- Email: [email protected]
Discussion on paper
Ves. & Piping 68 (1996) 229-230
ht. J. Pm.
0 19% Published Printed in Northern ELSEVIER
0308-0161(94)00054-2
DISCUSSION
by Elsevier Science Limited Ireland. All rights reserved 0308-0161/96/$15.00
ON PAPER
Discussion on paper by Wang, X. N. and Wang, X. C.: An improved simplified method for determining multiaxial relaxation-compared with the GLOSS method and the ASME N-47 method, ht. J. Pressure Vessels and Piping, 63 (1995) 63-70
R. Seshadri Faculty of Engineering
and Applied
Science, Memorial
University
PAPER DISCUSSION
of Newfoundland,
St. John’s,
Canada,
AIB
3X5
where h is the constraint or follow-up factor that can be related to the relaxation slope in the GLOSS diagram (Fig. 1) by the expression:
Wang & Wang have proposed another method for determining multiaxial stress relaxation in pressure components in an attempt to improve existing simplified methods. It is unfortunate that they have not referred to a previously published paper’ which is quite relevant to their discussions. In that a paper a comparison of the ASME N-47 and the GLOSS time-scaling methods with respect to multiaxial relaxation and creep damage assessment was provided. The isochronous GLOSS, or simply ISO-GLOSS method, was introduced and the notion of time-scaling was brought out. The ASME N-47 and the GLOSS methods are both based on a number of simplified assumptions in order to make them attractive to the designer and analyst. For instance, the ASME N-47 method requires a single linear elastic analysis in order to estimate the G-factor, while the GLOSS method requires two linear elastic analyses to determine h. It has been pointed out’ that the ASMENmethod: (1) excludes local stress concentration effects; (2) considers local follow-up effects by introducing the O-8G correction factor; (3) is akin to the GLOSS method in tha both h and G are time-scaling parameters. Given that there is significant scatter in creep data and there are difficulties in modelling material degradation, one needs to assess the trade-offs between model simplifications and the benefits of detailed analysis. Multiaxial relaxation can be described by the equation: da, -& + ABE~U: = 0
E A=--.--E, - 1
(2)
For small to moderate plastic or creep zone sizes (0 5 60” on the GLOSS diagram), the local region relaxation-locus is almost linear for a significant portion of the relaxation where most of the creep damage occurs. Therefore E, is a constant along the locus, and by virtue of eqn (2), h is also invariant. Stress relaxation occurs until
/
ELASTIC
&90,
uei
LINE
LOAD CONTROL
SECANT LINE
16
I/I’
I
8=0. 1 , x=1 I
DEFORMATION
Ff. NORMALIZED Fig. 229
1.
CONTROL
EQUIVALENT
STRAIN
Local region follow-up response on the GLOSS diagram.
Discussion
230
on paper
by Wang
and Wang
GLOSS
Diagram
I TIME, t Fig. 2. Multiaxial relaxation in the presence of primary stress.
the primary stress level is reached after which forward creep deformation would occur. The basic premise and subsequent assertions that the authors make are based on the assumption that statically indeterminate stresses relax asymptoticatly to load-controlled steadystate values. Creep action would persist if a deviatoric state of stress is present. Loadcontrolled steady-state stresses often have a deviatoric component; however, only forward creep would occur since these stresses are set up to equilibrate external tractions. The approximations underlying the GLOSS method are robust. With reference to Fig. 2, multiaxial relaxation occurs along the locus AC followed by forward creep along CD. For the initial period of the relaxation (AB) when a significant amount of creep damage occurs, the relaxation slope is linear. The aforementioned
approximations have been validated by several authors.‘.” The benefit of several elastic iterations as proposed by the authors, has therefore to be assessedin the light of the variability in material data and the difficulties in characterizing constitutive relationships in a degrading environment, as is often the case in an industrial plant. REFERENCES 1. Seshadri, R., Multiaxial relaxation and creep damage assessments-a comparison of ASMENand GLOSS time-scaling methods, ASME Journal of Pressure Vessel Technology,
115 (1993) 32-37.
2. Severud, L. K., Creep-fatigue assessment methods using elastic analysis results and adjustments, Pressure Vessels and Piping Conference, Honolulu, Hawaii, ASME PVP, 163 (1989).
3. Seshadri, R. & Mikulcik, E. C., On relating multiaxial and uniaxial stress relaxation in pressure components, Transactions
of the CSME,
13( l/2)
(1989).
ht. J. Pm.
0 19% Published Printed in Northern ELSEVIER
0308-0161(94)00054-2
DISCUSSION
by Elsevier Science Limited Ireland. All rights reserved 0308-0161/96/$15.00
ON PAPER
Discussion on paper by Wang, X. N. and Wang, X. C.: An improved simplified method for determining multiaxial relaxation-compared with the GLOSS method and the ASME N-47 method, ht. J. Pressure Vessels and Piping, 63 (1995) 63-70
R. Seshadri Faculty of Engineering
and Applied
Science, Memorial
University
PAPER DISCUSSION
of Newfoundland,
St. John’s,
Canada,
AIB
3X5
where h is the constraint or follow-up factor that can be related to the relaxation slope in the GLOSS diagram (Fig. 1) by the expression:
Wang & Wang have proposed another method for determining multiaxial stress relaxation in pressure components in an attempt to improve existing simplified methods. It is unfortunate that they have not referred to a previously published paper’ which is quite relevant to their discussions. In that a paper a comparison of the ASME N-47 and the GLOSS time-scaling methods with respect to multiaxial relaxation and creep damage assessment was provided. The isochronous GLOSS, or simply ISO-GLOSS method, was introduced and the notion of time-scaling was brought out. The ASME N-47 and the GLOSS methods are both based on a number of simplified assumptions in order to make them attractive to the designer and analyst. For instance, the ASME N-47 method requires a single linear elastic analysis in order to estimate the G-factor, while the GLOSS method requires two linear elastic analyses to determine h. It has been pointed out’ that the ASMENmethod: (1) excludes local stress concentration effects; (2) considers local follow-up effects by introducing the O-8G correction factor; (3) is akin to the GLOSS method in tha both h and G are time-scaling parameters. Given that there is significant scatter in creep data and there are difficulties in modelling material degradation, one needs to assess the trade-offs between model simplifications and the benefits of detailed analysis. Multiaxial relaxation can be described by the equation: da, -& + ABE~U: = 0
E A=--.--E, - 1
(2)
For small to moderate plastic or creep zone sizes (0 5 60” on the GLOSS diagram), the local region relaxation-locus is almost linear for a significant portion of the relaxation where most of the creep damage occurs. Therefore E, is a constant along the locus, and by virtue of eqn (2), h is also invariant. Stress relaxation occurs until
/
ELASTIC
&90,
uei
LINE
LOAD CONTROL
SECANT LINE
16
I/I’
I
8=0. 1 , x=1 I
DEFORMATION
Ff. NORMALIZED Fig. 229
1.
CONTROL
EQUIVALENT
STRAIN
Local region follow-up response on the GLOSS diagram.
Discussion
230
on paper
by Wang
and Wang
GLOSS
Diagram
I TIME, t Fig. 2. Multiaxial relaxation in the presence of primary stress.
the primary stress level is reached after which forward creep deformation would occur. The basic premise and subsequent assertions that the authors make are based on the assumption that statically indeterminate stresses relax asymptoticatly to load-controlled steadystate values. Creep action would persist if a deviatoric state of stress is present. Loadcontrolled steady-state stresses often have a deviatoric component; however, only forward creep would occur since these stresses are set up to equilibrate external tractions. The approximations underlying the GLOSS method are robust. With reference to Fig. 2, multiaxial relaxation occurs along the locus AC followed by forward creep along CD. For the initial period of the relaxation (AB) when a significant amount of creep damage occurs, the relaxation slope is linear. The aforementioned
approximations have been validated by several authors.‘.” The benefit of several elastic iterations as proposed by the authors, has therefore to be assessedin the light of the variability in material data and the difficulties in characterizing constitutive relationships in a degrading environment, as is often the case in an industrial plant. REFERENCES 1. Seshadri, R., Multiaxial relaxation and creep damage assessments-a comparison of ASMENand GLOSS time-scaling methods, ASME Journal of Pressure Vessel Technology,
115 (1993) 32-37.
2. Severud, L. K., Creep-fatigue assessment methods using elastic analysis results and adjustments, Pressure Vessels and Piping Conference, Honolulu, Hawaii, ASME PVP, 163 (1989).
3. Seshadri, R. & Mikulcik, E. C., On relating multiaxial and uniaxial stress relaxation in pressure components, Transactions
of the CSME,
13( l/2)
(1989).