Radiation-induced Stress Relaxation Research Articles

Radiation-induced stress relaxation in high temperature water of type 316L stainless steel evaluated by neutron diffraction

Weld beads on plate specimens made of type 316L stainless steel were neutron-irradiated up to about 2.5 × 10 25 n/m 2 ( E > 1 MeV) at 561 K in the Japan Material Testing Reactor (JMTR). Residual stresses of the specimens were measured by the neutron diffraction method, and the radiation-induced stress relaxation was evaluated. The values of σ x residual stress (transverse to the weld bead) and σ y residual stress (longitudinal to the weld bead) decreased with increasing neutron dose. The tendency of the stress relaxation was almost the same as previously published data, which were obtained for type 304 stainless steel. From this result, it was considered that there was no steel type dependence on radiation-induced stress relaxation. The neutron irradiation dose dependence of the stress relaxation was examined using an equation derived from the irradiation creep equation. The coefficient of the stress relaxation equation was obtained, and the value was 1.4 (×10 −6/MPa/dpa). This value was smaller than that of nickel alloy.

Effect of cold-work on the radiation-induced deformation of austenitic stainless steels

Austenitic stainless steels will be used in ITER as a major structural material. Although, initially supplied in annealed condition, cold-working is inevitable during construction or operation. Martensite would be introduced by cold-work in the case of unstabilized stainless steels. In this study, the effect of cold-work on the irradiation creep was examined under 17 MeV proton irradiation (2 × 10 −7 dpa/s) at 288 °C for stabilized (SUS 316L) and unstabilized (SUS 304) stainless steels. Radiation-induced stress relaxation was also evaluated using the creep data. The stress dependence of irradiation creep was different for the two steels; linear for SUS 316L and quadratic for SUS 304. This difference would influence the stress relaxation, which is faster in SUS 316L. The behavior of SUS 316L is consistent with computer simulation for evenly distributed network dislocations, while that of SUS 304 appears to originate from the dislocations localized in the vicinity of martensite boundaries.

Radiation-Induced Stress Relaxation of Welded Type 304 Stainless Steel Evaluated by Neutron Diffraction

Abstract The welded Type 304 stainless steel specimens were neutron-irradiated up to 2 × 1025 n/m2 (E > 1MeV) at 561K in the Japan Material Testing Reactor (JMTR). The residual stresses in the test specimens were measured by neutron diffraction, and the radiation-induced stress relaxation was evaluated from the change of stress distributions along the direction transverse to the weld bead. The magnitudes of σX (transverse to the weld) and σY (longitudinal to the weld) residual stresses decreased steadily with increasing neutron fluence. The irradiation dose dependence and the mechanism for the relaxation were examined using an equation derived from the irradiation creep equation. There was a steep stress relaxation, which might correspond to transient creep, at the early stage of neutron irradiation near the weld, and the different radiation-induced relaxation behaviors could be observed depending on the distance from the weld. The radiation-induced stress relaxation for welding-induced residual stress, which was generated under a three-dimensional restriction, indicated the tendency to be slower compared to the fundamental results for mechanically applied stress.

Cross-sectional TEM and X-ray examination of radiation-induced stress relaxation of peened stainless steel surfaces

Neutron irradiation-induced relaxation was emulated using proton irradiation in order to determine the expected amount of stress relaxation in the peened surface layer of a BWR core shroud during its 40 year lifetime. Samples of 304 SS were shot peened to induce a compressive residual stress, and then irradiated with 3.2 MeV protons at 288 °C to four dose levels spanning 0.1–2.0 dpa. One set of specimens was as-peened and a second was pre-injected with 25 appm He. Depth-dependent measurements of internal stress were conducted using successive steps of X-ray line broadening measurement and electropolishing. Results showed that the compressive stress state was progressively relaxed, but was maintained at some level for the majority of the 2 dpa target dose. Helium pre-injection did not significantly affect the relaxation, but the magnitude of thermally-induced relaxation was somewhat greater, although it was observed to be largely a transient, saturable process. A new cross-section technique was developed that allows multiple observations to be made in one specimen at all depths, both in and beyond the peen-damaged range. The as-peened microstructure varies strongly with depth, consisting of deformation twins and dense dislocation networks. The radiation-induced relaxation on the microstructural level was expressed primarily in modification and reduction of the dislocation structure. A comparison was made between the proton-induced relaxation of internal stresses and predictions based on neutron-induced relaxation of externally-applied stresses. The relatively good agreement indicates that proton irradiation is a valid emulation of neutron irradiation for this application.

Calculation of radiation-induced deformation of SUS 316 in the ITER-relevant condition

SUS 316 stainless steel will be used in ITER as a major structural material. However, accumulation of radiation-induced deformation data in the anticipated operation temperature range, centered at about 300 °C, is not adequate. Although it will be used mainly in annealed condition, cold-working and consequent introduction of network dislocations is inevitable at various parts of the structure. Computer simulation was performed to examine the radiation-induced deformation of SUS 316 steel, with emphasis on the effect of network dislocation density, for the ITER-relevant condition. Irradiation creep rates and radiation-induced stress relaxation behaviors were examined at 300 °C for various displacement rates, applied stresses and network dislocation densities. Rather complicated dependence on dislocation density has been predicted in both creep deformation and stress relaxation. This complexity results from a competitive redistribution of point defects to the operating deformation mechanisms and from the predominance among them that depends on the network dislocation density.

Evolution of thin-film morphologies in metals during ion beam bombardment

The evolution of thin-film morphologies during ion beam bombardment has been examined by a combination of experimental and molecular dynamics simulation methods. Surface roughness, stress and domain growth in amorphous and nanocrystalline films were investigated. The experiments provide support for the model of radiation-induced viscous flow. The coefficient of radiation-induced viscosity deduced from the experiments is insensitive to both the particular material and the irradiation particle. The computer simulations, using molecular dynamics, reveal that while flow may be caused by local melting of nanometer-sized regions along the track of the ion, in some cases the simple creation of point defects is sufficient to explain radiation-induced stress relaxation.

1135. 小型Cリング試験片による応力緩和評価法

A stress relaxation measurement method has been developed by using C-ring specimens, and a specimen size effect has been evaluated taking radiation-induced stress relaxation into consideration. C-ring specimens were stressed by forcing a wedge in the gap. Giving an appropriate eccentric configuration in the half of the ring opposite the gap, the stress gradient along the circumference was eliminated in the section and the stress level could be varied by changing the gap spacing. The validity of the C-ring test method was confirmed by thermally stress relaxation experiments at annealing temperatures from 300 to 600°C for 1min to 200h in carbon steel: considerable stress relaxation could be measured for all levels of applied stress even at relatively low annealing temperatures. The relaxation results obtained from the C-ring test were in good agreement with those from a uniaxial tensile stress relaxation test. The smaller C-ring specimen with about 40mm diameter, which is required for radiation-induced stress relaxation test, also showed adequate accuracy on stress relaxation at 600 to 830°C in stainless steel, compared with the large size C-ring specimen test.

Calculation of radiation-induced deformation in the ITER vacuum vessel

Numerical calculation was carried out to evaluate the radiation induced deformation at the blanket side of the vacuum vessel (120°C) and the rear portion of the blanket module (200°C) in ITER. The calculation was performed for solution-annealed 316 stainless steel mainly at 1×10 −9 dpa/s and 100 MPa. Enhanced transient creep characteristic of the low temperature irradiation is evident at 120°C. However, the total accumulated creep strain in the vacuum vessel is only below 0.01% for the lifetime irradiation so that serious consequence would not be anticipated. At the rear portion of the blanket the creep strain would be about 0.01% and not serious either. Radiation-induced stress relaxation at the vacuum vessel is only several per cent during the lifetime. At the rear portion of the blanket, on the other hand, the relaxation could be of the order of 10% and should not be completely neglected.

Calculation of radiation-induced stress relaxation

A numerical calculation based on point defect kinetics under stress was carried out to evaluate radiation-induced stress relaxation in a solution-annealed 316 stainless steel at low and medium temperatures (60 and 300°C). The calculation shows that the stress relaxation relative to the initial stress is almost independent of the initial stress under irradiation. At 1 × 10 −6 dpa/s, stress relaxation at 60°C is greater than that at 300°C and the stress almost disappears in a month of continuous irradiation. Even with a low damage rate of 1 × 10 −8 dpa/s, the stress relaxation at 60°C is considerable and exceeding that at 300°C and 1 × 10 −6 dpa/s during the first several months of irradiation. The results indicate the importance of the radiation-induced stress relaxation in experimental fusion reactors.

Stress relaxation of vitreous silica on irradiation

The radiation-induced stress relaxation which is observed on ion bombardment of vitreous silica is described as a viscoelastic behavior in which the apparent viscosity is reduced to ∼1014 Poise during irradiation and then increases rapidly by 4 or 5 orders of magnitude on cessation or interruption of irradiation. The bombarded layer appears to possess a viscosity ∼1019 Poise, lower than would be expected for normal vitreous silica. On electron bombardment the viscosity is also reduced, but not as greatly as an ion bombardment, yet sufficiently to result in the whole radiation-induced volume contraction being realized perpendicularly to the surface, as has been found for ion bombardment. The maximum elastic stored energy which can be realized is but a fraction of a calorie per gram, hence the reported values of 200 cal/g would seem to be associated with the fragmentation of the network responsible for the reduced viscosity.

Extrusion of quartz on ion bombardment: Further evidence for radiation-induced stress relaxation of the silica network

It is shown that although crystal quartz shows a highly aeolotropic uniaxial radiation expansion when it is disordered with energetic neutrons, plates of crystal quartz oriented on either principal axis exhibit a perpendicular expansion equivalent to the total volume expansion without developing porosity when subjected to bombardment with protons, deuterons, or helium ions having ranges about a micron. Thus a plastic flow must occur during ion bombardment. A mechanism for these courses of transformation is given: that the crystal structure breaks up into a mosaic of units elongated parallel to the optic axis in a vitreous matrix. The elongated units could account for the relative behavior of various aeolotropic properties, and the vitreous matrix could facilitate plastic flow.

Radiation Induced Stress Relaxation of Silica-Polydimethylsiloxane Composites

Abstract A study of G (scission) for silica filled dimethylsiloxane composites has been carried out by measurement of stress relaxation in a y-ray flux. The results show the same trend as those derived from a combination of sol-dose and swelling experiments. G (scission) is found to be strongly dependent on the weight fraction of filler in the composite, as well as on the particle size. A model involving reaction of physisorbed polymer chains with electrons and/or positive holes, generated within the silica by γ-radiation, is proposed to explain the increase in the scission yield. The model also explains the observation of a large negative activation energy on the basis of an increase in the rate of annihilation of the holes and electrons within the silica particles.

Radiation-Induced Stress Relaxation in Quartz and Vitreous Silica

Helium or proton beams, penetrating several microns or more, cause crazing on vitreous silica surfaces. In a lower energy range (beams up to 150 keV were used) there was observed upon irradiation a plastic flow stress relaxation that relieved the stress from the radiation-induced dilatation and resulted in the total volume change in vitreous silica or quartz occurring perpendicular to the surface! Implications of this radiation-induced stress relaxation seem fundamental to the radiation behavior of quartz, silica, and similar substances, and are discussed.