Ductile Fracture Resistance Research Articles

Derivation of ductile fracture resistance by use of small punch specimens

Conventional fracture mechanics cannot address the influences of stress triaxiality since the assumption of geometry independence is valid only in limited conditions. Various local approaches have been proposed to investigate the material’s fracture behaviours covering a broad range of loading and flaw shapes. However, unfortunately, standard test specimens have been difficult to obtain from operating facilities. This paper investigates the characterization of ductile fracture resistance ( J– R) curves by the small punch (SP) testing technique in conjunction with finite element analyses incorporating well-known damage models. In this context, basically, standard tensile tests and SP tests using tiny specimens are carried out. Furthermore, micro-mechanical parameters constituting the Gurson–Tvergaard–Needleman and Rousselier models are calibrated for typical nuclear materials based on experimental and estimated load–displacement data of their miniaturized specimens. Then, the J– R curves of larger 1 2 T-CT (compact tension) as well as standard 1T-CT specimens are predicted by detailed finite element analyses employing the calibrated parameters. Finally, the estimated J– R curves are validated by fracture toughness tests using the standard CT specimens. The estimated results suggested confidence in the use of the proposed method for ductile crack growth evaluation.

Dynamic strain ageing in prior cold worked 15Cr–15Ni titanium modified stainless steel (Alloy D9)

Dynamic strain ageing (DSA) behavior of a 20% prior cold worked titanium modified austenitic stainless steel (Alloy D9) was investigated in the temperature range 300–1023K at a constant (nominal) strain rate of 1.33×10−3s−1. Serrated plastic flow was observed in a narrow temperature region of 773–873K. Anomalies characteristic of DSA were observed in the variations of strength, ductility, and strain hardening parameters with temperature. Specifically, the difference between ultimate tensile stress and yield stress indicated that a single DSA mechanism is operative over the range of observations. Plastic strain energy density for uniform deformation showed a peak in the DSA temperature range, suggesting that in this material, DSA should enhance the ductile fracture resistance.

炭素鋼配管の軸方向き裂の延性進展抵抗(OS6-2 配管の欠陥評価,OS6 エネルギー機器の維持管理と健全性評価)

It is very important to develop a simplified flaw evaluation method that can be applied to carbon steel pipes with an axial flaw under the elastic-plastic fracture mode, since the JSME fitness for services code does not contain such a technique. So, this paper makes clear the ductile fracture resistance, J-R curves of an axial crack in STS410 carbon steel pipes commonly used in the primary loops of boiling water reactor plants, and derives the Z-factor for axial flaw evaluation based on the failure assessment diagram approach.

Parametrizing ductile tearing resistance by four parameters

The energy dissipation rate, R, is considered as a measure of resistance to crack extension in elasto-plastic fracture mechanics. It can be re-evaluated from J R test records of bend and tensile specimens. Three types of R(Δ a)-curves are identified. If crack initiation occurs close to or at maximum load, the energy dissipation rate is decreasing with crack extension and approaches a stationary value. This type of R(Δ a)-curves can be described by an exponential function with three parameters, namely the initial value, the final stationary value, and a transition length. The cumulative J R-curves for different specimen geometries can be derived by integration. The three parameters of the R(Δ a)-fit together with an integration constant, the initiation value, J i , characterize ductile fracture resistance both quantitatively and physically interpretable. Constraint effects on R-curves can be quantified in terms of these parameters. A procedure for transferring J R-curves from one geometry to another is proposed.

Low‐cost J‐R curve estimation based on CVN upper shelf energy

J‐R curve testing is costly and difficult. The results may also sometimes be unreliable. For less demanding structures, J‐R curve testing is therefore not practical. The only way to introduce tearing instability analysis for such cases is to estimate the J‐R curves indirectly from some simpler test. The Charpy‐V notch test provides information about the energy needed to fracture a small specimen in half. On the upper shelf this energy relates to ductile fracture resistance and it is possible to correlate it to the J‐R curve. Here, 112 multispecimen J‐R curves from a wide variety of materials were analysed and a simple power‐law‐based description of the J‐R curves was correlated to the CVNUS energy. This new correlation corresponds essentially to a 5% lower bound and conforms well with the earlier correlations, regardless of the definition of the ductile fracture toughness parameter.

J-R fracture properties of SA508-1a ferritic steels and SA312-TP347 austenitic steels for pressurized water reactor’s (PWR) primary coolant piping

J-R fracture resistance of SA508-1a and SA312-TP347 steels, which are both rather peculiar as PWR primary coolant piping materials, were evaluated for application of leak-before-break methodology to the design basis of nuclear power plant piping. Archive materials from various heats of both steel pipes showed apparent heat-to-heat variations in ductile fracture resistance at the operating temperature 316°C. The SA508-1a ferritic steels showed relatively good J-R curve properties although they varied with the microstructures depending on the manufacturing process and chemical compositions. On the other hand, ductile crack growth resistance of SA312-TP347 austenitic stainless steel was unexpectedly poor when carbon content is moderately high. It was found that coarse niobium carbides deteriorated the ductile fracture resistance, so that more rigorous specification is needed in carbon and niobium contents to improve fracture properties of Type 347 stainless steels.

The feasibility of small size specimens for testing of environmentally assisted cracking of irradiated materials and of materials under irradiation in reactor core

Environmentally assisted cracking (EAC) or, in other words, stress corrosion cracking (SCC) of in-core materials has become an increasingly important reason for the downtime and maintenance costs of nuclear power plants (NPPs). Use of small size specimens for stress corrosion testing of irradiated materials is necessary because handling of high dose rate materials is difficult and the availability of these materials is limited. A drawback of using small size specimens is that they do not in some cases fulfil the requirements of the relevant testing standards and sometimes their limited load-bearing capacity prevents corrosion fatigue tests and tests with static loading at reasonable K I values. The test results show that the ductile fracture resistance curves of a Cu–Zr–Cr alloy are, to some extent, independent of the specimen geometry and size. However, the curves of small specimens deviate from the curves of larger specimens at high J values (large plastic zone relative to the remaining ligament) or when the crack growth exceeds about 30% of the remaining ligament. The size dependency of the tested Cu–Zr–Cr alloy seems to be a consequence of decreasing stress triaxiality as the size of the specimen is decreased. The results of the SCC tests of sensitized SIS 2333 stainless steel (equal to AISI 304) specimens in simulated boiling water reactor (BWR) water show that the plastic deformation of the remaining ligament of the specimen has no significant effect on the environmentally assisted crack growth rate. This indicates that stress corrosion testing is not limited by the specimen size. The size dependency in SCC tests should be further studied by conducting tests using various specimen sizes.

Effect of specimen type and size on fracture resistance curve determination for CuCrZr alloy

The precipitation hardened copper alloy CuCrZr is one of the candidate materials for the first wall and divertor components of ITER. In order to validate the applicability of copper alloys in the structural components of ITER, the effect of neutron irradiation on the fracture toughness behaviour has to be studied. Fracture toughness testing of irradiated materials requires the use of miniaturised specimens and the verification of specimen size effects on fracture toughness test results. In the present study the effects of specimen type, size as well as temperature on the ductile tearing fracture resistance behaviour of CuCrZr alloy will be shown. The applicability of miniaturised specimens for fracture resistance curve determination is considered by studying the allowable crack extension with which the miniaturised specimens still have similar fracture resistance curves as full-size specimens. The crack length and J-integral values will be compared to ASTM standard requirements for the maximum allowable crack extension and J-integral values in the fracture resistance test.

Relationship between the structure and ductile fracture resistance of ferritic-pearlitic structural steels

The nucleation in structural steels of regions of ductile failure whose main micromechanism is the formation, development, and coalescence of microvoids is usually linked with nonmetallic inclusions, and the cracking resistanceδcdwith their dimensions, form, composition, number and the nature of distribution.

Effect of Cyclic Loading on Ductile Fracture Resistance

The influence of cyclic loading on ductile fracture toughness was examined from limited experimental data available in the literature. The review indicated that, in the absence of global compressive loads during the unloading cycle (R ≥ 0), the total crack extension generally is the sum of the monotonic component (Δamono) and the cyclic component (Δacyclic). The former can be obtained from the monotonic J-R curve and the latter from a fatigue-crack-growth curve. When large global compressive loads are developed during unloading (R<0), the review indicated that simply summing Δamono and Δacyclic gives an under prediction of the actual crack extension in load cycling tests. Thus, it appears that the local compressive strains lower a material’s normal monotonic fracture resistance in a region immediately ahead of the crack following the load reversal. Although few data are available in this regime of cyclic loading, it is likely that the magnitude of the loss of monotonic toughness depends on frequency of unloadings, R-ratio, and material. For ferritic steels susceptible to strain aging, temperature and strain rate also are likely to be important. The review also examined the applicability of various fracture mechanics parameters, including those based on deformation plasticity and those based on incremental plasticity, in quantifying cyclic-loading effects. While incremental plasticity parameters (for example T*) appear promising, they may be time-consuming and costly to develop. Until such advanced parameters are available, it is likely that empirical correlations involving deformation J will have to be developed to handle cyclic loading effects. Finally, the significance of the findings reported herein relative to the performance of flawed nuclear piping subjected to cyclic loading is discussed. It appears that circumferential cracks are of greater concern in seismic events than are axial cracks because the former may experience negative R-ratios.

Fracture toughness testing on ferritic alloys using the electropotential technique

Fracture toughness measurements as done conventionally require large specimens (5×5×2.5 cm) which would be prohibitively expensive to irradiate over the fluence and temperature ranges required for first wall design. To overcome this difficulty a single specimen technique for J integral fracture toughness measurements on miniature specimens (1.6 cm OD × 0.25 cm thick) was developed. Comparisons with specimens three times as thick show that the derived J IC is constant, validating the specimen for first wall applications. The electropotential technique was used to obtain continuous crack extension measurements, allowing a ductile fracture resistance curve to be constructed from a single specimen. Fracture toughness test results for HT-9 and 9Cr-lMo from 25–539°C are presented to illustrate the technique.

Use of the DWTT Energy for Predicting Ductile Fracture Behavior in Controlled-Rolled Steel Line Pipes

Abstract Prior to this time, it has been uncertain how ductile fracture toughness measurements of highly separated controlled-rolled steel pipes should be characterized to design pipelines against unstable ductile fracture propagation. Highly separated controlled-rolled steels are unique from conventional steels in that through-the-thickness splitting, or separations, can significantly effect the toughness.This paper will show that a better evaluation of the ductile fracture resistance of rising shelf controlled rolled steels than Charpy energy can be made by using the drop weight tear test (DWTT) energy at the service temperature. It has also been found that generally for higher toughness steels that a precracked notch DWTT specimen is needed to evaluate the propagation toughness. Both the pressed notch and precracked notch DWTT energies can be correlated to Charpy energy values, so that existing formulations for ductile fracture arrest in pipelines can be used with the DWTT energy. Comparisons of variou...

Fracture propagation control in gas pipelines: A survey of relevant studies

This paper reviews the problem of crack propagation in gas pressurised pipelines. After summarising the fracture control philosophy for pipelines, the main terminology and basic concepts are defined. A review of the brittle fracture problem summarises Battelle and British Gas studies relating fracture appearance and crack behaviour, and the background to the adoption of the DWTT fracture appearance criterion in specifications is detailed. The application of the strain energy theory to full-scale test results is discussed. A balance between strain energy stored in the pipe wall and fracture energy explains why brittle fractures arrest if the stress is below a critical level. The problem of ductile fracture propagation is reviewed and the results of full-size tests carried out by British Gas to provide toughness levels for fracture arrest are presented. The nature of ductile fracture propagation is discussed with reference to analogies with fracture initiation. Steady state, constant velocity, fractures imply specific shapes of the G (energy available)-velocity characteristic curves. In particular, a falling G with increasing fracture velocity is necessary. Some published theories to account for this effect are summarised and it is concluded that considerations of inertia in the fractured pipe wall provide the required relationship. Laboratory toughness tests for assessing brittle and ductile fracture resistance in relation to pipeline fracture behaviour are discussed. The relevance of fracture mechanics tests, particularly R curve analysis, is highlighted. The paper concludes with a list of areas of work which warrant further study.

The effect of the carbide phases on the crack grow of 0.5 pct Mo-B steels under impact, cyclic, and monotonically increasing loading

A study of the influence of carbide phases on the cracking resistance of as-quenched and of quenched and tempered 0.5 pct Mo-B steels was made using notched or notched and precracked specimens that were subjected to impact, cyclic, and monotonically increasing loading. The carbide influence on fracture, while limited in extent, was found to increase as load, loading rate, volume fraction, and particle size increase. The results for the asquenched condition showed that the susceptibility of these steels to crack initiation under impact loading at temperatures below - 100°F is greater when even a small amount of titanium carbide (less than 0.2 vol pet of 1 to 5 μm particles) is present than when none is present. At room temperature, this same carbide concentration has no influence on impact properties, fatigue-crack initiation (in the presence of a notch), fatigue-crack growth rate, or the ductile fracture resistance under monotonically increasing loading at slow strain rate. In the case of the quenched and tempered materials, the alloy containing a large amount of M23C6 (2 vol pct of 1 to 10 μm particles) exhibited behavior similar to that observed in the as-quenched materials containing titanium carbide. That is, the presence of M23C6 was associated with increased susceptibility to crack initiation for impact loading at low temperature. In addition, at room temperature this alloy had a reduced impact energy for crack propagation. For monotonically increasing loading at slow strain rate, this same carbide distribution had no influence on the net section stresses required to initiate stable or unstable crack growth. These stresses fall closely in line with, respectively, the yield stress and tensile strength of the material. The alloy containing M23C6 required less crack opening for a given crack extension—an effect most pronounced after maximum load. Finally, some attention is directed to the use of Charpy test data to assess fracture resistance for modes of loading other than impact.

Fracture of steels containing pearlite

The relative effects of pearlite and spherodite on ductile, cleavage, and fatigue failure are summarized. Neither the cleavage strength nor the fatigue endurance limit appear to depend directly on cementite contentper se. Spherodized steels cleave less readily than ferrite/pearlite steels. Ductile fracture resistance is lowered considerably by both types of cementite, pearlite being more deleterious. Ferrite/pearlite steels appear to exhibit slower fatigue crack growth rates at low stress intensity levels than high strength steels. At high stress intensity levels the behavior is reversed. Slip-incuded cracking of carbide lamellae appears easier than that of spherodized carbides. In ductile fracture situations the crack spreads progressively through a pearlite colony via preferential cracking of carbides and rupture of the intervening ferrite accompanied by large local shear strains. Fatigue fracture proceeds with formation of frequent branches, preferentially along the pearlite colony interface.