Rising Load Tests Research Articles

Threshold stress intensity factor for hydrogen-assisted cracking of CR-MO steel used as stationary storage buffer of a hydrogen refueling station

In order to determine appropriate value for threshold stress intensity factor for hydrogen-assisted cracking (KIH), constant-displacement and rising-load tests were conducted in high-pressure hydrogen gas for JIS-SCM435 low alloy steel (Cr-Mo steel) used as stationary storage buffer of a hydrogen refuelling station with 0.2% proof strength and ultimate tensile strength equal to 772 MPa and 948 MPa respectively. Thresholds for crack arrest under constant displacement and for crack initiation under rising load were identified. The crack arrest threshold under constant displacement was 44.3 MPa m1/2 to 44.5 MPa m1/2 when small-scale yielding and plane-strain criteria were satisfied and the crack initiation threshold under rising load was 33.1 MPa m1/2 to 41.1 MPa m1/2 in 115 MPa hydrogen gas. The crack arrest threshold was roughly equivalent to the crack initiation threshold although the crack initiation threshold showed slightly more conservative values. It was considered that both test methods could be suitable to determine appropriate value for KIH for this material.

Study of Embrittlement of the 2.25Cr-1Mo-V Steel Weld Metal by Hydrogen Charge and High Pressure Hydrogen Gas Environment

It is well-known that hydrogen would accumulate at internal defects of pressure vessels during shutdown and hydrogen embrittlement occurred. However it has not been clear that the effect of hydrogen gas environment for 2.25Cr-1Mo-V steel (22V) weld metal. For this reason, the purpose of this work is to identify and understand the potential for embrittlement of the 22V steel by hydrogen charge and high pressure hydrogen gas environment.Therefore, rising load test was carried out in this study to examine the effect of dissolved hydrogen by high temperature, high pressure hydrogen exposure and high pressure hydrogen gas environment on hydrogen embrittlement at room temperature (R.T.) and 150 ̊C. 22V forged steel base metal was used and welding was conducted by submerged arc welding (SAW) process. High and low toughness weld metals were prepared by changing PWHT condition and notch location in order to consider variation of product's weld metal.From rising load test results, hydrogen gas environment had effect on the embrittlement of the 2.25Cr-1Mo-V steel weld metal. In contrast, dissolved hydrogen had little effect on the embrittlement. Moreover, KIH value of high toughness weld metal (KIH value is about 90 MPa√m) was higher than that of low toughness (KIH value is about 70 MPa√m) at RT. However, KIH value of low toughness weld metal was the similar level (KIH value is about 110 MPa√m) of high toughness weld metal at 150 ̊C.

Structural integrity of an X-750 jet pump beam of a BWR by means of FITNET FFS procedure

In this paper, the J– R curves of the X-750 alloy, previously obtained in the rising load test environment, have been implemented in the structural integrity assessment of a jet pump beam of a boiling water reactor manufactured in this material. The analysis was performed with the FITNET FFS procedure considering the in-service loads on the beam. The critical crack lengths for the different scenarios studied have been obtained and, taking into consideration real crack length velocities, the lifespan of the beams were conservatively estimated.

Obtaining the J–Δ a curves of an X-750 alloy from rising load test results and iso- a curves obtained by means of finite elements model

The rising load test (RLT) allows the susceptibility of the X-750 alloy to intergranular stress corrosion cracking (IGSCC) to be assessed. It consists of a three-point bending test of notched and precracked small specimens, in which the response in air and in an aggressive environment (water at 93 °C), under displacement control, is compared. The test classifies each material depending on the value of the stress intensity factor for the maximum load, P max, and, also, on the time for the load to drop from P max to 1/2 · P max during propagation. It is, in summary, a measurement of toughness at initiation together with a measurement of crack growth rate. In this paper, the response to IGSCC of an X-750 alloy under different heat treatments by means of the RLT is presented. In addition, a finite element model has been developed to obtain the iso- a curves (without propagation) for different crack lengths. By combining the experimental curves with the results of the numerical model it is possible to obtain the J–Δ a curves ( R curves) of the X-750 material, in air and RLT environment, respectively. In this paper, a detailed analysis methodology is presented.

Investigation of the susceptibility of EUROFER97 in lead–lithium to liquid metal embrittlement (LME)

Liquid metal embrittlement (LME) is defined as the brittle fracture (loss of ductility) of usually ductile materials in the presence of a liquid metal. The sensitivity to LME is likely to increase with irradiation hardening as localised stresses can promote the aggressive action of a liquid metal. To investigate the mechanical response of irradiated materials in contact with a liquid metal, an instrumented hot cell has been developed. The testing machine installed inside allows mechanical testing of active materials in liquid lead lithium under well controlled chemistry conditions. Typical mechanical tests that can be carried out are slow strain rate tests (SSRT), constant load and rising load tests at temperatures from 150 °C to 500 °C. In this paper the first results of the SSRT tests with EUROFER97 in argon and lead–lithium at different temperatures with different strain rates will be presented. The SSRT test method has been chosen due to the accelerated nature of the test, i.e., during straining the oxide layer will be ruptured and wetting of the sample surface by the lead–lithium melt is promoted. The results collected up till now showed no sign of LME. Tests with longer pre-exposure times and tests with irradiated samples will be carried out in the next phase. A longer pre-exposure time can enhance wetting and so the susceptibility to LME. An increase of the yield stress due to irradiation can also enhance the susceptibility to LME.

水素を吸蔵した 2.25Cr-1Mo 厚肉圧力容器用鋼の水素脆化割れ進展挙動におよぼす試験片サイズの影響

By means of charging large thick specimens with hydrogen, we investigated the effects of specimen size on hydrogen-embrittlement cracking. Crack extension in a hydrogen-charged 3.5T-CT specimen extended over a longer duration than was the case for a 1.0T-CT specimen. However, the values of the lower-bound threshold stress intensity factor (KIH) for 1.0T-CT and 3.5T-CT specimens were similar to one another when these were determined with a short-term rising load (dK/dt=0.005 MPa•m1/2•s-1). We conducted numerical analysis on the hydrogen diffusion and accumulation around a crack tip, taking into consideration the hydrogen distribution in the specimen. This analysis demonstrated that the maximum hydrogen concentration for cracking can be reached under the conditions present during a short-term rising load test (dK/dt=0.005 MPa•m1/2/s). Thus, the results of the numerical analysis confirm that a minimum value of KIH equivalent to that of a heavy section steel can be obtained with a small fracture mechanics specimen. We also attempted to explain long-term crack extension characteristics, taking into consideration hydrogen dissipation from a specimen. The analysis predicts that when the mean hydrogen concentration falls below a certain level (e.g., about 1.6 ppm under certain assumptions), the value of KIH increases significantly. This increase in KIH occurs because when the necessary stress intensity factor for cracking increases as a result of a decrease in the mean hydrogen concentration, the gradient of the maximum hydrostatic stress distribution becomes moderate, especially when the applied stress intensity factor is more than about 48 MPa•m1/2. Finally, we propose a method for the prediction of the long-term crack extension behavior of a large thick specimen; the method takes into consideration the hydrogen dissipation curve and the effect on KIH of a decrease in the mean hydrogen concentration.

Hydrogen embrittlement, grain boundary segregation, and stress corrosion cracking of alloy X-750 in low-and high-temperature water

The nature of intergranular stress corrosion cracking (SCC) of alloy X-750 was characterized in low-and high-temperature water by testing as-notched and precracked fracture mechanics specimens. Materials given the AH, BH, and HTH heat treatments were studied. While all heat treatments were susceptible to rapid low-temperature crack propagation (LTCP) below 150 °C, conditions AH and BH were particularly susceptible. Low-temperature tests under various loading conditions (e.g., constant displacement, constant load, and increasing load) revealed that the maximum stress intensity factors (Kpmax) from conventional rising load tests provide conservative estimates of the critical loading conditions in highly susceptible heats, regardless of the load path history. For resistant heats, KPmax provides a reasonable, but not necessarily conservative, estimate of the critical stress intensity factor for LTCP. Testing of as-notched specimens showed that LTCP will not initiate at a smooth surface or notch, but will readily occur if a cracklike defect is present. Comparison of the cracking response in water with that for hydrogen-precharged specimens tested in air demonstrated that LTCP is associated with hydrogen embrittlement of grain boundaries. Equivalent activation energies for stage II LTCP rates (11.3 kcal/mol) and hydrogen diffusion (11.5 kcal/mol) indicate that hydrogen diffusion to the peak stress region ahead of a crack is the rate-controlling process. Auger analysis showed that variability in LTCP resistance is associated with phosphorus and sulfur segregation to grain boundaries. Above 150 °C, an increase in fracture resistance and decrease in the degree of hydrogen enrichment precludes rapid intergranular cracking. The stress corrosion crack initiation and growth does occur in high-temperature water (>250 °C), but crack growth rates are orders of magnitude lower than LTCP rates. The SCC resistance of HTH heats is far superior to that of AH heats as crack initiation times are two to three orders of magnitude greater and growth rates are one to two orders of magnitude lower.

The effect of crack blunting in liquid metal environments on KIEAC determined by the rising load test

KIEAC is the threshold stress intensity below which environmentally assisted crack growth does not occur. This parameter is equivalent to Klscc for stress corrosion cracking, or KILME for crack growth in molten metal environments. Two techniques frequently employed in the determination of KIEAC are the constant load and constant displacement tests. Although these tests have been used extensively in the past [1], they require substantially long testing times before reliable KIEAC values can be obtained. By using environments significantly more aggressive than those encountered in practice, the testing times can be reduced. However, this approach introduces considerable difficulties and uncertainties in the interpretation of data. In order to overcome the limitations of the constant load and constant displacement tests, McIntyre and Priest [2] proposed the use of an accelerated testing method, now commonly referred to as the rising load K~scc test. This technique is essentially identical to the procedure used for Kic fracture toughness testing [3], except that slower loading rates are used. A monotonically increasing load is applied to a pre-cracked fracture mechanics type specimen exposed to the environment of interest. The stress intensity for crack initiation is calculated from the initiation load, as determined from the point of deviation from the linear load-displacement record, and the corresponding crack length (Fig. 1). This stress intensity value is assumed to be equivalent to KIEAC.

Determination of the Loading Rate Needed to Obtain Environmentally Assisted Cracking in Rising Load Tests

Abstract A method is proposed to calculate the loading rate needed to induce environmentally assisted cracking (EAC) in precracked specimens tested under rising load. The procedure applies to materials that fracture by ductile tearing under inert conditions and requires some EAC data, although these may be obtained from one of several sources. A fundamental basis is provided to explain why the rising load test can serve as an accelerated test. Application to a medium strength steel in synthetic seawater and a pipeline steel in a carbonate/bicarbonate solution shows good results: EAC was exhibited and a close estimate of the threshold stress intensity factor for sustained load was obtained in less than one tenth the time required by conventional methods.

定荷重フラクトグラフィ試験によるＫＩ ＳＣＣ，ＫＩ Ｈおよび割れ進展速度の評価

The threshold stress intensity factor, KISCC for stress-corrosion cracking or KIH for hydrogen assisted cracking, is generally determined by one of the cantilever constant-load test, constant-displacement test and rising-load test. In the present paper, problems encountered with these tests, especially on the accuracy, test duration, and/or measurement of crack growth rate, were discussed first. Then the holding-load and fractography test was proposed as a way to improve the existing procedures. An outline of the new testing procedure is as follows: Several precracked specimens exposed in test environments or precharged with hydrogen are loaded constantly at different KI values for a relatively short predetermined test duration. The specimens are then unloaded and separated apart by cleavage fracture or fatigue loading. The amount of subcritical crack growth during the test, ΔaH, is measured by an electron microscopic observation of the fracture surface. The value of KISCC or KIH at the onset of the crack growth and the growth rate are determined from the relation of ΔaH and its rate per test duration to KI value.The test procedure was used to predict the values of KIH and growth rate for hydrogen assisted cracking of 1/2 Mo steel and 2-1/4Cr-1Mo steel. The results indicate that the usual test procedure estimates higher KIH than the proposed one. Thus, it is considered that the reexamination of the existing KISCC and KIH data is required.

Effect of specimen thickness on subcritical crack growth under sustained load

Subcritical crack growth under sustained load in annealed Ti - 4Al - 3Mo - 1V (117 k.s.i. yield strength) was measured using compact tension specimens of three thicknesses, 0.75, 0.25 and 0.13 inch. Crack growth terminating in failure occurred under constant load in all three specimens at initial stress intensities well below the critical stress intensity as measured in rising load tests. Time to failure at low initial stress intensity was considerably increased by increased mixed mode stress conditions; it was appreciable longer in 0.13 than in 0.25 inch thick specimens. However, time to failure was about the same in 0.75 inch thick specimens, tested under linear elastic plane strain conditions, as in 0.25 inch thick specimens. Specimens size affected the kinetics of crack growth and the shape of the crack surface developed under sustained load. Up to the point of final unstable failure, fracture occurred almost exclusively in material under triaxial stress, resulting in a considerable amount of crack front bowing.