Low Ductility Failure Research Articles

Effects of Loading Rate on Strength and Failure of Coal Rock from Hancheng

The effects of loading rate on splitting test, uniaxial compression strength, strength parameters in triaixal compression test, and creep failure of Hangcheng coal rock were investigated through indoor mechanical experiments. Experimental results shown that tensile strength, uniaxial strength and Youngs modulus of coal rock increased with the increase of loading rate and the increment is very small. Triaixal compression strength and loading rate has a linear relation. Brittleness was very marked under high loading rate. Ductility of coal rock in the steady creep stage diminished with the increase of loading rate, and failure mode of coal rock had a change from low ductility failure mode to ductile-to-brittle failure mode. The results of experimental researches can be used to determine the bottom hole flowing pressure of coalbed methane well.

Metallurgical Investigation on Low Ductility Failures of Cu-ETP Components

In the present paper, the investigation on low ductility failures occurred to ETP copper strips and connectors manufactured from cold stamping of copper tubes is presented. Optical microscopy, SEM/EDS for macro- and microfractography and local elemental analysis along with tensile testing were employed as the principal analytical techniques in the context of the present investigation. Casting defects and microstructural abnormalities associated to hydrogen induced cracking (H.I.C.) are the principal contributors of the examined failures.

Tensile, Compression and Fracture Properties of Thick-Walled Ductile Cast Iron Components

Abstract The article presents the outcome of a comprehensive program of tensile, compression and fracture toughness experiments, addressing thick-walled ductile cast iron inserts used for the production of three nuclear waste canisters. The resulting data are required as input to the assessment of the failure probability of the canisters. Moreover, these data are useful for the improvement of the casting technique as such. Although the same material specification is always used, material properties are found to show significant variation. Considerable attention is paid to linking the scatter in tensile properties to fractographic and microstructural observations. The main finding is that low ductility tensile test results can be primarily connected to the presence of specific casting defects, from which oxide films have the most detrimental effect. Another important observation is that compression experiments do not result in low ductility failure. During fracture testing, stable ductile crack propagation is observed. Basic fracture analysis of a tensile test is performed in order to better understand the effect of defect size, stress-strain behavior and fracture toughness on the ductility measured through tensile testing. Two opposing specimen size effects are observed.

Review of factors affecting condition assessment of nickel based transition joints

The high temperature performance of welds manufactured between ferritic low alloy steels and austenitic stainless steels is critical to power plant operation. The fabrication of these joints via established procedures using nickel based consumables should ensure adequate service life. However, low ductility failures as a consequence of creep cavitation at interfacial carbides have been reported in these welds within less than 10 years of operation. Accurate assessment of damage develop ment in plant welds is therefore required to prevent unplanned outages. Remaining life predictions can be made using post-exposure testing provided material is available. Non-destructive assessments are preferred and the potential exists for establishing techniques based on monitoring cavity density or strain accumulation.

The failure of the Dartmouth turbine casing

The 180 MW(e) Dartmouth turbine casing failed due to a water hammer transient in May 1990. This paper describes the failure of the turbine casing, which acts as a pressure vessel and was failed by the overload transient. The overload caused 12 separate low ductility failures at the stay vanes which form part of the pressure containment. This paper assesses the pressure transient in the turbine, the stress conditions which existed in the structure and outlines the fracture mechanics conditions of the failure.

Creep and fracture behaviour of English Electric Mark III transition welds: Part (b) Ex-service weldments

The microstructure and high temperature behaviour of an ex-service weldment have been assessed. Exposure at elevated temperature resulted in some carbide development in the niobium stabilised buttered layer. However, significant decarburisation in the low alloy steel was not detected. Cross-weld uniaxial creep testing invariably led to low ductility failures in the heat affected zone on the 2.25 Cr1Mo steel side of the weld. These failures were associated with the nucleation, growth and link-up of cavities on prior austenite grain boundaries. Assessment of tests interrupted at selected life fractions indicated that the development of cavitation followed a sensible trend with creep exposure. The cavitation behaviour of the transition weld was in general agreement with data obtained from creep tests undertaken on material heat treated to simulate heat affected zone structures. The level of creep damage in these welds can be assessed by evaluation of local strain accumulation or through monitoring the presence of cavities.

Assessment of notched tubular ferritic oxide-dispersion-strengthened components subjected to multiaxial creep loading at 1100°C

Notched tubular components of a ferritic oxide-dispersion-strengthened material, MA 956, were subjected to multiaxial creep loading at 1100°C by applying a constant internal pressure. The components proved to be extremely insensitive to circumferential notches of up to 80 percent of the wall thickness due to the high axial creep rupture strength of this material. However, the components were sensitive to both externally and internally axially aligned notches, and displayed similar stress rupture behaviour but consistently longer rupture lives than plane components at the same ligament stress level. Failure was found to be due to strain controlled cavitation in the ligament rather than as a consequence of creep crack growth from the notch. A direct current/potential drop method was shown to provide a reasonable indication of the development of cavitation in these tests. It is shown that the low ductility failure of notched MA 956 components is best described by a creep fracture mechanism rather than by fracture mechanics.

Effect of heat treatment on creep and fracture behaviour of 1·25Cr–O·5Mo steel

AbstractThe effect of tempering treatments on the microstructure and creep behaviour of multipass 1·25Cr–0·5Mo steel weldments has been evaluated. While tempering invariably reduced the hardness, significant changes in microstructure were only found after heat treatment at 750°C. In this case ferrite bands developed adjacent to the fusion boundary. Tempering increased creep deformation and reduced failure lifetime for base metal specimens. For crossweld testpieces, the susceptibility to low ductility failures in the heat affected zone was found to be linked to the development of creep cavities and cracks. Thus, brittle failure modes were a function of stress, temperature, microstructure, post weld heat treatment, and to a lesser extent, specimen geometry.MST/3069

Conditions under which CsI can cause SCC failure of zircaloy tubing

To determine whether CsI can be dissociated by the action of an oxygen potential and, therefore, can cause low-ductility failure of Zircaloy cladding due to stress corrosion cracking (SCC), out-of-pile creep rupture and burst tests were performed. The oxygen potential in the tube specimens was established by various metal/metal oxide systems or O/U ratios of the UO 2+ x . The test parameters were selected in such a manner that burst temperatures of about 700°C were obtained. The results of the isothermal creep rupture tests with UO 2+ x of various O U ratios showed that at initial oxygen potentials as high as −230 kJ/mol O 2 (which corresponds to an O U ratio of about 2.04 at 700°C), the CsI dissociates sufficiently to cause iodine-induced low-ductility cladding failure. When the oxygen potential was set by Mo/MoO 2 mixtures, sufficient dissociation of CsI took place at much lower oxygen potentials of about −400 kJ/mol O 2. With respect to the CsI dissociation both the formation of Cs oxides and the possible formation of complex compounds such as Cs uranate and Cs molybdate must be considered. Under these conditions, the I from the CsI behaved like elemental iodine. That is, iodine-induced low-ductility failure of the Zircaloy-4 tubing due to SCC occurred when the fission-product element iodine was present as either elemental I or as CsI.

Stress corrosion cracking of Zircaloy-4 cladding at elevated temperatures and its relevance to transient LWR fuel rod behaviour

In extensive out-of-pile experiments from 500 to 900° C it has been shown that, of all the volatile fission products in a LWR fuel rod, only iodine can cause low ductility failure of Zircaloy-4 tubing due to stress corrosion cracking up to about 800° C. The critical iodine concentration above which brittle cladding failure occurs was determined as a function of temperature in the absence and presence of UO 2 fuel. A comparison of these values with the amount expected in the fuel cladding gap during a LOCA transient shows that a clear influence of iodine on burst strain can be expected only up to 700° C. This is in agreement with the results of in-pile LOCA tests performed in the FR-2 reactor with high burnup fuel rods. Since the burst temperatures during a LOCA transient would generally be above 700° C, an influence of iodine on burst strain is not very probable in a LOCA. However, with respect to ATWS transients where the maximum cladding temperatures would be below 700° C, an influence of iodine on the mechanical properties of zircaloy can be expected.

Fracture mechanics analysis of iodine-induced crack growth in Zircaloy-4 tubing between 500 and 700°C

Low ductility failure of zircaloy tubing due to iodine-induced stress corrosion cracking (SCC) can occur up to about 700°C. The time-to-failure behavior of Zircaloy-4 cladding tubes containing iodine has been described by the elastic-plastic fracture mechanics model CEPFRAME for the temperature region 500 to 700°C. The model includes an empirically-determined computation method for the incubation period of crack formation, as a portion of the time-to-failure, as well as an elastic-plastic model for describing crack growth due to iodine-induced SCC. The total life time of the cladding tube is obtained by adding the crack initiation and crack propagation periods. The incubation period is a temperature-dependent function of both the depth of surface damage (both fabrication pits and machined notches) and the applied load, and is 40 to 90% of the time-to-failure. The elastic-plastic crack growth model is a modified version of the stress intensity K I-concept of linear-elastic fracture mechanics. The extensions of this concept take into account a plastic strain zone ahead of the crack tip, which effectively increases the crack depth, and in addition, a dynamic correction factor for the crack geometry which is essentially a function of the effective crack depth. Unstable crack growth is predicted to occur when the residual cross section reaches plastic instability. Model results show good agreement with experimental data of tube burst tests at 500, 600, and 700°C. The crack growth velocity at all three temperatures is a power function of stress intensity ahead of the crack tip; the exponent is 4.9. The model can estimate time-to-failure of as-received cladding tubes containing iodine within a factor of 2. Application of the model to temperatures below 500°C is possible in principle. Due to the increasing scatter in experimental data, the structural transformation of the cladding by recrystallization, and the growing importance of creep strain, CEPFRAME has an upper temperature limit of approximately 650°C. The model is suitable for use in computer codes describing LWR fuel rod behavior during reactor transients and accidents.

Determination of the critical iodine concentration for stress corrosion cracking failure of Zircaloy-4 tubing between 500 and 900°C

Iodine can influence the burst strain and time-to-failure of Zircaloy-4 tubing up to about 800°C. Low-ductility failure of zircaloy specimens containing iodine is characterized by little cladding deformation compared to reference specimens without iodine. However, the failure strain is significantly reduced only after a critical concentration of iodine within the tube specimens has been exceeded. Therefore, the critical iodine concentration resulting in stress corrosion cracking failure of as-received zircaloy tubing has been determined in the temperature range 500 to 900°C in out-of-pile creep rupture tests (times-to-failure <900 s). The test results show that the critical iodine concentration depends strongly on temperature, varying between 0.01 mg/cm3 at 500°C and 50 mg/cm3 at 900°C. The inserted iodine reacts completely with the zircaloy cladding to form gaseous ZrI4 and various condensed ZrIx, phases. Gaseous ZrI4 is probably responsible for crack initiation and crack propagation.

Creep-rupture properties of austenitic and nickel-based transition joints

Cross-weld tensile specimens were machined from welds made in 2·25 Cr-1 M 0 steel plate using either AISI 316 or Inconel 182 filler metals. Low-ductility failures, close to the ferritic steel/weld metal interface, were produced by iso-stress temperature-acceleration tests at either 62 or 100 MN m−2 over the temperature range 575°-640°C. The endurances of joints made using Inconel 182 weld metal were greater than those made using AISI 316 weld metal by factors of ∼2·5 and 3 for tests at 62 and 100 MN m−2, respectively. The fracture morphology has been studied by optical and scanning electron microscopy. By comparison with previous investigations, a change in the short- and long-term fracture mechanism of nickelbased joints is identified. The interfacial damage has been measured and is used to discuss the relative importance of creep-strength mismatch and thermal-expansion coefficient differences between the weld metal and ferritic steel in controlling the fracture of the joint.

銅の中間温度脆性と粒界移動キャビテーション

銅の中間温度脆性と粒界移動キャビテーション

Chemical interactions between simulated fission products and zircaloy-4 at elevated temperatures, including influence on ductility and time-to-failure

Compatibility behavior of simulated fission product elements and compounds with Zircaloy-4 was studied in the temperature range 500–1100°C. Tube burst experiments were also performed with these agents to determine their influence on the mechanical properties of Zircaloy for transients under LWR accident conditions. The test results show that there is a definite change in burst strain and time-to-failure for all the substances investigated, as compared to the He-reference specimens. However, only iodine or volatile iodine compounds result in low ductility failure of Zircaloy-4 cladding, up to about 800°C. In contrast, CsI exerts no influence. Selenium and tellurium give rise to chemical interactions with Zircaloy. partly leading to some reduction in burst strain.

Influence of iodine on the burst strain of Zircaloy-4 cladding tubes under simulated reactor accident conditions

The Stress Corrosion Cracking (SCC) behavior of short tubular Zircaloy-4 (Zry) specimens by the action of iodine was investigated out-of-pile between 600 and 1100°C under inert gas conditions. The Zry cladding tubes were used as-received, with an inner preliminary oxidation and pre-damaged, respectively. Moreover, the influence of the UO 2 oxygen potential on the SCC behavior was studied. The burst and creep-rupture tests (time-to-rupture ⩽15 min) clearly show that the deformation behavior of Zry cladding tubes below 850°C is influenced by the presence of iodine. A low ductility failure of Zry tubes takes place that is characterized by little deformation as compared to reference specimens without iodine. Moreover, in the isothermal, isobaric experiments, the time-to-failure of the iodine containing specimens is markedly shorter as compared to the iodine-free reference specimens. Internal preoxidation of the cladding tube or UO 2 in the specimens exert an additional influence on the mechanical properties of Zry. SEM examinations of the rupture surface of the cladding tubes show that in the presence of iodine and at burst temperatures below 850°C the cracks in the cladding material are mainly intergranular followed by ductile residual rupture.

Influence of composition and microstructure on hot workability of austenitic stainless steels

The behaviour of a steel during hot working is determined by composition, thermomechanical history, and processing parameters. An experimental assessment of hot ductility has been made using hot tensile tests to show the importance of compositional and microstructural variations in austenitic stainless steels. The effects of a substitutional solute (molybdenum), a precipitation-hardening solute (niobium), and delta ferrite formation are described. Low-ductility failures arising from intergranular cracking were most severe in coarse-grained material at low test temperatures. Crack growth was impeded by dynamic recrystallization, and ductility was much improved, the blunted cracks reorientating with increasing strain and growing in the same manner as voids nucleated at inclusions. Molybdenum and niobium strengthened the matrix and enhanced intergranular cracking. The significance of these findings in relation to hot-rolling schedules is considered, and the metallurgical desirability of using low soaking temperatures and high finishing temperatures in rolling austenitic stainless steels is emphasized.

Fracture of Zircaloy cladding under simulated power ramp conditions

Behaviour of Zircaloy cladding under stresses imposed by fuel dimensional changes is discussed and results presented which suggest that the correct choice of cladding properties, in particular cold-work level and texture, is important if the probability of low ductility failures in fuel pins is to be reduced. Evidence is presented, based on laboratory tests and post-irradiation examination of cladding, which leads to the conclusion that failure of Zircaloy can take place by stress-corrosion cracking in the presence of fission product iodine and it is further demonstrated that the process is strongly stress-dependent. Consideration is given to the influence of irradiation exposure, strain rate and adverse stress systems, for instance at inter-pellet ridges or fuel cracks, on the cracking susceptibility of Zircaloy cladding. The significance of the results is discussed in relation to a proposed stress-corrosion cracking mechanism.

Strain/Oxidation Interactions in Steels and Model Alloys

AbstractStrain/oxidation interactions in steels and model alloys The strains generated in oxide films on a number of materials of interest to the power generation industry are considered in relation to their operating conditions. Topics discussed the growth of nuclear canning alloys, oxide spalling, the formation of multilayer oxides during internal boiler corrosion and low ductility failures in austenitic stainless steels. The strains induced during thermal cycling are considered in detail and the temperature changes required to cause film craking are estimated and compared with observed spalling behaviour.

Mechanical sciences

To determine whether CsI can be dissociated by the action of an oxygen potential and, therefore, can cause low-ductility failure of Zircaloy cladding due to stress corrosion cracking (SCC), out-of-pile creep rupture and burst tests were performed. The oxygen potential in the tube specimens was established by various metal/metal oxide systems or O/U ratios of the UO2+x. The test parameters were selected in such a manner that burst temperatures of about 700°C were obtained. The results of the isothermal creep rupture tests with UO2+x of various OU ratios showed that at initial oxygen potentials as high as −230 kJ/mol O2 (which corresponds to an OU ratio of about 2.04 at 700°C), the CsI dissociates sufficiently to cause iodine-induced low-ductility cladding failure. When the oxygen potential was set by Mo/MoO2 mixtures, sufficient dissociation of CsI took place at much lower oxygen potentials of about −400 kJ/mol O2. With respect to the CsI dissociation both the formation of Cs oxides and the possible formation of complex compounds such as Cs uranate and Cs molybdate must be considered. Under these conditions, the I from the CsI behaved like elemental iodine. That is, iodine-induced low-ductility failure of the Zircaloy-4 tubing due to SCC occurred when the fission-product element iodine was present as either elemental I or as CsI.