Scholarly article on topic 'Fatigue and Fracture Resistance of 316H Stainless Steel With Prior Creep Damage'

Fatigue and Fracture Resistance of 316H Stainless Steel With Prior Creep Damage Academic research paper on "Materials engineering"

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{316H / "Fatigue Crack Growth" / "Fracture Toughness" / "Creep Damage" / Pre-Strain}

Abstract of research paper on Materials engineering, author of scientific article — Marco Rocchini, Catrin M. Davies, David W. Dean, Kamran M. Nikbin

Abstract 316H stainless steel is widely used in high temperature components for advanced gas cooled reactors. Some plats have been de-rated to temperatures below the creep regime, due to the presence of creep damage in some components. However, such damaged or defected components must still satisfy structural integrity criteria for fatigue crack growth and fracture toughness behaviour. Therefore work has been performed to examine the fatigue and fracture toughness resistance of prior creep damaged material. A large block of material has been globally creep damaged (GCD) at 550 °C to the onset of tertiary creep behaviour. In this work, half sized compact tension C(T) specimens have been extracted from 316H blocks which were both pre-compressed and prior creep damaged at 550 °C. Fatigue crack growth and fracture toughness tests were subsequently performed on them. Note that prior to creep, this block was pre-compressed (PC) to 8% plastic strain at room temperature in order to work harden the material and limit the influence of crack tip plasticity in subsequent creep crack growth tests. The results, when compared to those previously obtained from as-received (AR), pre-compressed (PC) and locally creep damaged (LCD) standard sized C(T) samples, show an overall reduction of the fracture energy J. However, the global creep damage method does not introduce substantial changes on the fatigue crack growth behaviour of the material.

Academic research paper on topic "Fatigue and Fracture Resistance of 316H Stainless Steel With Prior Creep Damage"

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Structural Integrity

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Procedia Structural Integrity 2 (2016) 879-886

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21st European Conference on Fracture, ECF21, 20-24 June 2016, Catania, Italy

Fatigue and Fracture Resistance of 316H Stainless Steel With Prior

Creep Damage

Marco Rocchinia*, Catrin M. Daviesa, David W. Deanb, Kamran M. Nikbina

"Mechanical Engineering Deparment, Imperial College London, South Kensington Campus, London SW7 2AZ, UK bEDF Energy, Barnett Bay, Barnwood, Gloucester GL4 3RS, UK

Abstract

316H stainless steel is widely used in high temperature components for advanced gas cooled reactors. Some plats have been de-rated to temperatures below the creep regime , due to the presence of creep damage in some components. However, such damaged or defected components must still satisfy structural integrity criteria for fatigue crack growth and fracture toughness behaviour. Therefore work has been performed to examine the fatigue and fracture toughness resistance of prior creep damaged material. A large block of material has been globally creep damaged (GCD) at 550 °C to the onset of tertiary creep behaviour. In this work, half sized compact tension C(T) specimens have been extracted from 316H blocks which were both pre-compressed and prior creep) damaged at 550 °C. Fatigue rrack growth and fracture toughness tests were subsequently performed on them. Note that prior to creep, this block was pre-compressed (PC) to 8% plastic strait at room temperature in osder to worm harden the material and limii the influence or crack tip plasticity in subsequent creep crack growth tsrts. The results, wron compared to those previously obtained front as-received (AR), pre-compressed (PC) and locally creep) damaged (LCD) standard sized C(T) ramples, show an overall reduction of Hie fracture energy J. However, the global creep damage methrd does not introduce substantirl changes on the fattgue crack growth behaviour of trite material.

Copytigtt © f016 The Autfors. Published by Elsevier 13.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility erf titles Sciestific Committee of ECF21.

Keywords: 35^6H; Fatigue Coack Growth; Fractare Toughness; Creep Damage, Pre-Strain

* Corresponding author. Tel: +44(0)2075947035 E-mail address: marco.rocchini14@imperial.ac.uk (M. Rocchini)

Copyright © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer review under responsibility of the Scientific Committee of ECF21.

10.1016/j.prostr.2016.06.113

1. Introduction

Type 316H stainless steel (SS) is widely used in the UK's advanced gas cooled reactors (AGR) high temperature components. Such components may undergo inelastic damage, which is a combination of plastic strain and creep damage. The former may be introduced during the manufacturing process, while the latter is due to the components operation at sustained periods at stress and temperature. It is important to determine how inelastic damage may influence the structural integrity of high temperature components. Mehmanparast A. (2012) and Mehmanparast et al. (2010, 2013a) have therefore examined the influence of prior plastic and creep strain/damage on the fracture toughness and FCG behaviour of 316H SS. Tests were performed on as-received (AR) and uniformly pre-compressed (PC) material in addition to material that had uniformly pre-compressed and then locally creep damaged (LCD) through the interruption of a creep crack growth (CCG) test on a compact tension, C(T), specimen. A drop in the fracture toughness has been found as a result of the pre-compression process. Tests on LCD samples showed lower fracture toughness (Jic) values compared to the PC material. FCG tests show that the LCD had substantial effects on the initiation and early stages of the FCG behaviour of the material. However, the obtained da/dN vs AK trends were insensitive to the pre-compression process. According to Gan (1982), 316H SS shows a strong reduction in tensile strain at failure and a rapid drop in the fracture energy, when a moderate increase in the yield stress is provided. These observations are consistent with results given in Albertini and Montagnani (1990), and Mehmanparast et al. (2013b) on 316H SS. In this work the tensile creep and compressive plastic pre-straining effects on the fracture resistance and fatigue crack growth behaviour of 316H SS have been examined.

2. Specimen Geometry and Manufacture

The influence of global creep damage on the FCG and fracture toughness behavior of type 316H stainless steel (taken from an ex-service steam header provided by EDF Energy) has been examined. The tensile and creep properties of the as-received (AR) and pre-compressed (PC) material have been previously characterised as detailed in Mehmanparast et al. (2013b). A large block of material, of dimensions 63.0*25.5*150.0 mm3, was uniformly crept at 550 °C and 300 MPa in order to introduce creep strain/damage into the material. The test was interrupted when an instantaneous creep strain rate of around twice the minimum creep strain rate was achieved in the block. Prior to creeping the sample, it was pre-compressed to 8% pre-compression plastic strain. Subsequently, standard sized (50 mm width) C(T) fracture samples were extracted from the crept material, here denoted the globally creep damaged (GCD) samples. FCG and fracture toughness tests were then performed on these samples.

Table 1. Fracture toughness and fatigue crack growth specimens' dimensions

Test ID Test Type W [mm] B [mm] Bn [mm] a / W

CT25 - GCD1 FCG 25.0 12.0 9.0 0.40

CT25 - GCD2 FCG 25.0 12.0 9.0 0.40

CT25 - GCD3 FCG 25.0 12.0 9.0 0.40

CT25 - GCD4 Jic 25.0 12.0 9.0 0.50

CT25 - GCD5 Jic 25.0 12.0 9.0 0.50

CT50 - AR1 Jic 50.0 25.0 20.0 0.50

CT50 - AR2 Jic 50.0 25.0 20.0 0.50

CT50 - PC1 Jic 50.0 25.0 20.0 0.50

CT50 - PC2 Jic 50.0 25.0 20.0 0.50

CT50 - LCD1 Jic 50.0 25.0 17.5 0.55

CT50 - LCD2 Jic 50.0 25.0 17.5 0.55

CT50 - GCD1 FCG 50.0 25.0 17.5 0.50

CT50 - GCD2 FCG 50.0 25.0 17.5 0.50

In this work, half sized C(T) samples (W = 25 mm) have been reconstituted from the broken half standard globally creep damaged C(T) samples to maximize the number of tests that can be performed on this material. Fracture toughness (Jic) and FCG tests have been performed on these half-sized samples. Table 1 summaries the dimensions of the half sized C(T) specimens examined here and the standard sized C(T) samples tested by Mehmanparast et al. (2012), where a0 is the initial crack length, W is the specimen width, B is the specimen's thickness and Bn is the net thickness between the side-grooves. Note that all FCG and fracture toughness tests were performed at room temperature.

The microstructure of the GCD material has been examined to assess the damaged state of the material. As shown in Figure 1, the sample's surface shows evidence of intergranular creep damage in the form of voids and micro-cracks, as elongated intergranular black areas. Furthermore, some macro-cracks along the grain boundaries have been observed, as illustrated in Figure 2. Vickers hardness tests have also been performed on the GCD material.

Figure 1. Image of GCD material's surface, taken from the optical Figure 2. Image of the GCD material's surface, taken from the optical microscope at 5x magnification. The elongated black areas along the microscope at 20 X magnification. grain boundaries represent creep damage.

3. Fatigue Crack Growth Testing

Room temperature fatigue crack growth (FCG) tests were performed following the ASTM E647 standard. To be consistent with the tests performed by Mehmanparast et al. (2012), all tests were conducted at the R-ratio of R = 0.1 and a frequency o f =10 Hz. The fatigue crack growth is generally correlated with the stress intensity factor range, AK, and is described by the Paris' law

3.1 Fatigue Crack Growth Propagation Monitoring Technique

Samples were initially pre-notched to an initial crack length a0 = 10 mm using an electro-discharge-machine (EDM). The unloading compliance technique was employed for measuring the instantaneous crack length during the FCG tests. The instantaneous crack length normalised by the specimen's width W can be found by

1.0012 - 4.9165^ + 23.057^ - 323.91u3 + 1798.3;U4 - 3513.2^

where a/W is the crack length normalised by the specimen's width and ^ is given by

V^e^m^e

where Ce is the elastic unloading compliance, Em is the effective Young modulus and Be is the effective thickness.

4. Fracture Toughness Testing

Room temperature fracture toughness (Jic) tests were performed on two GCD specimens. A single specimen approach was employed to quantify the fracture toughness of the material, following the ASTM E1820 standard.

4.1 Pre-Fatigue Cracking

Specimen were pre-fatigue cracked until a crack length normalized by the specimen's width a/W = 0.5 was attained in order to introduce sufficiently sharp crack tip into the samples and to obtain valid results from the fracture toughness tests. The elastic unloading compliance technique was employed to measure the instantaneous crack length during the pre-cracking, as detailed in Section 3.1. Furthermore, specimens were side-grooved in order to promote straight fronted ductile crack growth during the test. According to the ASTM E1820, the total depth of the side-groves is 0.255, falling within the range 0.10B< (Bn - B) < 0.25 B.

5. Influence of Plastic Pre-Compression and Tensile Creep Pre-Straining

5.1 Fatigue Crack Growth Tests

Figure 3 shows the variation of the crack length against the number of cycles for the performed FCG tests on the GCD material. Significantly different values of the crack incubation period and number of cycles at failure are shown, as also detailed in Table 2. After around 2000 cycles of crack incubation, specimen FCG - GCD1 failed at just under 16,000 cycles, reaching a final normalised crack length of 0.65. For specimen FCG - GCD3, the fatigue crack growth took just over 9000 cycles to initiate and specimen's failure was reached after 28,318 cycles, corresponding to a normalised crack length of 0.7. All specimens exhibit an accelerated cracking towards the end of the test.

The fatigue crack growth rate per cycle da/dN is correlated with the stress intensity factor range, AK, for the GCD material in Figure 4 for the test performed here on half-sized samples. These results are then compared to the results on standard sized C(T) samples on GCD and PC material in Figure 5. All GCD specimens exhibit a similar trend in the steady state Paris law region of the curve, which can be described by a power-law relationship. The fatigue crack growth rate data for the GCD material is also in good agreement with PC material. A power-law regression fit has been made for each data set within the linear steady state Paris law region (see Eqn (1)) and the constants obtained are given in Table 2.

Table 2. FCG test results for the GCD material. Columns show the crack incubation period in cycles, the number of cycles at failure, the

normalised initial and final crack length and the Paris law constants.

Specimen Incubation Period [cycles] N [cycles] ao / W a/ W C m

FCG - GCD1 2065 15906 0.4 0.65 1.75X10-10 4.04

FCG - GCD2 5415 23326 0.4 0.69 1.19X10-9 3.52

FCG - GCD3 9133 28318 0.4 0.70 3.80X10-9 3.19

Figure 3. Variation of the normalised crack length against the number of cycles for the GCD material.

Figure 4. Fatigue crack growth rate per cycle correlated with the stress intensity factor range for the GCD material.

Figure 5. Fatigue crack growth rate per cycle against stress intensity factor range for GCD and PC material.

5.2 Fracture Toughness Tests

The fracture resistance curves obtained from the GCD samples are shown in Figure 6 and 7 depicts a comparison of the J vs Ac data for the GCD, AR, PC and LCD specimens. The fracture toughness values calculated for all the analysed samples are summarised in Table 3. In order to compare the results with GCD, AR and PC material, the R-curves from the LCD specimens have been adjusted, as described in Mehmanparast A. (2012), to exclude the region influenced by extensive cracking but contained unbroken ligaments which were not valid considered in the Jic test. Similar trends are observed between the LCD and PC material, since the creep damage zone was largely excluded in the analysis. The GCD material shows an overall lower values of Jic, compared to all other samples. This can be due to the creep deformation and damage effects in the GCD specimens, which may lead to a reduction in the tensile failure strain of the material. Furthermore, 8% plastic pre-compression increases the material's yield stress and decreases its ductility, also contributing in the reduction in Jic.

Table 3. Critical J integral values for GCD, AR, PC and LCD material.

Specimen Jic [MPam]

JIC - GCD4 0.095

JIC - GCD5 0.133

JIC - AR1 0.440

JIC - AR2 0.300

JIC - PC1 0.190

JIC - PC2 0.226

JIC - LCD1 0.185

JIC - LCD2 0.200

Aa [mm]

Figure 6. Fracture toughness resistance curves obtained from the GCD material. The black dots represent the critical value of the J integral. The dashed line represents the blunting line, plotted according to ISO 12135. The dashed line parallel to the x-axes represents the limit value of the J integral, evaluated according to ASTM E1820.

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Aa [mm]

Figure 7. Comparison between the obtained fracture toughness resistance curves from the GCD, AR, PC and LCD material. The black dots represent the critical value of J. The dashed line represents the blunting line, plotted according to ISO 12135.

6. Conclusion

The influence of inelastic (plastic and creep) strain/damage on the subsequent fatigue crack growth FCG and fracture toughness behaviour of 316H stainless steel has been examined. Creep damage was globally introduced into a 8% pre-compressed 316H stainless steel. Half sized compact tension samples have been extracted to perform FCG and fracture toughness (Jic) tests at room temperature. The results have been compared to previously obtained data on this GCD material and from as-received, pre-compressed and locally creep damage material, all of which were performed on standard sized samples. The results have shown that the plastic pre-compression and tensile creep pre-strain have insignificant effects on the fatigue crack growth behaviour of the material at room temperature. However, inelastic deformation and damage has resulted in reduced fracture toughness, which is related to the reduction in tensile ductility of the pre-strained materials.

Acknowledgments

The authors would like to thank Dr Ali Mehmanparast for the provision of the experimental data on standard size C(T) specimens employed in this work. This paper is published with permission from EDF Energy.

References

Albertini, C., Montagnani M., 1990, Strain-rate dependent of residual strength and ductility of AISI 316 stainless steel after creep, fatigue and

irradiation, Res Mechanica 30(4), 361-375. ASTM, E647-08 : Standard Test Method for Measurement of Fatigue Crack Growth Rates. ASTM, E1820-15 : Standard Test Method for Measurement of Fracture Toughness.

Gan, D., 1982. Tensile and fracture properties of type 316H stainless steel after creep, Metallurgical and Material Transaction A 13(12) 2155-2163. ISO, 12135-2002: Metallic materials : unified method of test for the quantification of quasistatic fracture toughness.

Mehmanparast A., Davies C.M., Dean D.W., Nikbin K.M., 2010. Compressive pre-compression effects on the creep and crack growth behaviour of 316H stainless steel, in ASME PVP 18-22 July 2010, Proceeding of the ASME Pressure Vessels and Piping division, Bellevue, Washington, USA.

Mehmanparast, A., 2012. The influence of inelastic damage on creep, fatigue and fracture toughness, PhD thesis, Imperial College London. Mehmanparast, A., Davies, C.M., Dean, D.W., Nikbin, K.M., 2013. The influence of inelastic damage and tensile deformation on creep crack growth behaviour of type 316H stainless steel", in ASME PVP 14-18 July 2013, Proceeding of the ASME Pressure Vessels and Piping division, Paris, France.

Mehmanparast, A., Davies, C.M., Dean, D.W., Nikbin, K.M., 2013. The influence of pre-compression on the creep deformation and failure behaviour of type 316H stainless steel, Engineering Fracture Mechanics 110, 52-67.