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Crack Ansys 14 304

Stainless steel 304 has excellent corrosion resistance in a wide variety of environments and when in contact with different corrosive media. Pitting and crevice corrosion can occur in environments containing chlorides. Stress corrosion cracking can occur at temperatures over 60C.

Crack Ansys 14 304

Stainless steel 304 readily work hardens. Fabrication methods involving cold working may require an intermediate annealing stage to alleviate work hardening and avoid tearing or cracking. At the completion of fabrication a full annealing operation should be employed to reduce internal stresses and optimise corrosion resistance.

Abstract:The fatigue crack propagation of 304 austenitic stainless steel was studied both by experiments and numerical simulations. Two methods were applied to simulate the crack propagation: the extended finite element method (XFEM) and the cohesive zone model (CZM). Based on the XFEM, the direct cyclic solver was used to simulate the fatigue crack propagation. Based on the CZM, the VUMAT subroutine was used to describe the crack tip constitutive equation during fatigue crack propagation, and the mechanical properties of the crack tip were simulated. The effects of different frequency, f, and stress ratio, R, on the fatigue crack growth life were studied by XFEM and CZM separately and compared with the experimental results. Results show that the crack propagation path simulated by the XFEM agrees well with the experimental result, but the deviation of the fatigue life between the simulated results and the experimental results is large. The CZM model can predict the crack propagation life very well in comparison with the experimental data, but it has certain limitations because the crack propagation path is preset.Keywords: fatigue crack propagation; crack propagation life; extended finite element method; cohesive zone model

In order to illustrate the enhancement or shielding effect of the two parallel cracks more clearly, a single crack is modeled as the reference. The stress intensity factor at the single crack tip is denoted by KI0. The ratio of stress intensity factor of the parallel cracks to the stress intensity factor of the single crack, KI/KI0, is introduced to characterize the crack interactions. Specially, if the value of KI/KI0 is more than one, the crack interaction is considered to be enhanced, and if the value of KI/KI0 is less than one, the crack interaction is shielded.

As indicated before, for the two parallel cracks with different lengths under the fatigue loading, the long crack is usually regarded as to be more "dangerous". Thus, in the following analysis, we focus on the effect of the short crack on the stress intensity factors of the long crack.

With sufficient numerical results, the criterion diagram to determine the enhancement, shielding, or no interaction effect between two parallel cracks is obtained, as shown in Figure 7. To make the criterion expression more concise and universal, two dimensionless numbers H and S are introduced. Here, H represents the ratio of the normal distance to the half of the crack length, i.e., h/a, and S represents the ratio of the deviation distance to the half of the crack length, i.e., s/a. Specially, to determine the effect of the short crack on the long crack, a in H and S is the half of the short crack length, a2. Likewise, to determine the effect of the long crack on the short crack, a in H and S is the half of the long crack length, a1.

From Figure 7 it can be found that if the two cracks are close and share the same perpendicular bisector, i.e., s = 0, only the shielding effect exists. This result implies that for two cracks sharing the same perpendicular bisector, it would be too conservative and even irrational to simply merge them into a bigger crack by applying the enveloping method, or in other words, it is safe to just consider the long crack.

On the other hand, if the two cracks are close and collinear, i.e., h = 0, only the enhancement effect exists. Of course, when the two cracks are not close, either in deviation or in normal distance, their interactions can be neglected.

Crack growth paths in different specimens: (a) SC, (b) PC0.9S0, (c) PC1.0S0, (d) PC0.9S7, and (e) PC1.0S7. Points A-D are the four crack tips shown in Figure 1.

To prove this viewpoint, the changes of the stress fields around the cracks caused by crack interactions are obtained. Figure 13 shows the stress distributions in the vicinity of the crack tips for the single crack (SC), the two equal parallel cracks with s = 7 and h = 2.5 (PCS7), s = 17 and h = 2.5 (PCS17), and s = 0 and h = 2.5 (PCS0). In order to avoid the stress singularity at the crack tip, a circle with the center at tip A and the radius, r, of a1/10 is chosen, as shown in Figure 14, to compare the stress distributions in the vicinity of tip A for different crack configurations.

Von Mises stress distributions in the vicinity of the crack tips: (a) SC, (b) PCS7, (c) PCS17, and (d) PCS0. Points A-D are the four crack tips shown in Figure 1.

Thermal load resulting from heat action was found to be a common cause for stress concentration and failure of structures built from a 304 stainless steel material. Adnyana [3] recently conducted an interesting research concerned with the failure analysis of stainless steel heat exchanger tubes for use in a petrochemical plant, published with the Journal of Failure Analysis and Prevention. He investigated on a case where the shell and tube of the heat exchanger failed after a year of maintenance work. Metallurgical examination, chemical analysis, hardness testing and microscopic examination approaches were carried out by Adnyana [3] to identify the cause of failure. However, his study informed that the heat exchanger tubes exposed to high heat levels failed due to stress-corrosion cracking. The thermal stress exerted on the material was as a result of the consistent change in the temperature gradient at local points. Furthermore, Maharaj and Marquez [12] looked into the failure of a stainless steel pipe elbow used in the transportation of purge gas. The material type was an SA-312 TP04 stainless steel. It was found that the material failed due to local stress at the welded points, and can be accounted by the extreme steady-state piping vibration at welded points were the thermal stress was experienced. The microstructural and vibrational evaluation techniques were implemented for the evaluation for the possible cause of failure.

Fuller et al. [7] demonstrated that the failure analysis of an AISI 304 stainless steel shaft can be achieved using the conventional 14-step failure analysis approach. The approach involves mechanical testing, nondestructive testing, metallography, chemical analysis, but does not include detail transient thermal analysis coupled with the model structure. This study showed that the steel shaft failed at specific areas due to intergranullar stress cracking. The failure rate was rapid at heat affected zones.

The purpose of this paper was to study the failure behavior of SUS304 stainless steel in view of cooling water leakage in actual service conditions. The temperature field and thermal stress distribution in the thickness direction of SUS304 stainless steel under actual service conditions were further analyzed by the finite element method. The results show that chromium-rich carbides are precipitated at the grain boundaries, which may cause intergranular corrosion. The morphology of the corrosion grooves is ditch structure, which has high intergranular corrosion susceptibility. The expansion direction of the cracks is from the contact surface of cooling water to the high-temperature contact surface. Fracture is characterized by a river pattern and rock candy pattern. Fracture morphology is characterized by brittle rupture. The Cl contained in the corrosion products comes from the circulating cooling water. XRD patterns show that the corrosion products are composed of CaCO3 (the high-temperature contact surface) and Fe3O4 (the contact surface of cooling water), respectively. EDS results of corrosion products are consistent with XRD results. The results of finite element simulation show that the temperature field along the thickness direction of SUS304 stainless steel plate is in the range of 436.6-450 C. The peak of thermal stress is located in the middle of cross section. The cause of failure is acceleration of the initiation and extension of the cracks under the combined action of chloride ion stress corrosion, intergranular corrosion, and thermal stress. The form of crack fracture is mixture of transgranular and intergranular fracture, mainly transgranular fracture, which belongs to typical chloride ion stress corrosion.

A major concern in the design of engineering structures is the ability of components to maintain their integrity during their entire life service even when subjected to a combination of fluctuating loads and aggressive environments. The requirement for reliable structural integrity is particularly important for structures, involved in fundamental fields such as transportation, oil and gas production and energy generation. Fatigue and corrosion fatigue failures take place in components and structures as result of complex loading histories. There are different stages of fatigue damage in engineering components where defects may nucleate on initially smooth or undamaged sections, followed by microstructural crack formation, stable propagation and finally unstable crack propagation where catastrophic failure occurs.

Then, fatigue life of engineering components and structures can be defined as the period during which cracks initiate from defects and propagate. The largest fraction of fatigue life is spent in the crack propagation stage. However, when engineering structures operate under severe conditions, the problem of fatigue failure is raised, especially in presence of aggressive environments. Under such severe conditions, the crack initiation stage is dramatically reduced as the case of structures in marine environments. 350c69d7ab


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