open blg report

OPEN REPORT SCK•CEN-BLG-1009

Use of Miniaturized Compact Tension Specimens for Fracture Toughness Measurements in the Upper Shelf Regime

Electrabel/Tractebel-SCK·CEN Convention 2004 Task 1.1.4/2

E. Lucon, M. Scibetta, R. Chaouadi and E. van Walle CO 90.03.17.00

April, 2005

SCK•CEN

Boeretang 200 2400 Mol Belgium

OPEN REPORT OF THE BELGIAN NUCLEAR RESEARCH CENTRE SCK•CEN-BLG-1009

Use of Miniaturized Compact Tension Specimens for Fracture Toughness Measurements in the Upper Shelf Regime

Electrabel/Tractebel-SCK·CEN Convention 2004 Task 1.1.4/2

E. Lucon, M. Scibetta, R. Chaouadi and E. van Walle CO 90.03.17.00

April, 2005 Status: Unclassified ISSN 1379-2407

SCK•CEN

Boeretang 200 2400 Mol Belgium

Name Institute Number R. Chaouadi SCK•CEN, RMO 1

E. Lucon SCK•CEN, RMO 1

J.-L. Puzzolante SCK•CEN, RMO 1

J. Schuurmans SCK•CEN, RMO 1

M. Scibetta SCK•CEN, RMO 1

E. van Walle SCK•CEN, RMO 1

Secretariaat RMO 3

R. Gérard Tractebel Engineering 3

© SCK•CEN Belgian Nuclear Research Centre Boeretang 200 2400 Mol Belgium Phone +32 14 33 21 11 Fax +32 14 31 50 21 http://www.sckcen.be Contact: Knowledge Centre [email protected]

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http://www.sckcen.be/

BLG 1009 – Page 1

Abstract In the nuclear field, the importance of direct fracture toughness measurements on RPV materials has been nowadays widely recognized, as opposed to Charpy-based estimations. However, sample dimensions have to be kept small in order to optimize the use of available material (often in the form of previously broken Charpy specimens) or, in the case of new irradiations, make effective use of the limited space available inside irradiation facilities. One of the most appealing geometries for fracture toughness measurements is the miniature Compact Tension specimen, MC(T), which has the following dimensions: B = 4.15 mm, W = 8.3 mm, cross section 10 × 10 mm². Four MC(T) specimens can be machined out of a broken half Charpy, and in the case of irradiation ten MC(T) samples occupy approximately the same volume as a full-size Charpy specimen. The MC(T) geometry was already successfully applied and qualified for fracture toughness assessments in the ductile-to-brittle transition regime, using the Master Curve method (ASTM E1921-03). A further, comprehensive investigation is presented in this report, aimed at assessing the applicability of MC(T) specimens to measure fracture toughness in fully ductile (upper shelf) conditions. In this study, 18 1TC(T) and 20 MC(T) specimens have been tested at different temperatures from three RPV steels and one low-alloy C-Mn steel. The results obtained clearly show that MC(T) samples exhibit lower fracture toughness properties, both in terms of initiation of ductile tearing (according to various test standards) and resistance to ductile crack propagation (J-R curve). The reduction of tearing resistance might be attributed to work hardening prevailing over loss of constraint in the uncracked ligament in a side-grooved specimen, or to the inadequacy of J-integral to represent ductile crack extension in very small specimens. Both arguments will have to be verified with further investigations. Keywords Upper shelf fracture toughness measurement, miniature Compact Tension specimen, ductile tearing initiation, J-R curve, work hardening, loss of constraint.

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Table of contents Abstract ..........................................................................................................................................1 Keywords........................................................................................................................................1 1 Introduction .............................................................................................................................3 2 Background: size effects in fracture toughness.......................................................................3 3 Materials and experimental .....................................................................................................5 4 Comparison of J-R test results between 1TC(T) and MC(T) specimens ................................7

4.1 Critical (initiation) toughness values...............................................................................7 4.1.1 Comparisons among different single-specimen techniques ..................................13

4.2 Crack resistance (J-R) curves........................................................................................15 4.2.1 Multi-specimen curves (18MND5, T = 150 °C) ...................................................21

5 Use of a different test standard (ISO 12135:2002) and influence of the crack growth correction.......................................................................................................................................23 6 Use of a J-R curve scaling approach based on plastic constraint factors..............................30 7 Use of a revised procedure for the construction of the resistance curve and the determination of initiation toughness J0.2mm ..................................................................................36 8 "Real" initiation of ductile crack extension: stretch zone width (SZW) measurements .......39 Conclusions ..................................................................................................................................41 Recommendations for future work............................................................................................42 Acknowledgements......................................................................................................................43 References ....................................................................................................................................43

BLG 1009 – Page 3

1 Introduction

The knowledge of mechanical properties for structural materials of plant components is among the key points for producing reliable integrity assessments and accurate residual life predictions. Particularly when embrittlement and degradation of structural materials are present due to service exposure under neutron irradiation, elevated temperatures or aggressive environment, the actual mechanical behaviour of the components must be confidently assessed, possibly without being overconservative.

The evaluation of mechanical properties, with the sole exception of hardness measurements, is by definition a destructive technique, since it requires direct sampling from the component (whenever this is feasible). When direct sampling is possible and if the component has to be maintained in service, the sample size has to be small enough so that easy repair or even no repair at all is needed to allow further operation of the component. Furthermore, in the nuclear field, the space available inside irradiation facilities is normally quite limited and the exploitation of material has to be optimized. All these circumstances make the use of very small specimens, usually not complying with the requirements of tests standard, very appealing.

In the specific domain of fracture toughness, the importance of direct toughness measurements is nowadays widely recognized, rather than relying on indirect correlations with other mechanical properties, such as for instance Charpy impact test results. This has justified the intensive effort which SCK•CEN and Electrabel/Tractebel have been putting during recent years in the development and validation of various sub-sized or miniaturized specimen geometries to be used for fracture toughness measurements:

• the reconstituted precracked Charpy-V specimen (not a sub-size sample per se, but an option which allows reusing previously tested specimens and therefore optimize material consumption) [1];

• the miniaturized precracked Charpy-V specimen, MPCCv [2]; • the small-size cracked round bar, CRB [2]; • the miniature Compact Tension specimen, MC(T) [3].

The latter specimen has a cross section of approximately 10 × 10 mm², equivalent to that of a full-size Charpy specimen, and a thickness B = 4.15 mm (roughly corresponding to 1/6"). In the framework of past Conventions, its use has been validated in the ductile-to-brittle transition region, where the Master Curve approach has been successfully applied on both unirradiated [3,4] and irradiated [5] materials.

More recently, MC(T) specimens have been selected for two specific tasks of the Convention: the characterization of Tihange III materials irradiated in the BR2 reactor [6,7] and the characterization of the RPV cladding of the Lemoniz power plant [8]. In both instances, MC(T) specimens have been foreseen for the charaterization of the upper shelf toughness of the unirradiated as well as for the irradiated condition; namely, the use of these samples is particularly advantageous for what concerns the space occupied in the BR2 irradiation rig: as illustrated in [6], 13 MC(T) samples occupy less space than a full-size Charpy-V specimen.

However, an assessment of their applicability in fully ductile (upper shelf) conditions, by comparison with standard-type 1TC(T) samples, is necessary.

2 Background: size effects in fracture toughness

The influence of specimen dimensions (and configuration) on the results of fracture toughness tests has always been an intensely debated issue, since the early developments of fracture mechanics.

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If the material's behaviour is typical of lower shelf conditions, it is generally accepted that the critical value of toughness, expressed in terms of the linear elastic parameter KIc, is practically size-insensitive, provided that the validity requirements expressed for example in the ASTM E399-90 standard are met.

In the ductile-to-brittle transition region, the influence of sample size has a very significant effect on fracture, which can be synthetically summarized as follows: in a larger specimen the crack front samples more material and has therefore a higher probability of intercepting a cleavage initiation point which can trigger fracture; therefore larger samples tend to exhibit lower toughness. Moreover, thinner samples are prone to loss of constraint, and the loss of triaxiality in the stress/strain state along the crack front produces an apparent increase of toughness. Nowadays, the Master Curve methodology offers a powerful tool to represent the inherent statistical nature of cleavage in the transition region, and even allows to quantify size effects by a relatively simple normalization procedure.

As far as specimen size effects in case of fully ductile behaviour are concerned, indications are often contradictory when one examines the available literature. A few selected examples are given below.

In 1979, Ingham and Morland [9] tested 25 and 12.5 thick SEN(B) specimens of A533B Cl.1 steel, using the multi-specimen technique; they did not find any significant influence of specimen size on either initiation values or crack resistance curves.

In 1987, Ingham, Wardle and Bland [10] published fracture toughness data obtained on A533B Cl.1 at RT and 288 °C using plane-sided and side-grooved C(T) specimens with thicknesses ranging from 10 to 100 mm. Once again, the multi-specimen method was employed. Their results showed that size-independent J-R curves were only obtained for specimens thicker than 40 mm; in the case of samples with B = 10 and 20 mm, the slope of the J-R curve tended to increase with decreasing specimen size, although the effect was small and appeared to be minimized by side-grooving.

In 1991, Link and coworkers [11] remarked that "J-R curve size effects are not generally observed in any consistent trend". They reported that J-R curves measured from C(T) specimens with thicknesses ranging from 1/2T to 10T (12.5 to 254 mm) were independent of specimen size. However, when using other sample geometries (CCT, DEN, SEN) in conjunction with C(T) specimen, they found "classical effects of changing constraint by varying specimen thickness", meaning that reducing thickness reduced through-thickness constraint and led to an elevation of toughness; nevertheless, a "non-classical effect" was observed for two highly ductile materials (A736 and 304 SS), for which toughness was seen to decrease with specimen size.

In 1993, Heerens et al. [12] reported an evident thickness effect on J-R curves obtained from C(T) specimens with B = 25 and 50 mm of 20MnMoNi55 and an Al alloy (Al2024 FC); in all cases, the resistance curves of the thicker specimen fell below those of the thinner. The authors also contended that both the crack tip opening displacement δ5 and Ernst's modified J-integral Jm [13] can provide size-insensitivity up to larger amounts of crack extension than the conventional parameter J.

In 2002, Wardle [14] published a comparison of initiation toughness values and J-R curve data for a series of geometrically scaled C(T) and SEN(B) specimens of A533B Cl.1, with thicknesses ranging from 5 to 100 mm. He concluded that there is little effect of specimen size and geometry on initiation toughness when the specimen is side-grooved; moreover, even the thinnest specimens (B = 5 mm) could provide J-R curve data equivalent to larger specimens up to J levels of about 600 kJ/m² and crack extensions up to 40% of their initial remaining ligament.

BLG 1009 – Page 5

In 2003, Neimitz and coworkers [15] observed no influence of specimen thickness on J-R curves or fracture initiation values using SEN(B) specimens of typical RPV steels with thicknesses from 10 to 20 mm, provided that side-grooves are present. For plane-sided samples, a clear thickness effect can be seen, with two materials providing however completely opposite indications: increasing toughness with decreasing thickness for 40HNMA (equivalent to 4340H) and flatter resistance curves from thinner samples for 40H (equivalent to 5140 or 41Cr4).

In 2004, Ono, Kasada and Kimura [16] reported upper shelf fracture toughness results on a Japanese reduced-activation ferritic-martensitic steel, JLF-1, using C(T) with thicknesses = 1/4", 1/2" and 1". They observed a decrease of initiation toughness (JQ) and slope of the resistance curve with decreasing specimen thickness, which they explained in terms of increase of plane stress and plastic zone size at the crack tip of the specimen.

What is reported above is in no way meant to represent a comprehensive literature review

on the subject. Rather, it is intended to give the reader a 'feel' of the contradictory indications that can be found in the literature.

3 Materials and experimental Three well-known RPV steels (22NiMoCr37, 18MND5, A533B) and a typical low-alloy C-Mn steel (A106) have been used for this study. Their chemical composition is given in Table 1, which also includes relevant references from previous Convention tasks, which offer more details on the selected materials. The only material which has never been previously used is a plate of A533B Cl.1 with dimensions 14.45 × 2.78 × 0.115 m³, produced by Creusot-Loire for Siemens in 1993; the complete Certified Material Report is reproduced in Annex 1.

Table 1 - Chemical compositions (weight %, Fe balance) of the steels selected for this study.

Steel C Si P S Cr Mn Ni Cu Mo Al Ref. 22NiMoCr37 0.22 0.23 0.006 0.004 0.39 0.88 0.84 0.08 0.51 [17]

18MND5 0.18 0.25 0.007 0.002 0.17 1.6 0.64 0.13 0.5 0.02 [18] A533B Cl.1 0.183 0.231 0.0073 0.0031 0.17 1.49 0.642 0.062 0.479 0.022 Annex 1

A106 0.24 0.23 0.011 0.011 0.09 1.08 0.11 0.09 0.04 [19] For each material, Compact Tension specimens with thicknesses B = 25 mm (1TC(T)) and B = 4.15 mm (MC(T)) have been tested at RT and in some cases also at higher temperatures (150 and 290 °C). Every test has been analyzed using three different single-specimen techniques: Unloading Compliance (UC), Potential Drop (PD) and Normalization Data Reduction (NDR); additionally, two multi-specimen J-R curves have been obtained on 1TC(T) and MC(T) specimens of 18MND5 at 150 °C. The complete test matrix is given in Table 2. All the specimens were side-grooved up to 80% of their original thickness after fatigue precracking.

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Table 2 - Complete test matrix.

Specimens tested Steel T

(°C)1TC(T) MC(T)

22NiMoCr37RT 150 290

2 2 2

2 2 2

18MND5 RT 150 290

2 4 2

2 4 2

A533B Cl.1 25 290

2 2

2 2

A106 RT -1 2 TOTAL 18 20

The geometry of the MC(T) specimens is shown in Figure 1. The grooves on the outer side of the pin holes are used to accomodate the clip-gage, which is mounted externally, whereas in the case of bigger samples the arms of transducer are placed inside the machined notch. The clip-gage, in addition to measuring load-line displacement, is also used for measuring the voltage drop across the crack plane when the PD technique is applied. All tests have been performed on an INSTRON universal test bank, using a constant crossead displacement rate of 0.2 mm/min for MC(T) and 1 mm/min for 1TC(T). The cutting plans for all selected materials are collected in Annex 2.

Figure 1 - Drawing of the MC(T) specimen.

1 In the case of A106B, upper shelf fracture toughness tests on 1TC(T) specimens had already been performed in the framework of an ASTM E08 Round-Robin [ ]. 19

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For the analysis of all tests, the procedure prescribed by the ASTM E1820-01 standard has been followed; in addition, for a few selected tests, alternative procedures have been used (ISO 12135, use of alternative crack growth corrections, etc). Such investigations will be addressed in separate sections.

4 Comparison of J-R test results between 1TC(T) and MC(T) specimens A straightforward evaluation of the results obtained from the analyses performed in accordance with ASTM E1820-01 is presented hereafter. Individual test reports, which include detailed analyses according to UC, PD and NDR techniques are given in Annexes 3 and 4 (22NiMoCr37), Annexes 5 and 6 (18MND5), Annexes 7 and 8 (A533B) and Annex 9 (A106 - only MC(T) tests). It is to be noted that, in several cases and particularly for MC(T) specimens, the PD technique has provided poor-quality data which could not be used in further analyses.

4.1 Critical (initiation) toughness values For most test standards, the engineering approximation of ductile crack growth initiation is defined at the intersection of the J-R curve (fitted through all data points contained within a so-called "validity box") with a straight line, parallel to the blunting (construction) line and having an offset of 0.2 mm. In other words, initiation is assumed for practical purposes at 0.2 mm of ductile crack extension beyond blunting (apparent crack extension due to plastic stretching of the crack tip). This parameter is denominated JQ (or JIc if qualified according to a list of validity criteria) in ASTM E1820-01 and J0.2BL in ISO 12135. Additionally, often reference is made to the value of J corresponding to 0.2 mm absolute crack extension, i.e. blunting included; the use of this parameter avoids all controversies related to the analytical expression of the blunting line, which is significantly different between ASTM and ISO standards. This critical value is commonly indicated as J0.2mm and is presently not included in either ASTM E1820 or ISO 12135; it was, however, considered by the ESIS P2-92 procedure which can be considered the precursor of the ISO 12135 standard. Finally, the tearing modulus (TM) is defined as the slope of the fitting curve at the instersection point which defines JQ or J0.2BL; this parameter was included in an old and now discontinued ASTM standard (E813) and was meant to provide an indication of the steepness of the crack resistance curve at the point of crack initiation. Values of JQ (or JIc

2) and tearing modulus for all tests performed, calculated according to ASTM E1820-01 using UC, PD and NDR are presented in Table 3 (22NiMoCr37), Table 4 (18MND5), Table 5 (A533B) and Table 6 (A106). Values of J0.2mm are given in Table 7 to Table 10 for the four materials. In all cases, toughness is reported not only in terms of J-integral but

also as the corresponding value of stress intensity factor K (calculated as ( )E

JK21 ν−

= , with E

= Young's modulus and ν = Poisson's ratio).

2 Values of JQ which qualify as JIc (size-insensitive fracture toughness) according to ASTM E1820-01 are shown in bold in the tables, along with their corresponding K value.

BLG 1009 – Page 8

Table 3 – Values of critical toughness JQ and tearing modulus obtained from 22NiMoCr37.

T Specimen Specimen JQ/JIc KJq/KJIc TM JQ/JIc KJq/KJIc TM JQ/JIc KJq/KJIc TM(°C) type code (kN/m) (MPa√m) (MPa) (kN/m) (MPa√m) (MPa) (kN/m) (MPa√m) (MPa)

N1 440 315 221 - - - 418 307 217N2 373 290 274 411 305 209 370 289 250

Mean 406 303 247 411 305 209 394 298 233N9 - - - - - - 769 417 434N10 667 388 510 633 378 505 830 433 372

Mean 667 388 510 633 378 505 799 425 403N3 132 170 310 - - - 330 268 165N4 341 272 145 - - - 322 265 183

Mean 237 221 227 - - - 326 266 174N11 579 355 269 355 278 394 630 370 264N12 629 370 297 368 283 419 650 376 315

Mean 604 362 283 361 280 407 640 373 289N5 305 252 152 278 241 176 265 235 144N6 310 254 190 278 241 209 264 235 193

Mean 308 253 171 278 241 192 265 235 169N13 271 238 271 - - - 402 289 282N14 480 316 239 315 256 320 406 291 292

Mean 376 277 255 315 256 320 404 290 287

25

MC(T)

1TC(T)

150

MC(T)

1TC(T)

290

MC(T)

1TC(T)

UNLOADING COMPLIANCE POTENTIAL DROP NORMALIZATION

Table 4 – Values of critical toughness JQ and tearing modulus obtained from 18MND5.


M1 407 303 253 - - - 389 296 223M2 452 319 279 - - - 400 301 271

Mean 429 311 266 - - - 394 298 247M9 853 439 516 632 378 577 939 460 493M10 569 359 541 547 352 551 762 415 490

Mean 711 399 528 589 365 564 851 438 491M3 334 270 162 258 237 240 298 254 171M4 348 275 163 - - - 314 261 164

Mean 341 272 162 258 237 240 306 258 168M11 523 337 415 417 301 436 606 363 390M12 561 349 392 489 326 386 553 347 366

Mean 542 343 403 453 314 411 579 355 378M5 243 225 202 259 232 182 225 216 166M6 240 223 204 250 228 194 241 224 155

Mean 241 224 203 255 230 188 233 220 160M13 371 278 332 368 277 331 415 294 321M14 284 243 322 259 232 321 360 274 285

Mean 328 261 327 314 255 326 387 284 303


25

MC(T)

1TC(T)

150

MC(T)

1TC(T)

290

MC(T)

1TC(T)

BLG 1009 – Page 9

Table 5 – Values of critical toughness JQ and tearing modulus obtained from A533B Cl.1.


B1 349 281 331 388 296 245 331 273 306B2 298 259 292 401 301 169 253 239 296

Mean 323 270 312 395 299 207 292 256 301B6 505 338 420 394 298 445 525 344 414B7 393 298 432 371 289 442 559 355 355

Mean 449 318 426 382 294 443 542 350 385B3 262 234 182 263 234 192 240 224 145B4 241 224 172 279 241 122 271 237 71

Mean 251 229 177 271 238 157 256 231 108B8 305 252 256 259 232 274 356 272 224B9 295 248 245 247 227 275 360 274 188

Mean 300 250 251 253 230 274 358 273 206


25

MC(T)

1TC(T)

290

MC(T)

1TC(T)

Table 6 – Values of critical toughness JQ and tearing modulus obtained from A106B3.


A1 256 241 117 - - - 218 222 134A2 233 229 165 237 231 150 229 227 154

Mean 244 235 141 237 231 150 224 225 144A1_01-04 253 228 349 277 238 299 285 242 294W3_01-02 - - - - - - 275 238 318B4_01-03 330 260 273 286 242 331 275 238 -

Mean 291 244 311 281 240 315 278 239 306

NORMALIZATION

MC(T)

UNLOADING COMPLIANCE POTENTIAL DROP

25

1TC(T)

Table 7 – Values of critical toughness J0.2mm obtained from 22NiMoCr37.

T Specimen Specimen J0.2mm KJ02mm J0.2mm KJ02mm J0.2mm KJ02mm

(°C) type code (kN/m) (MPa√m) (kN/m) (MPa√m) (kN/m) (MPa√m)N1 289 255 - - 277 250N2 227 226 278 251 235 231

Mean 258 241 278 251 256 240N9 - - - - 291 256N10 223 224 250 237 354 283

Mean 223 224 250 237 323 270N3 83 134 - - 242 230N4 268 242 - - 229 223

Mean 176 188 - - 236 226N11 349 276 188 202 347 275N12 322 265 198 208 318 263

Mean 336 270 193 205 333 269N5 229 219 206 207 204 206N6 216 212 189 198 186 197

Mean 223 215 198 203 195 201N13 165 185 - - 226 217N14 287 245 171 189 223 216

Mean 226 215 171 189 225 216

MC(T)

1TC(T)

POT. DROPUNL. COMPL.

MC(T)

1TC(T)

MC(T)

1TC(T)

25

150

290

NORMALIZATION

3 For the tests on 1TC(T) specimens, see Note 1 on page 6.

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Table 8 – Values of critical toughness J0.2mm obtained from 18MND5.


(°C) type code (kN/m) (MPa√m) (kN/m) (MPa√m) (kN/m) (MPa√m)M1 272 248 - - 273 248M2 285 254 - - 260 242

Mean 278 251 - - 266 245M9 307 263 222 224 344 279M10 220 223 232 229 298 259

Mean 263 243 227 226 321 269M3 257 237 179 197 227 222M4 267 241 - - 242 229

Mean 262 239 179 197 234 226M11 244 231 197 207 287 250M12 269 242 244 230 286 249

Mean 257 236 220 219 286 250M5 179 193 195 202 175 191M6 177 192 186 197 191 199

Mean 178 193 191 199 183 195M13 212 210 210 209 259 233M14 171 189 158 181 237 222

Mean 191 199 184 195 248 227

25

150

290

NORMALIZATION

MC(T)

1TC(T)

POT. DROPUNL. COMPL.

MC(T)

1TC(T)

MC(T)

1TC(T)

Table 9 – Values of critical toughness J0.2mm obtained from A533B Cl.1.


(°C) type code (kN/m) (MPa√m) (kN/m) (MPa√m) (kN/m) (MPa√m)B1 220 223 273 248 217 222B2 201 213 314 266 173 198

Mean 211 218 294 257 195 210B6 265 244 208 217 275 249B7 211 218 207 216 324 270

Mean 238 231 207 216 300 260B3 195 202 193 200 191 199B4 183 195 228 218 241 224

Mean 189 198 210 209 216 212B8 198 203 166 186 240 224B9 196 202 159 182 260 233

Mean 197 203 163 184 250 228

NORMALIZATIONPOT. DROPUNL. COMPL.

MC(T)

1TC(T)

MC(T)

1TC(T)

25

290

Table 10 – Values of critical toughness J0.2mm obtained from A106B.


(°C) type code (kN/m) (MPa√m) (kN/m) (MPa√m) (kN/m) (MPa√m)A1 194 210 - - 161 191A2 159 189 168 195 161 191

Mean 177 200 168 195 161 191A1_01-04 193 200 167 186 177 191W3_01-02 - - - - 181 193B4_01-03 204 205 210 208 176 190

Mean 198 202 189 197 178 191

25

1TC(T)

MC(T)

NORMALIZATIONPOT. DROPUNL. COMPL.

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With reference to the validity of the critical toughness parameters, it has to be noted that not all the 1TC(T) specimens provide qualified JIc values; particularly at room temperature, the toughness is so elevated that the dimensions of the sample (thickness B and ligament length b)

are not large enough to satisfy the criteria of the standard (Y

QJbB

σ25, ≥ , with σY = yield stress at

test temperature). As far as MC(T) specimens are concerned, it's the limit for J-controlled crack growth

(15lim

YbJ σ⋅= ) which makes it impossible to obtain qualified JIc values from such tiny samples.

The overall results are summarized in Table 11 (JQ|JIc) and Table 12 (J0.2mm), where average values for the two tests performed under the same conditions (same material + temperature + specimen type + single-specimen method) are reported. The results obtained using the multi-specimen technique on 18MND5 at 150 °C are also included.

Table 11 - Average JQ/JIc results obtained from 1TC(T) and MC(T) specimens.

T(°C) MC(T) 1TC(T) MC(T) 1TC(T) MC(T) 1TC(T) MC(T) 1TC(T)RT 406.1 666.7 410.7 632.7 393.9 799.4150 236.6 604.0 361.4 326.0 639.6290 307.8 375.5 278.0 314.9 264.5 403.8RT 429.4 711.3 589.4 394.3 850.7150 341.1 542.2 257.5 453.2 305.7 579.4290 241.5 327.7 254.6 313.6 232.9 387.4

A106 RT 244.4 291.2 236.9 281.3 223.6 278.3 308.7 467.0RT 323.2 448.9 394.8 382.3 292.3 541.9290 251.4 299.8 271.0 252.9 255.6 357.9

UC

22NiMoCr37

PD

18MND5

A533B

Material NDR Multi-specimen

Table 12 - Average J0.2mm results obtained from 1TC(T) and MC(T) specimens.

T(°C) MC(T) 1TC(T) MC(T) 1TC(T) MC(T) 1TC(T) MC(T) 1TC(T)RT 257.7 223.0 278.1 249.7 256.1 322.6150 175.6 335.9 192.9 235.6 332.6290 222.8 226.2 197.7 171.1 194.8 224.6RT 278.5 263.4 227.1 266.4 320.9150 261.9 256.8 179.0 220.5 234.5 286.4290 178.1 191.3 190.6 184.1 183.1 248.1

A106 RT 176.7 198.5 168.2 188.7 161.0 177.7 188.0 200.5RT 210.6 237.8 293.8 207.3 195.2 299.7290 189.1 196.9 210.3 162.8 216.0 249.8

18MND5

A533B

Multi-specimenUC PD NDRMaterial

22NiMoCr37

The results obtained show a clear and systematic underestimation of initiation toughness measured by MC(T) specimens with respect to the values obtained from 1TC(T) samples. Considering the overall ratio between MC(T) and 1TC(T) average results reported in Table 11 and Table 12, we obtain:

• for JQ|JIc, 0.69 ± 0.1464 for UC, 0.84 ± 0.184 for PD, 0.59 ± 0.114 for NDR and 0.69 ± 0.170 for all results (including multi-specimen data);

• for J0.2mm, 0.93 ± 0.176 for UC, 1.10 ± 0.213 for PD, 0.79 ± 0.078 for NDR and 0.93 ± 0.196 for all results (including multi-specimen data).

4 One standard deviation.

BLG 1009 – Page 12

The tearing modulus of MC(T) specimens is also systematically lower than for 1TC(T) samples, whose J-R curves (see next chapter) are significantly steeper. The values presented in Table 11 and Table 12 are plotted in Figure 2 (JQ|JIc) and Figure 3 (J0.2mm), showing that the correspondence between specimen geometries is definitely better when J0.2mm is used as initiation parameter; most data points fall inside the ±25% interval around the 1:1 line. However, NDR shows the most significant deviation among the three single-specimen methods. From Figure 2, it appears possible to derive a reasonable estimation of a "standard" 1TC(T) JQ value based on a MC(T) test result, by using second-order polynomial fitting functions; NDR data exhibit the best relationship (R² = 0.96) between MC(T) and 1TC(T), whilst more scatter is observed with UC (where one outlier point is very evident) and particularly PD data (R² = -2.66!). It would be useful, however, to better substantiate such relationships by adding data points with lower J values (namely, below 200 kJ/m², where the difference between the two geometries seems to become negligible).

y = 0.004x2 + 0.5624xR2 = 0.9626

y = 0.0008x2 + 1.2861xR2 = 0.5804

y = 0.0003x2 + 1.1645xR2 = -2.6562

0

150

300

450

600

750

900

0 150 300 450 600 750 900

JQ/JIc from MC(T) (kJ/m²)

J Q/J

Ic fr

om 1

TC(T

) (kJ

/m²)

Unloading CompliancePotential DropNormalizationMulti-specimen

Figure 2 - Relationship between JQ|JIc values measured from 1TC(T) and MC(T) specimens.

BLG 1009 – Page 13

100

200

300

400

100 200 300 400

J0.2mm from MC(T) (kJ/m²)

J 0.2

mm

from

1TC

(T) (

kJ/m

²)

Unloading CompliancePotential DropNormalizationMulti-specimen

Figure 3 - Relationship between J0.2mm values measured from 1TC(T) and MC(T) specimens.

4.1.1 Comparisons among different single-specimen techniques The critical toughness values shown in Table 5 and Table 6 can also be evaluated in terms of comparisons among the three different single-specimen techniques used (UC, PD and NDR), in order to identify a possible systematic bias of one method with respect to the others. Such comparisons, in terms of both JQ/JIc and J0.2mm, are presented in Figure 4 (UC vs PD), Figure 5 (UC vs NDR) and Figure 6 (PD vs NDR). The following can be remarked.

1. UC tends to provide higher critical values than PD for 1TC(T) specimens, whereas the effect is much less pronounced (or even slightly reversed) for MC(T) samples (Figure 4).

2. UC and NDR yield generally consistent critical values, with a slight tendency for overestimation of NDR in the case of 1TC(T) tests; with a couple of exceptions, the agreement is excellent for MC(T) specimens (Figure 5).

3. As it should be expected from the previous two points, NDR values tend to be higher than PD values in the case of 1TC(T) specimens; once again, MC(T) results appear less biased (Figure 6).

Overall, we have found that MC(T) specimens provide more consistency among the three single-specimen techniques than standard 1TC(T) samples.

BLG 1009 – Page 14

0

250

500

750

1000

0 250 500 750 1000

JQ/JIc or J0.2mm from UC (kJ/m²)

J Q/J

Ic o

r J0.

2mm

from

PD

(kJ/

m²)

Squares: MC(T)Triangles: 1TC(T)

Empty symbols: J0.2mm

Filled symbols: JQ/JIc

Figure 4 - Comparison between critical toughness values measured using the UC and PD techniques. Dotted lines indicate ±25% with respect to the 1:1 (dashed) line.

0

250

500

750

1000

0 250 500 750 1000

JQ/JIc or J0.2mm from UC (kJ/m²)

J Q/J

Ic o

r J0.

2mm

from

ND

R (k

J/m

²)




Figure 5 - Comparison between critical toughness values measured using the UC and NDR techniques. Dotted lines indicate ±25% with respect to the 1:1 (dashed) line.

BLG 1009 – Page 15

0

250

500

750

1000

0 250 500 750 1000

JQ/JIc or J0.2mm from PD (kJ/m²)

J Q/J

Ic o

r J0.

2mm

from

ND

R (k

J/m

²)




Figure 6 - Comparison between critical toughness values measured using the PD and NDR techniques. Dotted lines indicate ±25% with respect to the 1:1 (dashed) line.

4.2 Crack resistance (J-R) curves The comparison between 1TC(T) and MC(T) specimens in terms of crack resistance curves is given in Figure 7 to Figure 9 (22NiMoCr37), Figure 10 to Figure 12 (18MND5), Figure 13 and Figure 14 (A533B Cl.1) and Figure 15 (A106), where J-∆a points are shown rather than regression curves. In the Figures, only curves based on the NDR analysis are presented. However, Figure 16 and Figure 17 show that for A106 at room temperature, J-R curves obtained using UC or PD exhibit the same trend. This holds true for the other materials as well. All calculations shown in Figure 7 to Figure 17 have been performed in accordance with ASTM E1820-01. It is observed that crack resistance curves measured on MC(T) specimens are consistently flatter than those obtained from 1TC(T) samples. For each material and test temperature, J-R curves start significantly deviating above approximately 200 kJ/m² in terms of J-integral and 0.2-0.3 mm in terms of ductile crack extension. These results are obviously consistent with the previous observation that critical toughness values are generally in better agreement when evaluated at 0.2 mm of total crack extension (J0.2mm) then when they correspond to 0.2 mm beyond crack tip blunting (JQ, corresponding to ∆a values larger than 0.5 mm). In most cases the two curves obtained under the same nominal conditions (material, specimen type and temperature) are in satisfactory agreement with each other.

BLG 1009 – Page 16

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2.5

Ductile crack extension (mm)

J-in

tegr

al (k

J/m

²)

N1 - MC(T)

N2 - MC(T)

N9 - 1TC(T)

N10 - 1TC(T)

Material: 22NiMoCr37T = 25 °C

0.2 mm

Figure 7 - J-R curves obtained from 1TC(T) and MC(T) specimens on 22NiMoCr37 at RT (NDR analyses).

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5


J-in

tegr

al (k

J/m

²)

N3 - MC(T)N4 - MC(T)N11 - 1TC(T)N12 - 1TC(T)


0.2mm

Figure 8 - J-R curves obtained from 1TC(T) and MC(T) specimens on 22NiMoCr37 at 150 °C (NDR analyses).

BLG 1009 – Page 17

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2.5 3 3.5


J-in

tegr

al (k

J/m

²)

4



0.2mm

Figure 9 - J-R curves obtained from 1TC(T) and MC(T) specimens on 22NiMoCr37 at 290 °C (NDR analyses).

0

200

400

600

800

1000

1200

1400

1600

0 0.5 1 1.5 2 2.5 3


J-in

tegr

al (k

J/m

²)

M1 - MC(T)

M2 - MC(T)

M9 - 1TC(T)

M10 - 1TC(T)

Material: 18MND5T = 25 °C

0.2 mm

Figure 10 - J-R curves obtained from 1TC(T) and MC(T) specimens on 18MND5 at RT (NDR analyses).

BLG 1009 – Page 18

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2.5 3


J-in

tegr

al (k

J/m

²)

M3 - MC(T)

M4 - MC(T)

M11 - 1TC(T)

M12 - 1TC(T)


0.2mm

Figure 11 - J-R curves obtained from 1TC(T) and MC(T) specimens on 18MND5 at 150 °C (NDR analyses).

0

200

400

600

800

1000

1200

0 1 2 3 4 5 6 7


J-in

tegr

al (k

J/m

²)

M5 - MC(T)M6 - MC(T)M13 - 1TC(T)M14 - 1TC(T)


0.2mm

Figure 12 - J-R curves obtained from 1TC(T) and MC(T) specimens on 18MND5 at 290 °C (NDR analyses).

BLG 1009 – Page 19

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2.5 3


J-in

tegr

al (k

J/m

²)

B1 - MC(T)

B2 - MC(T)

B6 - 1TC(T)

B7 - 1TC(T)

Material: A533BT = 25 °C

0.2 mm

Figure 13 - J-R curves obtained from 1TC(T) and MC(T) specimens on A533B at RT °C (NDR analyses).

0

100

200

300

400

500

600

700

800

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5


J-in

tegr

al (k

J/m

²)

B3 - MC(T)B4 - MC(T)B8 - 1TC(T)B9 - 1TC(T)

Material: A533BT = 290 °C

0.2mm

Figure 14 - J-R curves obtained from 1TC(T) and MC(T) specimens on A533B at 290 °C (NDR analyses).

BLG 1009 – Page 20

0

100

200

300

400

500

600

700

800

0 0.5 1 1.5 2 2.5 3 3.5 4


J-in

tegr

al (k

J/m

²)

A1 - MC(T)A2 - MC(T)A1_01-4 - 1TC(T)B4_01-3 - 1TC(T)W3_01-2 - 1TC(T)

Material: A106T = 25 °CNDR data

0.2mm

Figure 15 - J-R curves obtained from 1TC(T) and MC(T) specimens on A106 at RT (NDR analyses).

0

100

200

300

400

500

600

700

800

0 0.5 1 1.5 2 2.5 3 3.5 4


J-in

tegr

al (k

J/m

²)

A1 - MC(T)

A2 - MC(T)

A1_01-4 - 1TC(T)

B4_01-3 - 1TC(T)

Material: A106T = 25 °CUC data

0.2mm

Figure 16 - J-R curves obtained from 1TC(T) and MC(T) specimens on A106 at RT (UC analyses).

BLG 1009 – Page 21

0

100

200

300

400

500

600

700

800

900

1000

0 0.5 1 1.5 2 2.5 3 3.5


J-in

tegr

al (k

J/m

²)

4

A2 - MC(T)

A1_01-4 - 1TC(T)

B4_01-3 - 1TC(T)

W3_01-2 - 1TC(T)

Material: A106T = 25 °CPD data

0.2mm

Figure 17 - J-R curves obtained from 1TC(T) and MC(T) specimens on A106 at RT (PD analyses).

4.2.1 Multi-specimen curves (18MND5, T = 150 °C) As previously mentioned, two multi-specimen J-R curves have been obtained on 18MND5 at 150 °C, one for 1TC(T) and one for MC(T) specimens. These are shown in Figure 18, directly compared with data calculated using one of the single-specimen techniques (NDR). The individual J-∆a values used in the construction of the multispecimen curves are given in Table 13.

Table 13 - Values used in the construction of the multi-specimen curves (18MND5, 150 °C).

Specimen type

J (kJ/m²)

∆a (mm)

0.603 429.47 0.656 427.04 1.204 787.69 2.164 1004.17

1TC(T)

2.793 1212.940.509 304.24 1.325 517.12 1.917 602.42 MC(T)

2.142 648.72

BLG 1009 – Page 22

0

200

400

600

800

1000

1200

1400

0 0.5 1 1.5 2 2.5 3


J-in

tegr

al (k

J/m

²)

M3 - MC(T)M4 - MC(T)M11 - 1TC(T)M12 - 1TC(T)Multi-specimen - 1TC(T)Multi-specimen - MC(T)


0.2mm

Single-specimen: crack growth correctedMulti-specimen: crack growth uncorrected

Figure 18 – Single-specimen (NDR) and multi-specimen data obtained from 1TC(T) and MC(T) specimens on 18MND5 at 150 °C. The trend shown in Figure 18 for multi-specimen J-R curves substantially reflects the situation observed for single-specimen data. Therefore, no influence on the differences between 1TC(T) and MC(T) can be attributed to the choice of the experimental technique (multi- or single-specimen). It's also worth pointing out that the multi-specimen curve of 1TC(T) specimens is in much better agreement with the single-specimen (NDR) ones than in the case of MC(T) samples. This circumstance can be explained in terms of crack growth correction (a subject which will be more extensively treated in §5 below). The present version of the ASTM E1820-01 standard does not include a crack growth correction for the so-called "basic test method" (which corresponds to the multi-specimen approach5), whereas for single-specimen methods the plastic part of the J-integral is corrected for crack extension. Such correction is proportional to ∆a and inversely proportional to the current ligament length. As a consequence, for MC(T) specimens the correction is much more significant than for 1TC(T) samples, and the relevant multi-specimen J-R curve clearly departs from the NDR curves starting from crack extension values greater than ≈ 0.6 mm.

5 We should also emphasize, however, that the "basic test method" is specifically restricted to the evaluation of initiation toughness, and is not intended for the derivation of resistance curves.

BLG 1009 – Page 23

5 Use of a different test standard (ISO 12135:2002) and influence of the crack growth correction

The ISO 12135:2002 standard "Metallic Materials – Unified Method of Test for the Determination of Quasistatic Fracture Toughness" stems from the ESIS P2-92 procedure, which was developed during the 80's and 90's by the European Working Group TC1/4 lead by K.-H. Schwalbe (GKSS, Germany). While the actual experimental procedure and the basic J-integral calculations are virtually identical to ASTM E1820-01, the following aspects in the determination of upper shelf fracture toughness (critical values and resistance curves) are significantly different:

• form of the crack growth correction, which is not applied incrementally as in E1820; • analytical expression of the blunting (or construction) line, which is much steeper than in

the ASTM standard; • form of the data regression curve, which has three parameters (offset power law) rather

than two (simple power law); • qualification requirements for J0,2BL (which corresponds to JQ), which are more stringent

than in E1820, both in terms of Jmax and ∆amax. The consequences of such differences in the comparison between MC(T) and 1TC(T) specimens have been investigated by re-analising the 12 tests (6 per specimen type) performed on 22NiMoCr37 in accordance with the ISO 12135:2005 standard. Additionally, we have examined the influence of the crack growth correction on the results by comparing data calculated according to ASTM and ISO with and without crack growth correction. A third expression of crack growth correction, recently proposed by Wallin [20], has also been considered:

o

O

aWa

mmJ

J

−∆

⋅⎟⎠⎞

⎜⎝⎛+−

+=

111

(1)

where JO is the uncorrected value and m is the power of the J-R curve (to be evaluated by successive iterations). As previously mentioned, for any form of crack growth correction, the smaller the specimens the larger is the effect on J-integral values for crack extensions larger than ∼0.5 mm. For instance, using the expression given in the ISO standard:

( ) ⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−∆

−⋅=o

O aWaJJ

21 (2)

and assuming that 5.0=Wao and ∆a = 2 mm, the correction amounts to 33.5% for a MC(T)

specimen and only 5.6% for 1TC(T). It can therefore be questioned whether the available crack growth corrections actually "over-correct" in the case of very small samples. The results of all the analyses are presented in Table 14 and Table 15 (ASTM E1820, with and without correction), Table 16 and Table 17 (ISO 12135, with and without correction) and Table 18 (ASTM E1820 uncorrected + Wallin's correction).

BLG 1009 – Page 24

Details of all the analyses performed are given in Annex 10 (ASTM – without correction), Annex 11 (ISO – with correction), Annex 12 (ISO – without correction) and Annex 13 (ASTM – Wallin's correction). Table 14 - Results obtained at 0.2 mm crack extension beyond blunting on 22NiMoCr37 with and without crack growth correction (ASTM E1820-01 standard).

T Specimen Specimen JQ/JIc KJq/KJIc TM JQ/JIc KJq/KJIc TM(°C) type code (kN/m) (MPa√m) (MPa) (kN/m) (MPa√m) (MPa)

N1 439.5 307.1 216.8 456.5 321.1 346.7N2 370.3 289.2 249.9 410.7 304.5 343.5

Mean 404.9 298.1 233.3 433.6 312.8 345.1N9 769.2 416.8 434.1 882.4 446.4 394.6N10 829.7 432.9 371.5 857.7 440.1 398.4

Mean 799.4 424.8 402.8 870.0 443.2 396.5N3 329.9 267.9 165.3 356.8 278.6 275.2N4 322.0 264.7 182.8 341.9 272.7 193.3

Mean 326.0 266.3 174.1 349.3 275.7 234.3N11 629.6 370.1 264.2 636.8 372.2 299.5N12 649.6 376.0 314.7 660.7 379.2 343.4

Mean 639.6 373.0 289.4 648.8 375.7 321.4N5 265.0 235.0 144.1 281.2 242.1 248.6N6 264.0 234.5 193.3 281.3 242.1 284.9

Mean 264.5 234.8 168.7 281.2 242.1 266.7N13 401.8 289.3 282.2 409.0 291.9 295.5N14 405.7 290.8 292.1 407.4 291.4 316.7

Mean 403.8 290.0 287.1 408.2 291.6 306.1

ASTM - With correction ASTM - Without correction

25

MC(T)

1TC(T)

150

MC(T)

1TC(T)

290

MC(T)

1TC(T)

Table 15 - Results obtained at 0.2 mm total crack extension on 22NiMoCr37 with and without crack growth correction (ASTM E1820-01 standard).

T Specimen Specimen J0.2mm KJ02mm J0.2mm KJ02mm

(°C) type code (kN/m) (MPa√m) (kN/m) (MPa√m)N1 276.9 250.1 234.6 230.2N2 235.4 230.5 216.4 221.0

Mean 256.1 240.3 225.5 225.6N9 291.0 256.3 351.5 281.7N10 354.3 282.9 341.3 277.6

Mean 322.6 269.6 346.4 279.7N3 242.1 229.5 210.8 214.2N4 229.2 223.3 193.3 205.1

Mean 235.6 226.4 202.1 209.6N11 347.4 274.9 323.8 265.4N12 317.8 263.0 301.6 256.2

Mean 332.6 268.9 312.7 260.8N5 203.8 206.1 177.5 192.3N6 185.8 196.7 166.0 186.0

Mean 194.8 201.4 171.8 189.2N13 225.9 217.0 223.2 215.6N14 223.2 215.6 213.0 210.6

Mean 224.6 216.3 218.1 213.11TC(T)

ASTM - W/O corr.ASTM - With corr.

MC(T)

1TC(T)

MC(T)

1TC(T)

MC(T)

25

150

290

BLG 1009 – Page 25

Table 16 - Results obtained at 0.2 mm crack extension beyond blunting on 22NiMoCr37 with and without crack growth correction (ISO 12135:2002 standard).

T Specimen Specimen J02/BL KJ02/BL TM J02/BL KJ02/BL TM(°C) type code (kN/m) (MPa√m) (MPa) (kN/m) (MPa√m) (MPa)

N1 250.0 237.6 503.4 267.3 245.7 575.2N2 229.1 227.5 528.2 244.6 235.0 580.3

Mean 239.6 232.6 515.8 256.0 240.4 577.8N9 310.9 265.0 676.4 313.3 266.0 703.1N10 326.6 271.6 704.2 328.9 272.5 728.8

Mean 318.8 268.3 690.3 321.1 269.3 716.0N3 202.9 210.1 449.3 216.5 217.0 512.1N4 204.3 210.9 459.8 218.0 217.8 507.7

Mean 203.6 210.5 454.6 217.3 217.4 509.9N11 321.0 264.3 518.3 322.6 264.9 545.8N12 326.5 266.5 548.7 327.3 266.9 577.1

Mean 323.8 265.4 533.5 325.0 265.9 561.4N5 187.8 197.8 344.5 199.3 203.8 394.6N6 186.1 196.9 325.7 197.1 202.6 372.9

Mean 187.0 197.4 335.1 198.2 203.2 383.8N13 258.4 232.0 418.4 260.0 232.8 440.4N14 263.2 234.2 423.6 264.2 234.6 445.7

Mean 260.8 233.1 421.0 262.1 233.7 443.0

25

MC(T)

1TC(T)

150

MC(T)

1TC(T)

ISO - Without correction

290

MC(T)

1TC(T)

ISO - With correction

Table 17 - Results obtained at 0.2 mm total crack extension on 22NiMoCr37 with and without crack growth correction (ISO 12135:2002 standard).

T Specimen Specimen J0.2 KJ02 J0.2 KJ02

(°C) type code (kN/m) (MPa√m) (kN/m) (MPa√m)N1 191.6 208.0 196.6 210.7N2 174.0 198.2 180.2 201.7

Mean 182.8 203.1 188.4 206.2N9 214.6 220.1 212.9 219.3N10 221.2 223.5 219.6 222.7

Mean 217.9 221.8 216.3 221.0N3 159.0 186.0 163.5 188.6N4 159.1 186.1 164.8 189.4

Mean 159.0 186.0 164.1 189.0N11 236.8 227.0 234.1 225.7N12 236.4 226.8 233.0 225.2

Mean 236.6 226.9 233.6 225.4N5 155.6 180.0 160.4 182.8N6 155.9 180.2 160.6 182.9

Mean 155.7 180.1 160.5 182.9N13 204.7 206.5 202.4 205.3N14 206.3 207.3 204.4 206.4

Mean 205.5 206.9 203.4 205.9

25

MC(T)

1TC(T)

ISO - With corr. ISO - W/O corr.

150

MC(T)

1TC(T)

290

MC(T)

1TC(T)

BLG 1009 – Page 26

Table 18 - Results obtained on 22NiMoCr37 using Wallin's crack growth correction (ASTM E1820-01 standard).

T Specimen Specimen JQ/JIc KJq/KJIc TM J0.2mm KJ02mm

(°C) type code (kN/m) (MPa√m) (MPa) (kN/m) (MPa√m)N1 428.4 311.1 302.6 242.0 233.7N2 390.5 297.0 309.0 221.1 223.5

Mean 409.5 304.0 305.8 231.5 228.6N9 778.8 419.4 453.4 281.1 251.9N10 840.6 435.7 385.5 346.4 279.7

Mean 809.7 427.5 419.4 313.8 265.8N3 336.8 270.7 237.9 215.2 216.4N4 326.6 266.6 203.4 203.4 210.4

Mean 331.7 268.6 220.7 209.3 213.4N11 629.2 370.0 281.2 334.2 269.6N12 650.5 376.2 329.3 307.7 258.7

Mean 639.9 373.1 305.3 320.9 264.2N5 268.1 236.3 202.5 185.1 196.4N6 271.9 238.0 242.6 173.5 190.1

Mean 270.0 237.2 222.5 179.3 193.3N13 405.4 290.6 287.1 225.3 216.7N14 406.6 291.1 300.2 219.7 214.0

Mean 406.0 290.8 293.7 222.5 215.3

290

MC(T)

1TC(T)

25

MC(T)

1TC(T)

150

MC(T)

1TC(T)

Graphically, critical values calculated using all the different approaches are shown in Figure 19 and Figure 20. It can be observed that the values obtained using the ISO standard are significantly lower than ASTM critical parameters, due to the steepness of the construction line, and therefore show less discrepancy between MC(T) and 1TC(T) specimens, falling in proximity of the -25% line. Moreover, removing the crack growth correction typically slightly moves the data points upwards and to the right, without significantly affecting the comparison between MC(T) and 1TC(T) specimens; the effect appears similar for both ASTM and ISO standards. The influence of Wallin's correction is very similiar to the incremental one already implemented in E1820-01.

y = -0.0004x2 + 0.8146xR2 = 0.9438

100

200

300

400

500

600

700

800

900

100 200 300 400 500 600 700 800 900

JQ/JIc/J02BL from 1TC(T) (kJ/m²)

J Q/J

Ic/J

02B

L fro

m M

C(T

) (kJ

/m²)

ASTM - With correctionASTM - Without correctionASTM - Wallin's correctionISO - With correctionISO - Without correction

Figure 19 - Critical toughness values calculated at 0.2 mm crack extension beyond blunting for 22NiMoCr37.

BLG 1009 – Page 27

y = -0.0009x2 + 0.983xR2 = 0.7469

100

150

200

250

300

350

400

100 150 200 250 300 350 400

J0.2mm from 1TC(T) (kJ/m²)

J 0.2

mm

from

MC

(T) (

kJ/m

²)

ASTM - With correctionASTM - Without correctionASTM - Wallin's correctionISO - With correctionISO - Without correction

Figure 20 - Critical toughness values calculated at 0.2 mm total crack extension for 22NiMoCr37.

As far as crack resistance curves are concerned, the effect of crack growth correction is shown in Figure 21 to Figure 23 for ASTM data and in Figure 24 to Figure 26 for ISO data. As anticipated above, the correction starts to play a role only after at least 0.5 mm of ductile crack extension. For both ASTM and ISO standards, removing the correction has a much larger effect for MC(T) specimens, as expected, and therefore brings the J-R curves closer. Although the ranking between MC(T) and 1TC(T) does not significantly change, it might be contended that crack growth corrections should be less dependent on specimen size.

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2.5

Ductile crack growth (mm)

J-in

tegr

al (k

N/m

)

N1 - MC(T)

N2 - MC(T)

N9 - 1TC(T)

N10 - 1TC(T)


0.2 mm

BLACK SYMBOLS: NO CORRECTION

ASTM E1820 standard

BLG 1009 – Page 28

Figure 21 - Crack resistance data obtained on 22NiMoCr37 at RT using ASTM E1820-01, with and without crack growth correction.

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5


J-in

tegr

al (k

N/m

)



0.2mm


ASTM E1820 standard

Figure 22 - Crack resistance data obtained on 22NiMoCr37 at 150 °C using ASTM E1820-01, with and without crack growth correction.

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2.5 3 3.5 4


J-in

tegr

al (k

N/m

)



0.2mm


ASTM E1820 standard

Figure 23 - Crack resistance data obtained on 22NiMoCr37 at 290 °C using ASTM E1820-01, with and without crack growth correction.

BLG 1009 – Page 29

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2


J-in

tegr

al (k

N/m

)

.5

N1 - MC(T)

N2 - MC(T)

N9 - 1TC(T)

N10 - 1TC(T)


0.2 mm


ISO 12135 standard

Figure 24 - Crack resistance data obtained on 22NiMoCr37 at RT using ISO 12135:2002, with and without crack growth correction.

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5


J-in

tegr

al (k

N/m

)



0.2mm


ISO 12135 standard

Figure 25 - Crack resistance data obtained on 22NiMoCr37 at 150 °C using ISO 12135:2002, with and without crack growth correction.

BLG 1009 – Page 30

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2.5 3 3.5 4


J-in

tegr

al (k

N/m

)



0.2mm


ISO 12135 standard

Figure 26 - Crack resistance data obtained on 22NiMoCr37 at 290 °C using ISO 12135:2002, with and without crack growth correction. The statement that the available crack growth corrections might indeed "over-correct" the J-∆a data for a small specimen could also be substantiated by the observation that in some cases, for crack extensions around 1.5-2 mm, the corrected MC(T) data show a very slight but abnormal decreasing trend; see for instance the MC(T) curves at RT (Figure 21 or Figure 24) and 290 °C (Figure 23 or Figure 26), which is clearly not the case with 1TC(T) data. Over-correction might therefore explain the "unphysical" occurence of a decreasing J-integral with increasing crack size.

6 Use of a J-R curve scaling approach based on plastic constraint factors In an effort to find a more physical interpretation of the differences observed between MC(T) and 1TC(T) specimens, a model recently proposed by Brocks et al. [21] has been considered, based on the concept of energy dissipation rate. A concise summary of this approach is given below. The energy dissipation rate, R, has been proposed as the "true driving force" for elastic-plastic fracture mechanics by Turner [22], in alternative to J-integral, on the basis of its consistency with incremental plasticity. It characterizes the increment of non-recoverable mechanical work per unit of crack extension, and includes two contributions: the specific work of remote plastic deformation and the specific local work of separation. It has been claimed that solving the problem of the geometry dependency of J-R curves can only be achieved using the concept of dissipation rate [23]. If the J-R curve is known, R can be evaluated for a C(T) or SEN(B) specimen as a function of the plastic component of J (Jpl) and crack size from [24]:

BLG 1009 – Page 31

ηγ

η plpl J

dadJaWR +⎟⎟

⎠

⎞⎜⎜⎝

⎛ −= (3)

where η and γ are plastic factors given in the ASTM E1820-01. If eq.(3) is scaled to a different specimen size and then integrated, the plastic J-R curve for this new specimen size can be obtained. According to Brocks' model, therefore, all is needed is to scale R, which in turn allows scaling the conventional crack resistance curve (or at least its plastic component). The typical curve shape of R as a function of crack growth can be effectively represented by a decreasing exponential function [24]:

⎥⎦

⎤⎢⎣

⎡⎟⎠⎞

⎜⎝⎛ ∆−+= ∞ W

aRR λα exp1 (4)

where R∞ is the final stationary value, also known as "crack propagation energy" [25], α is a parameter which determines the initial value at ∆a = 0 and λ is another parameter which describes the rate of decay from the initial to the stationary value. Brocks' approach consists in scaling R-∆a curves by the following geometrical factor, in order to eliminate the geometry dependence:

⎟⎠⎞

⎜⎝⎛⋅−=

Wa

faWf oYoG )( (5)

with fY being the ratio of the plastic limit load of the cracked structure to the net section yield load. If eq.(4) is used to represent R, then the only parameter that needs to be scaled is R∞. Given this theoretical framework, the following has been done in this study using NDR test results obtained on 22NiMoCr37:

• Jpl vs ∆a data from MC(T) specimens have been converted into RMC(T) vs ∆a data using eq.(3);

• the latter have been fitted using eq.(4) in order to obtain R∞, α and λ;

• the predicted energy dissipation rate curve for 1TC(T), R1TC(T) vs ∆a, has been reconstructed using again eq.(4), where α and λ remain the same as for MC(T) specimens and R∞ is scaled by the ratio of the respective geometrical factors calculated according to eq.(5);

• R1TC(T) vs ∆a data are integrated by inverting eq.(3), in order to obtain a predicted Jpl-∆a curve;

• this latter is compared directly with the experimental (measured) plastic J-R curves for 1TC(T) specimens.

An example of the application of this procedure is given in Figure 27 (R and Jpl data for MC(T) samples) and Figure 28 (R and Jpl data for 1TC(T) samples, and comparison with measured data). More details about the individual analyses performed are given in Annex 14.

BLG 1009 – Page 32

0

500

1000

1500

2000

2500

0 0.5 1 1.5 2 2.5

Crack extension ∆a (mm)

Ener

gy d

issi

patio

n ra

te R

(kJ/

m²)

0

100

200

300

400

500

600

700J

pl (kJ/m²)

R

Jpl

Specimen N1 - MC(T) - RT

Figure 27 - Energy dissipation rate and plastic J-integral data for a MC(T) specimen (experimental data and fitting curves).

0

2000

4000

6000

8000

10000

12000

0 0.5 1 1.5 2

Crack extension ∆a (mm)

Ener

gy d

issi

patio

n ra

te R

(kJ/

m²)

0

300

600

900

1200

Jpl (kJ/m

²)

R-1T (predicted)Jpl (N9-1TCT)Jpl (N10-1TCT)Jpl-1T (predicted)

Figure 28 – Predicted energy dissipation rate and plastic J-integral curves for a 1TC(T) specimen and comparison with measured plastic J-R curves. The overall results, in terms of comparison between predictions and measurements, are shown in Figure 29 (RT), Figure 30 (150 °C) and Figure 31 (290 °C).

BLG 1009 – Page 33

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2.5


J pl (

kJ/m

²)

N1 - MC(T)

N2 - MC(T)

N9 - 1TC(T)

N10 - 1TC(T)


1TC(T) - Predicted (from MC(T) data)

1TC(T)

MC(T)(reference)

Figure 29 - Comparison between predicted and measured plastic J-R curves for 1TC(T) specimens of 22NiMoCr37 tested at RT.

0

300

600

900

1200

0 0.5 1 1.5 2 2.5 3 3.5 4


J pl (

kJ/m

²)




1TC(T)

MC(T)(reference)

1TC(T)

Figure 30 - Comparison between predicted and measured plastic J-R curves for 1TC(T) specimens of 22NiMoCr37 tested at 150 °C.

BLG 1009 – Page 34

0

150

300

450

600

750

900

0 0.5 1 1.5 2 2.5 3 3.5 4


J pl (

kJ/m

²)




1TC(T)

MC(T)(reference)

Figure 31 - Comparison between predicted and measured plastic J-R curves for 1TC(T) specimens of 22NiMoCr37 tested at 290 °C. The agreement between predictions and measurements, as illustrated in Figure 29 to Figure 31, varies: unsatisfactory at RT, acceptable at 150 °C and good at 290 °C. Brocks himself, actually, admits that "the scaling failed in some cases, which has to be further analyzed" and "the present procedure does not claim for generality ". Nevertheless, the analyses presented here justify the experimental results obtained, in that MC(T) specimens are indeed expected to produce significantly flatter J-R curves than 1TC(T) samples, being associated to a lower energy dissipation rate. The "intrinsic" difference between the two specimen geometries can also be illustrated in terms of normalized force/displacement curves, as shown in Figure 32 (RT), Figure 33 (150 °C) and Figure 34 (290 °C), once again for the 22NiMoCr37 steel. Here, the normalization of force and plastic load-line displacement data is that prescribed in Annex 15 of ASTM E1820-01 (Normalization Data Reduction Technique). In all cases, the same value of normalized plastic load-line displacement corresponds to higher normalized forces for MC(T) than 1TC(T) specimens.

BLG 1009 – Page 35

0

100

200

300

400

500

600

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Normalized plastic load-line displacement

Nor

mal

ized

forc

e (M

Pa)

N1 (MCT)N2 (MCT)N9 (1TCT)N10 (1TCT)

Figure 32 - Normalized force and plastic load-line displacement for the MC(T) and 1TC(T) specimens of 22NiMoCr37 tested at RT.

0

50

100

150

200

250

300

350

400

450

500

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35


Nor

mal

ized

forc

e (M

Pa)


Figure 33 - Normalized force and plastic load-line displacement for the MC(T) and 1TC(T) specimens of 22NiMoCr37 tested at 150 °C.

BLG 1009 – Page 36

0

50

100

150

200

250

300

350

400

450

500

0 0.05 0.1 0.15 0.2 0.25 0.3


Nor

mal

ized

forc

e (M

Pa)


Figure 34 - Normalized force and plastic load-line displacement for the MC(T) and 1TC(T) specimens of 22NiMoCr37 tested at 290 °C.

7 Use of a revised procedure for the construction of the resistance curve and the determination of initiation toughness J0.2mm

The ISO 12135:2002 standard prescribes the use of a three-parameter offset power law for the definition of the resistance curve:

yaJ ∆⋅+= βα (6)

where all fitting coefficients α, β and γ are calculated by a least-squares regression procedure with the following constraints: α,β ≥ 0 and 0 ≤ γ ≤ 1. In a recent study, Wardle [26] has stated that these fitting coefficients provide little physical meaning when applied to specimens of different sizes. He therefore proposed to attribute a specific physical meaning to the coefficient α, which should be interpreted as the elastic part of the J-integral which corresponds to purely elastic deformation of the specimen and is strictly associated to ∆a = 0 (J = α in eq.(6) when ∆a = 0). In this way, the J-integral in the crack resistance curve would reflect the form used for calculating J for an individual data point (J = Je + Jpl). Rather than being derived from a mathematical fitting procedure, Je = α should be calculated on theoretical grounds, using the K value corresponding to the limit load, where there is no physical crack extension:

⎟⎠⎞

⎜⎝⎛⋅⎟

⎟⎠

⎞⎜⎜⎝

⎛=

Wa

fE

WJ oKL

ye '2

2σ (7)

BLG 1009 – Page 37

where σy is the yield strength, E' is the plane strain Young's modulus and fKL is a quantity given by the combination of the stress intensity factor and the limit load factor. Based on these arguments, he claims that, by replacing α with Je from eq.(7) in eq.(6), less variation in the remaining two fitting coefficients (β and γ) is achieved; more specifically, the exponent of the regression curve becomes almost size-independent if the specimens are side-grooved. Furthermore, the size dependence of initiation toughness values (expressed in terms of J0.2mm only) can be rationalized by attributing it almost entirely to the elastic component Je. The practical implications of such a statement are quite important: if the plastic component of initiation toughness is almost size-independent, it could be effectively estimated from any specimen size, whereas the elastic component could be calculated directly based on specimen dimensions (W) and material's properties (σy and E'). In our case:

( )⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎠⎞

⎜⎝⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛−+⎟

⎟⎠

⎞⎜⎜⎝

⎛≈

Wa

fE

JfE

J oKL

yMCTmmKL

yTCTmm '2

3.85.0'2

25 2

)(2.0

2

)1(2.0

σσ (8)

for a 1TC(T) specimen with W = 25 mm and 5.0=Wao .

In this study, we applied Wardle's approach to our test results from 22NiMoCr37, calculated in accordance with the ISO 12135:2005 standard. Critical toughness values are shown in Table 19 (J0.2BL) and Table 20 (J0.2mm), including plastic values derived by subtracting the elastic component given by eq.(7). More details on the analyses performed are given in Annex 15 for MC(T) specimens and Annex 16 for 1TC(T) specimens. Table 19 - Results obtained at 0.2 mm crack extension beyond blunting using ISO 12135:2005 and Wardle's approach.

T Specimen Specimen J0.2BL KJ0.2BL TM J0.2BL KJ0.2BL TM J0.2BL-Je

(°C) type code (kN/m) (MPa√m) (MPa) (kN/m) (MPa√m) (MPa) (kN/m)N1 250.0 237.6 503.4 249.9 237.6 502.6 244.9N2 229.1 227.5 528.2 229.0 227.4 527.3 224.3

Mean 239.6 232.6 515.8 239.5 232.5 514.9 234.6N9 310.9 265.0 676.4 317.0 267.6 649.7 289.8N10 326.6 271.6 704.2 333.4 274.4 679.4 306.1

Mean 318.8 268.3 690.3 325.2 271.0 664.6 298.0N3 202.9 210.1 449.3 202.8 210.1 448.7 198.7N4 204.3 210.9 459.8 204.2 210.8 459.6 200.3

Mean 203.6 210.5 454.6 203.5 210.4 454.2 199.5N11 321.0 264.3 518.3 323.9 265.5 501.6 298.4N12 326.5 266.5 548.7 330.1 268.0 530.5 304.2

Mean 323.8 265.4 533.5 327.0 266.8 516.1 301.3N5 187.8 197.8 344.5 187.7 197.8 344.0 183.5N6 186.1 196.9 325.7 186.1 196.9 325.4 181.8

Mean 187.0 197.4 335.1 186.9 197.3 334.7 182.6N13 258.4 232.0 418.4 263.2 234.2 401.3 237.9N14 263.2 234.2 423.6 264.9 234.9 407.7 239.1

Mean 260.8 233.1 421.0 264.0 234.5 404.5 238.5

290

MC(T)

1TC(T)

25

MC(T)

1TC(T)

150

MC(T)

1TC(T)

ISO 12135:2005 ISO + Wardle model

BLG 1009 – Page 38

Table 20 - Results obtained at 0.2 mm total crack extension using ISO 12135:2005 and Wardle's approach.

T Specimen Specimen J0.2 KJ02 J0.2 KJ02 J02-Je

(°C) type code (kN/m) (MPa√m) (kN/m) (MPa√m) (kN/m)N1 191.6 208.0 191.8 208.1 186.7N2 174.0 198.2 174.1 198.3 169.5

Mean 182.8 203.1 183.0 203.2 178.1N9 214.6 220.1 221.2 223.5 194.0N10 221.2 223.5 228.7 227.3 201.5

Mean 217.9 221.8 224.9 225.4 197.7N3 159.0 186.0 159.0 186.0 154.9N4 159.1 186.1 159.2 186.1 155.3

Mean 159.0 186.0 159.1 186.1 155.1N11 224.0 220.8 227.8 222.6 202.3N12 236.4 226.8 242.5 229.7 216.5

Mean 230.2 223.8 235.1 226.2 209.4N5 155.6 180.0 155.6 180.1 151.4N6 155.9 180.2 155.9 180.2 151.6

Mean 155.7 180.1 155.8 180.2 151.5N13 204.7 206.5 208.4 208.4 183.2N14 206.3 207.3 210.0 209.2 184.3

Mean 205.5 206.9 209.2 208.8 183.7

ISO + Wardle model

290

MC(T)

1TC(T)

ISO 12135:2005

25

MC(T)

1TC(T)

150

MC(T)

1TC(T)

A close examination of Table 19 and Table 20 reveals that our results do not confirm Wardle's statements. Plastic components of initiation toughness are still very strongly size-dependent, as confirmed by Figure 356. In addition, the use of the revised curve fitting procedure, in spite of its more than reasonable assumptions, appears to even slightly worsen the agreement between MC(T) and 1TC(T) specimens (data points are shifted away from the 1:1 line).

100

150

200

250

300

350

100 150 200 250 300 350

J from 1TC(T) (kJ/m²)

J fr

om M

C(T

) (kJ

/m²)

ISO free fit

Wardle 2-par fit

J-Jelastic (Wardle fit)

J02J02/BL

Figure 35 - Results of the application of Wardle's approach on 22NiMoCr37. Dotted lines correspond to ±25% from the 1:1 dashed line.

6For the sake of honesty, we must add that Wardle clearly states that he observed quasi size-independence only for "conventional laboratory sized C(T) specimens (i.e. 25 < W < 100 mm)".

BLG 1009 – Page 39

8 "Real" initiation of ductile crack extension: stretch zone width (SZW) measurements

The results shown so far have demonstrated that, above approximately 200 kJ/m², where 'engineering' approximations of crack initiation (JQ or J0.2BL) are measured, MC(T) and 1TC(T) specimens deliver significantly different results. Differences are much smaller at 0.2 mm of total crack extension (including blunting). But what happens at the instant of "real" ductile crack initiation, i.e. when crack tip blunting is exausted and coalescence of voids starts to generate new fracture surfaces? To answer this question, we have applied the procedure outlined in Annex A of ISO 12135:2002 for the determination of the "real" initiation parameters Ji and δi, based on the measurement of the local stretch zone width (SZW) on the fracture surface of the broken specimen using calibrated photographs taken in a SEM (Scanning Electron Microscope). One MC(T) and one 1TC(T) specimen tested at RT from 18MND5 have been used for the measurements. The initiation parameters are calculated from best-fit curves fitted through all data points having ∆a > ∆aSZW, at an abscissa value corresponding to the measured ∆aSZW. The results obtained for the specimens of 18MND5 tested at RT are summarised in Table 21; the assumption has been made that the ∆aSZW value measured on one specimen of a given size can also be attributed to the other specimen of the same size. Since in several instances (especially for MC(T) specimens) we observed that the best-fit curve did not suitably follow the experimental data, we also estimated Ji by averaging the J values of the two points closer to the ∆a = ∆aSZW vertical line (i.e. the last one with ∆a < ∆aSZW and the first one with ∆a > ∆aSZW). These estimates are given in the right section of Table 21; the values in the central section are calculated strictly in accordance with ISO 12135:2002. More details on the SZW measurements and on the individual analyses are given in Annexes 17 and 18, respectively. Table 21 - "Real" initiation values measured in accordance with Annex A of ISO 12135:2002 on the specimens of 18MND5 tested at RT.

Specimen Specimen ∆aSZW

type code (mm) UC PD NDR UC PD NDRM1 168.4 - 174.5 127.6 - 121.1M2 186.4 - 168.3 146.6 - 121.9

177.4 - 171.4 137.1 - 121.5M9 371.7 312.7 378.5 360.8 305.9 297.4M10 301.0 324.5 364.5 292.5 319.8 309.6

336.3 318.6 371.5 326.7 312.8 303.5

Ji (kJ/m²) - From data

MC(T)

1TC(T)

Average values

Average values

Ji (kJ/m²) - From fit

0.119

0.318

Both approaches, however, suffer from the uncertainties associated to single-specimen techniques in the very early stages of crack propagation. A different approach makes use of the unique relationship which Shih [27] established between J and crack-tip opening displacement (CTOD), well beyond the validity limits of LEFM:

yn

JdCTODσ

= (9)

BLG 1009 – Page 40

where dn is a dimensionless constant which is plotted in [28] as a function of 1/n (n = strain hardening exponent) for different ratios of σy/E. Assuming plane strain and for 18MND5 at RT (n = 9, σy/E = 0.025), we obtain dn ≈ 0.48. During crack tip blunting, CTOD corresponds to twice the (apparent) crack extension; specifically, at initiation:

SZWi aCTOD ∆⋅= 2 (10) Substituting eq.(10) into eq.(9) and solving for Ji yields:

n

ySZWi d

aJ

σ∆= 2 (11)

which gives, using the ∆aSZW values of Table 21: Ji = 255 kJ/m² for MC(T) specimens, and

Ji = 680 kJ/m² for 1TC(T) specimens. These values are much larger than those given in Table 21 and, especially in the case of 1TC(T) specimens, appear somewhat overestimated. This will require a further re-examination of the SZW measurements. Nevertheless, Table 21 and eq.(11) clearly show that crack tip blunting is much larger for 1TC(T) than MC(T) specimens. As a consequence, initiation values from MC(T) samples are clearly higher of those calculated for 1TC(T), the difference being higher than for the typical critical parameters discussed in previous chapters. In summary, it appears that, with respect to a standard-size specimen, in a MC(T) sample the crack initiates sooner and propagates more easily.

BLG 1009 – Page 41

Conclusions A comprehensive study has been conducted on the applicability of miniature Compact Tension specimens (with thickness B = 4.15 mm) for fracture toughness measurements in the upper shelf regime. Results obtained from MC(T) specimens have been compared at different temperatures with data measured from standard-size 1TC(T) samples of four well-characterized materials (three RPV steels and one C-Mn steel). As a general conclusion, we observed that MC(T) specimens consistently and systematically underestimate the elastic-plastic fracture toughness as measured from 1TC(T) samples, both in terms of ductile crack initiation ("real" initiation and its engineering estimates) and crack resistance curve. The same situation has already been encountered in the fracture toughness characterization of the base and weld materials of Tihange III using MC(T) and PCCv specimens [28]. One of the possible explanations is related to work hardening in the region ahead of the crack tip, which would occur on a much larger scale in a very small specimen, and induce a significant decrease of tearing resistance [29]. In the absence of side-grooves, however, it's loss of constraint which prevails over work hardening and causes generalized plastic yielding of the whole specimen even before actual crack initiation: this was observed on a plain-sided MC(T) sample tested within a previous investigation [28]. Another hypothesis is that J-integral is not an adequate parameter for representing ductile crack extension for MC(T) specimens (as shown by the very early violation of the J limit criterion), and use of alternative parameters, such as CTOD (crack tip opening displacement) or CTOA (crack tip opening angle), would improve the agreement with larger specimens. More detailed findings are given below.

In terms of engineering approximation of ductile initiation at 0.2 mm beyond crack tip blunting (JQ), critical toughness values from MC(T) specimens have been found from 16% to 42% lower than for 1TC(T) samples, depending on the single-specimen technique (UC, PD or NDR). If the J-integral value at 0.2 mm of total crack growth (including blunting) is considered (J0.2mm), the underestimation provided by MC(T) specimens is significantly smaller (7% on the average).

It appears feasible to use empirical correlations to estimate 1TC(T)-equivalent critical values based on MC(T) results, although for some of the techniques (UC and PD) the scatter is significant.

From the tests performed, it appears that results obtained from MC(T) specimens using the three single-specimen techniques show more consistency than in the case of 1TC(T) samples.

Crack resistance curves measured from MC(T) are systematically lower and start to deviate from the 1TC(T) J-R data above 200 kJ/m² and beyond 0.2-0.3 mm of ductile crack extension.

The two multi-specimen curves obtained on 18MND5 at 150 °C show the same trend as the single-specimen data; for MC(T) specimens, the significant difference with respect to the single-specimen curve is to be attributed to the absence of a crack growth correction in the "basic method" of ASTM E1820-01.

If the ISO 12135:2002 standard is used to analyse the results, less discrepancy (around 25%) is found between the two specimen types, on account of the lower values of the critical toughness parameter (J0.2BL); this is mainly due to the much steeper slope of the blunting (construction) line used in the calculations.

BLG 1009 – Page 42

The influence of the crack growth correction, in its various formulations (ASTM, ISO or a recent proposal by Wallin), has been found negligible on critical toughness values (JQ or J0.2BL), but removing it completely does bring the J-R curves of MC(T) and 1TC(T) specimens closer. However, some evidence has been found that presently available crack growth corrections might indeed "over-correct" MC(T) data for crack extensions greater than 1.5-2 mm.

A more physical interpretation of the differences observed between MC(T) and 1TC(T) specimens has been found, using a recently proposed J-R curve scaling model, based on the concept of energy dissipation rate. Although the agreement between measured and predicted (i.e. scaled from MC(T)) resistance curves for the 1TC(T) geometry varies from case to case, it is clear that MC(T) specimens are indeed supposed to provide flatter J-R curves.

A clear difference between the two investigated specimen sizes has been observed also in terms of normalised force/plastic displacement curves (using the normalization procedure of the NDR technique, as prescribed in ASTM E1820-01).

We also considered a revised curve fitting procedure, based on the offset power law of ISO 12135:2002, which attributes a physical meaning to the offset coefficient: elastic component of the J-integral, associated to zero crack extension. As far as our test results are concerned, the use of this procedure somewhat worsens the agreement between MC(T) and 1TC(T) data. The statement that, using this approach, the plastic component of J0.2mm becomes almost size-insensitive could not be confirmed.

Stretch Zone Width (SZW) measurements, conducted on one MC(T) and one 1TC(T) specimen of 18MND5 tested at room temperature, have clearly shown that initiation of ductile tearing occurs at much lower J-integral values on the miniature samples. In terms of initiation toughness Ji, evaluated in accordance with Annex A of ISO 12135:2002, the ratio between MC(T) and 1TC(T) is around 0.5.

It can therefore be concluded that MC(T) specimen data obtained in upper shelf conditions, although clearly conservative with respect to larger samples, have to be treated with care. Recommendations for future work Based on the indications obtained from this study, a number of additional investigations can be proposed for a future extension of this activity. ⇒ In order to investigate the behaviour of both specimen geometries in the very early stage of

ductile crack extension (close to tearing initiation) and at the same confirm the measurements already performed on 18MND5 tested at RT, J-R tests interrupted at about 100 kJ/m² could be performed on both MC(T) and 1TC(T) samples, followed by SZW.

⇒ Several years ago, the precracked Charpy-V (PCCv) geometry was successfully qualified at SCK•CEN for upper shelf measurements by comparison with 1TC(T) tests [30]. In an attempt to identify the actual size limit which corresponds to the deviation from the reference J-R curve, elastic-plastic fracture tests on C(T) specimens with B = 10 mm (same thickness as PCCv) and tests on PCCv specimens with B = 4.2 mm (same thickness as MC(T)) could be performed. These results could also provide useful input for a proposal of revision of the limit for J-controlled crack growth (Jlim or Jmax) within the ASTM E1820-01 and/or ISO 12135:2002 standards.

BLG 1009 – Page 43

⇒ To clarify the possible role of work hardening in determining the lower tearing resistance of the MC(T), the following actions are proposed on one or two different materials:

o J-R testing of a 1TC(T) specimen; o machining of a 1/2TC(T) specimen from the plastically deformed section of the

previous specimen, followed by the relevant J-R test; o comparison of the result of the latter test with previously available MC(T) results.

Should the result agree with MC(T) resistance curves, in spite of the much larger size of the 1/2TC(T) sample, it would consitute proof of the decisive influence of work hardening. Another approach would be to test specimens with a high crack size-to-width ratio (a/W ∼ 0.8), where plastic deformation would spread quickly over a large portion of the uncracked ligament.

⇒ If the cause of the discrepancy between MC(T) and 1TC(T) specimens lies in the inadequacy of the J-integral to represent crack extension in a small specimen (as would be suggested by the clear violation of the Jmax limit for MC(T) data), another option would be to use an alternative parameter, such as CTOD (Crack Tip Opening Displacement) or CTOA (Crack Tip Opening Angle). The use of Ernst's modified J-integral, Jm [13], could also be considered.

⇒ Finally, Finite Element Analyses (FEA) could be performed in order to confirm the experimental observations, using the Local Approach model.

Acknowledgements The collaboration of the scientific and technical staff of the RMO department is gratefully acknowledged, with a special mention for R. Mertens (specimen preparation), L. Van Houdt (J-R tests and preliminary analyses) and A. Leenaers (SZW measurements). This work is partially sponsored by Electrabel/Tractebel Engineering, whose support is kindly appreciated. References [1] E. van Walle, Reconstitution: Where Do We Stand?, in “Effects of Irradiation on Materials:

17th International Symposium”, ASTM STP 1270, 1995. [2] E. Lucon, M. Scibetta, R. Chaouadi and E. van Walle, ASTM 1418, 2002, pp.3-17. [3] M. Scibetta, E. Lucon and E. van Walle, Int. J. of Fracture 116: 231-244, 2002. [4] M. Scibetta, E. Lucon and E. van Walle, SCK•CEN Report BLG-888, Sep 2001. [5] M. Scibetta, E. Lucon and E. van Walle, 22nd ASTM Symposium on Effect of Radiations

on Materials, June 2004, Boston (also SCK•CEN-P-25, Sep 2003). [6] E. Lucon, M. Scibetta, R. Chaouadi and E. van Walle, SCK•CEN Report R-3845, Apr

2004. [7] E. Lucon, J. Wagemans and M. Wéber, SCK•CEN Report R-4049, Nov 2004. [8] M. Scibetta et al., SCK•CEN Report, in preparation. [9] T. Ingham and E. Morland, UKAEA ND-R-408(R), Nov 1979. [10] T. Ingham, G. Wardle and J.T. Bland, UKAEA ND-R-1333(R), August 1987. [11] R.E. Link, J.D. Landes, R. Herrera and Z. Zhou, ESIS/EGF9, 1991, pp.707-721. [12] J. Heerens, K.-H. Schwalbe and C. Nix, ASTM STP 1171, 1993, pp.429-472. [13] H.A. Ernst, ASTM STP 803, 1983, pp.191-213. [14] G. Wardle, ASTM STP 1418, 2002, pp.48-66.

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[15] A. Neimitz, I. Dzioba, J. Galkiewicz and R. Molasy, Eng. Fr. Mech. 71 (2004) 1325-1355. [16] H. Ono, R. Kasada and A. Kimura, J. of Nucl. Mat. 329-333 (2004) 1117-1121. [17] R. Chaouadi and M. Scibetta, SCK•CEN Report R-3292, Dec 1998. [18] E. Lucon, R. Chaouadi, M. Scibetta and E. van Walle, SCK•CEN Report BLG-944, Apr

2003. [19] E. Lucon, M. Scibetta and E. van Walle, SCK•CEN Report BLG-915, Sep 2002. [20] K. Wallin and A. Laukkanen, Eng. Fr. Mech. 71 (2004) 1601-1614. [21] W. Brocks, P. Anuschewski and D. Hellmann, Int. J. of Fract. 130: 455-469, 2004, pp.455-

469. [22] C.E. Turner, Proceeedings of ECF 8, Vol.II, 1990, pp.933-949 and 951-968. [23] J.D.G. Sumpter, Eng. Fr. Mech. 64 (2004), pp.17-37. [24] D. Memhard, W. Brocks and S. Fricke, Fat. Fract. Eng. Mat. Struct. 16 (1993), pp.1109-

1124. [25] W. Klemm, Proc. 1989 ASME Pressure Vessels and Piping Conference, 1989, pp.99-104. [26] G. Wardle, Report WCL/R03/01, Jul 2003. [27] C.F. Shih, Journal of the Mechanics and Physics of Solids, Vol.29, 1981, pp.305-326. [28] T.L. Anderson, "Fracture Mechanics – Fundamentals and Applications", Second Edition,

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