Closed end burst testing of zircaloy canning tubes

JOURNAL

OF NUCLEAR

CLOSED

MATERIALS

END

31 (1969) 111-113.

BURST

TESTING

0

NORTH-HOLLAND

OF ZIRCALOY

PUBLISHINGI CO., AMSTERDABI

CANNING

TUBES

0. KRISTER ~~LLSTR~M Steel

Research Center, Sandvik Received

Steel

works,

30 September

Burst testing of Zircaloy canning tubes is normally specified to ensure a sound material with adequate transverse ductility out of pile. Often the maximum circumferential expansion is required to exceed a minimum value in closed end burst testing. However, the lack of a rigorously specified testing procedure allows for quite variable results. Two procedures for closed end burst testing are here being compared. One of them, referred to as method A, implies deformation by pressing a liquid into the closed end tube sample by means of a pressure system containing some compressed air. On deforming, the Zircaloy tube starts bending almost immediately and is free t,o do so. In the other procedure, method B, deformation is performed by pressing liquid into the tube sample at a constant rate of 1 cma/min using a stiff pressure system. That is, the possibilities to store elastic energy in the volume of liquid, in the constructional details under pressure, and in the pressure gauge are minimized. The elastic coefficient obtained for the sealed off system without tube sample is 500 kg/ems. A telescopic rod is placed inside the tube to prevent the tube from bending without restricting its change in length. Four tubes with internal diameter 1.2 cm and wall thickness 0.08 cm were annealed to different levels of ductility and 14 tube samples, 20 cm long, were cut out of each tube. Half the number of samples from each tube were tested with method A and the other half with method B. The maximum circumferential ex-

Sandviken,

Sweden

1968

TABLE 1 Maximum

circumferential expansion burst testing (%).

in closed end

pansions obtained are listed in table 1, where reported individual mean values and standard deviations are based on about seven tests. Method B evidently yields values a factor 1.8 greater than method A. The standard deviations are about the same, which implies that the relative error is less with method B than with method A. The increased ductility in method B is partly due to the higher stiffness of the pressure system. When, as in method A, the pressure drops in the later stage of the burst test with a system of low stiffness the volume of the

pump and of the other constructional details is contracting and pockets of air are expanding, thus increasing the rate of liquid transport into the sample. This implies a higher deformation rate which supposedly gives lower ductility 1). A stiff system as in method B, on the other hand, yields a more constant deformation rate, thus increasing the ductility. The main part of the increased ductility in method B, however, is certainly due to the telescopic rod keeping the sample straight. The tendency of the tube to bend during burst testing is a kind of plastic instability, specific for anisotropic materials like Zircaloy. The 111

112

0.

KRISTER

gaLLSTRaM

end,

axial strain increment axis (&). For an anisotropio material with a yield locus as in fig. la there is a positive axial strain increment

subjected to internal pressure, the ratio between tangential stress and axial stress is 2 : 1. The

component and the tube increases in length. A material with a yield locus as in fig. lb has

hydrostatic stress component has no effect on plastic deformation and subtraction of this

a negative

following

of

analysis

offered. In a thin

walled

stress component,

this tube

phenomenon with

closed

from the principal

is

stresses,

the radial, the tangential, and the axial stress, gives the deviator stresses. The sum of these deviator stresses is zero, which makes it possible to represent them in a plane triaxial diagram. A state of stress is defined by a vector in such a diagram. Diagrams with deviator stress vectors representing the state of stress in closed end burst testing are shown in fig. 1. Plastic deformation of material occurs when the deviator stress vector reaohes the yield locus characteristic for the material, shown as an elliptical loop in fig. 1. During plastic deformation constant volume of the metal is maintained. This makes the sum of the logarithmic strain increments in the three principal directions zero, and the effective strain increment can be represented by a vector in the same plane triaxial diagram. The strain increment vector of the deformation will be perpendicular to the yield locus at the point where the latter is reached by the stress vector 2). The yield locus of an isotropic material is circular and a tube of such a material will not change its length during closed end burst testing, because its strain increment

vector

should

be

normal

to

the

axial strain increment

component

and the tube decreases in length during closed end burst testing. The normal texture of Zircaloy, with predominantly

radial basal poles,

yields a yield locus of the type shown in fig. lb. The very first deformation might be homogeneous due to strain hardening, i.e. the tube stays straight, but, as soon as this hardening rate decreases a little, a slight deviation from straightness will not correct itself. The internal pressure will increase somewhat the axial stress on the convex side of the tube and correspondingly decrease the axial stress on the concave side. The direction of the stress vector in the deviator stress diagram will vary about that for a straight tube under internal pressure, fig. 2. With the yield locus indicated G; ds,

‘dE,

Fig. 2. The state of stress in a bent tube is varying. The yield locus is reached only on the concave side.

Yield locus

Tangential

Radial (a)

Fig. 1.

(b)

Triaxial plane diagrams with the deviator stress vector a, the strain increment vector da, and the yield loci of anisotropio materials.

CLOSED

END

BURST

113

TESTING

place.

Only

in a straight

tube

is the stress

in fig. 2, only the state of stress on themncave side will reach the yield locus and the de-

ratio 2: 1 as is in fact intended

formation proceeds only at this side. This means more shortening of the concave side and increased bending. After further deformation

closed end burst testing. The telescopic rod exerts a negligible force on the tube as long as the latter is straight.

and bending a second kind of plastic instability

In conclusion, either or both of two factors considerably increase the transverse ductility

is reached, when a bubble starts growing locally on the concave side where the wall has been thinned, finally leading to failure. Bending thus gives inhomogeneous deformation, which inherently means low ductility. In a straight tube homogeneous deformation will occur until the final bubble starts growing, and even this latter deformation is more homogeneous, because the bubble grows symmetrically around the tube. Thus the strain hardening capacity of the material all around the circumference is contributing to a high ductility. Bending also changes the stress ratio, especially where the deformation takes

Author’s note added in proof: Closed end burst testing is a more severe test giving inherently lower circumferential expansion than open end burst testing as shown by K. P. Steward, B. A. Cheadle, Trans AIME 239 (1967) 504.

by specifying

and improve the reproducibility in closed end burst testing of Zircaloy tubes: (1) constant deformation rate and (2) straight tube sample leading to homogeneous deformation and constant stress ratio. These requirements are best provided for by a stiff pressure system and an internal telescopic rod.

References 1) R. F. Steidel

and C. E. Makerov,

no. 247 (1960) 2)

R.

Hill,

Plasticity

p. 36 and 320

AS!CM Bulletin

57 (Oxford

U.P.,

London,

1950)