Delayed hydride cracking (DHC) is an important crack initiation and growth mechanism in Zr-2.5Nb alloy pressure tubes of CANDU nuclear reactors. DHC is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth, and fracture of a hydrided region at a flaw tip. In-service flaw evaluation requires analyses to demonstrate that DHC will not initiate from the flaw. The work presented in this paper examines DHC initiation behavior from V-notches with root radii of 15μm, 30μm, and 100μm, which simulate service-induced debris fretting flaws. Groups of notched cantilever beam specimens were prepared from two unirradiated pressure tubes hydrided to a nominal hydrogen concentration of 57wt.ppm. The specimens were loaded to different stress levels that straddled the threshold value predicted by an engineering process-zone (EPZ) model, and subjected to multiple thermal cycles representative of reactor operating conditions to form hydrides at the notch tip. Threshold conditions for DHC initiation were established for the notch geometries and thermal cycling conditions used in this program. Test results indicate that the resistance to DHC initiation is dependent on notch root radius, which is shown by optical metallography and scanning electron microscopy to have a significant effect on the distribution and morphology of the notch-tip reoriented hydrides. In addition, it is observed that one tube is less resistant to DHC initiation than the other tube, which may be attributed to the differences in their microstructure and texture. There is a reasonable agreement between the test results and the predictions from the EPZ model.

1.
Dutton
,
R.
,
Nuttall
,
K.
,
Puls
,
M. P.
, and
Simpson
,
L. A.
, 1977, “
Mechanisms of Hydrogen Induced Delayed Cracking in Hydride Forming Materials
,”
Metall. Trans. A
0360-2133,
8A
, pp.
1553
1562
.
2.
Cheadle
,
B. A.
,
Coleman
,
C. E.
, and
Ambler
,
J. F. R.
, 1987, “
Prevention of Delayed Hydride Cracking in Zirconium Alloys
,”
Zirconium in the Nuclear Industry: Seventh International Symposium
, pp.
224
240
, ASTM Paper No. STP 939.
3.
Canadian Standards Association
, 2005 “
Technical Requirements for In-Service Evaluation of Zirconium Alloy Pressure Tubes in CANDU Reactors
,” CSA Standard N285.8-05.
4.
Sagat
,
S.
,
Shi
,
S. Q.
, and
Puls
,
M. P.
, 1994, “
Crack Initiation Criterion at Notches in Zr-2.5Nb Alloys
,”
Mater. Sci. Eng., A
0921-5093,
176
, pp.
237
247
.
5.
Sagat
,
S
,
Lee
,
W. K.
and
Bowden
,
J. W.
, 1994, Chalk River Laboratories of Atomic Energy of Canada Ltd., and Ontario Hydro Technologies, unpublished.
6.
Sagat
,
S.
,
Newman
,
G. W.
, and
Scarth
,
D. A.
, 2002, “
Crack Initiation by Delayed Hydride Cracking at Sharp Notches in Zr-2.5Nb Alloys
,”
Proceedings of the International Conference for Hydrogen Effects on Material Behavior and Corrosion Deformation Interactions
, Jackson Lake Lodge, Moran, WY, Sept. 22–26.
7.
Cui
,
J.
,
Shek
,
G. K.
,
Scarth
,
D. A.
, and
Lee
,
W. K.
, 2004, “
Delayed Hydride Cracking Initiation at Simulated Secondary Flaws in Zr-2.5Nb Pressure Tube Material
,”
Proceedings of the 2004 ASME Pressure Vessels and Piping Conference
, San Diego, CA, July 25-29, PVP-Vol.
474
, pp.
53
65
.
8.
Scarth
,
D. A.
, and
Smith
,
E.
, 1999, “
Developments in Flaw Evaluation for CANDU Reactor Zr-Nb Pressure Tubes
,”
Proceedings of the 1999 ASME Pressure Vessels and Piping Conference
, Boston, MA, August 1–5, PVP-Vol.
391
, pp.
35
45
.
9.
Scarth
,
D. A.
, and
Smith
,
E.
, 2000, “
The Use of Failure Assessment Diagrams to Describe DHC Initiation at a Blunt Flaw
,”
Proceedings of the 2000 ASME Pressure Vessels and Piping Conference
, Seattle, WA, July 23–27, PVP-Vol.
412
, pp.
63
73
.
10.
Smith
,
E.
, and
Scarth
,
D. A.
, 2001, “
Extending Fracture Mechanics for Cracks to the Behavior of Notches
,”
Proceedings of the 2001 ASME Pressure Vessels and Piping Conference
, Atlanta, GA, July 22–26, PVP-Vol.
423
, pp.
11
24
.
11.
Scarth
,
D. A.
, and
Smith
,
E.
, 2002, “
The Effect of Plasticity on Process-Zone Predictions of DHC Initiation at a Flaw in CANDU Reactors Zr-Nb Pressure Tubes
,”
Proceedings of the 2002 ASME Pressure Vessels and Piping Conference
, Vancouver, Canada, August 4–8, PVP-Vol.
437
, pp.
19
30
.
12.
Scarth
,
D. A.
, and
Smith
,
E.
, 2003, “
Improved Failure Assessment Diagrams to Describe Delayed Hydride Cracking Initiation at a Blunt Flaw
,”
Proceedings of the 2003 ASME Pressure Vessels and Piping Conference
, Cleveland, OH, July 21–24, PVP-Vol.
462
, pp.
103
116
.
13.
Metzger
,
D. R.
, and
Sauve
,
R. G.
, 1996, “
A Self-Induced Stress Model for Simulating Hydride Formation at Flaws
,”
Proceedings of the 1996 ASME Pressure Vessels and Piping Conference
, Montreal, Canada, July 21–26, PVP-Vol.
326
, pp.
137
144
.
14.
Weatherly
,
G. C.
, 1981, “
The Precipitation of Gamma-Hydride Plates in Zirconium
,”
Acta Metall.
0001-6160,
29
, pp.
501
512
.
15.
Sagat
,
S.
,
Coleman
,
C. E.
,
Griffiths
,
M.
, and
Wilkins
,
B. J. S.
, 1994, “
The Effect of Fluence and Irradiation Temperature on Delayed Hydride Cracking in Zr-2.5 Nb
,”
Zirconium in the Nuclear Industry: Tenth International Symposium
, pp.
62
79
, ASTM Paper No. STP 1245.
16.
Shek
,
G. K.
, 1998, “
The Effect of Material Properties, Thermal and Loading History on Delayed Hydride Cracking in Zr-2.5Nb Alloys
,” Ph.D. thesis, University of Manchester, Manchester, UK.
17.
Rodgers
,
D. K.
, 1990, Chalk River Laboratories of Atomic Energy of Canada Ltd., unpublished.
18.
Kawa
,
D. M.
, 2005, Kedward, Kawa and Assoc. Ltd., Winnipeg, Canada, unpublished.
19.
Eadie
,
R. L.
,
Metzger
,
D. R.
, and
Léger
,
M.
, 1993, “
The Thermal Rachetting of Hydrogen in Zirconium-Niobium—An Illustration Using Finite Element Modelling
,”
Scr. Metall. Mater.
0956-716X,
29
, pp.
335
340
.
20.
Tada
,
H.
,
Paris
,
P. C.
, and
Irwin
,
G. R.
, 2000,
The Stress Analysis of Cracks Handbook
,
3rd ed.
,
ASME
,
New York
.
21.
Perovic
,
V.
,
Weatherly
,
G. C.
, and
Simpson
,
C. J.
, 1983, “
Hydride Precipitation in α/β Zirconium Alloys
,”
Acta Metall.
0001-6160,
31
(
9
), pp.
1381
1391
.
22.
Warr
,
B. D.
,
Perovic
,
V.
,
Lin
,
Y. P.
, and
Wallace
,
A. C.
, 2002, “
Role of Microchemistry and Microstructure on Variability in Corrosion and Deuterium Update of Zr-2.5Nb Pressure Tube Material
,”
Zirconium in the Nuclear Industry: 13th International Symposium
, pp.
313
318
, ASTM Paper No. STP 1423.
23.
Hardie
,
D.
, and
Shanahan
,
M. W.
, 1975, “
Stress Reorientation of Hydrides in Zirconium-2.5% Niobium
,”
J. Nucl. Mater.
0022-3115,
55
, pp.
1
13
.
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