Yoshio Urabe

Further Experimental Verification of Warm Prestressing Effect Under Pressurized Thermal Shock (PTS)

Fracture tests for the verification of WPS (warm prestressing) effect were carried out by using large flat specimens with very low toughness. Tensile and bending loads and thermal shock were applied simultaneously to the specimens with the realistically postulated flaw and the two times larger one in order to make the maximum K{sub I} cross the lower bound of K{sub IC} data. During the tests, loading was controlled to simulate the shape of K{sub I} versus temperature curve for the postulated PTS transient. Both the specimens did not break within the scatter band of K{sub IC} when K{sub I} was decreased during cooling. K{sub I} values at fracture by reloading were beyond the upper bound of K{sub IC}. That is, the effectiveness of WPS was directly demonstrated for the PTS transients. Also, K{sup I} values at fracture can be predicted by Chell`s theory. As the test results, Japanese PWRs have sufficient temperature margin against PTS.

Verification of Warm Prestressing Effect Under a Pressurized Thermal Shock (PTS) Event

Fracture tests for the verification of WPS (warm prestressing) effect were carried out by using large flat specimens and big compact specimens with low toughness. In the case of monotonical K[sub I] increasing during cooling, the specimen broke within the scatter band of K[sub IC]. On the other hand, when K[sub I] was decreasing during cooling, the specimens did not break even if K[sub I] values were beyond the scatter band of K[sub IC]. That is, WPS effect was confirmed even for the low toughness steel like reactor pressure vessel wall under neutron irradiation. Also, K[sub I] values at fracture can be predicted by Chells theory. By applying WPS effect and the predictive equations for irradiation embrittlement for Japanese PWR reactor steels to the PTS integrity analysis, much more temperature margin can be expected.

Low Cycle Fatigue Behavior and Seismic Assessment for Elbow Pipe Having Local Wall Thinning

One of the concerned technical issues in the nuclear piping under operation is pipe wall thinning caused by flow accelerated corrosion. Recently, it has been reported that the elbow section is more suspicious on pipe wall thinning by erosion–corrosion. Some researchers including authors have been studied static and fatigue strength of elbows with local wall thinning. However, still more experiment and analysis data are needed to clarify the technical issues. Accordingly, further experiments and their evaluations were carried out by the authors. This paper presents the influences of size and location on fatigue life. Also as one of the application of the test results, safety margin of elbows with wall thinning against seismic loading is discussed. Low cycle fatigue tests were conducted using elbow specimens made of STPT410 steel with local wall thinning. The local wall thinning was machined on the inside of elbow specimens in order to simulate erosion/corrosion metal loss. The local wall thinning areas were located at three different areas, called extrados, crown, and intrados. Eroded ratio (eroded depth/wall thickness) is 0.5 and 0.8 and eroded angle is 90 deg and 180 deg. The elbow specimens were subjected to cyclic in-plane bending under displacement control (±20 mm) without and with internal pressure of 3 MPa using a universal testing machine. Fatigue life was defined as fatigue crack penetration through the thickness and crack penetration was watched by naked eyes during the test through the protection window made of a transparent plastic plate. Obtained main conclusions are as follows: (1) Existence of local wall thinning in extrados does not have an important effect on fatigue life. Especially, fatigue crack does not initiate at the extrados where the extreme local wall thinning exists (eroded ratio = 0.8 and eroded angle = 180 deg). (2) Regardless of existence of internal pressure, fatigue crack initiates at the crown where local wall thinning does not exist for an elbow with local wall thinning at extrados. This conclusion should be confirmed using eroded elbow specimens under more high pressure. (3) Even if the eroded ratio and the eroded angle reached up to 0.8 and 180 deg, the elbows with local wall thinning have high safety margin against seismic loading, comparing to ASME Boiler and Pressure Vessel Code Section 3 allowable seismic stress criteria.

Dynamic fracture analysis of a transverse wedge-loaded compact specimen

The J-integral method cannot be applied to the elastic plastic dynamic crack propagation, because unloading and inertia force may take place. From this point of view dynamic elastic plastic scheme using J-integral is developed. Using this dynamic finite element program an MRL type specimen is analyzed. As the result, the property of path-independence of the J-integral under the existence of inertia force and unloading is confirmed. Dynamic effects are considerably small in the MRL type specimen. Also the influence of plastic zone on the crack arrest toughness is shown. Finally the present result is compared with the request of ASTM 2nd round robin test program for crack arrest toughness.

Estimation of Low-Cycle Fatigue Life of Elbow Pipes Considering the Multi-Axial Stress Effect

The stress states of elbow pipes are complex and different from those of straight pipes. Mansons universal slope method cannot predict the low-cycle fatigue lives of elbow pipes under combined cyclic bending and internal pressure. In this work, fatigue tests and finite element analysis showed that the multi-axial stress factor (i.e., ratio of axial stress to hoop stress) is quite high at elbows. This paper proposes a revised Mansons universal slope method that considers the multi-axial stress factor to predict the low-cycle fatigue lives of elbows under combined cyclic bending and internal pressure with considerably high accuracy.

Procedure of crack shape determination by reversing DC potential method

Abstract On-line monitoring of a crack and evaluation of component integrity are needed for maintaining the safety of a plant. The authors have been developing this kind of system using the Reversing DC Potential Method (RDCPM). The crack shape estimation based on RDCPM is performed by comparing the measured potential difference with the analytical potential difference. In this paper the simplified method for determining the crack shape is discussed and its application to a pipe is shown.

Low Cycle Fatigue Behaviors of Elbow With Local Wall Thinning Under Combined Bending and Internal Pressure

Low-cycle fatigue tests were conducted using elbow specimens with local wall thinning in order to investigate the influences of position of local wall thinning on the low-cycle fatigue behaviors of elbows. Local wall thinning was machined on the inside of the elbow in order to simulate metal loss from erosion corrosion. The local wall thinning was located in three different areas. The elbow specimens were subjected to cyclic in-plane bending under displacement control with internal pressure of 9 MPa. In addition, three-dimensional elastic-plastic analyses were also carried out using the finite element method. As a result, the crack penetration area and the crack growth direction were successfully predicted by the analyses.Copyright

Simplified J-Integral Evaluation Method for Evaluation of Reactor Pressure Vessel in the Upper-Shelf Region

It is well-known that the Upper-Shelf Energy of reactor pressure vessel (RPV) steels is reduced due to neutron irradiation and J-integral (J-applied) is used for the structural integrity evaluation in upper-shelf region. It is very time consuming to calculate the J-integral in detail by using 3-dimensional elastic-plastic FE analysis. U.S. Regulatory Guide 1.161[1] applies a simplified J-applied calculation method based on elastic calculation of the stress intensity factor, KI . However, this method for thermal stress can be applied only for constant cooling rates. From this point of view, a simplified J-applied evaluation method which can be applied to design transients and has high accuracy has been studied. In order to develop the new evaluation method, KI based on 1-dimensional elastic analysis, KI based on 3-dimensional elastic FE analysis and J-applied based on 3-dimensional elastic-plastic FE analysis are calculated and compared with each other. An accurate J estimation method for design transients from 1-dimensional elastic analysis results is proposed and the severest transient in each condition of each RPV group is evaluated in this paper.Copyright

Ductile crack growth under a PTS event

Abstract A step B test was carried out as one of the test items using large scale flat plate specimens within the Japanese PTS research project. Its experimental goal was the investigation of the crack behavior in the upper shelf region under combined mechanical and thermal loading. The experimental data were described in detail. By means of numerical simulation of the experiment, time histories of temperature distribution through the thickness, deformation and strain were obtained and compared with experimental data. Also the ductile stable crack growth by the J resistance concept was compared with experimental results. As a result, the stable crack growth of the step B test was conservatively estimated by J resistance curves obtained by 1TCT specimens. Currently, the reason of overestimation is not so clear. Triaxiality near the crack tip may be one of the reasons for the overestimation

Low Cycle Fatigue Evaluation of Pipe Bends With Local Wall Thinning Considering Multi-Axial Stress State

Low cycle fatigue tests and finite element (FEM) analysis were conducted using 100A pipe bend specimens made of STPT410 carbon steel with and without local wall thinning local wall thinning was machined on the inside of the elbow and was prepared at extrados, crown, and intrados. The parameters of the wall thinning were same (the wall thinning ratio = 0.5, the wall thinning angle = 180 deg, and the wall thinning length = 100 mm) in the all test cases. The pipe bend specimens were subjected to the prescribed cyclic in-plane bending displacement with constant internal pressure of 0–12 MPa. Also, low cycle fatigue tests using sound pipe bend specimens were carried out for comparison. According to the test results, low cycle fatigue strength of wall thinned pipe bend specimens was not so different, regardless of location of wall thinning. Low cycle fatigue strength of the pipe bend specimens was beneath the best fit fatigue curve and its reason can be explained quantitatively by a proposed cumulated damage rule introducing ductility exhaustion considering multi-axial stress state. The validity of the new proposed cumulative damage rule was also confirmed by the another sample analysis using other reference data obtained by pre-overloaded in-plane cyclic bending tests.