.4 MPa {radical}m. The refined lamellar structure contained long and straight alternating {alpha}{sub 2} and {gamma} platelets with an extremely fine interlamellar spacing (0.1 {mu}m) and {alpha}{sub 2}-to-{alpha}{sub 2} spacing (0.22 {mu}m). The mechanical properties of the alloy have been correlated with the unique microstructures developed by hot extrusion.
Date: July 1, 1995
Creator: Liu, C.T.; Maziasz, P.J.; Schneibel, J.H.; Sikka, V.K.; Wright, J.; Walker, L.R. et al.
Partner: UNT Libraries Government Documents Department

On the development of life prediction methodologies for the failure of human teeth

Description: Human dentin is known to be susceptible to failure under cyclic loading. Surprisingly, there are few reports that quantify the effect of such loading, considering the fact that a typical tooth experiences a million or so loading cycles annually. In the present study, a systematic investigation is described of the effects of prolonged cyclic loading on human dentin in a simulated physiological environment. In vitro stress-life (S/N) data are discussed in the context of possible mechanisms of fatigue damage and failure.
Date: September 18, 2002
Creator: Nalla, R.K.; Imbeni, V.; Kinney, J.H.; Marshall, S.J. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Optical and analytical electron microscopy of ductility-dip cracking in Ni-base filler metal 52 -- Initial studies

Description: Microcharacterization studies were performed on weld-metal microstructures of a Ni-base filler metal. Specimens were taken from the fusion zone and the weld-metal heat-affected zone of transverse- and spot-Varestraint welds. The filler metal was first deposited onto a steel substrate by hot-wire, gas tungsten arc welding before specimen removal. Optical microscopy indicates the crack morphology is intergranular and is along high-angle, migrated grain boundaries. At low magnifications, scanning electron microscopy reveals a relatively smooth fracture surface. However, at higher magnifications the grain faces exhibit microductility. Analytical electron microscopy reveals high-angle, migrated grain boundaries decorated with MC (Ti, Cr) and M{sub 23}C{sub 6} (Cr, Ni, Fe) precipitates ranging from 10 to 200 n. Auger electron spectroscopy of pre-strained Gleeble specimens fractured in situ revealed internal ductility-dip cracks decorated with magnesium aluminate (MgAl{sub 2}O{sub 4}) spinel particles (1,000 nm).
Date: January 1, 1998
Creator: Cola, M.J. & Teter, D.F.
Partner: UNT Libraries Government Documents Department

Effect of sample test volume and geometry on the tensile mechanical behavior of SiC/SiC continuous fiber ceramic composites. Final report

Description: The development of a silicon carbide-type fiber from an organometallic precursor has led to a major resurgence of interest in fiber-reinforced ceramic matrix composites. By combining this high strength fiber with a variety of ceramic matrices it has been possible to achieve tough composites offering significant potential advantages over monolithic ceramics and carbon-carbon for high temperature applications. A continuous-fiber ceramic matrix composite (CFCC) typical of materials proposed for such industrial applications as power generation, heat recovery and chemical production as well as biomedical and environmental applications was tested in uniaxial tension using a universal test machine. Test parameters investigated included: test mode (load versus displacement), test rate (0.003 mm/s, 0.03 mm/s, 50 N/s and 500 N/s), specimen geometry (straight-sided versus reduced-gauge section) and type of specimen volume (long/thin versus short/fat). Typical properties include an average elastic modulus 130 {+-} 10 Gpa, an average proportional limit stress of 45 {+-} 20 Mpa, an average ultimate tensile strength of 180 {+-} 20 MPa and an average modulus of toughness of 8.4 {+-} 2 (x10{sup 5})J/m{sup 3}.
Date: September 1, 1998
Creator: Sankar, J.; Kelkar, A.D. & Neogi, J.
Partner: UNT Libraries Government Documents Department

The strain-rate sensitivity of high-strength high-toughness steels.

Description: The present study examines the strain-rate sensitivity of four high strength, high-toughness alloys at strain rates ranging from 0.0002 s-1 to 200 s-1: Aermet 100, a modified 4340, modified HP9-4-20, and a recently developed Eglin AFB steel alloy, ES-1c. A refined dynamic servohydraulic method was used to perform tensile tests over this entire range. Each of these alloys exhibit only modest strain-rate sensitivity. Specifically, the strain-rate sensitivity exponent m, is found to be in the range of 0.004-0.007 depending on the alloy. This corresponds to a {approx}10% increase in the yield strength over the 7-orders of magnitude change in strain-rate. Interestingly, while three of the alloys showed a concominant {approx}3-10% drop in their ductility with increasing strain-rate, the ES1-c alloy actually exhibited a 25% increase in ductility with increasing strain-rate. Fractography suggests the possibility that at higher strain-rates ES-1c evolves towards a more ductile dimple fracture mode associated with microvoid coalescence.
Date: January 1, 2006
Creator: Dilmore, M.F. (AFRL/MNMW, Eglin AFB, FL); Crenshaw, Thomas B. & Boyce, Brad Lee
Partner: UNT Libraries Government Documents Department

Fatigue Crack Propagation from Notched Specimens of 304 SS in elevated Temperature Aqueous Environment

Description: Fatigue crack propagation (FCP) rates for 304 stainless steel (304SS) were determined in 24 degree C and 288 degree C air and 288 degree C water using double-edged notch (DEN) specimens of 304 stainless steel (304 SS). Test performed at matched loading conditions in air and water at 288 degree C with 20-6- cc h[sub]2/kg h[sub]2O provided a direct comparison of the relative crack growth rates in air and water over a wide range of crack growth rates. The DEN crack extension ranged from short cracks (0.03-0.25 mm) to long cracks up to 4.06 mm, which are consistent with conventional deep crack tests. Crack growth rates of 304 SS in water were about 12 times the air rate. This 12X environmental enhancement persisted to crack extensions up to 4.06 mm, far outside the range associated with short crack effects. The large environmental degradation for 304 SS crack growth is consistent with the strong reduction of fatigue life in high hydrogen water. Further, very similar environmental effects w ere reported in fatigue crack growth tests in hydrogen water chemistry (HWC). Most literature data in high hydrogen water show only a mild environmental effect for 304 SS, of order 2.5 times air or less, but the tests were predominantly performed at high cyclic stress intensity or equivalently, high air rates. The environmental effect in low oxygen environments at low stress intensity depends strongly on both the stress ratio, R, and the load rise time, T[sub]r, as recently reported for austenitic stainless steel in BWR water. Fractography was performed for both tests in air and water. At 288 degree C in water, the fracture surfaces were crisply faceted with a crystallographic appearance, and showed striations under high magnification. The cleavage-like facets on the fracture surfaces suggest that hydrogen embrittlement is the primary cause ...
Date: August 1, 2002
Creator: Wire, G. L. & Mills, W. J.
Partner: UNT Libraries Government Documents Department

Development of ASTM standards in support of advanced ceramics -- continuing efforts

Description: An update is presented of the activities of the American Society for Testing and Materials (ASTM) Committee C-28 on Advanced Ceramics. Since its inception in 1986, this committee, which has five standard producing subcommittees, has written and published over 32 consensus standards. These standards are concerned with mechanical testing of monolithic and composite ceramics, nondestructive examination, statistical analysis and design, powder characterization, quantitative microscopy, fractography, and terminology. These standards ensure optimum material behavior with physical and mechanical property reproducibility, component reliability, and well-defined methods of data treatment and material analysis for both monolithic and composite materials. Committee C-28 continues to sponsor technical symposia and to cooperate in the development of international standards. An update of recent and current activities as well as possible new areas of standardization work will be presented.
Date: February 1, 1998
Creator: Brinkman, C.R.
Partner: UNT Libraries Government Documents Department

Mechanical properties of oxide dispersion strengthened (ODS) molybdenum alloys

Description: Oxide dispersion strengthened molybdenum, Mo-ODS, developed by a proprietary powder metallurgy process, exhibits a creep rupture life at 0.65T{sub m} (1,600 C) of three to five orders of magnitude greater than unalloyed molybdenum, while maintaining ductile fracture behavior at temperatures significantly below room temperature. In comparison, the creep rupture life of the Mo-50Re solid solution strengthened alloy at 1,600 C is only an order of magnitude greater than unalloyed molybdenum. The results of microstructural characterization and thermal stability and mechanical property testing are discussed.
Date: March 1, 1998
Creator: Bianco, R. & Buckman, R.W. Jr.
Partner: UNT Libraries Government Documents Department

Intergranular Crack Propagation Rates in Sensitized Type 304 Stainless Steel in an Oxygenated Water Environment

Description: Intergranular stress-corrosion crack (IGSCC) propagation rates were measured in three heats of sensitized Type 304 stainless steel (SS) as a function of sensitization in an environment of high-purity water with 8 ppm oxygen, using a fracture mechanics approach. Specimens were sensitized using controlled furnace heat treatments and the degree of sensitization was measured by the electrochemical potentiokinetic reactivation (EPR) method. Active loading tests were performed on standard specimens over a range of intensities. Crack lengths were determined by compilance measurements using in-situ high-temperature clip gage or LVDT methods, optical metallography on the side faces of the specimen, and fractography of the cracked surface after completion of the tests. The optical metallography measurements did not provide useful estimates of crack lengths, because large variations in IGSCC propagation across the thickness of the specimens occurred. The effects of the degree of sensitization on the IGSCC propagation rate are obscured by the data scatter. However, it seems clear that these variables do not lead to order-of-magnitude changes in the crack propagation rate.
Date: December 1, 1983
Creator: Park, J. Y. & Shack, W. J.
Partner: UNT Libraries Government Documents Department

The effect of low dose rate irradiation on the tensile properties and microstructure of austenitic stainless steel.

Description: To assess the effects of long-term, low-dose-rate neutron exposure on mechanical strength and ductility, tensile properties were measured on 12% and 20% cold-worked Type 316 stainless steel. Samples were prepared from reactor core components retrieved from the EBR-II reactor following final shutdown. Sample locations were chosen to cover a dose range of 1-56 dpa at temperatures from 371-440 C and dose rates from 0.5-5.8 x10{sup -7} dpa/s. These dose rates are approximately an order of magnitude lower than those of typical EBR-II test sample locations. The tensile tests for the 12% CW material were performed at 380 C and 430 C while those for the 20% CW samples were performed at 370 C. In each case, the tensile test temperature approximately matched the irradiation temperature. To help understand the tensile properties, microstructural samples with similar irradiation history were also examined. The strength and loss of work hardening increase the fastest as a function of irradiation dose for the 12% CW material irradiated at lower temperature. The decrease in ductility with increasing dose occurs more rapidly for the 12% CW material irradiated at lower temperature and the 20% cold-worked material. Post-tensile test fractography indicates that at higher dose, the 20% CW samples begin a shift in fracture mode from purely ductile to mainly small facets and slip bands, suggesting a transition toward channel fracture. The fracture for all of the 12% cold-worked samples was ductile. For both the 12% and 20% CW materials, the yield strength increases correlate with changes in void and loop density and size.
Date: September 17, 2002
Creator: Allen, T. R.; Tsai, H.; Cole, J. I.; Yoshitake, T.; Akasaka, N.; Donomae, T. et al.
Partner: UNT Libraries Government Documents Department

Creep rupture testing of alloy 617 and A508/533 base metals and weldments.

Description: The NGNP, which is an advanced HTGR concept with emphasis on both electricity and hydrogen production, involves helium as the coolant and a closed-cycle gas turbine for power generation with a core outlet/gas turbine inlet temperature of 750-1000 C. Alloy 617 is a prime candidate for VHTR structural components such as reactor internals, piping, and heat exchangers in view of its resistance to oxidation and elevated temperature strength. However, lack of adequate data on the performance of the alloy in welded condition prompted to initiate a creep test program at Argonne National Laboratory. In addition, Testing has been initiated to evaluate the creep rupture properties of the pressure vessel steel A508/533 in air and in helium environments. The program, which began in December 2009, was certified for quality assurance NQA-1 requirements during January and February 2010. Specimens were designed and fabricated during March and the tests were initiated in April 2010. During the past year, several creep tests were conducted in air on Alloy 617 base metal and weldment specimens at temperatures of 750, 850, and 950 C. Idaho National Laboratory, using gas tungsten arc welding method with Alloy 617 weld wire, fabricated the weldment specimens. Eight tests were conducted on Alloy 617 base metal specimens and nine were on Alloy 617 weldments. The creep rupture times for the base alloy and weldment tests were up to {approx}3900 and {approx}4500 h, respectively. The results showed that the creep rupture lives of weld specimens are much longer than those for the base alloy, when tested under identical test conditions. The test results also showed that the creep strain at fracture is in the range of 7-18% for weldment samples and were much lower than those for the base alloy, under similar test conditions. In general, the weldment specimens showed more of ...
Date: January 17, 2012
Creator: Natesan, K.; Li, M.; Soppet, W.K. & Rink, D.L. (Nuclear Engineering Division)
Partner: UNT Libraries Government Documents Department

AEM investigations of primary water SCC in nickel alloys

Description: The microstructure of nickel alloys, particularly the grain boundary composition and intergranular precipitates, plays an important role in high temperature primary water stress corrosion cracking (SCC) performance. Analytical electron microscopy (AEM) was used to examine SCC cracks in Alloys 600 and X-750 to investigate the role of grain boundary precipitates, dislocations and oxides in primary water SCC (PWSCC). Analysis of oxides by AEM and ESCA/Auger indicates that the crack tip oxides are different than the oxides formed on the outer surfaces. Comparison of heats with good and poor SCC resistance has identified metallurgical features that affect cracking. These AEM results show that the mechanism of PWSCC in nickel-base alloys does not involve void formation or blunting of the crack tip near intergranular carbides. The role of grain boundary composition, the interaction of cracks with carbides and other intergranular precipitates, and observations from AEM examinations ahead of the crack tip are discussed in relation to the mechanism of SCC.
Date: August 1, 1997
Creator: Fish, J. S.; Perry, D. J.; Lewis, N.; Thompson, C. D. & Yang, W. J. S.
Partner: UNT Libraries Government Documents Department

Biaxial loading and shallow-flaw effects on crack-tip constraint and fracture toughness

Description: A program to develop and evaluate fracture methodologies for the assessment of crack-tip constraint effects on fracture toughness of reactor pressure vessel (RPV) steels has been initiated in the Heavy-Section Steel Technology (HSST) Program. Crack-tip constraint is an issue that significantly impacts fracture mechanics technologies employed in safety assessment procedures for commercially licensed nuclear RPVs. The focus of studies described herein is on the evaluation of two stressed-based methodologies for quantifying crack-tip constraint (i.e., J-Q theory and a micromechanical scaling model based on critical stressed volumes) through applications to experimental and fractographic data. Data were utilized from single-edge notch bend (SENB) specimens and HSST-developed cruciform beam specimens that were tested in HSST shallow-crack and biaxial testing programs. Results from applications indicate that both the J-Q methodology and the micromechanical scaling model can be used successfully to interpret experimental data from the shallow- and deep-crack SENB specimen tests. When applied to the uniaxially and biaxially loaded cruciform specimens, the two methodologies showed some promising features, but also raised several questions concerning the interpretation of constraint conditions in the specimen based on near-tip stress fields. Fractographic data taken from the fracture surfaces of the SENB and cruciform specimens are used to assess the relevance of stress-based fracture characterizations to conditions at cleavage initiation sites. Unresolved issues identified from these analyses require resolution as part of a validation process for biaxial loading applications. This report is designated as HSST Report No. 142.
Date: January 1, 1994
Creator: Bass, B.R.; Bryson, J.W.; Theiss, T.J. & Rao, M.C.
Partner: UNT Libraries Government Documents Department