Effect of loading rate upon fracture behavior of ferritic steel under large scaled yielding

Dynamic loading to ferromagnetic materials and large scaled yielding result in peak or valley and non-linear curve, respectively, on the Direct Current Potential Drop (DCPD) versus Crack Opening Displacement (COD) plots, which make it difficult to determine the crack initiation point. In this work high intensity of current up to 100 A was applied to the specimens of SA106Gr.C ferritic steel and the crack growth behavior was directly monitored by a high speed camera to obtain the crack initiation point. The effects of loading rate up to 1200 mm min⁻¹ upon the fracture resistance were explored. As the results, it has been shown that, although no substantial difference was seen in the load–COD plots, the crack initiation and then J_i and J–R curve were quite sensitive to the loading rate. That is, under the loading rate of 300 mm min⁻¹ the material showed the worst fracture resistance than under static loading and even under the higher loading rates of 600 and 1200 mm min⁻¹. Also applying the high speed camera and high current source have been proved to be an effective way to find out the accurate crack initiation point and to compensate the pulse of DCPD due to the ferromagnetic effect.     

Short circumferential through-wall cracked pipes subjected to stretch-bending load

The methods for assessment of elastic–plastic fracture behaviour of cracked components include the net section plastic collapse concept, the J-integral approach, and the two-parameter R-6 failure assessment diagram, Revision 3. These failure assessment methods are usually used to obtain fracture behaviour prediction with a reasonable degree of accuracy without carrying out complicated full-length numerical fracture analysis. In the current work, fracture experiments on stainless steel pipes with short circumferential through-wall cracks under stretch-bending load were conducted. Stretch-bending load refers to the loading situation where axial load is generated that is proportional or related to the applied bending load. The J-integral values derived from the experimental load-point load–displacement data under stretch-bending and pure bending conditions are compared to investigate the effect of axial load on the J–resistance curves. The results show clear dependence of crack resistance force on axial load for short circumferential cracks. Crack resistance force decreased noticeably for increased stretch-bending loading compared to pure bending loading.     

Influences of the dynamic strain aging on the J–R fracture characteristics of the ferritic steels for reactor coolant piping system

The leak-before-break (LBB) design of the piping system for nuclear power plants has been based on the premise that the leakage due to the through-wall crack can be detected by using leak detection systems before a catastrophic break. The piping materials are required to have excellent J–R fracture characteristics. However, where ferritic steels for reactor coolant piping systems operate at the temperatures where dynamic strain aging (DSA) could occur, the fracture resistance could be reduced with the influence of DSA under dynamic loading. Therefore, in order to apply the LBB design concept to the piping system under seismic loading, both static and dynamic J–R characteristics must be evaluated.Materials used in this study are SA516 Gr.70 for the elbow pipe and SA508 Cl.1a for the main pipe and their welding joints. The crack extension during the dynamic and the static J–R tests was measured by the direct current potential drop (DCPD) and the compliance method, respectively. This paper describes the influences of the dynamic strain aging on the J–R fracture characteristics with the loading rate of the pipe materials and their welding joints.     

The use of acoustic emission to determine characteristic dynamic strength and toughness properties of steel

The dynamic J integral at crack initiation (J_id) and dynamic yield stress (σ_yd) are useful parameters to characterize elastic-plastic material behaviour under rapid loading rates. The critical step for evaluating J_id and σ_yd under the condition of the three point impact bending test is the detection of the crack initiation and of the yield point in the impact load–deflection curve, respectively. This paper presents an acoustic emission (AE) based method to determine the ductile crack initiation and additionally the beginning of yield. The experimental techniques used to evaluate σ_yd and J_id include both instrumented pendulum impact tests with the AE transducer within the striker (tup) and medium rate three point bend (TPB) tests with additional AE transducers on the specimen surface. Results obtained from the tests indicate that the AE method is capable of detecting general yielding and the onset of ductile crack growth (initiation). Different types of pulse shaped AE signals can be observed. They were connected with characteristic features during the loading process.     

Experimental investigation on ductile stable crack growth emanating from wire-cut notch in AISI 4340 steel

An experimental investigation on the stable crack growth (SCG) behaviour in AISI 4340 using CT type specimen with a sharp slit (0.05 mm) under mode I and mixed modes (I and II) loading is presented. The slit was made in the specimen through wire cutting technique. Different combinations of loading angle and ratio of original crack length to specimen width (a₀/W) are examined. Data concerned with direction of initial crack extension, load–load line displacement (L–LLD) diagrams, initiation and maximum loads, range of stable crack growth, crack tip blunting, crack front geometry, fracture surfaces and their scanning electron micrographs are obtained. A noticeable blunting effect is observed prior to crack initiation. Although the crack initiates from a straight front, a considerable front tunnelling effect occurs as the crack extends. Under mixed mode, the crack extension takes place initially almost along a straight path, inclined with the main crack. The loading angle and initial crack length affect the initiation (P_i) and maximum (P_max) loads significantly, but the ratio between P_max and P_i remains almost constant. The direction of initial stable crack extension due to mixed mode loading is determined throughout an elastic finite element analysis. There is a good agreement between the experimental and predicted results.     

Low cycle fatigue and ductile fracture for Japanese carbon steel piping under dynamic loadings

Dynamic fracture behavior of circumferentially cracked pipe is important to evaluate the structural integrity of nuclear piping from the viewpoint of the LBB concept under seismic conditions. Fracture tests have been conducted for Japanese carbon steel (STS410) circumferentially through-wall cracked pipes that are subjected to monotonic or cyclic bending loads at room temperature. In the monotonic-loading tests, the maximum load to failure increases slightly with increasing loading rate. The failure cycles can be expressed simply by ratio of the load amplitude to the plastic collapse load. Fracture analysis has been also conducted to model the pipe tests. A new equation for calculating ΔJ for a circumferentially through-wall cracked pipe subjected to bending has been proposed. The failure cycles under cyclic loads are satisfactorily evaluated using an elastic-plastic fracture mechanics parameter ΔJ.     

Low cycle fatigue of welded joints: new experimental approach

To take into account the reduced fatigue strength of welded joints, a reduction life factor applied on fatigue curves (J_f value) was introduced into the RCC-MR [Design and Construction Rules for Mechanical Components of FBR Nuclear Islands, AFCEN, 1993], for the design and construction of fast breeder reactors. To better assess this factor, previous work showed that mechanical behavior of a welded assembly is influenced by the geometry of the weld and by the interaction of the different cyclic plastic behavior of the two materials: base metal (BM) and weld metal (WM). A new procedure (named FFAST) was performed on welded joint specimens extracted from butt-welded pipe connections (uniaxial tensile–compressive load). An innovative experimental approach is proposed to study the local mechanical behavior of the welded joint specimens and then determine the J_f parameter. The main advantage of the method is to avoid problems due to the relative stiffness of weld part versus the BM part of the specimen. A continuous recording of the stress and strain in the weld allows an estimation of the mechanical behavior and finally the fatigue life of the joint. Observations of the crack surface show two different crack initiation zones near the weld depending on the load level. Calculations of the tests and comparison with experimental results are presented. These studies make it possible to assess in a practical way the J_f design method. It appears that J_f value cannot be considered as a single value for it is influenced by several factors depending on the weldment and on the load level.     

Effect of dynamic strain aging on the leak-before-break analysis in SA106 Gr.C piping steel

The characteristics of dynamic strain aging (DSA) on material properties used in leak-before-break (LBB) analysis were discussed. Using these material data, the effect of DSA on the LBB analysis was estimated through the evaluation of leakage-size crack and flaw stability in SA106 Gr.C piping steel. Also, the results were represented as a form of ‘LBB allowable load window'. In the DSA temperature region, the leakage-size crack length was smaller than that at other temperatures and it increased with increasing tensile strain rate. In the results of flaw stability analysis, the lowest instability load appeared at the temperature corresponding to the minimum J–R curve which was caused by DSA. The instability load depended on the loading rate of J–R data, and decreased with increasing tensile strain rate at the plant operating temperature. These are due to the strain hardening characteristic and strain rate sensitivity of DSA. In the ‘LBB allowable load window', the LBB allowable region at the temperature and loading conditions where DSA occurs was decreased by about 30% compared with that in other conditions.     

Evaluation of microcrack resistance curves by in-situ measurements

The paper deals with the influence of the microstructure on the crack resistance behaviour of a bainitic pressure vessel steel. The size, shape, distribution and volume fraction of the carbides were changed by heat treatment. An in-situ electron microscopy technique allows the direct observation of the deformation process near the crack tip and the continuous measurement of δ–Δa curves under loading. These microcrack resistance curves are discussed in connection with sets of micrographs of the crack tip region for increasing load levels. It could be concluded, that both the crack tip blunting and the elastic-plastic fracture initiation parameter were determined by the microstructure.     

JR curves of the low alloy steel 20 MnMoNi 5 5 with two different sulphur contents in oxygen-containing high temperature water at 240°C

J_R curves of the low alloy steel 20 MnMoNi 5 5 with two different sulphur contents (0.003 and 0.011 wt.%) were determined at 240°C in oxygen-containing high temperature water as well as in air. The tests were performed by the single-specimen unloading compliance technique at load line displacement rates from 1 × 10⁻⁴ down to 1 × 10⁻⁶ mm s⁻¹ on 20% side-grooved 2T CT specimens in an autoclave testing facility at an oxygen content of 8 ppm and a pressure of 7 MPa under quasi-stagnant flow conditions.In the case of testing in high temperature water, remarkably lower J_R curves than in air at the same load line displacement rate (1 × 10⁻⁴ mm s⁻¹) were obtained. A decrease in the load line displacement rate as well as an increase in the sulphur content of the steel caused a reduction of the J_R curves. At the fastest load line displacement rate a stretch zone could be detected fractographically on the specimens tested in air and in high temperature water and consequently J_i could be determined. When testing in high temperature water, the J_i value of the higher sulphur material type decreases from 45 N mm⁻¹ in air to 3 N mm⁻¹, much more than that of the optimized material type from 51 N mm⁻¹ in air to 20 N mm⁻¹ at 1 × 10⁻⁴ mm s⁻¹.     

Yielding- and fracture behaviour of ferritic steels in the transition region of quasistatic to dynamic loading

This paper presents results of investigations on the influence of loading rate on yielding and fracture behaviour of ferritic steels. The range of loading rates was below a certain level at which a special stress wave analysis is required. Concerning the yielding behaviour it was found, that the yield strength can be predicted by the model of thermally activated flow. The strain hardening dσ/dε appeared to be independent of strain rate, if adiabetic heating can be neglected. Concerning the fracture behaviour it is demonstrated that the K_lc-T-curves are shifted to higher temperatures with increasing loading rate. The temperature shift could be correlated with the strain rate sensitivity m = d lnσ/d lnε. The ductile/brittle transition temperature increases with incrasing loading rate. For the upper shelf region crack resistance curves as a function of loading rate are presented. It is shown that the crack length can be determined using the key-curve-method. A slightly increasing tendency of the evaluated J_R-curves was found.     

The importance of the triaxiality modified J concept for the assessment of pressurized components with surface flaws

With the progress of stable crack growth of surface flaws observed in panels or pressure vessels a canoe-shaped crack front is formed. The crack propagation in the longitudinal direction is more pronounced that in the wall thickness direction. Therefore, the canoe effect is important with respect to a leak-before-break assessment because the actual through crack length is influenced by this effect. Based on the J integral concept crack initiation and crack propagation in ductile materials are described by J resistance curves which were found to be dependent on the constraint effect of the specimen geometry. Prediction of local crack growth by taking a conservative (flat) J_R-curve into account results in a nonconservative estimate of the axial extension of the surface crack [W. Brocks, H. Veith and K. Wobst, in K. Kussmaul (ed.), Fracture Mechanics Verification by Large Scale Testing, Mech. Eng. Publication Limited, London, 1991]. This means that the influence of local constraint effects on crack resistance has to be considered.Ductile crack growth of semi-elliptical surface cracks in side-grooved specimens F(SCTsg) under tension made from German standard steel StE 460 will be reported on. The development of the canoe effect of an SCTsg specimen was also analysed by a finite element simulation of ductile crack growth which was modelled by using the node shift and node release technique and controlled by crack mouth opening displacement versus crack growth curves from the experiment. The simulation allows the determination of local J_R-curves in dependence on the local multiaxility of the stress state to verify the constraint modified J concept. It is demonstrate that the slope of the J_R-curves decreases with increasing multiaxiality of the stress state near by the crack front.     

Large stable crack growth in fracture mechanics specimens

According to the J concept, information is reported about the crack resistance behaviour up to 8 mm crack growth of side-grooved CT-25 as well as CCT-25 specimens made from German standard steel StE 460. Numerical simulations controlled by J_R curves make the calculation of J from the stresses and strains of specimen models during large crack growth feasible. These data allow a comparison to standards and rules describing the evaluation of J from experiments. Using stress, strain and displacement fields from a plane-strain finite-element analysis, the extended J concept is discussed concerning larger ductile crack growth. Additionally, the distribution of other fracture mechanics parameters such as the crack tip opening displacement (CTOD) and the crack tip opening angle (CTOA) are presented for larger crack growth.     

Ductile fracture parameters of RPV-materials

Ductile fracture material parameters have been determined for a reactor pressure vessel material to characterize its fracture resistance in the upper shelf toughness regime. Three different methods (the multiple specimen unloading (MSU), direct current potential drop (DCPD) and single specimen partial unloading compliance (SSPUC) methods) have been applied to test different CT-specimen geometries at temperatures between 25 and 300°C.It is shown that there are principle differences between J-R-curves measured by different experimental procedures, because of different methods for the measurement of crack lengths and crack growth. For instability analyses, using a complete J-R-curve, these differences seem to be negligible. For the determination of critical material parameters at or close to initiation of stable crack growth these differences may cause systematic errors tending to higher values for DCPD as compared to MSU-results and to lower values for SSPUC respectively.Procedures can be defined to evaluate comparable critical material parameters from the different experimental procedures, if J_i is known in a good approximation allowing to consider only the real crack extension without blunting, or if in addition the real (or realistically modelled) blunting and the effective blunting of the specific method are known. The differences in material parameters will depend quantitatively on the type of material and its toughness (slope of J-R-curve). They may be in the range of the experimental scatter observed in testing and seem to be negligible, but their systematic character should be kept in mind, e.g. when ranking different materials according to their critical parameters determined by different methods.     

Fracture initiation and propagation in a cylindrical shell containing an initial surface flaw

The paper deals with the problem of fracture initiation, propagation, and arrest in a pressurized cylindrical vessel which contains an initial surface flaw. It is assumed that the flaw has the most unfavorable geometry and orientation, namely, it is a relatively long part-through axial crack.First we consider the problem of a crack which is sufficiently ‘shallow’ so that the plastic deformations are confined to the neighborhood of the crack border and part of the net section near the inner wall is still elastic. The plasticity-corrected stress intensity factor obtained from this analysis is the controlling load factor in failure considerations related to fatigue crack propagation, stress corrosion cracking, and static fracture (with the use of fracture toughness, COD, or a K_R curvetype failure criterion).The problem of relatively deep crack with fully-yielded net ligament is then considered. Plastic deformations are also assumed to spread around the crack ends through the entire wall thickness. A perfectly plastic strip model (with an eight order shell theory) is used to calculate the plastic zone size and the crack opening displacement along the crack border. Previous studies indicate that for the analysis of the type of stable and subsequent unstable crack propagation problems under consideration, the crack opening displacement δ is a more suitable load factor than the stress intensity factor K, or the crack extension force G. Thus, in this paper a ‘crack opening stretch’ type material characterization will be used.After the rupture of the net ligament under the crack, the axial crack propagation is accompanied by the depressurization of the vessel caused by leakage. From this point on the fracture problem is coupled with the related fluid mechanics or gas dynamics problem where the primary unknowns are the pressure and the crack length as functions of time. In the present study it is assumed that the volume of the vessel is finite and the crack propagation is quasi-static (this assumption, which is necessary to keep the problem within manageable proportions, is justified by the relatively low crack velocities, i.e. v_c < 0.25 c₂, c₂ being the shear wave velocity).     

The growth of fatigue cracks in the creep range from the point of view of ductile fracture

This paper deals with experimental and numerical investigations of crack growth in 1% CrMoV steels in the creep temperature range. For the load and displacement controlled fatigue tests with predominantly plastic deformation amplitudes, centre cracked panels and CT-specimens of different thicknesses were used.For describing the crack growth per load cycle the crack tip displacement δ_t and the J-integral were applied, whereby the question arises whether the various fracture concepts are applicable unrestrictedly or where they meet their limits of validity.Detailed theoretical-numerical calculations using the finite element method were expected to yield information about the path independence of the J-integral. Special importance was therefore attached to investigate the J-integral, both cyclical and with reference to the instantaneous state of deformation, by comparing the different J-integral values (ΔJ, J) with the experimental method of Dowling/Begley.     

Constraint effects on fracture toughness of impact-loaded, precracked Charpy specimens

Impact-loaded, precracked Charpy specimens often play a crucial role in irradiation surveillance programs for nuclear power plants. However, the small specimen size B = W = 10 mm limits the maximum value of cleavage fracture toughness J_c that can be measured under elastic—plastic conditions without loss of crack tip constraint. In this investigation, plane strain impact analyses provide detailed resolution of crack tip fields for impact-loaded specimens. Crack tip stress fields are characterized in terms of J—Q trajectories and the toughness-scaling model which is applicable for a cleavage fracture mechanism. Results of the analyses suggest deformation limits at fracture in the form of b > MJ_c/σ₀, where M approaches 25–30 for a strongly rate-sensitive material at impact velocities of 3–6 m s⁻¹. Based on direct comparison of the static and dynamic J values computed using a domain integral formulation, a new proposal emerges for the transition time, the time after impact at which interial effects diminish sufficiently for simple evaluation of J using the plastic η factor approach.     

Influence of hydrogen content on fracture toughness of CWSR Zr–2.5Nb pressure tube alloy

In this work, influence of hydrogen and temperature on the fracture toughness parameters of unirradiated, cold worked and stress relieved (CWSR) Zr–2.5Nb pressure tube alloys used in Indian Pressurized Heavy Water Reactor is reported. The fracture toughness tests were carried out using 17 mm width curved compact tension specimens machined from gaseously hydrogen charged tube-sections. Metallography of the samples revealed that hydrides were predominantly oriented along axial–circumferential plane of the tube. Fracture toughness tests were carried out in the temperature range of 30–300 °C as per ASTM standard E-1820-06, with the crack length measured using direct current potential drop (DCPD) technique. The fracture toughness parameters (J_Q, J_Max and dJ/da), were determined. The critical crack length (CCL) for catastrophic failure was determined using a numerical method. It was observed that for a given test temperature, the fracture toughness parameters representing crack initiation (J_Q) and crack propagation (J_Max, and dJ/da) is practically unaffected by hydrogen content. Also, for given hydrogen content, all the aforementioned fracture toughness parameters increased with temperature to a saturation value.     

Time dependence of J-integral

Tests performed within the framework of earlier RWTÜV projects together with results obtained elsewhere with regard to the time dependence of fracture mechanics data show that time effects reduced the toughness of materials, according to the nature of the test (extremely slow load rate or hold times with sustained load).

Reduction in toughness has an effect on the following:

&#x02022; - decrease in critical material data (J₀, δ_i)

&#x02022; - levelling off of the crack resistance curve J = J(Δa) and in consequence a decrease of tearing modulus.

This tendency is confirmed quantitatively by recent test results. These tests were performed with the material 15 Mn Ni 63 at room temperature with hold times under sustained load and according to the appropriate standards (without hold times). The tests show that hold times cause additional stable crack growth. The resulting J − Δa curve is lower and less sloping than the curve obtained in a standardized test. The time effect should be taken into account in a safety analysis.     

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.