Fracture energies of Zircaloy-4

Fracture properties of Zircaloy-4 were determined as a function of the energies required for failure. Charpy V-notch fracture samples were prepared from nuclear grade Zircaloy-4. The fracture mechanism was studied as a function of orientation, loading rate, and temperature. Yield stress, tensile streets, and integrated energy values were determined from the resulting fracture force-displacement curves. Fracture toughness values were found to be independent of orientation but strongly dependent upon temperature and loading rate; K-values decreased with increasing temperature, dynamic values were higher than static. Structure of the force-displacement curves revealed four distinct segments: (a) elastic deformation, (b) plastic deformation, (c) fracture, and (d) tearing after crack arrest. Elastic energy (a) values decreased with temperature, while plastic energy (b) values increased with temperature. Cracking energy (c) values decreased with increasing temperature for statically loaded specimens but increased with increasing temperature for dynamically loaded specimens. Tearing energy (d) values were strongly dependent upon temperature and orientation. Two orientations (T-S and L-S) exhibited a gradual increase in tearing energy with increasing temperatures very characteristic of ductile materials. The other two orientations (T-L and L-T) had a temperature transition curve reminiscent of a ductile-brittle transition material. Sharp temperature transition resulted from the onset of crack arrest. A sudden transition dip was observed in the tearing energy which was a function of loading rate and orientation.     

Constitutive description of creep behavior of M-4Al-1Ca alloy

Creep behavior of an advanced magnesium alloy AX41 (4 wt.% Al, 1 wt.% Ca, Mg balanced) was investigated in temperature interval from 343 to 673 K and stresses from 2 to 200 MPa. Compressive creep experiments with stepwise loading were used in order to obtain stress dependence of the creep rate in interval from 10⁻⁹ to 10⁻³ s⁻¹ for a given temperature. All stress dependences can be well described by the Garofalo sinh relationship with natural exponent n = 5. An analysis of the parameters of this relationship has shown that lattice diffusion controls creep at all experimental conditions. While climb-controlled creep mechanism is decisive at lower stresses and higher temperatures, glide-controlled mechanisms act at higher stresses and lower temperatures. A typical power-law breakdown is observed at intermediate stresses and temperatures. __________ Translated from Problemy Prochnosti, No. 1, pp. 36–39, January–February, 2008.     

High-temperature internal friction in polycrystalline Zircaloy-4

The high-temperature internal friction spectrum of polycrystalline Zircaloy-4 is investigated in detail, for a wide range of frequencies. Two internal friction maxima are observed. The lower-temperature peak is interpreted in terms of a relaxation mechanism produced by the sliding of particle-free grain boundaries. The higher-temperature peak is attributed to the sliding of boundaries blocked by small precipitates. Values for the activation enthalpy and the preexponential factor for diffusion along grain boundaries are given, and the viscosity coefficient associated with grain-boundary sliding is determined as a function of temperature.     

Effect of microstructural texture on the creep behavior

The two steels were prepared with the same composition but different rolling processes. The equiaxed grain of TMCP EH36 steel was produced by thermo-mechanical control rolling (TMCP) with an accelerated cooling process. The banded structure of SM490C steel was produced using the conventional hot rolling process. After creep the results show that the apparent activation energy and apparent stress exponent in the band structure of SM490C steel are much higher than that in the equiaxed grain of TMCP EH36 steel. The second phase distribution and morphology have an important effect on the creep behavior in this study.     

Nonlinear creep behavior of viscoelastic polycarbonate

A study of the tension behavior of polycarbonate (PC) under room temperature and various sustained loads is presented. Time-dependent axial elongations and transverse contractions of the specimen were simultaneously measured at nine different stress levels, from 15.89 to 59.4 MPa, and modeled according to a time-stress superposition principle. The test duration was only one hour. It was shown that creep compliance vs. log time curves at different stresses can be horizontally shifted to form a smooth master curve for one year at a reference stress of 30.97 MPa. Moreover, the stress shift factors for axial extension creep curves and transverse contraction creep curves are found to be identical for the stress levels considered, and this is verified using the Poisson's ratio measurements.     

Near-tip behavior of deflected creep cracks

Near-tip displacement fields of a creep crack which exhibited moderate deflection from its initially mode I condition have been measured using the stereoimaging technique. From the measured displacement fields, near-tip strains and crack opening displacements (CODs) are obtained and compared with existing asymptotic solutions for stationary, deflected cracks. The comparison reveals that the near-tip strain field and CODs of a stationary deflected creep crack in stainless steel (creep exponent of 8) are of the Riedel-Rice type. The degree of mode mixity is also adequately predicted for the deflected crack. The results for stainless steel are compared with previous results for a glass-ceramic (creep exponent of 1.5), to assess the range of applicability of the RR field. Discrepancies between theory and experiment are discussed in terms of the dominant creep mechanism, which is dislocation creep for the stainless steel and grain boundary sliding for the glass-ceramic.     

Atomic-scale interface structure of a Cr-coated Zircaloy-4 material

Highly adherent, thin Cr coatings on Zr-based nuclear fuel claddings can be potentially used for the development of accident-tolerant fuels in light water reactors. To guarantee the successful implementation of Cr-coated Zr alloys as cladding tubes in nuclear power plants, the adhesive strength of the Cr coatings must be assessed. The interface between Cr and Zr was characterized via high-resolution transmission electron microscopy. We observed the formation of nanometer-thick Zr(Fe, Cr)₂ poly-type, structured Laves phases at the interfacial region that display both C14 and C15 lattice symmetries. Although the crystallinity was preserved throughout the interfacial region, different atomic configurations were observed for all the interfaces studied. In most cases, coherent or semicoherent crystallographic relationships were observed, ensuring the adhesive strength of the coating.     

Crack propagation behavior under creep conditions

The creep crack propagation behavior of a Cr–Mo–V rotor steel has been investigated at 538°C using time-dependent fracture mechanics concepts. The creep crack propagation lives and the creep deflection rates of double-edge-notched (DEN) specimens were estimated using previous data from compact-type specimens. The predicted crack growth lives and deflection rates compared favorably with the experimental data. When plotted as a function of the C _t parameter, the experimentally determined creep crack propagation rates of DEN specimens were found to be in agreement with those of compact and center-crack-tension specimens. These results provide further experimental verification for the validity of the C _t parameter for characterizing creep crack growth behavior. Some discrepancies between the predicted and the observed behavior are attributed to primary creep deformation behavior which was not considered in estimating the value of C _t .     

Low cycle deformation behaviour of Zircaloy-4 at elevated temperatures

The LC deformation behaviour of Zry-4 at 400°C and 600°C was examined by means of tension/compression experiments conducted in load and in strain control respectively. The main results were compared to those obtained at comparable conditions on the stainless steel type AISI 304. For both the materials the influence of the stress ratio R = σ_min/σ_max (where within one test σ_max > 0 was kept constant) upon the lifetime T_f at low and high homologeous temperature T_h was examined. Whereas at the lower T_h for R < 0 the lifetime decreased with decreasing R, the opposite was true at the higher T_h. The explanation of the influence of R upon t_f suggests that at high temperatures the fatigue damage rate Å_f drops below the rate for creep damage Å_c Two cases are considered. If the above damage mechanisms are sequentially independent the resulting damage rate Å ≈? Å_c and hence Å_c is the failure (rate) determining mechanism. In the case that the mechanisms are sequentially dependent then Å ≈? Å_f. TEM investigations conducted on Zry-4 cycled at 600° C have shown that the typical dislocation pattern revealed is a band structure consisting of dense dislocation walls separating denuded zones. The habit and the crystallographic characteristics of the band structure resemble the structure associated with PSBs observed in fee metals. The comparison of the values of the saturation stress τ_s and the wall spacing d for different fee and hep metals shows that there is a proportionality between τ_s and 1/d which is independent of stress and temperature.     

Experimental and numerical study of creep and creep rupture behavior of PA6

Polyamide 6 (PA6) is a semi-crystalline polymer utilized in many structural components working under steady load. At room temperature, PA6 exhibits time dependent (viscoplastic) deformation. The aim of this work is to investigate the mechanical response and the crack growth of PA6 under creep conditions.The experimental database consists of tests carried out on smooth and notched round bars. Load versus displacement curves were recorded for monotonic tests tensile at various crosshead speeds. Then, creep tests at constant load were performed allowing the record of the creep strain history according to the applied load.Microscopic observations highlight the initial spherulitic structure.Smooth and notched specimens were utilized in order to identify and to validate material coefficients dedicated for analytical modeling. The non linear fracture mechanics for creeping solids was applied to results on “pre-cracked” specimens. For this kind of loading, use of the load parameter C^∗ is recommended.By plotting C^∗ values versus time to failure, a unique correlation was obtained. The knowledge of this master curve allows lifetime assessment of PA6 cracked bodies.     

Simulation of the creep damage behavior of thin film/substrate systems by bending creep tests

The purpose of this paper is to understand the creep damage properties of thin film/substrate systems by bending creep tests. To this aim, a numerical study has been performed on the creep damage development of the thin film/substrate systems by implementing the Kachanov–Rabothov damage law into finite element models. The work may shed some light on the influence of the modulus ratio of the substrate to the thin film, the thickness of the thin film and the bending load. Finite element method (FEM) results show that three obviously damaged zones are found. The first is at the edge of the loading pin, the second is at the interface between the film and the substrate ahead of the loading pin edge, the last is at the edge of the supporting pin A. The influence of the modulus ratio of the substrate to the thin film on the bending creep damage is not obvious at the preliminary stage of creep time. However, with the lapse of time, the damage rate decreases with the increase of modulus ratio of the substrate to the thin film. The change of the thickness of the thin film and the bending load also influence the creep damage behavior of the thin film/substrate systems.     

Microstructure and electrochemical properties of Zircaloy-4 under Fe ion irradiation

To investigate the microstructure and electrochemical properties of Zircaloy-4 induced by Fe ion irradiation with the energy of 150 keV at liquid nitrogen temperature, transmission electron microscope analysis (TEM) was employed to analyze the surface layer of the samples irradiated at a dose ranging from 1×10¹³ to 1×10¹⁶ ions/cm², and potentiodynamic polarization measurements were used to evaluate the corrosion resistance of Zircaloy-4 in a 1 N H₂SO₄ solution at room temperature. TEM analyses show that Fe ion irradiation lead to a structural change and amorphous phase formation on the surface of the samples. Moreover, it is indicated from the corrosion tests that with an increase of the irradiation dose, the passive current density increases at first and then decreases rapidly, while the natural corrosion potential goes down at first and then up rapidly. The critical point, where the corrosion properties are transformed from a damaging stage to protecting stage, is at the damage level of 3.19 dpa. Finally, the mechanism for the change of corrosion resistance of the Zircaloy-4 samples is discussed.     

Hydrogen in Zircaloy-4: effects of the neutron irradiation on the hydride formation

X-ray diffraction patterns of neutron irradiated Zircaloy-4 samples were obtained at the Brazilian Synchrotron Light Laboratory (LNLS) to study the effects of the fast neutron fluxes and post-irradiation thermal treatments on the zirconium hydride evolution. The high intensity and resolution of the synchrotron beam allowed to detect the (111)_δ and (220)_δ diffraction peaks of the ZrH_1.5 + x (0 ≤ x≤ 0.16) δ-phase in unirradiated Zircaloy-4 samples having a hydrogen concentration as low as 0.2 at.% (20 wppm). Then, irradiated samples, that were taken from the Zircaly-4 core components of the argentine HWPR Atucha I Nuclear Power Plant (CNA-1) were studied with that radiation to detect the diffraction peaks of ZrH_1.5 + x δ-phase particles precipitated in samples having hydrogen isotope concentration lower than 2 at.% (220 wppm). A significant increment of the (111)_δ and (220)_δ peak areas were observed at room temperature after post-irradiation thermal treatments at 600 °C during 4 h. These results indicate that after the annealing a hydrogen concentration between 35 to 70 wppm, which were apparently absent in the irradiated samples has precipitated at room temperature as zirconium hydrides. That amount was estimated from a (220)_δ peak area versus the bulk H concentration regression line, made with unirradiated samples. These results get support to a hydrogen trapping hypothesis proposed in previous works. In addition, it has been shown that the zirconium hydride that precipitate in the irradiated samples have the equilibrium δ-ZrH_1.5 + x cubic crystalline structure.

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Development of substructure in Zircaloy-4 during LCF at 873 K

Transmission electron microscopy techniques were used to study the substructure developed in Zircaloy-4 during strain controlled Low Cycle Fatigue (LCF) test at 873 K. A complete microstructural analysis on samples cycled with a total strain range of 0.01 and total strain rate 2×10^–3s^–1 was made at a different number of cycles. This study revealed that the dislocations arranged themselves into a band structure which remains essentially constant since the first cycle up to fracture. This substructure stability agrees with the stress response observed of this alloy at 873 K. A mechanism to account for the observed structure is proposed.     

Mismatch effect in creep properties on creep crack growth behavior in welded joints

The finite element method based on ductility exhaustion model was used to systematically investigate the mismatch effect in creep properties on creep crack growth (CCG) behavior in welded joints. The crack-tip damage, stress states, CCG paths, CCG rate and rupture life were calculated for different configurations of creep properties between weldment constituents under the same load level, and the creep life assessment and design for welded joints were discussed. The results show that when the zone containing the crack is softer than at least one of the other two surrounding materials or both, the creep crack propagates straight along the initial crack plane. Otherwise, it will form a second crack in the soft material near interface. These simulation results were confirmed by the experimental observations in the literature, and the mechanism was analyzed. The harder surrounding materials can lead to higher CCG rate and shorter rupture life due to the higher constraint given from them. The early initiation and propagation of the second cracks increase CCG rate and reduce rupture life, and the incubation time of the second cracks in soft materials near interfaces should be accurately determined in the creep life assessment and design for the welded joints. A proper mismatch design with harder material containing crack and softer surrounding material can improve CCG properties of welded joints (decreasing CCG rate and prolong rupture life).