J.P. Strizak

Time-dependent strain-controlled fatigue behavior of annealed 2 14 Cr-1 Mo steel for use in nuclear steam generator design☆

Experimental results are reported from fully reversed strain-controlled push-pull fatigue tests of specimens from several heats of isothermally annealed commercial 2 14 Cr-1 Mo steel. The tests were conducted in air over the temperature range 316 to 593°C. Fatigue life depended on strain range, temperature, strain rate, cyclic wave form, and the particular heat of material involved. The fatigue life was reduced when either tensile or compressive strain hold times were introduced each cycle; however, the effect was more pronounced for compressive hold times at low strain ranges. Increasing the compressive hold period increased the reduction in fatigue lifetime; however, the reduction was most pronounced after short hold times and at higher temperatures. With increasing hold time there was some indication of diminished variation in lifetime due to heat-to-heat variations.

The Effect of Neutron Irradiation on the Structure and Properties of Carbon-Carbon Composite Materials

Carbon-based materials are an attractive choice for fusion reactor plasma facing components (PFCs) because of their low atomic number, superior thermal shock resistance, and low neutron activation. Next generation plasma fusion reactors, such as the international thermonuclear experimental reactor (ITER), will require advanced carbon-carbon composite materials possessing extremely high thermal conductivity to manage the anticipated severe heat loads. Moreover, ignition machines such as ITER wilt produce high neutron fluxes. Consequently, the influence of neutron damage on the structure and properties of carbon-carbon composite materials must be evaluated. Data from an irradiation experiment are reported and discussed here. Fusion relevant graphite and carbon-carbon composites were irradiated in a target capsule in the high flux isotope reactor (HFIR) at Oak Ridge National Laboratory (ORNL). A peak damage dose of 1.58 dpa (displacements per atom) at 600°C was attained. The carbon materials irradiated included nuclear graphite grade H-451 and one-, two-, and three-directional carbon-carbon composite materials. Dimensional changes and strength are reported for the materials examined. The influence of fiber type, architecture, and heat treatment temperature on properties and irradiation behavior are reported. Carbon-carbon composite dimensional changes are interpreted in terms of simple microstructural models.

The effect of mercury on the fatigue behavior of 316 LN stainless steel

Abstract Design of the mercury target system components for the Spallation Neutron Source (SNS) requires data on high- and low-cycle fatigue behavior, and the program in progress includes determining the effects of mercury on the fatigue behavior of type 316 LN stainless steel, the primary material of choice for the target vessel. Uniaxial, load-controlled fatigue tests with R =−1 and (minimum stress/maximum stress) have been conducted in air and mercury at room temperature employing constant amplitude, sinusoidal loading at frequencies from 0.1 to 700 Hz. Stress amplitude versus fatigue life ( S – N curves) data at 10 Hz for both air and mercury show a sharp knee at approximately 1 million cycles indicating a fatigue endurance limit in either air or mercury around 240 Mpa. Mean stress ( R =0.1) lowers the endurance limit to 160 MPa. At relatively low frequency, both frequency and environment (mercury) had some impact on fatigue life of type 316 LN stainless steel at high-stress levels (i.e., stresses considerably above the apparent fatigue limit). Although testing at a high frequency of 700 Hz, showed a decrease in fatigue life in air compared with that at 10 Hz, a significant increase in specimen temperature was observed in air due to self-heating. No pronounced effects of waveform have yet been found, but data are limited.

The effect of mean stress on the fatigue behavior of 316 LN stainless steel in air and mercury

Abstract Design of the mercury target system components for the spallation neutron source (SNS) requires data on high- and low-cycle fatigue behavior. The research and development program in progress includes determining the effects of mercury on the fatigue behavior of type 316 LN stainless steel, the primary material of choice for the target vessel. Uniaxial, load-controlled, fully reversed tension–compression R=−1 (minimum stress/maximum stress) fatigue tests have been conducted in air and mercury at room temperature employing constant amplitude sinusoidal loading at frequencies from 0.2 to 10 Hz. Stress amplitude versus fatigue life data (S–N curves) for both air and mercury show a sharp knee at approximately 1 million cycles indicating a fatigue endurance limit in either air or mercury around 240 MPa. Tensile mean stress (R=0.1) lowers the endurance limit to 160 MPa. Lower frequency and mercury environment had some impact (degradation) on fatigue life of type 316 LN stainless steel at high stress levels (i.e., stresses considerably above the apparent fatigue limit). Test results for high mean stress conditions (R=0.3, 0.5, and 0.75) at a cyclic frequency of 10 Hz exhibited further reductions in the endurance limit.

Influence of mercury environment on the fatigue behavior of spallation neutron source (SNS) target container materials

Abstract The high-cycle fatigue behavior of 316 LN stainless steel (SS), the prime candidate target-container material for the spallation neutron source (SNS), was investigated in air and mercury at frequencies from 10 to 700 Hz with a R ratio of 0.1. A decrease in the fatigue life of 316 LN SS in air was observed with increasing frequency. However, little influence of frequency on fatigue life was found in mercury. An increase in the specimen temperature at 700 Hz seems to be the main factor that contributed to the decrease of the fatigue life in air, relative to that at 10 Hz. However, because of the cooling effect of mercury, only a small temperature increase was found at 700 Hz, and, therefore, there was little frequency influence in mercury. At 10 Hz, a shorter fatigue life of 316 LN SS was measured in mercury than in air at stresses greater than yield strength, which may have resulted from liquid metal embrittlement (LME). At lower stresses, no difference in fatigue lives between mercury and air was detected at 10 Hz. At 700 Hz, the fatigue life in mercury was longer than in air. The fatigue endurance limit measured at both frequencies in mercury and in air was approx. 350 MPa.

Effects of mercury on fatigue behavior of Type 316 LN stainless steel: application in the spallation neutron source

Abstract The high-cycle fatigue behavior of Type 316 stainless steel (SS), the prime candidate target-container material for the spallation neutron source (SNS), was investigated in air and mercury at frequencies of 0.2 and 10 Hz with a R ratio of −1, and at 10 and 700 Hz with a R ratio of 0.1. Here R equals the ratio of the applied minimum to maximum loads during fatigue experiments. A decrease in the fatigue life in mercury was observed, relative to that in air, at 0.2 Hz. Correspondingly, intergranular fracture was found on the fracture surfaces of specimens tested in mercury at 0.2 Hz, which is a typical fracture mode caused by liquid metal embrittlement (LME). Heating by mechanical working was observed during fatigue tests at 10 Hz and a R of −1, and at 700 Hz and a R of 0.1, which resulted in great increases in specimen temperatures and shorter fatigue lives for large stress amplitudes (⩾210 MPa), relative to those in mercury. However, in the fatigue tests at 10 and 700 Hz, the fatigue lives in air with cooling and those in mercury seemed to be comparable, indicating little influence of the mercury. Thus, both specimen self-heating and LME need to be considered in understanding fatigue behavior of Type 316 SS in air and mercury.