D.A. Koss

The influence of hydride blisters on the fracture of Zircaloy-4

Abstract The fracture behavior under near plane-strain deformation conditions of Zircaloy-4 sheet containing solid hydride blisters of various depths has been examined at 25 and 300 °C. The study was based on material with either model ‘blisters’ having diameters of 2 and 3 mm or a continuous layer of hydride; in all cases, the substrate material contained discrete hydride precipitates. The fracture strains decrease rapidly with increasing hydride blister/layer depth to levels of about 100 μm deep, and then remain roughly constant. For a given blister depth, the material is significantly more ductile at 300 °C than at room temperature although measurable ductility is retained even at 25 °C and for large blister depths. The material is somewhat more ductile if the hydride is in the form of a blister than in the form of a continuous layer (rim). The hydride blisters/layers are brittle at all temperatures, and crack shortly after yielding of the ductile substrate. Consequently, both experimental evidence and analytical modeling indicate that fracture of the sheet is controlled by the crack growth resistance of the substrate at 25 °C. At elevated temperatures, the hydride particles within the substrate are quite ductile, inhibit crack growth, and failure eventually occurs due to a shear instability.

Ductile failure as a result of a void-sheet instability : experiment and computational modeling

The ductile fracture of circumferentially notched HY-100 steel specimens tested at high stress triaxiality is characterized by a fracture surface with a zig-zag profile created by large elongated voids separated by inclined sheets of microvoids. The failure path suggests that a local deformation instability, triggered by the growth of MnS inclusion-nucleated voids, may be responsible for this form of fracture. Measurements show that the failure strains are small and decrease slowly with increasing stress triaxiality as compared to a more global void coalescence. Micro-mechanical modeling by finite element analysis of a geometry based on the observed metallurgical microstructure shows a distinct localization of plastic flow. The localization of plastic flow leads to a void-sheet failure prediction and the experimental results are correctly characterized by the computational model.

Modeling the ductile fracture process of void coalescence by void-sheet formation

Ductile fracture of engineering alloys frequently occurs by a mechanism of void coalescence in which void-sheets form between the primary voids. Based on the microstructural features that control failure of HY-100 steel, computational modeling has been performed to examine the deformation localization behavior between primary voids and to predict ductile fracture by the void-sheet coalescence mechanism. Elongated inclusion-initiated voids are simulated as two distinct, hole-like voids on a plane inclined to the stress axis based on the inclined nature of the fracture surface. Consistent with experimental behavior, the micro-mechanical model identifies a strong tendency for strain localization between the voids (and therefore void-sheet failure) but only at a high degree of stress triaxiality. Furthermore, based on the formation of a secondary void population, the analysis also predicts with reasonable accuracy both the magnitude and stress-state dependence of the experimentally determined failure strains.

Temperature, strain rate, stress state and the failure of HY-100 steel

Abstract The influence of temperature and strain rate on the deformation and failure behavior of HY-100 steel has been examined as a function of stress state using notched and un-notched axisymmetric tensile specimens. Behavior over the range of temperatures/strain rates from −85°C and 1 s −1 to 27°C and 10 −3 s −1 shows an equivalence of decreasing test temperature or increasing strain rate on deformation behavior in a manner that can be predicted by the thermally activated flow theory. Over the entire range of temperatures/strain rates, the influence of stress state on failure is such that two void coalescence mechanisms control failure; at low stress triaxialities, relatively equiaxed voids grow to impingement, while at high triaxialities, a void-sheet process intervenes linking elongated MnS-initiated voids by a shear instability. The failure strains decrease rapidly with increasing stress triaxiality ratio in a similar manner for all temperatures and strain rates except for an intermediate stress triaxiality condition where the void-sheet mode of failure extends to lower stress triaxialities under cryogenic test conditions.

Failure of Hydrided Zircaloy-4 Under Equal-Biaxial and Plane-Strain Tensile Deformation

The fracture behavior of unirradiated Zircaloy-4 containing either solid hydride blisters or hydrided rims has been examined for the contrasting conditions of equal-biaxial and plane-strain tensile deformation at three temperatures (25, 300, and 375 C). Cold-worked and stress-relieved Zircaloy-4 sheet containing hydride blisters shows nearly identical failure strains in equal-biaxial and plane-strain tensile deformation for a wide range of blister or rim depths. In all cases, failure strains decrease rapidly with increasing hydride blister or rim thickness, especially in the 100 {micro}m range. Test temperature has a significant effect on ductility with failure strains at 300 and 375 C being much greater than at room temperature. The results indicate that the ductility of material containing hydride rims/blisters greater than 3040 {micro}m deep is limited by crack growth, which occurs in a mode I manner at 25 C but in a mixed mode I/II manner at 300 C (and at higher failure strain levels).

Tensile specimen geometry and the constitutive behavior of Zircaloy-4

The influence of tensile specimen geometry on the deformation behavior of flat Zircaloy-4 tensile specimens has been examined for gauge length-to-width ratios that range from 1:1 to 4:1. Specimen geometry has only minor effects on the values of the yield stress, tensile strength, apparent uniform strain at maximum load, and strain-hardening exponent. However, in all geometries but the 4:1 configuration, diffuse necking occurs before maximum load. As a result, strain distributions at maximum load are uniform only in the 4:1 geometry. The elongation to failure is also affected by specimen geometry with the shorter gauge sections exhibiting much higher total elongation values, due in large part to the concomitant specimen necking behavior.

Mechanical Property Testing of Irradiated Zircaloy Cladding Under Reactor Transient Conditions

Specimen geometries have been developed to determine the mechanical properties of irradiated Zircaloy cladding subjected to the mechanical conditions and temperatures associated with reactivity-initiated accidents (RIA) and loss-of-coolant accidents (LOCA). Miniature ring-stretch specimens were designed to induce both uniaxial and plane-strain states of stress in the transverse (hoop) direction of the cladding. Also, longitudinal tube specimens were also designed to determine the constitutive properties in the axial direction. Finite-element analysis (FEA) and experimental parameters and results were closely coupled to optimize an accurate determination of the stress-strain response and to induce fracture behavior representative of accident conditions. To determine the constitutive properties, a procedure was utilized to transform measured values of load and displacement to a stress-strain response under complex loading states. Additionally, methods have been developed to measure true plastic strains in the gauge section and the initiation of failure using real-time data analysis software. Strain rates and heating conditions have been selected based on their relevance to the mechanical response and temperatures of the cladding during the accidents.

Fracture Toughness of Hydrided Zircaloy-4 Sheet Under Through-Thickness Crack Growth Conditions

The susceptibility of fuel cladding to failure in the case of a postulated reactivity-initiated accident may be determined by crack initiation within a hydride blister or rim and subsequent crack growth through the thickness of the cladding. This study has determined the fracture toughness of hydrided cold- worked stress relieved Zircaloy-4 sheet subject to through-thickness crack growth at both 25 and 300°C. The experimental approach utilizes a novel procedure in which a narrow linear strip of brittle hydride blister across the specimen width creates a well-defined precrack upon initial loading. The subsequent crack growth resistance is then characterized by four-point bending of the specimen and an elastic-plastic frac- ture mechanics analysis. At room temperature, the through-thickness fracture toughness K q is sensitive to the orientation of the hydride platelets and K q 25 MPam for crack growth through a mixed in-plane/out- of-plane hydride field. In contrast, K q is much higher 75 MPam when the hydride platelets are oriented predominantly in the plane of the sheet and therefore normal to both the crack plane and the crack growth direction. At 300°C, the material exhibits greater ductility as the hydride particles within the matrix resist fracture such that K q 83 MPam, despite the much lower flow stress of the material.

The effect of pre-strain and strain-path changes on ductile fracture: experiment and computational modeling

Abstract Based on experiments and computational modeling, this study examines the failure behavior of structural steel, HY-100, which has been pre-strained at a high stress triaxiality and subsequently failed at a lower stress triaxiality. Both tensile tests of circumferentially notched specimens and the associated fractography show that even a small pre-strain at high stress triaxiality promotes an extension of the low ductility, ‘void-sheet’ mode of failure to lower stress triaxialities. Thus, there is a decrease in the failure strain compared to that if the material is deformed only at the lower stress triaxiality. These results imply that the pre-strain damage nucleates elongated voids whose growth is critical to the development of void-sheet failure. Micro-mechanical modeling using finite element analysis confirms that localization of plastic flow should occur between elongated ‘hole shaped voids’, despite their rather small initial cross-section size (2.5 μm) and comparatively large spacing (70 μm). Furthermore, employing a local failure criterion, the computational analysis predicts failure strains which are in good agreement with those observed after the pre-strain and strain-path change.

Fiber pushout and interfacial shear in metal-matrix composites

The mechanical response of a fiber/matrix interface is very important in determining the strength and the fracture behavior of metal-matrix composites. As a means of examining interfacial shear behavior, the use of the “thin-slice” fiber pushout test is becoming increasingly common. However, recent thin-slice pushout tests suggest interfacial failure processes depend not only on intrinsic factors (e.g., interfacial bond strength and toughness and matrix plasticity), but also on extrinsic factors (e.g., specimen configuration, thermally induced residual stresses, and the mechanics associated with the test). In light of these factors, this article briefly describes the contrasts in the mechanics of fiber pullout and fiber pushout. In addition, selected aspects of thin-slice fiber pushout behavior are examined to illustrate the physical nature of the interfacial shear response and its dependence on both intrinsic and extrinsic factors.