The Experts below are selected from a list of 4551 Experts worldwide ranked by ideXlab platform
Jonathan A. Salem - One of the best experts on this subject based on the ideXlab platform.
-
The Effects of Salt Water on the Slow Crack Growth of Soda Lime Silicate Glass
2016Co-Authors: Bronson D. Hausmann, Jonathan A. SalemAbstract:The Slow Crack Growth parameters of soda-lime silicate were measured in distilled and salt water of various concentrations in order to determine if stress corrosion susceptibility is affected by the presence of salt and the contaminate formation of a weak sodium film. Past research indicates that solvents effect the rate of Crack Growth, however, the effects of salt have not been studied. The results indicate a small but statistically significant effect on the Slow Crack Growth parameters A and n. However, for typical engineering purposes, the effect can be ignored.
-
Slow Crack Growth and fracture toughness of sapphire for the international space station fluids and combustion facility
2013Co-Authors: Jonathan A. SalemAbstract:The fracture toughness, inert flexural strength, and Slow Crack Growth parameters of the r- and a-planes of sapphire grown by the Heat Exchange Method were measured to qualify sapphire for structural use in the International Space Station. The fracture toughness in dry nitrogen, K(sub Ipb), was 2.31 +/- 0.12 MPa(square root of)m and 2.47 +/- 0.15 MPa(squre root of)m for the a- and r-planes, respectively. Fracture toughness measured in water via the operational procedure in ASTM C1421 was significantly lower, K(sub Ivb) = 1.95+/- 0.03 MPa(square root of)m, 1.94 +/- 0.07 and 1.77 +/- 0.13 MPa(square root of)m for the a- , m- and r-planes, respectively. The mean inert flexural strength in dry nitrogen was 1085 +/- 127 MPa for the r-plane and 1255 +/- 547 MPa for the a-plane. The power law Slow Crack Growth exponent for testing in water was n = 21 +/- 4 for the r-plane and n (greater than or equal to) 31 for the a-plane. The power law Slow Crack Growth coefficient was A = 2.81 x 10(exp -14) m/s x (MPa(squre root of)m)/n for the r-plane and A (approx. equals)2.06 x 10(exp -15) m/s x (MPa(square root of)m)/n for the a-plane. The r- and a-planes of sapphire are relatively susceptible to stress corrosion induced Slow Crack Growth in water. However, failure occurs by competing modes of Slow Crack Growth at long failure times and twinning for short failure time and inert environments. Slow Crack Growth testing needs to be performed at low failure stress levels and long failure times so that twinning does not affect the results. Some difficulty was encountered in measuring the Slow Crack Growth parameters for the a-plane due to a short finish (i.e., insufficient material removal for elimination of the damage generated in the early grinding stages). A consistent preparation method that increases the Weibull modulus of sapphire test specimens and components is needed. This would impart higher component reliability, even if higher Weibull modulus is gained at the sacrifice of absolute strength of the component. The current specification frequently used for the preparation of sapphire test specimens and components (e.g., a "60/40" scratch-dig finish) is inadequate to avoid a short finish.
-
Estimation and Simulation of Slow Crack Growth Parameters from Constant Stress Rate Data
Fracture Mechanics of Ceramics, 2005Co-Authors: Jonathan A. Salem, Aaron S WeaverAbstract:Closed form, approximate functions for estimating the variances and degrees-of-freedom associated with the Slow Crack Growth parameters n,D,B, and A* as measured using constant stress rate (“dynamic fatigue”) testing were derived by using propagation of errors. Estimates made with the resulting functions and Slow Crack Growth data for a sapphire window were compared to the results of Monte Carlo simulations.
-
Test Standard Developed for Determining the Slow Crack Growth of Advanced Ceramics at Ambient Temperature
1998Co-Authors: Sung R. Choi, Jonathan A. SalemAbstract:The service life of structural ceramic components is often limited by the process of Slow Crack Growth. Therefore, it is important to develop an appropriate testing methodology for accurately determining the Slow Crack Growth design parameters necessary for component life prediction. In addition, an appropriate test methodology can be used to determine the influences of component processing variables and composition on the Slow Crack Growth and strength behavior of newly developed materials, thus allowing the component process to be tailored and optimized to specific needs. At the NASA Lewis Research Center, work to develop a standard test method to determine the Slow Crack Growth parameters of advanced ceramics was initiated by the authors in early 1994 in the C 28 (Advanced Ceramics) committee of the American Society for Testing and Materials (ASTM). After about 2 years of required balloting, the draft written by the authors was approved and established as a new ASTM test standard: ASTM C 1368-97, Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Flexural Testing at Ambient Temperature. Briefly, the test method uses constant stress-rate testing to determine strengths as a function of stress rate at ambient temperature. Strengths are measured in a routine manner at four or more stress rates by applying constant displacement or loading rates. The Slow Crack Growth parameters required for design are then estimated from a relationship between strength and stress rate. This new standard will be published in the Annual Book of ASTM Standards, Vol. 15.01, in 1998. Currently, a companion draft ASTM standard for determination of the Slow Crack Growth parameters of advanced ceramics at elevated temperatures is being prepared by the authors and will be presented to the committee by the middle of 1998. Consequently, Lewis will maintain an active leadership role in advanced ceramics standardization within ASTM. In addition, the authors have been and are involved with several international standardization organizations including the Versailles Project on Advanced Materials and Standards (VAMAS), the International Energy Agency (IEA), and the International Organization for Standardization (ISO). The associated standardization activities involve fracture toughness, strength, elastic modulus, and the machining of advanced ceramics.
-
Slow Crack Growth of indent Cracks in glass with and without applied stress
Materials Science and Engineering: A, 1992Co-Authors: Sung R. Choi, Jonathan A. SalemAbstract:Abstract The Slow Crack Growth behavior occurring in dynamic fatigue and post-indentation fatigue with no applied stress was compared for indented soda-lime glass specimens in six different environments at room temperature. A significant difference between the fatigue behaviors in the two methods was found, indicating that the post-indentation fatigue (Slow Crack Growth) is unique but that such an approach is not appropriate to evaluate fatigue parameters for use in life prediction. The existence of a plateau region in the post-indentation Crack Growth-time data was attributed to a fatigue limit for each test environment, as verified by the aging strength data.
Gilbert Fantozzi - One of the best experts on this subject based on the ideXlab platform.
-
Slow Crack Growth behaviour of hydroxyapatite ceramics
Biomaterials, 2017Co-Authors: Chahid Benaqqa, Malika Saadaoui, Jérôme Chevalier, Gilbert FantozziAbstract:Among materials for medical applications, hydroxyapatite is one of the best candidates in orthopedics, since it exhibits a composition similar to the mineral part of bone. Double torsion technique was here performed to investigate Slow Crack Growth behaviour of dense hydroxyapatite materials. Crack rate, V, versus stress intensity factor, KI, laws were obtained for different environments and processing conditions. Stress assisted corrosion by water molecules in oxide ceramics is generally responsible for Slow Crack Growth. The different propagation stages obtained here could be analyzed in relation to this process. The presence of a threshold defining a safety range of use was also observed. Hydroxyapatite ceramics appear to be very sensitive to Slow Crack Growth, Crack propagation occurring even at very low KI. This can be explained by the fact that they contain hydroxyl groups (HAP: Ca10(PO4)6(OH)2), favouring water adsorption on the Crack surface and thus a strong decrease of surface energy in the presence of water. This study demonstrates that processing conditions must be carefully controlled, specially sintering temperature, which plays a key role on VKI laws. Sintering at 50 °C above or below the optimal temperature, for example, may shift the VKI law towards very low stress intensity factors. The influence of ageing is finally discussed.
-
Slow Crack Growth in Zirconia Ceramics with Different Microstructures
Fracture Mechanics of Ceramics, 2002Co-Authors: Jérôme Chevalier, Laurent Gremillard, R. Zenati, Y. Jorand, C. Olagnon, Gilbert FantozziAbstract:A review of Slow Crack Growth experiments conducted in the same laboratory with the same technique (Double Torsion) is presented for zirconia ceramics with different microstructures: (i) a single crystal cubic zirconia as a model brittle material, (ii) different 3%mol. Yttria Stabilised TZP ceramics with grain sizes in the range of 0.3 to 1 µm with or without grain boundary glassy phase, (iii) a 10%mol. Ceria Stabilised Zirconia with a grain size of about 2 pm and a very high transformation toughening capability, (iv) a multiplex zirconia based composite.
Norman Brown - One of the best experts on this subject based on the ideXlab platform.
-
Slow Crack Growth-Notches-Pressurized Polyethylene Pipes
Polymer Engineering & Science, 2007Co-Authors: Norman BrownAbstract:A general method for predicting the lifetime of a polyethylene structure that fails by Slow Crack Growth was applied to the case of externally notched pressurized pipes. An analysis of experimental data indicated that the residual stress must be taken into account. The critical notch depths associated with a given lifetime were calculated as a function of pipe size, PENT value of the resin, and temperature. The results were tabulated to serve as practical guide lines for deciding whether a pipe should be discarded if the notch is too deep. The current 10% of the thickness rule now used by industry was found to be invalid. POLYM. ENG. SCI., 47:1951–1955, 2007. © 2007 Society of Plastics Engineers
-
A sensitive mechanical test for Slow Crack Growth in polyethylene
Polymer Engineering & Science, 1997Co-Authors: Zhiqiang Zhou, Norman BrownAbstract:Slow Crack Growth (SCG) in a wide variety of polyethylenes has been investigated by a constant tensile load test (the PENT test) for a single edge notched specimen. The PENT test is very sensitive to the changes in molecular structure and morphology of polyethylene. The resistance to SCG depends on the density of the tie molecules and the strength of the crystals.
-
Abnormal Slow Crack Growth in polyethylene
Polymer, 1997Co-Authors: Norman BrownAbstract:The normal Slow Crack Growth behaviour of polyethylenes consists of a ductile-brittle transition where the transition to brittle fracture occurs below a critical stress and after a critical time. The curves of stress vs. failure time depend on temperature, but all curves can be shifted vertically and horizontally by well established shift functions. It has been observed that there are some polyethylenes whose stress vs. time to failure curves do not shift in the normal way and in some cases the time for brittle fracture does not change with temperature. SEM micrographs do not reveal any difference between the fibrillated fractured surfaces from the normal and abnormal specimens. Whereas the normal Slow Crack Growth behaviour has been attributed to the disentanglement of the molecules by sliding without chain scission, it is proposed that the abnormal fracture behaviour may involve a disentanglement mechanism that consists of a combination of chain sliding and scission.
-
The critical molecular weight for resisting Slow Crack Growth in a polyethylene
Journal of Polymer Science Part B: Polymer Physics, 1996Co-Authors: Narumi Ishikawa, Norman BrownAbstract:An ethylene-hexene copolymer was fractionated into five fractions and the density of short-chain branches was measured for each fraction. The Slow Crack Growth behavior was measured on each fraction by sandwiching the small amount of fractionated resin of about 0.2 g between polyethylene grips. The resistance to Slow Crack Growth was negligible for the three fractions whose Mw was less than 1.5 × 105. For the fourth fraction with Mw greater than 1.5 × 105, the resistance to Slow Crack Growth was very high, being greater than that for the whole resin even though its density of short-chain branches was less than that of the whole resin. It is concluded that a molecular weight greater than 1.5 × 105 is required to create the number of tie molecules that is necessary to produce a high resistance to Slow Crack Growth in this particular copolymer. © 1996 John Wiley & Sons, Inc.
-
A fundamental theory for Slow Crack Growth in polyethylene
Polymer, 1995Co-Authors: Norman BrownAbstract:Abstract The following theoretical equation has been obtained for measuring the rate of Slow Crack Growth in polyethylene in terms of the Crack opening displacement rate δ: δ = α y (1−y 2 ) 2 ηd o E 2 α 2 c K 4 Here δy is the yield point, K is the stress intensity, η is the intrinsic viscosity of the fibrils in the craze, E is Young's modulus, δc is the stress to produce a craze, d0 is the primordial thickness from which the craze originates and γ is Poisson's ratio. The theoretical equation agrees with the experimental observation: δCK4e-−Q/RT Thus, for the first time, the dependence of δ on stress and notch depth have been derived in fundamental terms and the physical parameters that constitute the factor C have been identified. The intrinsic viscosity η can be calculated from the theory using specific experimental data. For example at 42°C, the fibrils in a craze in a homopolymer have an intrinsic viscosity of 3 × 1011 Pas. This is much larger than the melt viscosity of the amorphous region, which is about 105–106 Pas. Thus, the resistance of polyethylene to Slow Crack Growth is governed by the crystals and not by the amorphous region.
Richard E. Tressler - One of the best experts on this subject based on the ideXlab platform.
-
Slow Crack Growth in sapphire fibers at 800 to 1500 c
Journal of the American Ceramic Society, 1993Co-Authors: Stephen A. Newcomb, Richard E. TresslerAbstract:Sapphire fibers with (near) c-axis orientation were tested in tension over a range of strain rates (10−5 to 0.5 min−1) at elevated temperatures (800° to 1500°C). The strength of the fibers was dependent on the strain rate. Slow Crack Growth was confirmed as the degradation process by direct inspection of the fracture surfaces and estimation of fracture stresses from measured flaw sizes. The Slow Crack Growth parameter, N, decreased with increasing temperature. At 1400°C a threshold in strength was observed; the threshold stress intensity factor was estimated to be ≅ 1 MP·m1/2 at 1400°C. Thermally activated bond rupture (e.g., lattice trapping model) is postulated as the mechanism responsible for the Slow Crack Growth.
-
Slow Crack Growth in Sapphire Fibers at 800° to 1500°C
Journal of the American Ceramic Society, 1993Co-Authors: Stephen A. Newcomb, Richard E. TresslerAbstract:Sapphire fibers with (near) c-axis orientation were tested in tension over a range of strain rates (10−5 to 0.5 min−1) at elevated temperatures (800° to 1500°C). The strength of the fibers was dependent on the strain rate. Slow Crack Growth was confirmed as the degradation process by direct inspection of the fracture surfaces and estimation of fracture stresses from measured flaw sizes. The Slow Crack Growth parameter, N, decreased with increasing temperature. At 1400°C a threshold in strength was observed; the threshold stress intensity factor was estimated to be ≅ 1 MP·m1/2 at 1400°C. Thermally activated bond rupture (e.g., lattice trapping model) is postulated as the mechanism responsible for the Slow Crack Growth.
Sung R. Choi - One of the best experts on this subject based on the ideXlab platform.
-
Slow Crack Growth of a Pyroceram Glass Ceramic Under Static Fatigue Loading—Commonality of Slow Crack Growth in Advanced Ceramics
Journal of Engineering for Gas Turbines and Power, 2014Co-Authors: Sung R. Choi, D. Calvin Faucett, Brenna SkelleyAbstract:An extensive experimental work for Pyroceram™ 9606 glass-ceramic was conducted to determine static fatigue at ambient temperature in distilled water. This work was an extension and companion of the previous work conducted in dynamic fatigue. Four different applied stresses ranging from 120 to 170 MPa was incorporated with a total of 20–23 test specimens used at each of four applied stresses. The Slow Crack Growth parameters n and D were found to be n = 19 and D = 45 with a coefficient of correlation of rcoef = 0.9653. The Weibull modulus of time to failure was in a range of msf = 1.6 to 1.9 with an average of msf = 1.7±0.2. A life prediction using the previously-determined dynamic fatigue data was in excellent agreement with the static fatigue data. The life prediction approach was also applied to advanced monolithic ceramics and ceramic matrix composites based on their dynamic and static fatigue data determined at elevated temperatures. All of these results indicated that a SCG mechanism governed by a power-law Crack-Growth formulation was operative, a commonality of Slow Crack Growth in these materials systems.
-
Slow Crack Growth of a Pyroceram Glass Ceramic Under Static Fatigue Loading: Commonality of Slow Crack Growth in Advanced Ceramics
Volume 6: Ceramics; Controls Diagnostics and Instrumentation; Education; Manufacturing Materials and Metallurgy, 2014Co-Authors: Sung R. Choi, D. Calvin Faucett, Brenna SkelleyAbstract:An extensive experimental work for Pyroceram™ 9606 glass-ceramic was conducted to determine static fatigue at ambient temperature in distilled water. This work was an extension and companion of the previous work conducted in dynamic fatigue. Four different applied stresses ranging from 120 to 170 MPa was incorporated with a total of 20–23 test specimens used at each of four applied stresses. The Slow Crack Growth parameters n and D were found to be n = 19 and D = 45 with a coefficient of correlation of rcoef = 0.9653. The Weibull modulus of time to failure was in a range of msf = 1.6 to 1.9 with an average of msf = 1.7±0.2. A life prediction using the previously-determined dynamic fatigue data was in excellent agreement with the static fatigue data. The life prediction approach was also applied to advanced monolithic ceramics and ceramic matrix composites based on their dynamic and static fatigue data determined at elevated temperatures. All of these results indicated that a SCG mechanism governed by a power-law Crack-Growth formulation was operative, a commonality of Slow Crack Growth in these materials systems.
-
Test Standard Developed for Determining the Slow Crack Growth of Advanced Ceramics at Ambient Temperature
1998Co-Authors: Sung R. Choi, Jonathan A. SalemAbstract:The service life of structural ceramic components is often limited by the process of Slow Crack Growth. Therefore, it is important to develop an appropriate testing methodology for accurately determining the Slow Crack Growth design parameters necessary for component life prediction. In addition, an appropriate test methodology can be used to determine the influences of component processing variables and composition on the Slow Crack Growth and strength behavior of newly developed materials, thus allowing the component process to be tailored and optimized to specific needs. At the NASA Lewis Research Center, work to develop a standard test method to determine the Slow Crack Growth parameters of advanced ceramics was initiated by the authors in early 1994 in the C 28 (Advanced Ceramics) committee of the American Society for Testing and Materials (ASTM). After about 2 years of required balloting, the draft written by the authors was approved and established as a new ASTM test standard: ASTM C 1368-97, Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Flexural Testing at Ambient Temperature. Briefly, the test method uses constant stress-rate testing to determine strengths as a function of stress rate at ambient temperature. Strengths are measured in a routine manner at four or more stress rates by applying constant displacement or loading rates. The Slow Crack Growth parameters required for design are then estimated from a relationship between strength and stress rate. This new standard will be published in the Annual Book of ASTM Standards, Vol. 15.01, in 1998. Currently, a companion draft ASTM standard for determination of the Slow Crack Growth parameters of advanced ceramics at elevated temperatures is being prepared by the authors and will be presented to the committee by the middle of 1998. Consequently, Lewis will maintain an active leadership role in advanced ceramics standardization within ASTM. In addition, the authors have been and are involved with several international standardization organizations including the Versailles Project on Advanced Materials and Standards (VAMAS), the International Energy Agency (IEA), and the International Organization for Standardization (ISO). The associated standardization activities involve fracture toughness, strength, elastic modulus, and the machining of advanced ceramics.
-
Accelerated Testing Methodology Developed for Determining the Slow Crack Growth of Advanced Ceramics
1998Co-Authors: Sung R. Choi, John P. GyekenyesiAbstract:Constant stress-rate ("dynamic fatigue") testing has been used for several decades to characterize the Slow Crack Growth behavior of glass and structural ceramics at both ambient and elevated temperatures. The advantage of such testing over other methods lies in its simplicity: strengths are measured in a routine manner at four or more stress rates by applying a constant displacement or loading rate. The Slow Crack Growth parameters required for component design can be estimated from a relationship between strength and stress rate. With the proper use of preloading in constant stress-rate testing, test time can be reduced appreciably. If a preload corresponding to 50 percent of the strength is applied to the specimen prior to testing, 50 percent of the test time can be saved as long as the applied preload does not change the strength. In fact, it has been a common, empirical practice in the strength testing of ceramics or optical fibers to apply some preloading (
-
Slow Crack Growth of indent Cracks in glass with and without applied stress
Materials Science and Engineering: A, 1992Co-Authors: Sung R. Choi, Jonathan A. SalemAbstract:Abstract The Slow Crack Growth behavior occurring in dynamic fatigue and post-indentation fatigue with no applied stress was compared for indented soda-lime glass specimens in six different environments at room temperature. A significant difference between the fatigue behaviors in the two methods was found, indicating that the post-indentation fatigue (Slow Crack Growth) is unique but that such an approach is not appropriate to evaluate fatigue parameters for use in life prediction. The existence of a plateau region in the post-indentation Crack Growth-time data was attributed to a fatigue limit for each test environment, as verified by the aging strength data.
