Cw Marschall

Effect of Dynamic Strain Aging on Fracture Resistance of Carbon Steels Operating at Light-Water Reactor Temperatures

This paper reviews the phenomenon of dynamic strain aging in carbon steels and considers its effects on the fracture behavior of carbon-steel pipes and pressure vessels in light-water reactors operating at elevated temperatures near 290°C (550°F). Dynamic strain aging is a phenomenon in which aging occurs simultaneously with plastic straining. It occurs over a range of temperatures that depends on strain rate. In tensile tests, it is manifested by increased tensile strength, increased strain-hardening rate, serrated stress-strain curves, and decreased ductility. Evidence is presented to show that the occurrence of dynamic strain aging can significantly lower the fracture resistance of carbon steels. This lowering of fracture resistance may be manifested in several ways: (1) J I c is lower at light-water reactor (LWR) temperatures than at room temperature, (2) the tearing modulus is lower at LWR temperatures than at room temperature, and (3) stable ductile crack growth may be interrupted by unstable ductile fracture at LWR temperatures but not at room temperature. The paper examines probable causes of dynamic strain aging and describes methods for identifying which steels are susceptible to it.

Comparison of static and dynamic strength and J-R curves of various piping materials from the IPIRG-1 program☆

Abstract Material property tests were conducted on nuclear piping steels at 288°C at both quasi-static and dynamic displacement rates. The dynamic rates were similar to those estimated to occur in pipe tests that simulate seismic loading. The results suggest that quasi-static material property data are adequate for in-service flaw evaluation and leak-before-break analyses for stainless-steel pipes. However, dynamic strength and toughness data appear to be needed for such evaluations in some carbon-steel pipes.

Experience in Using Direct Current Electric Potential to Monitor Crack Growth in Ductile Metals

The direct-current electric potential (d-c EP) method is receiving increasing attention as an alternative to the unloading compliance method for monitoring crack initiation and growth during fracture toughness testing of ductile metals. Advantages of the d-c EP method include uninterrupted tests, continuous monitoring of crack extension, ability to accurately measure relatively large amounts of crack growth, and ability to be used at high displacement rates in many materials. The principal shortcoming of the d-c EP method (as with most other methods) is the uncertainty in defining the point of crack initiation in some tests. This paper describes Battelles experience in using the d-c EP method to monitor crack initiation and growth in compact (tension) specimens machined from various pipes used in cooling systems of nuclear reactors. Among the materials investigated are carbon steel pipes (base metal and weld metal) and extremely ductile austenitic stainless steel pipes (base metal and weld metal). Discussed in the paper are: (1) estimation of the crack-initiation point from d-c EP data, (2) ability of the d-c EP method to accurately predict large amounts of crack growth in highly ductile metals. (3) modification of the Johnson equation to improve the accuracy of the d-c EP method for large crack growth, and (4) use of the d-c EP method at high displacement rates.

Use of the Direct-Current Electric Potential Method to Monitor Large Amounts of Crack Growth in Highly Ductile Metals

The direct-current electric potential method is receiving increasing attention for monitoring crack extension in J-resistance curve testing. Among its advantages over the unloading-compliance method are: (1) no time-consuming unloadings are required, (2) a continuous record of crack extension versus displacement can be obtained, and (3) the method can be used at higher strain rates where unloading compliance cannot be used. Despite the advantages of the direct-current electric potential method, questions persist regarding its ability to monitor large amounts of crack growth in highly ductile materials where large displacements and large amounts of plastic strain occur. This paper presents details of an experiment conducted on a 3T planform-size compact specimen of 25.4 mm thickness to assess the ability of the direct-current electric potential method to accurately measure crack extension in a highly ductile material. The material selected was Type 304 austenitic stainless steel. It was found that the Johnson expression, often used to calculate crack extension from direct-current electric potential data, significantly underestimated the actual amount of crack extension. However, a simple modification of the Johnson expression resulted in excellent agreement between calculated and measured crack extensions.

Extrapolation of C(T) Specimen J-R Curves

In many structural flaw evaluations, frequently it is necessary to predict the maximum load where a ductile tearing failure mode is expected. In such a calculation, large ductile crack growth needs to be accounted for to predict the maximum load, especially for large structures and/or lower toughness materials. One difficulty in such an evaluation is that the amount of crack growth obtainable in C(T) specimens that can be machined from the material (e.g., from a pipe) is relatively small. Because of this, it is necessary to extrapolate the J-R curves to the larger amount of crack growth. No generally accepted method exists for such extrapolations. This paper discusses methods used to evaluate J-R curves extrapolation techniques. As part of this study, several materials were evaluated using C(T) specimens of various sizes but with the same thickness. This is frequently the case for piping, whereas for a pressure vessel or other heavy-wall structure the specimens will frequently not even be the same thickness as the structure. The results in this study showed more geometry effects with the J M -R curve than expected.

A Method for Determining the Crack Arrest Fracture Toughness of Ferritic Materials

This paper summarizes the results from a round robin test program that was conducted from 1983 through 1985 to evaluate a proposed ASTM standard test method for determining the crack arrest fracture toughness of ferritic materials. The round robin attracted a total of 27 participants from the United States, Canada, Europe, and Japan, each of whom agreed to test three specimens of A514 bridge steel, three specimens of A588 bridge steel, and six specimens of A533 Grade B, Class 1 reactor pressure vessel steel. Twenty-one partic ipants had completed their testing and forwarded test results when this paper was written, which provided a data base consisting of 54 test results for each of the bridge steels at - 30°C, 70 test results for the reactor steel at 10°C, and 65 test results for the reactor steel at 25°C. This paper summarizes the test procedure followed in the round robin, discusses the results obtained, and provides the rationale for the modifications to the test method that were recommended in light of the experience gained from this test program. The test procedure has since been revised and is now proposed as the ASTM Test for Determining the Plane-Strain Crack-Arrest Fracture Toughness, K I a , of Ferritic Steels (E 1221).

Battelle’s Columbus Laboratories Reactor Vessel Surveillance Service Activities

This paper describes the current methodology in use at Battelle’s Columbus Laboratories for obtaining and processing pressure vessel surveillance data, extrapolating mechanical behavior, and calculating reactor coolant pressure-temperature operating curves. Recent results from the Arkansas Nuclear One—Unit 2 (ANO-2) benchmark cavity dosimetry experiment are reported. The results indicate that it is possible to calculate the flux in ex- vessel locations with accuracies on the order of 10-15 percent. Also, end of life metallurgical predictions for the Poolside Facility (PSF) Blind Test materials are compared with experimental data.

Observations in Conducting J-R Curve Tests on Nuclear Piping Materials

This paper describes some of Battelles experiences in developing J-R data for nuclear piping materials. Compact specimens were machined from both carbon steel and stainless steel pipe and subjected to testing at elevatedtemperatures, similar to temperatures encountered in nuclear reactor coolant piping. In many cases, the specimens displayed toughness levels above those considered valid by ASTM standard methods of testing. Furthermore, cracks were grown by about 50% of the original ligament, well in excess of the 10% limit imposed by ASTM Test Method for Determining J-R Curves (E 1152-87). Topics discussed in the paper include: (1) monitoring crack extension using the direct-current electric potential method, (2) specimen thickness changes ahead of the growing crack, (3) observations of fracture appearance and crack growth direction in ferritic versus austenitic steels, (4) observations of ductile-crack jumps in carbon steels tested at 288°C (550°F), (5) unusual effects of partial unloadings in testing carbon steels at 288°C (550°F), and (6) difficulties with certain ASTM definitions in testing highly ductile materials.