32 research outputs found
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Effects of notches on elevated-temperature, low-cycle fatigue behavior of Type 204 stainless steel. [593/sup 0/C]
The results of an investigation into the effects of geometric stress concentrations on the elevated-temperature low-cycle fatigue behavior of Type 304 stainless steel are presented. The objective of this study was to develop a data base that could be used to verify the ASME Code Case 1592-8 design method for predicting the creep-fatigue behavior of structural components which contain discontinuities. In continuous-cycling tests, the local strains at the notch root determined from the Code Case procedure result in fatigue-life estimates that are conservative when compared to the experimental values. For tests containing a hold time at the peak tensile strain, the Code Case procedure results in local strains that are nearly identical to the experimentally obtained local strains, if the local stress is assumed to relax to the nominal value during the hold time
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Stress corrosion cracking of candidate materials for nuclear waste containers
Types 304L and 316L stainless steel (SS), Incoloy 825, Cu, Cu-30%Ni, and Cu-7%Al have been selected as candidate materials for the containment of high-level nuclear waste at the proposed Yucca Mountain Site in Nevada. The susceptibility of these materials to stress corrosion cracking has been investigated by slow-strain-rate tests (SSRTs) in water which simulates that from well J-13 (J-13 water) and is representative of the groundwater present at the Yucca Mountain site. The SSRTs were performed on specimens exposed to simulated J-13 water at 93{degree}C and at a strain rate 10{sup {minus}7} s{sup {minus}1} under crevice conditions and at a strain rate of 10{sup {minus}8} s{sup {minus}1} under both crevice and noncrevice conditions. All the tests were interrupted after nominal elongation strains of 1--4%. Examination by scanning electron microscopy showed some crack initiation in virtually all specimens. Optical microscopy of metallographically prepared transverse sections of Type 304L SS suggests that the crack depths are small (<10 {mu}m). Preliminary results suggest that a lower strain rate increases the severity of cracking of Types 304L and 316L SS, Incoloy 825, and Cu but has virtually no effect on Cu-30%Ni and Cu-7%Al. Differences in susceptibility to cracking were evaluated in terms of a stress ratio, which is defined as the ratio of the increase in stress after local yielding in the environment to the corresponding stress increase in an identical test in air, both computed at the same strain. On the basis of this stress ratio, the ranking of materials in order of increasing resistance to cracking is: Types 304L SS < 316L SS < Incoloy 825 {congruent} Cu-30%Ni < Cu {congruent} Cu-7%Al. 9 refs., 12 figs., 7 tabs
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Creep-fatigue life prediction for different heats of Type 304 stainless steel by linear-damage rule, strain-range partitioning method, and damage-rate approach
The creep-fatigue life results for five different heats of Type 304 stainless steel at 593/sup 0/C (1100/sup 0/F), generated under push-pull conditions in the axial strain-control mode, are presented. The life predictions for the various heats based on the linear-damage rule, strain-range partitioning method, and damage-rate approach are discussed. The appropriate material properties required for computation of fatigue life are also included
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Prediction of environmental and strain-rate effects on the stress corrosion cracking of austenitic stainless steels
The stress corrosion cracking (SCC) susceptibility of austenitic stainless steels in high-temperature water is controlled by environmental variables (e.g., dissolved oxygen, corrosion potential, impurities), microstructure (e.g., degree of sensitization), and strain rate. A phenomenological model based on the slip-dissolution mechanism and elastic-plastic fracture mechanics is presented to quantitatively describe the effects of both environment-related parameters and strain rate on SCC in constant extension rate tests. The model predictions are in good agreement with the results of tests performed on Types 304, 316, and 316NG stainless steel at different strain rates in a wide variety of environments relevant to boiling-water reactors
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Phenomenological model for stress corrosion cracking of Types 316NG and 316 stainless steel. [BWR]
A phenomenological model for both intergranular and transgranular stress corrosion cracking (SCC) in constant extension rate (CERT) tests is discussed. The model is based on an estimate of crack-tip strain rate obtained by use of a J-integral approach, a slip dissolution model for SCC, and a J-integral fracture criterion. The model is used to derive correlations between the SCC parameters (such as time to failure and average crack growth rate) and applied (nominal) strain rate. The model is extended to determine similar relationships between the SCC parameters and average near-crack-tip strain rate. The model predictions are compared with CERT test results obtained for Type 316NG stainless steel (SS) in water containing dissolved oxygen and impurities at 289/sup 0/C over a range of strain rates between 10/sup -5/ and 10/sup -7//s. Good agreement is obtained between the theoretical description and experimental results. The SCC behavior of Type 316NG SS is also compared with that of Type 316 SS to determine the relative SCC susceptibility of the two materials
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Role of atmospheric corrosion of aluminum alloys in viability of intrinsic-surface methods for tagging military hardware
A primary requirement for authentication of tags for military equipment desigated as treaty-limited items (TLIs) is that the surface topograhy of the tag area be maintained after exposure to a variety of atmospheric conditions over many years. This report summarizes the chemical and physical properties of atmospheric as they relate to localized corrosion of aluminum and aluminum alloys. The role of impurity species that exacerbate corrison, and that hence may interfere with tag verification, is discussed. Because exposure times for the tag materials are much longer than those practical in laboratory experiments, it is important to understand the kinetics of processes occurring in these alloys and the viability of various protection schemes. General principles and limitations of testing in natural atmospheres and in the laboratory are discussed. Corrosion results indicate that the tag surface must be protected, and a tag protection scheme is proposed
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Creep-fatigue interactions in an austenitic stainless steel
A phenomenological model of the interaction between creep and fatigue in Type 304 stainless steel at elevated temperatures is presented. The model is based on a crack-growth equation and an equation governing cavity growth, expressed in terms of current plastic strain and plastic strain rate. Failure is assumed to occur when a proposed interaction equation is satisfied. Various parameters of the equations can be obtained by correlation with continuously cycling fatigue and monotonic creep-rupture test data, without the use of any hold-time fatigue tests. Effects of various wave shapes such as tensile, compressive, and symmetrical hold on the low-cycle fatigue life can be computed by integrating the damage-rate equations along the appropriate loading path. Microstructural evidence in support of the proposed model is also discussed
Effects of wave shape and ultrahigh vacuum on elevated temperature low cycle fatigue in type 304 stainless steel
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Effects of nominal and crack-tip strain rate on IGSCC susceptibility in CERT tests
Constant extension rate tests have been performed on sensitized Type 316 stainless steel in oxygenated water (8 ppM O/sub 2/) containing chloride ion impurities (0.5 ppM) over a range of strain rates from 10/sup -5/ to 2 x 10/sup -7/ s/sup -1/. The susceptibility to IGSCC (as quantified by parameters such as crack length at failure) increases with a decrease in strain rate. A model consistent with the observed and postulated crack growth behavior and with a fracture criterion is presented and used to derive power laws that relate the IGSCC susceptibility parameters and strain rate. The predicted strain rate exponents are in agreement with the experimental results of this and other studies. The correlations between IGSCC susceptibility and strain rate can be used to predict susceptibility to cracking outside the range of conditions used in the laboratory. In addition, it is shown that the average crack-tip strain rate in CERT experiments can be estimated by use of a J-integral approach. It is observed that the average crack growth rate is proportional to the square root of the estimated average crack-tip strain rate. The experimentally observed correlation is in good agreement with that deduced from a slip-dissolution model proposed by Ford
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Development of mixed-conducting oxides for gas separation
Mixed-conducting oxides have been used in many applications, including fuel cells, gas separation membranes, sensors, and electrocatalysis. The authors are developing a mixed-conducting, dense ceramic membrane for selectively transporting oxygen and hydrogen. Ceramic membranes made of Sr-Fe-Co oxide, which has high combined electronic and oxygen ionic conductions, can be used to selectively transport oxygen during the partial oxidation of methane to synthesis gas (syngas, CO + H{sub 2}). The authors have measured the steady-state oxygen permeability of SrFeCo{sub 0.5}O{sub x} as a function of oxygen-partial-pressure gradient and temperature. At 900{degrees}C, oxygen permeability was {approx}2.5 scc{center_dot}cm{sup {minus}2}{center_dot}min{sup {minus}1} for a 2.9-mm-thick membrane and this value increases as membrane thickness decreases. The authors have fabricated tubular SrFeCo{sub 0.5}O{sub x} membranes and operated them at 900{degrees}C for >1000 h during conversion of methane into syngas. The hydrogen ion (proton) transport properties of yttria-doped BaCeO{sub 3} were investigated by impedance spectroscopy and open-cell voltage measurements. High proton conductivity and a high protonic transference number make yttria-doped BaCeO{sub 3} a potential membrane for hydrogen separation