APLIKÁCIA SKÚŠKY TYPU SMALL PUNCH TEST PRI HODNOTENÍ DEGRADÁCIE VLASTNOSTÍ MATERIÁLOV TLAKOVEJ NÁDOBY REAKTORA APLIKÁCIA SKÚŠKY TYPU SMALL PUNCH TEST PRI HODNOTENÍ DEGRADÁCIE VLASTNOSTÍ MATERIÁLOV TLAKOVEJ NÁDOBY REAKTORA A SMALL PUNCH TEST METHOD APPLICATION FOR THE EVALUATION OF REACTOR PRESSURE VESSEL MATERIAL PROPERTIES DEGRADATION

Various standard tests are available for the determination of mechanical properties such as tensile strength, ductile-brittle transition temperature and fracture toughness, which are required for the structural integrity assessment of NPP components. In order to study irradiation damage, small samples are required because there are limitations on a sample size in irradiated material testing evaluation. SP samples have a great advantage in reducing the radioactivity of the irradiated materials for their small volume. APLIKÁCIA SKÚŠKY TYPU SMALL PUNCH TEST PRI HODNOTENÍ DEGRADÁCIE VLASTNOSTÍ MATERIÁLOV TLAKOVEJ NÁDOBY REAKTORA


Introduction
Presently, there has been a number of investigations on the evaluation of mechanical properties of various alloys using the small punch test method [1-3, 6].
Small punch testing methods are very popular now due to the: q low consumption of the original testing material, q relatively simple evaluation, q possibility of using universal and cheap testing machines, q uniformity of specimen for the several type of testing procedures.
However, the details of the method employed by individual research teams have differed somewhat from one to the other according to: q specimen size and shape, q punching tool (ball or rod), q configuration of testing holder, q number of specimen for one type of material, etc.
Most of research teams used this method for mechanical properties of metallic materials evaluation, but relatively few investigations have dealt with irradiated specimen [6].
Various standard tests are available for the determination of mechanical properties such as tensile strength, ductile-brittle transition temperature and fracture toughness, which are required for the structural integrity assessment of NPP components. In order to study irradiation damage, small samples are required because there are limitations on a sample size in irradiated material testing evaluation. SP samples have a great advantage in reducing the radioactivity of the irradiated materials for their small volume. This method has been involved in the VÚJE project "New Surveillance Specimen Program for Jaslovské Bohunice V-1 nuclear power plant (NPP)" (NSSP). This paper describes the basic information about a testing procedure, metallographic preparation of test samples and comparison between results from standard tests and this new testing method.
The SP procedure has also advantage for the determination of material properties, which are highly dependent on the local microstructure (e.g. heat affected zone). Figure 1 shows the set up for the small punch test. The principle of our testing procedure is penetration of the disk sample by hemispheric rod [5]. Disk shaped sample has 8 mm in diameter and 0.5 mm in thickness ( Figure 2). The specimen holder consists of a lower and upper die and holder body. Using this specimen holder, the specimens are prevented from cupping upward during punching and therefore, the plastic deformation is concentrated in the region below the punch rod. Using a relatively simple system with recorders of the load and deflection values, we can obtain the following data of basic mechanical properties: a) The yield stress and ultimate tensile strength at room temperature, which are correlated well with the parameters P y and P max , respectively. P y and P max are the loads corresponding to the yield and maximum strength of testing material.

Outline of the test
b) Ductile-brittle transition temperature (DBTT) measured by a Charpy test can be predicted from the results of temperature dependence of small punch energy (SP energy) determined from the area under the load -deflection curve.
c) Fracture toughness J IC can be evaluated using equivalent fracture strain ε ෆgf .

Apparatus
The testing is performed on a universal test machine INOVA TSM 10, equipped with load recorder, crosshead speed recorder, and data recorder for registration of load-deflection curves. Special fixture is used for the fixing of specimen and realization of SP test. For the low-temperature test, special temperature chamber cooled with liquid nitrogen vapor or liquid nitrogen is used [8].

Tensile properties
A typical load-deflection curve is shown in the figure 3. The loading force P at initial localized plastic strain is P y and load maximum, P max , can be related to the yield strength and the ultimate tensile strength, respectively. The empirical linear relationships can be expressed by equation: where k, q are constants obtained from comparing small punch results and results from the standard tensile test on the reference material.

SP Energy and SP ductile-brittle transition temperature (SPDBTT)
SP energy is estimated from the area under the load-deflection curve up to the fracture load or maximum deflection. Numerical integration method is used for the estimation. SPDBTT is defined by temperature at the energy level where SP max and SP min are energies at the upper and lower shelves, respectively. SP energy was analytically related with the ductilebrittle transition behavior by several authors. SPDBTT is approximately linear correlated with DBTT measured by a Charpy test using formula: where ␣ is correlation coefficient [6].

Fracture toughness
According to Mao et al.
[3] ␦*/t 0 is related to the equivalent fracture strain ε ෆgf by equation: where t* is the minimum thickness at the fracture portion and t 0 is initial thickness of specimen, ␤ is a constant, and ␦* is the maximum deflection at fracture. J IC can be estimated by a correlation between ε ෆgf and J IC obtained for testing material.

Materials
The experimental materials were base material, weld metal and heat affected zone of the RPV 15Ch2MFA steel. From this type of material we prepared samples, which hat been irradiated in power reactor in Jaslovské Bohunice V-2 NPP unit 3 [4, 9].

Sample preparation
The testing results are extremely depended on the quality of specimen preparation. Samples are polished by one of the following methods: q manually polish on the polishing papers for final thickness, q grinding on the horizontal machine and then manually polish on the papers. We have tested a new effective polishing method of the SP sample preparation, using the metallographic polishing machine on the special abrasive disks, which is illustrated on the figure 4. Three special holders are used for the fixation of four samples in each holder.

Testing conditions
Yield strength and ultimate tensile strength are evaluated by ambient temperature. The fixing torque is less than 5 Nm. Crosshead speed was 2 mm/min. For the evaluation of the ductilebrittle temperature, we used special cooling chamber. Testing was performed in the range temperatures from 77 K to ambient temperature. Temperature precision is controlled by system of the three thermocouples. Crosshead speed was higher (3 mm/min) than for tensile testing to achieve the constant temperature during the single test. For the evaluation of the fracture toughness value, we used cross section measurement of the sample after the penetration. Fracture thickness portion and initial thickness of specimen were measured by image analyzer procedures.

Correlations and examples of the initial state RPV materials results for NSSP
Correlations of SP parameters with the tensile properties for the base material and weld metal are described in Ref. [6]. Both SP and standard tests were done at room temperature. P y and P max correlate well with yield strength and ultimate tensile strength, respectively.  Example of the temperature dependence of SP energy for the base metal RPV is shown on the figure 5. In this case, SPDBTT is defined as the temperature corresponding to the middle energy between the energy at the intersection of the two fitting curves and energy at 50 K. Transition temperatures for materials from NSSP Table 2 Value of ε ෆgf was calculated using equations from chapter 2.3 using both formulas. For the weld metal RPV, J IC obtained from standard test versus ε ෆgf calculated from these equations is plotted in figure 6. This curve will be used for evaluation of the J IC shift after irradiation in the NSSP [3].

Conclusions
From the experiments follow these main conclusions: q SP test methods are suitable for basic mechanical parameters like tensile properties, evaluation of DBTT or fracture toughness. q The great advantage of SP test methods is a low consumption of experimental material and possibility to use this method for irradiation experiments in power reactors. q Geometry of specimen is convenient for properties evaluation of the heat-affected zone.
Main goal for the future is to standardize SP method for industrial applications. The conference will proceed in plenary session and in these four sections: Section No. 1: Crisis management, crisis planning Section No. 2: Fire prevention and safeguarding services Section No. 3: Security management -people and property protection Section No. 4 : Human factor in crisis management The conference will be held at the University of Žilina.
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