Towards the Standardization of Small Specimen Test Techniques For Fusion Applications

Closed for proposals

Project Type

Coordinated Research Project

Project Code

F13017

CRP

2170

Approved Date

5 October 2016

Status

4 - Closed

Start Date

17 March 2017

Expected End Date

16 March 2021

Completed Date

31 January 2022

Participating Countries

China
Germany
Japan
Spain
United Kingdom of Great Britain and Northern Ireland
United States of America

Description

Fission neutrons for materials testing have been available for decades in hundreds of experimental reactors worldwide; an extensive database for irradiated materials is available. Unfortunately, experimental Fusion reactors for materials testing do not exist. Testing facilities with a 14 MeV neutron source for irradiating candidate materials under Fusion-reactor conditions and offering control of the temperature of the irradiated material have been subject of development for four decades, but now have become an urgent need and crucial feature in world Fusion roadmaps. The available volumes for testing will be though reduced; in the design of IFMIF (International Fusion Materials Irradiation Facility) or its simplified versions the Japanese A-FNS or the European IFMIF/DONES, a maximum of 500 cm3 will allow the irradiation of structural materials at the needed dpa values. The optimization of the limited testing space makes indispensable the use of small specimens.The limited testing volume with needed neutron fluxes in accelerator driven Fusion relevant neutron sources drove the development of small specimens for fusion applications. The first review of the state-of-the-art dates from 1983. The development has continued in a steady manner in various laboratories worldwide yielding similar results, but without a standard procedure. Since 1983, more than 10 specific Symposia have taken place, mainly organized by ASTM, but no harmonization of small specimens test techniques has been accomplished so far.The nuclear industry, and in particular Fusion, has found that the lack of common uses in Small Specimen Test Techniques (SSTT) is preventing them to be able to compare and exchange data in an optimal manner. This would avoid duplication of experiments, need of material to be tested, etc. This CRP seeks to coordinate and encourage focused efforts in the production of guidelines for SSTT based on common agreed best practices on main test techniques (tensile, creep, low cycle fatigue, fracture toughness, fatigue crack growth rate) for reference structural Fusion materials. The final goal of this effort is a full standardization of the SSTT

Objectives

The overall objective of this research proposal is to provide a set of guidelines for SSTT based on common agreed best practices on main test techniques (tensile, creep, low cycle fatigue, fracture toughness, fatigue crack growth rate) for reference structural Fusion materials as first step of a full standardization of the SSTT.

Specific objectives

To analyse Small Specimen Test Techniques Results available data focusing on Fusion structural reference materials (RAFM steels) to produce a comprehensive reference database

To establish reference guidelines for fatigue crack growth rate tests using small specimens for the selected materials

To establish reference guidelines for tensile tests using small specimens for the selected materials

To establish reference guidelines for creep tests using small specimens for the selected materials

To establish reference guidelines for fracture toughness tests using small specimens for the selected materials

To establish guidelines for common practice in the use of SSTT

To establish reference guidelines for low cycle fatigue tests using small specimens for the selected materials

To produce meaningful Round Robin tests for establishing best practices in the field

Impact

International fusion roadmaps envisage the commissioning of the next generation fusion plants (referred as DEMO as short for demonstration reactor) by 2040-2050. The licensing process requires the qualification and validation of the candidate materials exposed to high neutron flux and fluence in plasma-near locations. The main pillars in the international material qualification strategies constitute of extensive irradiation campaigns in fission material test reactors (MTR), verification in dedicated fusion material test facilities matching the energetic harsher fusion neutron spectrum (such as IFMIF, DONES, A-FNS) and accompanying modelling programmes.
SSTT is essential for optimizing (in terms of number of tests, quality and reliability) the irradiation campaigns and thus maximizing the data that can be achieved in limited irradiation volumes. This holds for both, MTRs and fusion neutron sources. Conventional specimen sizes, such as recommended for example by current ASTM standards, pose huge issues in terms of cooling or assigning a well-defined test temperature or neutron dose.
Standardization of SSTT will not only guide the selection of specimens for dedicated material test facilities (back-up the currently selected types or suggest modifications), but will as well retroactively enable validation of data already gathered in neutron experiments.
Limitations in miniaturization are of different origins, which include the non-homogeneous nature of the material itself or specific material properties. Continuously decreasing length-scale of a “standard size” specimen (and adapting the applied load appropriately) runs into two problems: the remaining volume is no longer representative statistically (e.g. too few grains) or even worse like in fracture toughness, due to an intrinsic dependence on (crack) length, geometrical self-similar specimen do not represent similar stress-strain states, i.e. mini FT samples might exhibit a (too) large plastic zone to make it a valid test.
The second type of limitations arise from the test technique applied, where typical issues are related to limits in resolution or to a feed-back of the measurement on the test itself (e.g. clip gauges).
Some conventional test techniques, such as contact type strain extensometry, cannot be applied to small specimen due to space limitations and bending deformation issues. Non-contact techniques are strongly in demand to be developed. Some engineering design criteria (addressing the exhaustion of ductility) need the complete true stress-strain curve and specifically necking strain and reduction of area. This CRP shall contribute to improved strain and ductility data from both ends, from new measurement technologies and new FEM-guided analyses methods.
Specific impact for fusion neutron sources planned or under construction.
The Round Robin campaigns carried out under this CRP include tests on some reference specimens of some key projects (BA-IFMIF, DONES or A-FNS). Output of this CRP likely creates significant input at several levels: it contributes to the validation of the reference samples, demonstrates applicability of test techniques and evaluation of results, indicates limits of the materials to be used (specific to RAFM-steels) and as well addresses and discusses (material or test specific) limits in the generation of valid data.

Relevance

Small Specimen Testing Techniques and accepted guidelines are indispensable for generating valid material data bases for next step fusion plants engineering and commissioning.
SSTT standardization mandatory vs essential:
To obtain approvals for fusion material testing and generation of valid mechanical data using SSTT, standardization accepted by an international organization is strictly speaking not “mandatory”, but an essential step to ease and smoothen the upcoming processes. To break it down: the use of an international code framework (RCC or ASME) is essential. To have fusion materials “nuclear code qualified” as a material annex to codes is essential. In the course of approving (material) data, tests following standards are essential. A “recent” experience was the acceptance of EUROFER as material Annex [A3.19AS and A9.J19AS] to RCC-MRx. This paves the way to conformity analyses to get an approval for EU TBM (test blankets modules) in ITER operation in front of some notified body. The approval process with AFCEN has been very tedious, difficult and time consuming. As some tests that contribute to EUROFER data base are not gathered under agreed standards (standard specimen size), the validity of results had to be proven on a case by case basis. Therefore, to generate SSTT accepted guidelines as a step towards any kind of standard(ization) shall be considered as a huge progress in the fusion technology community.
The Round Robin exercise
The Round Robin (RR) tests shall make a solid case: through-out all test matrices on five different test techniques it is performed with a minimum of two reference materials, reducing the risk to become trapped with some specificity of one material. F82H and EUROFER, in particular, have been developed over more than two decades and are widely characterized in non-irradiated state. The benefit of the material selection is not only that already comprehensive data bases exist (each, partly gained on standard and small specimen), but as well that they are the reference materials for ITER TBM and strong candidates for DEMO, respectively in Japan and EU. Other quality features of the RR test include the number of independent participating institutes and organisations, the internationality (Japan, US, China; Germany, Spain, UK), their over decades developed experience as well as the density of the test matrices, where a minimum of two and up to 6 participants contribute to the individual test techniques.
The tensile test as example:
Tensile testing is specific: it provides multiple values (strength, ductility measures and the true stress vs. true strain curve) that are needed for the bulk of design criteria that rule “monotonic loading” in structural analyses. Data from tensile tests as well serve as input to evaluation methodologies of other test techniques such as the Weibull stress analysis in fracture toughness (part of “local approach” and applied in this CRP as well).
The Round Robin experiments planned in this CRP will provide statistical data, e.g., bias and precision, of the miniature tensile test technique. In short, preparatory assessment in this CRP support to judge reliability (i.e., repeatability and reproducibility) of the generated data. This is an essential part in standardization of SSTT by ASTM or other regulatory authorities. In view of next steps, the draft guidelines proposed in this CRP lean on existing standards such as ASTM.
The developmental of non-contact strain measurement technique proposed and applied in the RR definitely shall become a common and general tool well beyond the fusion community and shall have application in a wide range in industrial fields.

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