Wedge Opening Loading Specimens Research Articles

Accelerated crack growth experiments of SS304H for dry storage canister in substitute ocean water – Effect of temperature

Spent nuclear fuels (SNFs) are expected to be stored in dry storage canisters (DSCs) for much longer time than earlier anticipated due to a lack of permanent deep geological repository. As a result, DSCs located in marine environment may undergo chloride induced stress corrosion cracking (SCC) during long-term storage of SNFs. We report here the effect of test temperature on accelerated crack growth experiments carried out on sensitized austenitic stainless steel SS304H in substitute ocean water using fracture mechanics approach. Wedge-opening loading specimens were selected for measurement of crack growth rates using direct-current potential drop technique. Average crack growth rates of 0.975 × 10−10 ± 9.528 × 10-12, 3.258 × 10−10 ± 9.551 × 10-11 and 1.580 × 10-9 ± 2.593 × 10−10 m/s were obtained for 22, 37 and 60 °C, respectively resulting in an activation energy of 60.9 kJ/mol corresponding to diffusion of hydrogen in steel. Intergranular crack propagation was noted along with formation of chromium-rich carbide precipitates at grain boundaries.

Experimental study and numerical simulation on the SSCC in FV520B stainless steel exposed to H2S+Cl− Environment

Constant displacement loading tests using wedge opening loading specimens were carried out in aqueous hydrogen sulfide solution containing sodium chloride to investigate the susceptibility of stress corrosion cracking (SCC) of FV520B precipitation hardening martensitic stainless steel. Results of the SCC tests indicated that the stress corrosion critical stress intensity factor (KISCC) dramatically decreased in the corrosion medium investigated and decreased with the increasing of H2S concentration. Microstructures of fracture surfaces were analyzed using a scanning electron microscope (SEM) with an energy dispersive X-ray spectroscopy (EDS). The fracture surface was typical of sulfide stress corrosion fracture. In addition, large amount of intermittent arc-crack on the side surfaces around the tip of main crack formed even no main crack propagated.A sequentially coupling finite element analysis (FEA) program was utilized to simulate the stress field and calculate the diffused hydrogen concentration distribution of specimen exposed to the corrosion medium investigated. The FEA results indicated that corrosion pit affected the stress and diffusion hydrogen distribution around the corrosion pit where large stress gradients formed. Side surface cracks initiated from those corrosion pits and propagated under the synergy of stress and hydrogen. The effect of the corrosion pit on hydrostatic stress distribution was limited in superficial zone near the side surface, thus side surface cracks propagated along the hoop direction rather than along the direction of specimen thickness. Based on the morphology observation and FEA results, it can be concluded that the SCC mechanism of FV520B steel was hydrogen embrittlement mainly and combination of anodic dissolution. Simultaneously, corrosion pitting was the precondition of side surface crack formation while the stress induced hydrogen diffusion was the dominant factor.

Defect size dependence on threshold stress intensity for high‐strength steel with internal hydrogen

ABSTRACTA series of tensile tests were carried out using hydrogen‐precharged specimens of SAE52100. Two types of tensile specimens were used: (i) smooth specimens and (ii) specimens having various shapes of artificial defects with sizes of about 35–500 µm. In the smooth specimens, fracture origins were non‐metallic inclusions with the size of 10–30 µm (e.g. Al2O3∙(CaO)x, TiN and TiC). In both types of specimens, hydrogen charging drastically decreased the tensile strength. The fracture toughness determined for the small defects at the fracture origin, that is, the threshold stress intensity KTH calculated from the defect size and the applied tensile stress at the failure of the specimen σf, showed a defect size dependence, where area denotes the area of the domain defined by projecting the defect on a plane normal to the cylindrical axis of the specimen. Namely, the threshold value was reduced with a decrease in the defect size, which is similar to the crack size dependence in the ΔKth for small fatigue crack. The values of KTH for both the non‐metallic inclusions and artificial defects were much smaller than those for large cracks measured by the standard WOL (Wedge Opening Load) and CT (Compact Tension) specimens.

Role of Heat Treating on the Intergranular Stress Corrosion Cracking of Alloy 600

It has been recognized in Ni-base alloys that the grain boundary chemistry and its microestructural heterogeneities are important factors in the material environmental degradation. Therefore, in this study, we investigated the role of the heat treatments and the obtained microstructure in the IGSCC susceptibility of alloy 600 in environments of pressurized water reactors (PWR). The role of microstructure on the intergranular cracking resistance (IGC) of alloy 600 was also investigated using a modified wedge opening loading specimens which were annealed at 930, 800 and 600°C and also exposed to high purity water pressurized with hydrogen at 300°C. It was found the microstructure induce relatively low crack growth rates associated with the development of significant plastic deformation at the crack tip. In addition, some common features were found between the IGSCC performance and the pre-fatigue cracking conditions. For this purpose, the fatigue crack growth properties of this alloy were also evaluated using the ASTM E 606–92.

Fatigue crack growth behaviour of 4130X steel in H2S environment

Fatigue crack growth rates of 4130X steel used for compressed natural gas vessels were investigated in this paper. Considering the operating conditions, corrosion fatigue tests at a low frequency of 0·0067 Hz, in H2S saturated, H2S unsaturated and air environments were conducted on modified wedge opening load specimens by using a home made low cycle fatigue test system. Curve fitting was applied to the fatigue test data of da/dN–ΔK according to Paris formula. A correlation study between fracture surface and stress intensity factor range was conducted and K values for three stages in different environments were characterised quantitatively. The results show that da/dN in H2S environment is more than 20 times faster than in an air environment. When the H2S concentration reaches a certain range, the increase of da/dN becomes slower than that of the H2S concentration. da/dN differs by 2·4 times while the concentration differs by 11 times. The corrosive environment accelerates the fatigue failure.

Microstructural response on the cracking resistance of alloy 600

Precipitation of chromium rich carbides promotes the development of a Cr-depleted zone which in turn provided a weak path for the intergranular crack propagation. The role of low temperature anneals on the intergranular cracking resistance (IGC) of alloy 600 was investigated using modified wedge opening loading specimens heat treated at 930, 800 and 600 °C and exposed to high purity water pressurized with hydrogen at 300 °C. Mill annealing at 930 °C did lead to IGC susceptible microstructures. In this condition the alloy 600 exhibited the least crack growth rates (d a / d t) of the order of 1.86 × 10 − 12 m/s and characterized the substantial work hardening ahead of the crack front. In contrast, annealing at 600 °C (HT600) resulted in increasing IGC susceptibilities. Under these conditions, crack growth rates, d a / d t, as high as 7.10 × 10 − 10 m/s were found (HT600). Accordingly, significant interactions between the slip bands and the crack path lead to crack bifurcation into the slip planes and cavity formation.

Effect of Anhydrous Ammonia Composition on its Ability to cause Stress Corrosion Cracking in Steels

AbstractStress corrosion tests have been performed on pressure vessel steels in ammonia liquid and vapour, using the slow strain rate test and pre-cracked Wedge Opening Load (WOL) specimens. The materials. tested were BS 1501–224 Grade 30 weld imd parent plate, BS 1501–151 Grade 26 parent plate, STE 47 weld and parent plate and DIN 17135–ASt 52. Discriminant analysis was used to relate the severity of cracking in the slow strain rate test to the concentrations of oxygen, carbon dioxide and water present in the ammonia. The WOL specimens were not effective in predicting stress corrosion cracking in the liquid ammonia environment.Oxygen promoted cracting at a level of ≥5 ppm in commercial grade ammonia, but may be as low as 1 ppm when carbon dioxide and water are very low. Water, hydrazine, ammonium carbonate, bicarbonate and carbamate were effective inhibitors whereas an oil film was not. All of the steels tested suffered cracking but welded BS 1501–224 Grade 30 and STE 47 were the most susceptible. Cracki...

Effect of crack size on acoustic emission related to moving dislocations

Acoustic emission from a crack vicinity in a uniformly stressed solid is correlated with the crack size based on a continuous theory of dislocations. It is assumed that the total number of acoustic emissions is directly proportional to the total number of nonuniformly distributed moving dislocations in the plastic zone near the crack. The resultant equation of the relationship between acoustic emission and crack size can be expanded into a power series of applied stress. The first-order approximation is consistent with that of a macroscopic fracture mechanics consideration on the emission related to the plastic zone size. The theoretical calculation is verified with experimental results obtained from a flawed carbon steel vessel by a multichannel acoustic emission monitoring system and agrees reasonably well with the experimental data. Further calculations by utilizing the theory and by analyzing available experimental data of acoustic emissions from center-surface-flaw specimens and wedge opening load specimens of various crack sizes also show agreement between the theory and the experimental data.

Effect of specimen type and crack orientation on fracture toughness

The effect of specimen type on the room temperature plane strain fracture toughness of three aluminum alloys: 7079-T6,7075-T6, and 7001-T75, is presented. The difference in K IC as evaluated with center notch, single edge notch, slow bend, surface flaw and wedge opening loading specimens is discussed. The orientation of the crack plane and crack propagation direction with respect to the working direction, or grain orientation, of the alloys is considered. The possible influence of crack plane and propagation direction on the resulting plane strain fracture toughness value is discussed. The surface flaw specimens seem to yield the highest K IC value followed by slow bend, single edge notch and center notch specimens. The slow bend and surface flaw specimens were the most sensitive to material variability. Differences in orientation between crack propagation direction and material grain morphology are shown to effect K IC . For 7001-T75 and 7079-T6, the order of decreasing fracture resistance with respect to crack propagation direction is short transverse, short transverse and longitudinal. For 7075-T6, the order is long transverse, short transverse, and longitudinal.

State of the art of the WOL specimen for [formula omitted] fracture toughness testing

A comprehensive review of the development and the current state-of-the-art for the WOL (wedge opening loading) specimen for plane strain fracture toughness testing is described. Fracture toughness data ( K Ic ) were determined for a wide range of temperatures for several steels representing the strength range of 50–175 ksi yield strength. Various WOL and other types of specimens were employed. The consistency of K Ic data for various geometrical proportions and sizes of WOL specimens, as well as other types of specimens, is demonstrated. A newly developed modification of the ‘ T’ type WOL specimen to facilitate two pin loading is described. It is concluded that the state-of-the-art is sufficiently well advanced so that a standard ASTM recommended practice for plane strain fracture toughness testing with compact tension specimens can be prepared.