Author: Z. PanPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
Metallographic observations of delayed hydride cracking from simulated flaws in zr-2.5nb pressure tube during thermal cycling
Author: Z. PanOverload Fracture of Flaw Tip Hydrides in Zr-2.5Nb Pressure Tubes
Author: GK. ShekPublisher:
ISBN:
Category : Crack initiation
Languages : en
Pages : 30
Book Description
CANDU Zr-2.5Nb pressure tubes are susceptible to a crack initiation mechanism known as Delayed Hydride Cracking (DHC), which is a process that involves hydrogen diffusion, hydride precipitation, hydride region formation, and fracture at a flaw tip. An overload occurs when the hydrided region is loaded to a stress higher than that at which this region is formed. Service-induced flaws are present in some pressure tubes, which can act as crack initiation sites. Most experimental data to assess DHC initiation are obtained under constant loading conditions in which hydride formation and fracture occur at the same load, and therefore they are not suitable to assess crack initiation under overload condition. A series of step-wise increasing load experiments was performed on unirradiated Zr-2.5Nb pressure tube samples to determine the fracture stress of hydrides formed at notches with 15 ?m root radius under different hydride formation stresses and thermal histories. Crack initiation in the overload tests was detected by the acoustic emission technique. The notch tip hydride morphologies were examined by optical and scanning electron microscopy. Test results indicated that the resistance to overload fracture was dependent on the hydride formation stress and thermal histories, which affected the notch tip hydride size, density, and distribution. Overload tests were performed at different temperatures, and a transition temperature to high resistance to overload fracture was observed.
Factors Controlling Hydrogen Assisted Subcritical Crack Growth in Zr-2.5Nb Alloys
Author: LA. SimpsonPublisher:
ISBN:
Category : Cracks
Languages : en
Pages : 22
Book Description
The delayed failure of cold-worked Zr-2.5Nb pressure tube. material has been studied using static load tests on compact-tension specimens containing hydrogen within the range ~ 10 to 400 ?g/g. The experimental approach was to measure crack velocity (V) as a function of crack tip stress intensity factor (K), temperature and hydrogen content, relate these data to fractographic and metallographic observations, and compare the results with recent models of hydrogen embrittlement. Slow crack growth was observed at all temperatures between 25 and 325 °C and at K values between ~ 10 and 50 MPam. Below 250°C, the V-K relationships exhibited two-stage behavior; at K > 15 to 20 MPam, the crack velocity was only weekly dependent on stress intensity, whereas at smaller K values, the crack velocity decreased rapidly with K, an indication of a threshold value of K ~ 5 to 10 MPam. The crack velocity increased with increase in temperature, although because of scatter in the data, this could not be expressed quantitatively. At 250°C and above, slow crack growth was not reproducible except after a thermal cycle. The thermal cycle produced a region of reoriented hydrides concentrated at the crack tip which significantly reduced the incubation period for crack growth and confirmed the important role of the hydride phase in the fracture process. Fractography showed that the features of the slow growth fracture were similar at all temperatures studied. The main observations were of ductile striations, or stretch zones, parallel to the crack front, with brittle, plate-like regions, some of which contained cleavage features, between the striations. A fracture mechanism is suggested which involves the repeated precipitation of hydride at the crack tip, followed by crack advance through this embrittled region, and crack arrest in the more ductile matrix, leading to discontinuous crack growth. This general mode of crack growth has been considered in a recent model for embrittlement in hydride-forming materials, the predictions of which show good agreement with the results from this study.
Initiation of Delayed Hydride Cracking in Zr-2.5Nb Micro Pressure Tubes
Author: Ravi Kumar SundaramoorthyPublisher:
ISBN:
Category :
Languages : en
Pages : 280
Book Description
Pressure tubes pick up hydrogen while they are in service within CANDU reactors. Sufficiently high hydrogen concentration can lead to hydride precipitation during reactor shutdown/repair at flaws, resulting in the potential for eventual rupture of the pressure tubes by a process called Delayed Hydride Cracking (DHC). The threshold stress intensity factor (KIH) below which the cracks will not grow by delayed hydride cracking of Zr-2.5Nb micro pressure tubes (MPTs) has been determined using a load increasing mode (LIM) method at different temperatures. MPTs have been used to allow easy study of the impact of properties like texture and grain size on DHC. Previous studies on MPTs have focused on creep and effects of stress on hydride orientation; here the use of MPTs for DHC studies is confirmed for the first time. Micro pressure tube samples were hydrided to a target hydrogen content of 100 ppm using an electrolytic method. For DHC testing, 3 mm thick half ring samples were cut out from the tubes using Electrical Discharge Machining (EDM) with a notch at the center. A sharp notch with a root radius of 15 mu m was introduced by broaching to facilitate crack initiation. The direct current potential drop method was used to monitor crack growth during the DHC tests. For the temperature range tested the threshold stress intensity factors for the micro pressure tube used were found to be 6.5-10.5 MPa.m1/2 with the value increasing with increasing temperature. The average DHC velocities obtained for the three different test temperatures 180, 230 and 250oC were 2.64, 10.87 and 8.45 x 10-8 m/s, respectively. The DHC data obtained from the MPTs are comparable to the data published in the literature for full sized CANDU pressure tubes.
Delayed hydride crack initiation from simulated bearing pad fretting flaws in the rolled joint of pressure tube p3b15
Author: Z. L. PanPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
Results are presented of mechanical tests on the standard brazed bearing pad and three types of anti-crevice corrosion bearing pad. the tests were performed at conditions representative of static and impact loads on the bearing pads during handling and in-reactor service.
Delayed Hydride Cracking in Zirconium Alloys in Pressure Tube Nuclear Reactors. IAEA TECDOC Series
Author: International Atomic Energy AgencyPublisher:
ISBN:
Category :
Languages : en
Pages : 199
Book Description
This report describes all of the research work undertaken as part of the IAEA coordinated research program on hydrogen and hydride induced degradation of the mechanical and physical properties of zirconium based alloys, and includes a review of the state of the art in understanding crack propagation by Delayed Hydride Cracking (DHC), and details of the experimental procedures that have produced the most consistent set of DHC rates reported in an international round-robin exercise to this date.
Correlation between delayed hydride cracking velocity and critical crack length from small specimens
Author: B. J. S. WilkinsPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
Electrical and mechanical transients were used to simulate acoustic emission (ae) signals expected during a crack-growth experiment by temperature-pressure cycling of a pressure tube. electric noise simulated heater noise, while mechanical transients from pencil-lead breaks and ballpoint-pen impacts on a pressure tube, respectively simulated ae from crack growth and mechanical noise in an operating fuel channel. simulations were used, before cycling, to assess the method's capability to discriminate against noise, calibrate instruments for sensitivity, and interpret ae events observed during cycling. digital signal processing (dsp) techniques, using correlation and frequency analyses, were applied to digitized transients. results show that: (i) a spatial filter on the ae instrument provides about 60 db discrimination against extraneous electrical and mechanical noise, (ii) ae bursts from ballpoint-pen impacts and pencil-lead breaks, on a pressure tube, contain low (
Delayed hydride cracking (dhc) velocity (axial) at 250 degrees celsius for zr-2.5nb chepetski pressure tube materials
Author: R. ChoubeyDelayed hydride cracking (dhc) velocity (axial) at 250 degrees c for zr-2.5nb cezus pressure tube materials
Author: R. ChoubeyDelayed Hydride Cracking in Zirconium Alloys in Pressure Tube Nuclear Reactors
Author: International Atomic Energy AgencyPublisher:
ISBN: 9781280180798
Category :
Languages : en
Pages :
Book Description