Richard E. Link

Evaluation of the Effect of Biaxial Loading on the T o Reference Temperature Using a Cruciform Specimen Geometry

A series of 12 cruciform geometry fracture toughness specimens has recently been tested using A533B base plate obtained from the decommissioned Shoreham plant pressure vessel. Specimens were tested at −100°C, placing them in the lower ductile to brittle transition of this ferritic structural steel. The overall objective of this work is to compare the results of these biaxial cruciform tests to the results of standard and shallow crack fracture toughness tests to assess the effect of biaxial loading on the measured master curve and the To reference temperature as defined by ASTM E 1921. Previous work done at Oak Ridge National Laboratory (ORNL) appeared to demonstrate an increase in the To reference temperature due to the presence of the biaxial stress field established in the cruciform test geometry. Because of the cost of the ORNL tests, only a few specimens could be run, and full statistical support of the “biaxial effect” could not be demonstrated. A second goal is to demonstrate that smaller size specimens, and hence lower cost tests, can be used to evaluate the magnitude of the biaxial effect in nuclear reactor pressure vessel materials. This report presents a brief overview of the test procedure, presents the test results, and compares the results to the database available on standard and shallow crack fracture toughness results available for the Shoreham plate material.

Towards Crack Arrest Testing Using Miniature Specimens

Crack arrest is an important concept that can be useful to guarantee the safety of reactor pressure vessels. In case of a pressurized thermal shock, a postulated crack can initiate and arrest due to a decrease in driving force combined with an increase in material toughness due to the thermal and neutron embrittlement gradient along the thickness. Due to the size of the specimen and the difficulty in obtaining valid results according to the ASTM E1221-06 standard, “Standard Test Method for Determining Plane-Strain Crack-Arrest Fracture Toughness, KIa, of Ferritic Steels,” current efforts to develop this technique for irradiated materials are very limited. However, advances in dynamic fracture modeling and elastic-plastic fracture mechanics open the possibility of using miniature crack arrest specimens. We have developed a stiff setup to perform crack arrest tests on precracked Charpy (PCCv) specimens. Different strategies were evaluated to provide enough reduction in driving force to produce arrest. Special attention is given to the starter notch and different starter notch preparations are investigated: precracking, chevron, and brittle weld. Testing configurations were found that guarantee crack arrest with sufficient remaining ligament. The dynamic loading condition is investigated experimentally using strain gauges. Finite element calculations of an arresting crack are also performed in order to provide more insight of the loading after the crack arrest event. Although it is not yet possible to derive the arrest fracture toughness from the PCCv testing, results are encouraging and open new perspectives in the field of crack arrest determination.

Round-robin Analysis of Standard Data Sets for Fracture Toughness Evaluation in ASTM E1820

A set of standard data sets from typical fracture toughness tests was used in an analytical round-robin to determine whether the proposed data sets were suitable for use as adjuncts to ASTM E1820-09, “Standard Test Method for the Measurement of Fracture Toughness.” Four laboratories participated in the round-robin and submitted analyses of all of the data sets. The results showed good agreement among the laboratories on the individual J-integral calculations and crack size estimates. However, small differences in the estimated crack extension during the tests led to large uncertainty in the calculated fracture toughness of up to 20 % of the mean value of the fracture toughness from all of the laboratories for the data set. A synthesized test record that eliminated the nonlinear behavior commonly observed in unloading compliance test records was developed and distributed to the participants. There was excellent agreement among the results from all participants for the synthesized data set. The standard data sets were determined to be suitable for validating analysis programs for calculating fracture toughness in accordance with ASTM E1820. The uncertainty in the calculated fracture toughness could be reduced by prescribing in the test method exactly which data points in an unload are to be used for estimating the crack length, rather than letting the user decide which data points to include.

Crack Mouth Opening Displacement-Based η Factors for SE(B) Specimens

Three-dimensional finite element analysis of SE(B) fracture toughness specimens with crack sizes ranging from a/W=0.1–0.6 were performed to develop ηCMOD values for estimating Jpl in fracture toughness tests. The analyses included specimens with smooth sides and 20 % sidegrooves. The results for ηCMOD were 4–9 % lower than previously published results based on two-dimensional plane strain analysis. A simple linear expression for ηCMOD(a/W) was proposed for introduction into ASTM E1820 for calculating Jpl in SE(B) specimens with 0.05≤a/W≤0.7.

Crack Arrest Testing Using Small Wide Plate SE(T) Specimens

A greatly scaled-down version of the wide plate crack arrest test has been developed to characterize the crack arrest performance of a high strength, low alloy (HSLA) steel alloy in the upper region of the ductile-brittle transition. The specimen is a single-edge notched, 152 mm wide by 19 mm thick by 910 mm long plate subjected to a strong thermal gradient and a tensile loading. The thermal gradient is required to arrest the crack at temperatures high in the transition region, close to the expected service temperature for crack arrest applications in surface ships. Strain gages are placed along the crack path to obtain crack position and crack velocity data, and this data, along with the applied loading is combined in a “generation mode” analysis using finite element analysis to obtain a dynamic analysis of the crack arrest event. A prior investigation demonstrated the feasibility of the technique and this investigation was devised to obtain a larger dataset of crack arrest toughness values of an HSLA-100 steel plate to better characterize the upper transition crack arrest performance of this alloy. Brittle crack initiation, significant cleavage crack propagation, and subsequent crack arrest was achieved in all eight of the tests conducted in this investigation. The cleavage cracks were observed to propagate at nearly constant velocity, typically in the range of 350–400 m/s before arresting abruptly. A crack arrest master curve approach was used to characterize the fracture toughness and a crack arrest reference temperature, TKIA=−136°C was determined for the HSLA-100 plate. This corresponded to a shift in the crack initiation reference temperature of +36°C. The shift in reference temperature was much less than that observed for low Ni steel alloys.