MECHANICAL PROPERTIES AND TESTING TECHNIQUES

Transcription

1 MECHANICAL PROPERTIES AND TESTING TECHNIQUES Marta Serrano IAEA Training Workshop on Assessment of Degradation Mechanisms of Primary Components in Water Cooled Nuclear Reactors: Current Issues and Future Challenges CIEMAT, 29 September 2 October 2014

2 Contents Introduction Testing techniques Tensile Impact Drop weight Fracture toughness Summary

3 Introduction Why tests are needed to asses the performance of materials? The capability of a material to meet design and service requirements is determined by its mechanical and physical properties. Physical properties are those typically measured by methods not requiring the application of an external mechanical force (or load). Typical examples of physical properties are density, magnetic properties (e.g., permeability), thermal conductivity and thermal diffusivity, electrical properties (e.g., resistivity), specific heat, and coefficient of thermal expansion. Mechanical properties, are described as the relationship between forces (or stresses) acting on a material and the resistance of the material to deformation (i.e., strains) and fracture Mechanical testing of engineering materials may be carried out for a number of reasons: Provide engineering design data, as well as acceptability, the main purpose of which is to check whether the material meets the specification Simulate the service conditions of a material, so that the test results may be used to predict its service performance.

4 Introduction Why tests are needed to asses the performance of Pressure Vessel material? The integrity of pressurized components has to be assured to avoid accidents There are two extreme situations. Catastrophically due to brittle fracture before the crack has propagated right through the wall, with complete depressurisation and loss of the contained fluid. Leak before break (fatigue): Ductile crack propagation right through the wall, and leakage of the contained fluid takes place without disintegration of the vessel.

5 Introduction Why test are needed to asses the performance of REACTOR Pressure Vessel material? Catastrophic brittle facture has to be avoided through the entire life of the vessel RPV peculiarity: long term operation under a neutron radiation environment radiation damage and its effect on mechanical properties For the total operating life on the Reactor Pressure Vessel (60-80 years) the mechanical characteristics that made a material suitable to operate at a particular stressors (load, temperature, pressure,..) have to be maintained and assessed.

6 Introduction Why tests are needed to asses the performance of NUCLEAR REACTOR pressure vessel material? Fracture mechanics approach Defects are allowed Critical size Safe operation = Not brittle fracture K I < K IC K IC,P K IC Material Irradiated material RPV The fracture toughness is affected by irradiation K I Stressor, geometry Temperature

7 Introduction What tests are needed to asses the performance of REACTOR Pressure Vessel material? Determination of the Fracture Toughness Curve by mechanical tests of a IRRADIATED MATERIAL Surveillance programmes To now in advance the status of the vessel

8 Introduction What tests are needed to asses the performance of REACTOR Pressure Vessel material? Ideally fracture toughness test of irradiated material are needed but FT specimens are not often included within the surveillance capsule Lack of space within the surveillance capsule to hold large fracture toughness specimens to obtain valid results according to the standards existing on the time when the surveillance programmes where designed Only charpy, tensile specimens are included in most of the surveillance capsules Fracture toughness reference curve K IC = A+B exp( C ( T- Reference temperature) ) RT NDT T k RT T0

9 Introduction How tests are performed with IRRADIATED MATERIAL? ANL In-cell fracture toughness tester. Hot cells at Argonne National Laboratory ANL mechanization of specimens.

10 Introduction How tests are performed with IRRADIATED MATERIAL? CIEMAT CIEMAT Semi-hot cell CIEMAT

11 TESTING TECHNIQUES

12 Testing techniques Acceptance tests for RPV steels are Tensile Charpy Pellini drop-weight Test performed within surveillance programmes Tensile Charpy Fracture toughness (not always) All this tests and the use of the results are normalized in the design codes (ASME, KTA, RCC- M..) and associated testing standards

13 TENSILE TEST

14 Tensile Test It is done by applying an axial load to an normalized specimen with a constant strain rate, until failure During the tests both load and strain (or displacement) are measured It is a quite simple test, but sometimes preparatory work can becomes very time consuming: Selection of specimen geometry Selection of griping system.. MTS load cell Epsilon axial extensometer

15 Tensile test To perform the test at temperature different from room temperature: Environmental chamber: Temperature < room temperature Furnace: Temperature > room temperature

16 Stress (MPa) Tensile test The yield strength YS or yield point of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically. A plastic strain of 0.2% is usually used to define the offset yield stress. Ultimate tensile strength (UTS), is the maximum stress that a material can withstand before necking, which is when the specimen's cross-section starts to significantly contract Reduction of area is obtained by measuring the original cross-sectional area of the specimen and relating it to the cross-sectional area after failure. Elongation. The increase in gauge length related to the original length times 100 is the percentage of elongation UTS YS Strain (%)

17 CHARPY IMPACT TEST

18 Charpy impact test Involve striking a standard specimen with a controlled weight pendulum travelling at a set speed. Higher/Lower temperature Energy absorbed by the specimen h 2 h 1 Potential energy=mgh 1 Potential energy=mgh 2 M. Serrano, MUNECO Int. School, June 2012, La Cristalera, SPAIN

19 Energy (J) Charpy impact test Charpy test Absorbed energy T41J Room Temperature Temperature (ºC)

20 Charpy impact test Ductile apperance (%) Charpy test - Percent of shear fracture T50% Temperature (ºC)

21 Charpy impact test Lateral expansion (mm) Charpy test - Lateral expansion T0.9mm Temperature (ºC)

22 Load Fracture toughness Instrumented Charpy test Intrumented charpy test Characteristic loads P gy P max P c P a P_gy: load at general yield P_m: maximum load P_u: load at unstable fracture P_a : Arrest load Time

23 Impact test Instrumented impact test E Ductile Transition Brittle T

24 Fracture toughness Instrumented Charpy test Crack arrest Master Curve K Ia = 30+70exp(0.019(T-T KIa )) K Ia follows a log-normal distribution T KIa = T Fa4kN + 11ºC Planman 1997

25 Load Fracture toughness Instrumented Charpy test Dynamic fracture toughness determination by instrumented impact test Pre-cracked specimens [IAEA CRP8 RR] 2 2 K (1 ) 2W c Fc S c( pl) Kc f J c J el J pl a / W 3 B B E B b N W N 0 F max F gy F c F a

26 FRACTURE TOUGHNESS

27 Fracture toughness test All fracture toughness test have several common features Specimen geometries Size to assure plain strain Pre-crack by fatigue Basic instrumentation Load, displacement The machine is the same as for tensile testing Most common specimen geometry are compact tension and bend specimens Anderson 1995

28 K IC test Fracture toughness tests Plain strain is necessary condition for a valid K IC, but also the specimen must also behave in linear elastic manner The pre-cracked specimen is loaded to failure at a constant displacement rate (mm/min). The resulting load displacement curve can be type I, II, or III Definition of critical load PQ P Q =P 5 for Type I P Q is defined at pop-in for Type II P Q =Pmax for Type III K Q B P Q W f a W K IC =K Q only if 0.45 a 0.55 W 2 KQ Plain B,astrain 2.5 YS P 1.10P max Q

29 Load (kn) Fracture toughness tests T 0 determination- ASTM E1921 Fracture toughness tests of pre-cracked specimens in the transition range. PS a f 3 2 W B K 0 e W P a f K 0 e WBBN W J e 1 E 2 K 2 e J P A B N b p 0 J C J e J p Displacement (mm) K JC J C 1 E 2 K JC(limit) Eb YS 2 KJC(1T) 20 KJC1 20 B

30 Fracture toughness Master Curve One temperature testing K 1 n 4 4 K JC(i) 20 i1 0 r 20 T 1 / 4 Ln2 K K JC(med) 0 0 T ensayo K ln JC(med) Multi-temperature testing exp Ti T exp T N N 0 i i 1 i T0 i1 4 K 20 exp0.019t T JC(i) exp T i i T = 1 valid y =0 invalid

31 Fracture toughness Master Curve Determination of T 0 for JRQ material with pre-cracked charpy specimens (CIEMAT data) K JC(med) = exp(0.019(t-t 0 ))

32 Fracture toughness tests J IC test The R curve (J.vs.crack growth) for J IC measurement can be generated by Multiple specimens: a serie of nominally identical specimens are loaded to various levels and then unloaded. Each specimen is broken after the tests and the crack extension is measures Single specimen technique. The crack growth is monitoring during the test by elastic compliance method or the direct current potential drop technique Regardless of the method for monitoring the crack growth, the J value is computed for each point of the R curve J el K 2 1 E J J el J pl 2 J pl(i) J pl(i1) (i BNb 1) (i1) A pl(i) A pl(i1) x1 (i1) a (i) J IC =J Q only if 0.45 a 0.70 W 2 JQ B,b 0 25 a Y (i1) b Y (i1) Jlimit b0 15 M. Serrano, MUNECO Int. School, June 2012, La Cristalera, SPAIN

33 Fracture toughness Blunting ,0 0,5 1,0 1,5 2,0 2,5

34 Sumary The mechanical property needed to assess the integrity of the RPV is the fracture toughness Fracture toughness valid data followings old standard imply the tests of large specimens Impractical for surveillance programs Fracture toughness is obtained via charpy tests Tensile tests are performed to have reference data but are not required by code New approaches allows to obtain valid fracture toughness data by testing small specimens

35 You can have the best equipment but always you need the best technicians