GROWTH – DEDICATED CALL – 10/00
TOPIC IV.3
Minimally invasive method for toughness, tensile and creep measurements
1. CONFORMITY WITH THE WORK PROGRAMME
This topic falls under the Competitive and Sustainable Growth Programme, generic
activity Measurement and Testing. Specifically, it is related to Objective GROW-20006.2.1 Methodologies to Support Standardisation and Community Policies for which
expressions of interest have been called.
It is also relevant to:
Generic Activity 1.1.3-5, Materials and Their Technologies for Production and
Transformation, Objectives 5.1, Cross-cutting generic materials technologies, and 5.4,
Expanding the limits and durability of structural materials.
Generic Activity 1.1.3-6, Measurements and Testing, Objectives 6.2.3: Measurements
and testing methodologies in support of quality.
2. KEYWORDS
Tensile properties, toughness, creep, non-invasive sampling, residual life, life extension,
metals, ceramics, heat affected zones and welds.
3. SUMMARY OF OBJECTIVES AND JUSTIFICATION
Objectives
This project has the overall aim of selecting and evaluating a minimally invasive
method for toughness, tensile and creep measurements on specimens of metal alloys,
welds and brittle materials (e.g. advanced coatings) of industrial components based on
the testing of small coupons, the so called small punch (SP) test.
The objectives of this project are:
i.
to compare the existing variants of test systems that are now available,
ii.
to define test conditions and test procedures to give robust and repeatable results,
iii.
to validate the tensile, toughness and creep testing procedure by inter-laboratory
exercise,
iv.
to produce test guidelines for tensile, toughness and creep in formats suitable for
consideration by CEN committees,
v.
to examine the industrial use of small punch testing by a comparison with results
obtained with the other more common test techniques (invasive, destructive, large
scale), for a class of materials of wide industrial relevance,
vi.
to demonstrate by a co-ordinated group of industrial users the non invasive nature
of sampling, on their components of interest, by comparing different sampling
devices,
vii.
to explore new directions for use of the method, namely to measure characteristics
of heat affected zones in welds, of bonding layers of coatings, and examining the
possibility to develop sampling systems in robotic form for remote sampling in
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dangerous areas (nuclear, chemical hazards).
Some aspects are different in high temperature and low temperature testing, indeed
distinct codes of practice will have to be formulated for creep and for low temperature
(tensile/toughness) applications. However, today's trend goes toward a unified approach
to the problem: exploitation policy towards standardisation bodies, non destructive
industrial component sampling qualification, modelling by finite elements, targets of
material classes and components are identical items to be considered in the two ranges
of temperature.
Justification
In most cases, component integrity is hardly evaluated, with traditional and well
standardised mechanical test techniques, because there is not enough amount of material
sampled non invasively from components to allow for determination of mechanical
properties. At least in the particular case of creep damage, replicas can reveal cavities
non destructively. But evidence has grown indicating that this is just a late index of life
consumption (therefore useful, but only suitable to suggest short periods of further
operation, not allowing long life predictions); it is unsuitable to be used for materials
than do not develop creep cavities, which is the case not only of most non ferrous
alloys, but even of several classes of high strength steels used in power plants.
As a result, present standards for integrity and remnant life analysis of components must
mostly assume that relevant materials data cannot be available. This leads to strongly
conservative procedures and safety factors, or to preheating periods (with additional fuel
consumption and unnecessary pollution), and premature component removal and
replacement. All such inconveniences are merely the consequence of non availability of
material data, characteristics of the component in the actual state of material condition.
The small punch test technique has grown to answer the needs of personalised material
data for evaluating:
- integrity of long service components and plant life prediction,
- convenience of repairing and replacing welds on old components,
- cost of component deterioration, cost of normal service, peak service, plant cycling,
- safe periods of plant life extensions..
In addition to these integrity analyses and life extension scope, information on the load
carrying properties of particular regions in structural elements realised by miniaturised
test techniques is also urgently required by users, namely to provide confidence in the
performance of engineering coatings, heat affected zones in welds, repaired regions of gas
turbine blades, etc.
Though the technique is reasonably established, at least for a number of applications,
there are still several variants, results are not always coherent, and it is not endorsed yet
by international certification. There is also need of an action at international level to
validate and standardise this methodology, possibly in view of its inclusion into test lists
in safety regulations (PED related and remanent life codes). Indeed the need for this
project has been raised by EPERC (European Pressure Equipment Research Council),
and in particular with the endorsement of TG2 of EPERC, linking CEN (as a client of
R&D, under M&T area) to EPERC Technical Task Forces; however the new potential
interested sectors lie outside the EPERC environment: gas turbines (aero and land),
weld technology, steel producers, machinery (particularly for coatings).
4. BACKGROUND
In a SP test, a specimen with the shape of a small disk is punched by a spherical ball, up
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to failure. Load-displacement records are used to derive estimates of tensile
paramenters and, from test at several temperatures, of FATT (Fracture Appearance
Transition Temperature); in testing at high temperature creep rupture strength is also
measured. The SP test shows strong potential and is the only method existing at present
capable of providing experimental determinations of several mechanical characteristics
of service exposed materials of components. Moreover, it is non destructive (in the
sense that it does not require post sampling repair nor de-rating of operation conditions
for subsequent component in service) and provides direct values (i.e. not indirectly
deducted from other measurements, such as X ray, cavitation, US methods, magnetic
parameters etc. etc.). Nevertheless, there is not jet a proof that this technique can
provide results which are as accurate and repeatable as the more commonly used test
methods. It was invented in MTI (USA) in 1981, explored in Japan in the early '80s,
much progressed by EPRI in America since 1987, and firstly introduced in Europe in
1992, in tests on steels. Industrial use is still limited, due to its limited validation, need
of characterisation of SP data analyses for each class of steels (e.g. the findings in Japan
on nuclear steels cannot be simply transferred to the EU forged steels of steam
plants…), lack of standard codes for SP testing and sampling. While results so far
obtained seem often encouraging, new good ideas for improving the results have been
raised only quite recently and therefore as yet are poorly explored; among the others:
- adding FE modelling,
- Local Approach theory,
- use of special equipment (videocameras, holography),
- new fields of application (superalloys, coatings, heat affected zones in welds, in
which case there are no other competitive techniques)
The current state of SP technology worldwide is the following:
EPRI approach to FATT determination for rotors provided reasonably good
results for that class of steels,
a Copernicus SP project (EC project, 1994-1997) explored the SP potentials for
creep: test configuration was developed, but no creep SP code, 2 steels (same
class) explored only, activation energy correctly estimated but no idea how to
derive important creep parametres, such as Norton coefficients and creep ductility,
an innovative EPRI method (including videocamera and FEM) was recently
proposed, as well as possible use of other types of SP test and analysis routes
based on modelling by the Local Approach (ESIS), but these ideas were not
explored sufficiently,
EC-DG12 was recently running an activity on an interferometry technique
(called ESPI) applied to SP tests for measuring deflection (at room temperature,
aimed at improving alignement, potentially suitable as direct measurement of
strains as input to advanced SP data analysis, i.e. Local Approach),
studies relevant to test coatings are reported in literature, mostly exploratory
(numerical simulations and feasibility), or with preliminary results only,
other European SP development works (in UK, Italy and CZ) in the fields of creep
and toughness are ongoing, but due to their small dimension (in budget and in
R&D collaborations) they run separately, and are therefore mostly limited to
looking at systematic correlations to standard creep and toughness data,
despite the mentioned barriers (including lack of validated sampling procedures
and code acceptation) the industrial use of SP tests continues to grow broader
(EPRI on rotors, European utilities and research organisations on rotors and boiler
drums and pipes, Japan on nuclear materials, two companies in EU offering
sampling service experience).
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However, despite this industrially driven interest, there is currently a lack in
understanding of how the results of the test depend on test variables, and there is no
agreed test procedure for conducting tests. In summary, SP industrial use appears very
promising but still limited, due to its novelty, limited validation, lack of standard codes
for SP testing and sampling. Room exists for substantial improvement of results: for
example, good results were reported in a number of applications of SP tests to different
kind of steels (rotors and nuclear pressure vessels), some incorrect estimates were found
for toughness and tensile properties for other alloys (Copper steels) utilising the
traditional empirical method of data analysis. Hence the need of optimising and
verifying the test system and test method, of deriving material specific correlations in
the formulas for data analysis, and of exploring and developing better (improved, less
empirical, scientifically sound, advanced, innovative) methods of performing the SP
tests and SP data analysis.
5. ECONOMIC AND SOCIAL BENEFITS
5.1 Economic benefits
There is a large market in Europe for activities related to plan refurbishements, reconversions, revisions to nuclear plant, life extension. Moreover, very old plants exist in
East Europe and NIS countries, including nuclear power stations (it is well known that the
market there is already open to EU business, and enormous), which might take advantage
of sophisticated methodologies as SP.
The following is an example for the power production, though similar considerations
apply to chemical and petrochemical industry.
- In EU countries, one day of loss of production for a large power station means a cost
of 200 Keuro. SP tests can provide a measure of the embrittlement (i.e. the FATT) for
critical components, non invasively, during a shut down for maintenance.
- SP sampling takes 2-3 days of work in situ, costing about 7-15 Keuro (2 persons,
travel cost depends from location of plant and SP experts origin); moreover a FATT
determination from SP tests has a cost of 7-8 Keuro, including determination of
tensile properties (basic SP version; cost of getting SP tensile and toughness data with
advanced SP versions will be about twice; SP creep data can have a cost around 10
Keuro).
- With lack of SP data for FATT, conservative embrittlement behaviour has to be
assumed, leading generally to the need of spending some time (2 days) for pre-heating
the large components (to minimise occurrence of large thermal stresses at
temperatures below the FATT, i.e. in the brittle condition for the material).
- This leads to a penalty of 400 Keuro for the plant: this penalty will occur at each cold
start: maybe once per year. On the other hand, SP measure of embrittlement may be
taken valid for the next years, SP testing is not needed each year.
- In conclusion, on the basis of a five year period, the penalty avoided by SP tests
would be 2 Meuro, quite impressive with respect to the cost of routine SP analysis,
even for the most expensive – most sophisticated versions of the SP variants (FE,
videocamera, Local Approach).
The need to improve SP method was emerging from EPERC, mainly TTF5 (on Service
Integrity and Life Extension), including end users, weld institutes, component
constructors, service providers in the field of NDE and maintenance, research
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organisations and universities having core business based on use of test data produced
in their labs or allied labs (see enclosed references).
The interest is great, and the know how too, so that the capability of the best European
industries to utilise the potential offered by the SP technology should provide a step of
increased competitiveness. As a consequence of this project the European industrial
sectors dealing with such component integrity and life business will move in advance
with respect to North America and East Asia. This advantage will not remain for more
than a few years, however, as the SP culture will unavoidably diffuse worldwide, even
if this project will take protective actions (such as restricting the delivery of the
correlations, patenting software packages ..). Nevertheless, the step in safety and
efficient use of plants is judged permanent, as well as the benefits in extending the life
and reducing negative impact to the society of the large plants (avoiding fuel
consumption for long pre-heating, general pollution due to production and premature
mounting of new components replacing old ones still in good condition).
5.2 Social benefits
Owing to the following benefits:
- safer plant operation,
- reduced need of new components,
- intelligent use of existing plant components in safe and economical manner,
SP might become a widespread technique, for a large variety of medium and small service
companies that at present have the problem of choosing high-level, economically critical
techniques, or very poor but cheap NDE methods in routine works.
Privatisations of several safety-relevant industrial sectors (power, petrochemical and
chemical) tend to make critical the existence of a good technological know how in
service companies. This include labour losses too. SP technology might be a good factor
against this unpleasant trend, favouring service company business, SME existence in
this field, while maintaining a high level know-how (because SP technology is high
level, culture intensive, even if it is cheap).
6. SCIENTIFIC AND TECHNOLOGICAL OBJECTIVES
6.1 Workplan
The project should be aimed at fully developing the small punch test method, up to full
industrial use i.e. defining strong guidelines how to use this technology for components;
and to explore, verify and set up the advanced versions and applications. The
workprogramme should be essentially made of:
1. selection of several types of materials (ferritics, austenitics, aged, embrittled, of
nuclear, petrochemical and fossil power production) and definition of common
working style in SP tests by a pool of labs,
2. SP creep tests and conventional creep tests (if not pre-existing) for comparison,
3. SP tests for tensile, impact and toughness properties, and again tests on normal
specimens for comparison,
4. advanced SP methods including finite element modelling and Local Approach, special
emphasis on innovative SP testing applications,
5. analysis of SP component sampling by the existing cutting apparatus (mechanical
cutter and spark erosion systems), qualification of the sampling operation,
experimental demonstration that no post-sampling repair is needed for components,
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6. exploitation activity for promoting acceptability of the SP methodology for materials
characterisation and plant integrity analyses,
7. technology implementation plan with marketability and new fields perspectives.
As part of technology implementation plan, aspects of remote sampling in dangerous
areas (nuclear, chemical hazards) should be considered (possible robotics science
applied to the current SP sampling devices), as well as SP potential for characterisations
and qualifications of blade repairs and coating methods, and for fast determination of new
casts of high strength produced economically by Electron Beam melting techniques.
6.2 Outputs to be achieved
The deliverables should be:
correlation factors between SP and traditional test data, specific for austenitic
steels, low alloy piping steels, forged rotor steels, low alloy vessel, nickel and
cobalt base alloys,
codes of practice for SP tests, at high and low temperature (i.e. creep, tensile,
toughness),
codes for SP sampling in industrial environments,
brochures, codes, and credentials sent to European certification and safety
authorities, to favour acceptance into codes of the SP sampling for steel
components and the SP testing technology.
7. TIME SCALE
Although no rigid timescale requirements apply to this project, based on the described
objectives, the whole project should be completed within three years maximum.
8. IMPORTANT ADDITIONAL INFORMATION
The need of this project for the pressure equipment field was mentioned already. In
addition to make available appropriate materials data for integrity and residual life
analyses of plant components, and to provide useful data to research on damage theories
(for real components at intermediate state of damage, i.e. not from testing retired parts,
almost limited to end of life conditions), the following sectors will also be of interest
for this SP technology:
a) failure analysis of any type of products - machining the conventional (large)
specimens for the standard tests is normally incompatible with the dimensions and
shapes of the available parts,
b) for thin parts in steels or superalloys (e.g. blades, including gas turbines, both land
based and aero) the zone to be characterised is very small and a traditional (large)
specimen would be poorly representative (e.g. impact tests on Charpy specimens for
Foreign Object Damage assessment cannot sense the surface damaged region of the
blade),
c) very localised regions must also be tested when needing material data specific for the
Heat Affected Zone of welds (alternative to the questionable traditional use of weld
simulations by heat treatments),
d) small trial melts might be produced by Electron Beam, which is more economical
than conventional casting of normal (large) ingots, and from such small casts SP
samples can be machined and tested, as first screening in material R&D,
e) SP is the best candidate to measure strengths of coatings: the brittleness of the coating
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itself (e.g. ceramic), the bonding layer, the interdiffusion region often responsible of
important loss of cohesion.
References
Related projects and actions.
This interest is in part a follow up of a previous small exploratory RTD project, concluded a
year and half ago, in the frame of a COPERNICUS EC programme, where definite correlations
(isostress rupture curves, Norton coefficients, activation energy, ductility …) to the results from
the conventional uniaxial creep tests were not produced but the possibility to work in that
direction was explored, with promising results. The project (SPCREEP) was limited to a couple
of steels, in the field of creep only: therefore not covering the tensile/toughness parts, nor the
new extensions here proposed on coatings, nor addressing sampling qualification aspects.
THERMIE might have benefit, both for getting the possibility of the SP technique to investigate
HAZ with new materials, and also ranking the strength characteristics of different casts (the SP
technique was indeed developed in Japan for economic and fast screening of new casts,
economically casted in small quantity by electron beam, instead of large ingots from normal
furnace melts).
It is mentioned that there is an EC project (going to be finished in this time) aimed at developing
an optical interferometry method for measuring deflections in SP specimens. This ancillary
technology might be considered in the new suggested project.
In addition to networking inside PLAN (Structural Mechanics Cluster C, Innovative Test
Techniques), the other links would be to COST and the JRC European Networks HYDANET,
NESC, AMES (and the above mentioned EPERC).
Outcomes from some exploratory thinking over potential for robots for SP sampling in critical
dangerous areas might be reported to the PLAN projects area covering robotics.
A few literature references on small punch testing
Foulds, J.R., and Jewett, C.W., 1991, Miniature Specimen Test Technique for Estimating
Toughness, EPRI GS-7526, Project 1957-10, Fin. Rep.
Bulloch, J.H. and Hickey, J.J., Miniature Specimen Testing of Critical Components in Steam
Rising Plants, Proc. Conf. Press. Vessel & Piping, ASME, Minneapolis, USA, June 1994, PVP
Vol. 288, pp. 147-153.
Bicego, V., Lucon, E., and Crudeli, R., Integrated technologies for life assessment of primary
power plant components, Proc. of Int. Symp. on Materials ageing and component life extension,
Eds. Bicego, Nitta and Viswanathan, 1995, EMAS, Vol. I, 295-305.
F. Dobes, K. Milicka, B. Ule, T. Sustar, V. Bicego, S. Tettamanti, R.H. Kozlowski, J. Klaput,
M.P. Whelan, K. Maile, C. Schwarzkopf, Miniaturised disk-bend creep test of heat-resistant steels
at elevated temperatures, Engineering Mechanics, ISSN 1210-2717, Vol.5, 1998, N°3, 157-160.
Kameda, J., Bloomer, T.E., and Sakurai, S., Oxidation/Carbonization/Nitridation and in service
Mech. Prop. Degradation of CoCrAlY coatings in land based GT blades, Journal of Thermal
Spray Technology, Vol. 8(3), 1999, 440-446.