It may serve as a useful tool for assessing the component integrity, for instance the weldment,. HAZ and high ... layout of miniature test specimens in boat sample, testing procedures and co-relationship .... values of mechanical properties using the load-deflection data. .... 'Application of miniature specimen technique to.
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MINIATURE SPECIMEN TECHNIQUE AS AN NDT TOOL FOR ESTIMATION OF SERVICE LIFE OF OPERATING PRESSURE EQUIPMENT Kundan Kumar, K. Madhusoodanan and B.B. Rupani Reactor Engineering Division, Bhabha Atomic Research Centre
This paper was selected as the Best Poster Paper at the International Conference & Exhibition on Pressure Vessel and Piping-”OPE-2006-CHENNAI” held during February 7-9, 2006
Abstract Measurement of mechanical properties of the material of an equipment is required, to estimate its safe operating life. The Bhabha Atomic Research Centre has developed ‘Boat Sampling Technique’ to obtain a boat-shaped sample from the surface of the equipment without affecting its integrity. The samples removed can be used for preparation of miniature specimens for various types of tests like tensile, bend, fatigue, impact etc. Surface sampling technique, using specimens derived from boat sample can be considered as non-destructive, as the removal of sample does not affect the integrity of the equipment. It may serve as a useful tool for assessing the component integrity, for instance the weldment, HAZ and high stressed regions, which requires adequate strength, toughness and ductility properties. This paper highlights development of Boat Sampling Technique, layout of miniature test specimens in boat sample, testing procedures and co-relationship with conventional tests. It also highlights the methodology to obtain samples from Core Shroud of Boiling Water Reactors under irradiation environment.
Introduction
The material of an operating pressure equipment, viz.
pressure vessel, piping etc. undergoes various degradation mechanisms, depending on its environment and service conditions. There have been continuous efforts, to miniaturize test specimens for various reasons like scope for deriving more number of specimens from the sample removed, reduction in size
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of test equipment, reduction in radiation exposure, saving in waste handling requirements etc. Development of miniature specimen testing technique involves two aspects : namely, development of methods for preparation of miniature specimens and development of techniques to extract useful mechanical properties from such specimens.
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The Bhabha Atomic Research Centre (BARC) has developed an in-situ sampling technique, which can be used to scoop material from operating components [1]. Miniature specimens can be prepared from the scooped sample and the same can be tested for determining the mechanical properties of the operating component. The scooping method is known as Boat Sampling Technique (BST) because the scooped sample has the shape of a boat. This paper highlights development of BST, layout of miniature test specimens in boat sample, preparation of test specimens, miniature specimen testing procedures and co-relationship with conventional procedures.
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sample is obtained without plastic deformation or thermal degradation of the parent material. BST mainly consists of a Sampling Module and a handling manipulator. The Sampling Module, shown in Fig. 1, consists of a cutter, a feeding device, a driving system and a device for controlling thickness of sample. The height of the cutter, above the surface being sampled, can be adjusted using a thickness controlling device, to vary sample thickness up to a maximum of 3 mm.
Miniature Specimen Technology Materials are subjected to various types of tests like tensile, impact and fatigue-fracture characterization. Sub-sized conventional tests, which are essentially a scaled down version of conventional testing, utilize specimens of similar geometry loaded in a similar manner, to produce results equivalent to that obtained from larger specimens. Miniature specimen tests are employed for determination of residual service life of the operating component, by extrapolating the results of evaluation of small specimen. For this purpose, a compact and non-invasive material sampling technique can be adopted for obtaining a small sample of appropriate size from the operating plant, without affecting its integrity. The following major steps are involved in miniature specimen technology:
. . .
Retrieval of representative metallic sample by BST Preparation of representative test specimen from the boat sample Modelling of test process and procedure for correlation to conventional tests.
Retrieval Of Metallic Sample BST is used, to remove boat-shaped sample from the area of concern of the equipment. Using this technique,
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Fig. 1 : Sampling module
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Sampling process The sampling process, shown in Fig. 2, involves spinning the cutter about its axis of symmetry, while moving it slowly in a perpendicular direction into the parent material with the help of a feeding mechanism. During this operation, the cutter grinds out material from the parent material and the cutter shell follows the path cleared by its leading edge, through the material. In this process, kerf of approximately 1 to 1.2 mm width is produced. However, the actual kerf width may increase due to asymmetry in the operating conditions.
Fig. 3 : Geometry of boat sample
The rate at which the cutter can be fed into the material being cut, is greatly affected by the thickness of sample being cut. Typically, samples from soft material can be obtained in five minutes, whereas, in the case of hard material, the time required is about three hours.
Fig. 2 : Sampling process
Contour of sample The samples obtained by BST have the shape of a boat in side view and are of elliptical shape in front view. Boat shape has been selected for ease of sample retrieval and to preserve the integrity of the equipment being examined. The geometry of the sample is shown in Fig. 3 and its typical dimensions for 3 mm thick samples are given in Table 1. Due to the sampling operation, the scooped region generated in the base material, is equivalent to the sample thickness plus the kerf.
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Fig. 4 : Geometry of scooped region
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The contour of scooped region of the parent material is shown in Fig. 4 and corresponding dimensions are given in Table 2. Fig. 5 shows some samples obtained from a welded stainless steel plate during mock-up trials and the corresponding scooped region is shown in Fig. 6. Preparation Of Miniature Test Specimen From Boat Sample Fig. 5 : Sample from SS plate
Fig. 6 : Sample from HAZ of welded SS plate
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Material characterization of operating equipment can be done, by removing boat sample from the area of concern and preparing miniature test specimen using it. The schematic layout of miniature test specimens that can be obtained from a boat sample is shown in Fig. 7. The layout is based on the consideration of cutting allowances and cutting sequence, which are to be followed for preparing a proper test specimen. Fig.8 shows geometrical configurations of these miniature test specimens. Figs. 9 to 11 show photographs of some of the miniature test specimens of SS 304 material, prepared from boat sample.
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Fig. 7 : Layout of the various types of miniature specimens in a boat sample
Fig. 8 : Miniature specimens for various tests
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Modelling Of Test Process And Procedure For Correlation To Conventional Tests
Fig. 9 : Tensile specimen
Tensile test Geometry and surface condition of the sample are very important in tensile testing. In order to reduce the scatter in measurements, dimensional control is very important and the specimen has to be polished well before testing. Parameters like ratio between thickness and grain size, ratio between width and thickness, ratio between length and width are very important, for which the following three guidelines are to be followed: Gauge length ≥ 5.65
Area of gauge section
Thickness ≥ 10 x Grain size Width ≤ 8 x Thickness
(i) (ii) (iii)
In miniature specimen testing, since it is not possible to monitor the extension of the gauge portion alone, movement of crosshead is used to monitor elongation. Keeping the stiffness of machine very high as compared to the specimen, reduces the error. Free cross head speed is of the order of 10-3/s, giving a strain rate of the order of 10-4/s, which is comparable with that used for estimation of yield stress in conventional testing. Fig. 10 : Small punch test specimen (3 mm diameter)
Fig. 11 : Tensile fatigue test specimen (3 mm diameter)
Fracture toughness test Small punch test can be used to estimate mechanical properties like yield stress, tensile strength, uniform elongation, fracture toughness and the Ductile Brittle Transition Temperature (DBTT) of the material. Load is applied on the specimen mounted in a die, using a hardened steel ball or a punch with a hemispherical tip. Load and corresponding deflection are recorded during the test, as shown in Fig. 12. A number of methodologies have been developed, to estimate the values of mechanical properties using the load-deflection data. One of the methodologies that can be employed for reactor vessels is given below [2]. Yield stress, σy = β
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(1)
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Where, ‘Py’ is the load corresponding to intersection of elastic portion (elastic bending stage) and the uniform elongation portion (plastic bending stage) of load vs deflection curve, ‘β’ is a material constant and ‘h’ is the specimen thickness.
Where, ‘f p’ is the plastic part of displacement, corresponding to load P. Based on the flow curve equation from initial yield to the end of region of uniform elongation given by, σ = K∈n Ep can be written as, ln Ep =
(4)
1 1 ln K + 1 − ln σ n n
(5)
Where, ‘K’ is strength coefficient and ‘n’ is strain hardening exponent. Corresponding to the five points on the region of uniform elongation, the values of σ and Ep are calculated. Then using equation (5), values of K and n are estimated by linear regression. The ultimate tensile strength is given by equation (6) and uniform elongation is given by equation (7). Fig. 12 : Deformation mode during small punch test
σUTS = (n/e)n K
Five points from the uniform elongation portion are selected and corresponding to each point, values of equivalent plastic modulus, ‘Ep’ and true stress σ are calculated using equations (2) and (3).
Ep =
(2)
Where, ‘P’ is the load, ‘r’ is specimen radius, ‘f’ is total displacement and ‘n’ is the Poisson’s ratio of the material. σ=β
(3)
Uniform elongation, δu = en-1
(6)
(7)
Area under the load-deflection curve up to a point corresponding to the sudden drop in load is estimated and is designated as SP-energy. It is calculated at different temperatures and SP-DBTT is defined as the temperature corresponding to the average of upper and lower shelf energy values. Parameters like presence of a notch, strain rate, punch tip shape and grain size can affect small punch test results. As the grain size increases, scatter in result will also increase. Since the effective strain rate in small punch test (10-3 to 10-4/s) is much lower than in a Charpy (CVN) test (106 to 107/s), estimated DBTT is also low and both are related as per equation (8).
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SP-DBTT = α x CVN-DBTT
(8)
Where, a is a constant, normally in the range 0.35 to 0.45. Since small punch tests produce a biaxial stress state that is favourable for brittle cracking, elastic plastic fracture toughness JIC can be estimated using the test [3]. It is based on a linear relationship between JIC and the equivalent fracture strain ∈qf. Equivalent fracture strain is calculated using the relationship (9). ∈qf = a(*/h)b
(9)
Where, ‘d*’ is the deflection corresponding to sharp drop in load, and ‘a’, ‘b’ are material constants. Ductile fracture toughness, JIC can be estimated using the relation (10) [4] JIC = c∈qf-d
(10)
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Fatigue test The advantage of using an axial type specimen for evaluating the fatigue properties is that unlike a bend type specimen, the fatigue characteristics of the material are not affected by the size of the specimen. A dogbone type specimen, which gives a stress concentration gradient of 1.21, is used to avoid the effect of any stress gradient and for ease of manufacture. It is necessary to polish the specimen well before testing. Design of the gripping system is very critical in this test. A servohydraulic testing machine is used to do the fatigue test at a typical stress ratio of 0.1 and frequency of 25 Hz under load control using a sinusoidal form wave [6]. Stress amplitude = Stress concentration factor x Load amplitude/Specimen cross section Stress amplitude is plotted with the corresponding number of cycles to failure and is used as the fatigue design curve for the material.
Where, ‘c’ and ‘d’ are material constants. Impact test
Implications Involved In Miniature Specimen Technology
Chapy impact test is done, to estimate the DBTT of the material. As the specimen size is reduced, the estimated value of DBTT is also reduced from that for full size specimen. The relationships (11) and (12) are used in the test to estimate DBTT [5].
Boat sampling technique has many positive features as explained earlier. It is particularly suitable for evaluating mechanical properties near weld joint. It has some limitations, which are however minor in nature and can be overcome by suitable modeling.
DBTTfull size = DBTTsubsize + C
Surface sampling technique
(11)
Where C is a material constant ∆DBTTfull size = ∆DBTTsubsize
(12)
Hence, in order to estimate the absolute value of DBTT, it is necessary to establish the value of the constant C by testing full sized and sub sized specimens of the same material.
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The sampling technique involved, is a type of surface sampling, assumed to represent the full thickness of the component. Guidelines are required, to select the most appropriate location to sample and the most reliable sampling techniques. The choice of location essentially requires the same decision that must be made, for estimation of residual service life:
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Sample material in the location that most effectively represents the most relevant to the
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assessment being carried out, or Sample material in the less damaged area in order to get a true representation of the component material’s original strength.
likely that small-scale testing will exhibit more scatter than conventional tests. Typical variation in nominally identical tests needs to be investigated. Conclusion
The most effective sampling techniques will be more a matter of practical experience. “Less invasive” sampling generally requires particular techniques and often implies dedicated instrumentation, that needs to be applied to the component, to be sampled. Therefore, limitations like access for sampling devices need to be considered. For particularly inaccessible locations, these considerations need to be extended to cover the capture and removal of the detached sample, the extraction of the sampler and in the worst cases, recovery of the situation if sampling fails. Simulation of conventional testing Comparisons are to be carried out with surveillance specimen or fresh materials of similar grades. The actual material might have undergone some degradation mechanisms, which may affect the modeling. Specimen size effects In some cases small-scale or miniature testing may be the representative of the component being investigated, particularly where the component operates in thin sections. Where small-scale testing is attempting to simulate thicker section behaviour, however, specimen size must be considered in the light of structurally significant microstructural dimensions. A further size effect, which is particularly important for small-scale conventional testing and to some extent for small punch, is the accuracy with which specimen alignment can be achieved. Misalignment giving rise to additional bending or torsion loads on specimens will be likely to increase experimental scatter. Reproducibility of results
Surface sampling can be considered as non-destructive technique, which is very useful for determination of residual service life of operating equipment. In the absence of any standards for miniature testing, different versions of test techniques and different geometries of specimen are adopted and test results are compared with conventional testing technique for qualification. A proper modeling is required, to derive equivalence for the loads applied in the various test geometries and interchangeability of results obtained, by different variants of the same technique or different techniques. In addition to this, load equivalence may vary from one class of material to another, so that, significant experience and background information is needed for interpretation of test result. In the first phase, empirical relationships would be developed utilizing test results. In the second phase, optimization of test procedure and size relationship would be established, to reduce variation in test results, so that, it can be treated as standard for taking safety related decisions. Acknowledgements The authors are grateful to Mr. R.K. Sinha, Director, Reactor Design and Development Group, BARC and Mr. Dilip Saha, Head, Reactor Engineering Division, BARC for their support and guidance. We are also grateful to Mr. S.P. Prabhakar and Mr. R.K. Modi, Head, Special System Design Section, Division of Remote Handling and Robotics, BARC for development of handling manipulator for sampling TAPS core shroud. We are also grateful to Mr. E. Ramadasan, Head, IMCS, PostIrradiation Examination Division, BARC for development of testing techniques for irradiated materials.
Because of the various factors already discussed, it is
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References 1.
2.
3.
4.
Kundan Kumar, T.V. Shyam, J.N. Kayal. B.S.V.G. Sharma and B.B. Rupani, ‘Development of Boat Sampling Technique’, BARC Report No. BARC/2002/I/013. Yuanchao Xu, Guangshen Ning et. al., ‘Application of miniature specimen technique to material irradiation tests and surveillance for reactor components’, International Journal of Pressure Vessels and Piping, 77(2000) 715-721. Suzuki, M., Eto, M., et. al., ‘Small specimen test technique for the evaluation of toughness degradation’, Journal of Nuclear Materials, V-191194(1992) 1023-1027.
5.
6.
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Kumar A.S., Louden B.S., et al ‘Recent Improvements in size effects correlations for DBTT and upper shelf energy of ferrite steels’, ASTM STP 1024, 1993, pp. 47-61. Mao X., ‘Small punch test to predict ductile fracture toughness J IC and brittle fracture toughness KIC’, Scripta Metallurgica, V 25(1991), pp. 2481-2485. Nunomura, S., Noguchi, S., et al, ‘Two micro fatigue test methods for irradiated materials’, Small Specimen Test Techniques applied to Nuclear Reactor Vessel Thermal Annealing and Plant Life Extension” ASTP STP 1204, 1993, pp. 275-288.
About the Authors After graduating from the 37th Batch of Orientation Course of BARC Training School, Mr. Kundan Kumar, B.Sc.(Mechanical Engineering) from B.I.T. Sindri, joined RED in 1994. He has developed Integrated garter spring repositioning system and Sliver sample scraping tools for pressure tubes of Indian PHWRs. He has also developed boat sampling technique for pressure vessel components like core shroud of Tarapur Atomic Power Station. Presently he is working on development of futuristic inspection tools for coolant channels, viz. Hydrogen Equivalent Assessment Tool (HEAT), Replica Tool, Multi-head Scraping Tool etc. He is also associated with development of Tool Delivery System, which is a tool handling and manoeuvring system for inspection tools for coolant channels of Indian PHWRs.
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After graduating from the 30th Batch of Orientation Course of BARC Training School, Mr. K Madhusoodanan, M.Tech. in Design Engineering, joined RED in 1987. He has developed analytical codes for assessment of fitness for service of pressure tubes of Indian PHWRs. Presently he is working on development of annulus leak monitoring system for AHWR and innovative tools and technology for in-situ measurement of mechanical properties and hydrogen content of pressure tubes.
After graduating from the 17 th batch of BARC Training School, Mr. B.B. Rupani, M.E. in Machine Design, joined Dhruva project in 1974 and after successful completion of Dhruva project, he was transferred to Reactor Engineering Division (RED) in 1985. Presently he is heading Reactor Coolant Channel Section of RED. He has developed various innovative inspection and rehabilitation tools and techniques for life management of coolant channels of Indian Pressurized Heavy Water Reactors (PHWRs). Presently he is responsible for development of coolant channels of Advanced Heavy Water Reactor (AHWR) and development of technologies required for Zr-2.5%Nb pressure tubes of Indian PHWRs. He is a recipient of VASVIK award in “Mechanical & Structural Sciences & Technology-2000”.
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