G.E. Lucas

The development of small specimen mechanical test techniques

Abstract The current program plan for the development of materials for fusion reactors requires testing candidate materials in both fission reactors and high energy neutron sources. Because of the volume limitations of available facilities, both current and near term, and because of the relatively large number of materials and test conditions that will need to be explored, it is essential that test techniques be developed to extract mechanical property information from small volume specimens. A variety of such test techniques are under development at the University of California, Santa Barbara. These include instrumented microhardness, bulge, shear punch, indentation creep and load relaxation and miniaturized fracture tests for obtaining strength, ductility, time-dependent flow, and fracture behavior information on specimens as small as TEM discs.

The effects of intermediate temperature irradiation on the mechanical behavior of 300-series austenitic stainless steels

Abstract Data in the literature on the microstructures and mechanical properties of solution annealed 300-series austenitic stainless steels irradiated under conditions relevant to the design of near term fusion reactors show consistent trends for a variety of alloys and environments. The yield stress increases rapidly approaching a saturation value of about 850 ±100 MPa, essentially identical to the ultimate stress. Visible irradiation-induced features, which are predominantly helium bubbles and dislocation loops, can account for only about 50 to 70% of these increases. The uniform elongation is reduced to less than 1% due to the decrease in strain hardening and severe flow localization following irradiation. The fracture toughness also decreases to values near or below 50 MPa m 1 2 . A simple critical strain model is shown to be qualitatively consistent with the observed toughness trends. These results suggest that more rigorous fracture models can be developed to permit quantitative predictions of toughness changes based on a variety of measurements made on small specimens.

The effect of plastic strain on the evolution of crystallographic texture in Zircaloy-2

Abstract The evolution of crystallographic texture during plastic deformation was investigated in Zircaloy-2 using X-ray and metallographic techniques. Inverse pole figures, the resolved fraction of basal poles, and the volume fraction of twinned material, were determined as a function of plastic strain for several strain paths and initial textures at 298 K and 623 K. Incremental transverse platic strain ratios ( R ) were mesured as a function of plastic strain. Texture rotation occurs early in the deformation process, after as little as 1.5% plastic strain. For compressive plastic strains, the resolved fraction of basal poles increases in the direction parallel to the strain axis. For tensile plastic strains, the resolved fraction of basal poles decreases in the direction parallel to the strain axis. The rate of change of the resolved fraction of basal poles with plastic strain is a function of the initial resolved fraction of basal poles. The texture rotation can be explained by considering the operation of the principal tensile twinning systems, {1012}〈1011〉.

Shear punch tests for mechanical property measurements in TEM disc-sized specimens

Abstract A shear punch test has been developed to extract strength and ductility information from thin sheet samples as small as transmission electron microscopy (TEM) disc specimens. The test is based on driving a cylindrical punch through a clamped specimen, and instrumenting the punch to obtain a load-displacement curve of the punching process. A number of correlations have been developed between features of the load-displacement curve and mechanical properties such as yield strength, ultimate tensile strength, work hardening exponent and reduction in area.

Parametric analysis of the disc bend test

A study was undertaken to determine the response of disc bend test data to variations in the relative size of the spherical penetrator, receiver hole and specimen thickness. Four materials - annealed and cold-worked copper and austenitic stainless steel — were lapped to three thicknesses, 0.1–0.25 mm. Specimens from each material were tested in a disc bend apparatus in which the penetrator diameter could be varied from 1 to 1.6 mm and the receiver hole diameter from 1.3 to 1.9 mm. Uniaxial tensile tests on each material were also conducted. It was found that load displacement data were most sensitive to thickness; “yield” load, maximum load and penetrator displacement to failure all increased to the greatest extent with increasing specimen thickness. Good correlations between these values and uniaxial tensile properties were found for all combinations of ball, hole and specimen size; but the data scatter was slightly reduced for the thickest specimens and the largest ball size.

Analysis of cleavage fracture potential of martensitic stainless steel fusion structures: Part I. Micromechanical models and material properties

Abstract The major disadvantage of martensitic stainless steels for structural applications in fusion reactors is currently considered to be their potential for low temperature brittle cleavage fracture. This study attempts to review the current understanding of cleavage fracture in steels and the role of microstructure in dictating material resistance to this type of fracture. A parametric analysis of cleavage fracture in a surrogate steel, A533B, is made and the results are used in conjunction with general cleavage fracture theory to establish some potential guidelines for future research in developing the martensitic stainless steels.

The influence of irradiation on fracture and impact properties of fusion reactor materials

Abstract This paper reviews the existing data on the effects of neutron irradiation on the fracture and impact properties of the important fusion reactor structural materials: austenitic and ferritic/martensitic steels and recent data on vanadium alloys and other low activation materials. The mechanisms which are believed to cause irradiation-induced changes in fracture properties are also discussed, and recommendations for future research are considered.

The mumechanical mechanisms of cleavage fracture in martensitic stainless steels

Abstract Coupled measurements of tensile and blunt and sharp notch fracture properties were made on a prototypical heat of HT-9, which is a 12% Cr martensitic stainless steel. Based on these data, it was found that cleavage fracture is controlled by a critical stress-critical distance criteria. The mucleavage fracture stress is 2400 MPa, consistent with the observed martensite lath packet size; and the critical distance is ~ 55 μm which is consistent with observed prior austenite grain size.

Analysis of cleavage fracture potential of martensitic stainless steel fusion structures: Part II. Fracture analysis procedures for flawed fusion structures

Abstract The potential for brittle cleavage fracture is a major concern for martensitic stainless steels which are candidates for fusion reactor structural materials. This study attempts to identify for flawed fusion structures the pertinent fracture resistance or failure parameters and the relationships between these parameters and the basic materials properties which govern cleavage fracture. Several procedures for relating test data to failure prediction, including Charpy-V-notch transition temperature referencing and two-parameter interpolation procedures, are considered; and results are discussed with respect to possible research paths for martensitic stainless steel alloy development.

Proton irradiation creep of Zircaloy-2

Abstract Irradiation creep tests were conducted in vacuum on textured Zircaloy-2 foil specimens bombarded with 4.75 MeV protons at a current density of 3.5 μA/cm 2 . Tests were performed at 375°C in the stress range 103–241 MPa. Both cold-worked, stress-relieved and fully recrystallized materials were used. Radiation hardening and enhanced irradiation creep behavior were both observed. Radiation hardening effects were more pronounced in the recrystallized material than the cold-worked material. Anisotropie irradiation creep behavior was also observed, and it was found to be similar in magnitude to the thermal creep anisotropy exhibited at the same test conditions. Finally, the observed proton irradiation creep rates were somewhat larger than those which might be expected for neutron irradiation creep under the same conditions.