INCEFA-PLUS Project: The Impact of Using Fatigue Data Generated From Multiple Specimen Geometries on the Outcome of a Regression Analysis

The INCEFA-PLUS project (INcreasing Safety in NPPs by Covering gaps in Environmental Fatigue Assessment) aims to generate and analyse Environmentally-Assisted Fatigue (EAF) experimental data studying parameters such as mean strain, hold times and surface finish. To understand the implications of these parameters for environmental fatigue assessments, these tests were carried out at 300 °C in air and light water reactor primary coolant environments (at 230 °C and 300 °C). Over the duration of this project around 230 fatigue data points were generated by different organisations using a common testing methodology, but with differing specimen geometries. Of these 230 data points, 23 were obtained from tests done using hollow specimen designs. Recent work comparing the fatigue lives of hollow to those of solid specimens indicates that on average the use of hollow specimens results in reduced fatigue lives. This has been explained in terms of the additional hoop and radial strains applied to the specimen due to the internal pressure of the hollow specimen. Given the examples published in the literature on the topic, the comparison of data generated using hollow and solid specimen geometries within the INCEFA-PLUS database has been a particular concern. This paper aims to explore the differences between hollow and solid specimen geometries within the INCEFA-PLUS database, highlight the potential risks of including both geometries in a single analysis, and discusses the approach taken by the project to mitigate the identified risks. The work presented in this paper details three approaches for the data obtained from hollow specimens: 1) exclude the data, 2) include the data as is, or 3) include the data with a correction on the strain amplitude. The strain amplitude correction will be based on the theoretical basis presented in Gill et al. [1], and extended to account for the different hollow specimen geometries used across the INCEFA-PLUS programme. This work demonstrates the robustness of the data analysis performed on the INCEFA-PLUS database to the use of differing specimen geometries. It also develops an explanation for the apparent difference in fatigue life between tests conducted on hollow and solid specimens under test conditions that are nominally the same. Furthermore, this paper builds on the mechanistic understanding presented in Gill et al. [1] and generalises across several Laboratories.

Applicability of Hollow Cylindrical Specimens to Environmental Assisted Fatigue Tests

When an environmental assisted fatigue (EAF) test is performed, the surface of a specimen has to contact with simulated LWR water. Autoclave equipment with a solid specimen is a commonly used method for EAF testing. It, however, takes time to setup a specimen into the autoclave and also it is necessary to strive to directly measure the displacement of the specimen. Another EAF testing method, which has been utilized by several research institutes, is to use a hollow cylindrical specimen. The hollow cylindrical specimen does not need the autoclave equipment and can be easily assembled to the fatigue test facility. This paper introduces the EAF test using the hollow cylindrical specimen, comparison between fatigue lives by hollow cylindrical specimens and solid specimens and gives some technical notes to use the hollow cylindrical specimen.

Variations in Measured Fatigue Life in LWR Coolant Environments due to Different Small Specimen Geometries

Small specimen fatigue testing is challenging in simulated LWR coolant environments at elevated temperatures and pressures. Two approaches to isothermal uniaxial testing in such environments have been developed: use of an autoclave to contain the environment around the specimen, which is conventionally of a solid design (e.g. circular cross-section, parallel sided gauge length); and use of a thin-walled hollow or tubular specimen, where the coolant environment passes through the bore of the specimen. It is often assumed that fatigue lives measured using these two specimen designs are equivalent. However, recent isothermal strain-controlled fatigue endurance tests on a single heat of Type 304L stainless steel at Amec Foster Wheeler — on behalf of Rolls-Royce — have indicated a significant difference in life from testing of these different specimen designs in high temperature PWR coolant, with hollow specimens consistently giving shorter lives. This paper presents those test results, and identifies a range of possible reasons for the differences in fatigue life through consideration of relevant literature and laboratory examination of failed specimens. These new test results have potentially significant implications for test programmes in which solid specimen test results in air are compared to hollow specimen results in LWR environments, and for fatigue databases that include results from testing of both specimen types. The use of a conversion factor, to be applied to fatigue lives from hollow specimens tests to allow comparison to solid specimen test results, is discussed. Further work to investigate the relevance of findings to further heats of material and to a wider range of loading conditions is identified.

Long term leachate evolution during flow-through leaching of a vault backfill (NRVB)

Some of the illustrative concepts for the disposal of intermediate-level waste in a geological disposal facility in the UK employ a cementitious backfill around the waste packages. The concept for higher strength rocks would use a highly alkaline backfill composed of Portland cement (now known as CEM I), hydrated lime and limestone flour, referred to as Nirex reference vault backfill (NRVB).This paper reports a study of the extensive leaching of cured NRVB in a range of generic leachant compositions (deionized water, 0.1 M and 1 M NaCl solutions) under flow-through conditions using a flexible wall permeameter. The experiments were designed to run for up to two years and to pass at least 1000 volumes of leachant (defined as the cumulative leachate volume produced/NRVB solid specimen volume) through the NRVB samples. Results for the pH evolution profiles of the leachates and the microstructural analysis of the unleached and leached samples are presented.

Rheological Behavior of Various Al Alloys during Solidification

This paper is concerned with the tensile behavior of various Al alloys during solidification obtained by using an initially solid specimen heated locally until it becomes fully liquid and then partially solidified at a controlled cooling rate. It is shown that for Al-Cu as well as for Al-Si-Mg alloys, a similar behavior is observed with a sharp transition on the stress-solid fraction curve when the coalescence solid fraction of the dendrites is reached. Below the transition fracture occurs along liquid films for very low stresses whereas beyond this transition, ductile fracture is observed leading to higher stresses.

Accurate Measurement of Opening Displacement of Notch with Moiré Interferometry by Changing Applied Load

Moiré interferometry is a very sensitive optical method to measure the in-plane displacement of a solid specimen, and is useful to measure small deformation of solids. But, there often appears measurement error caused by the residual strain of the grating that is pasted on a specimen in Moiré interferometry. The present paper applies a method to eliminate the error brought by the residual strain of the grating. The method measures the change of the order of Moiré interference fringes with varying the tensile force applied to the specimen, and obtain the true opening displacement of the notch. The present paper uses the method mentioned above to measure the opening displacement of a bifurcated notch in a plate specimen

Microstructure and Aging Behavior of AZ91 Mg Alloy Fabricated by Inclined Cooling Plate

Semi-solid AZ91 magnesium alloys were produced using a cooling plate followed by quenching in the mold. The cast and T6 heat treated microstructures were microscopically characterized and subjected to hardness tests. It was found that the resultant microstructure in water quenched semi-solid specimen were finer than that in conventional permanent mold cast specimen. The hardness tests also showed that the hardness of quenched semi-solid cast specimen was higher than that of permanent mold cast specimen. This was attributed to large amount of precipitates which results from fine precipitation of eutectic phase at intergranular areas during the aging treatment after its complete dissolution.