1

Failure Behavior of Elbows with Local Degradation under Excessive Seismic Load

Izumi NakamuraNational Research Institute for Earth Science and Disaster Prevention, Japan

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BackgroundDegradation such as cracks or wall thinning may occur to aged piping systems. It is important to understand the effect of degradation on the failure behavior and dynamic characteristic of piping systems to maintain the

Crack Wall thinning

Large Earthquake

Very few studies have been done on degraded pipes under seismic load.

plants in safe condition under seismic events.

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ObjectiveTo clarify how a pipe element and a piping system with degradation behave under seismic load

Experimental and analytical research program has been conducted. (1996 - 2005)

Pipe element test

Piping system test

FEM analysis

Displacement-controlled bending test on pipe elements

Shake table test on simple piping system models

Post analysis of tests to establish a reliable analytical model

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Pipe element testsPurpose :To clarify the failure mode of pipes with

degradation under high-level cyclic bending loadBend pipe element testStraight pipe element test

Degradation

Specimen

Cyclic in-plane/out-of-plane bending test

Cyclic four-point bending test

Degradation condition: Crack or Wall thinning

Degradation condition: Wall thinning

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Time history of input displacementPipe element test -- Displacement controlled cyclic bending test

(Not excitation test)

Straight pipe element test

-40

0

40

0 10 20 30 40 50 60D

isp.

[mm

]

Time[sec]

(a) Sinusoidal(F=1.0Hz, 26cycles)

(b) Random

-40

0

40

0 10 20 30 40 50 60

Dis

p.[m

m]

Time[sec]

Bend pipe element test

-100

0

100

0 20 40 60 80 100 120

Dis

p.[m

m]

Time[sec]

(F=1.0Hz, 46cycles)

(F=0.2Hz, 20cycles)

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Result of pipe element testBend pipe -- In-plane bending

Fatigue failurewith ratchet deformation

Straight pipe -- wall thinning

Fatigue failurewith ratchet deformation

Crack penetration

Straight pipe -- crack

Failure modes of pipes with degradation under cyclic bending load

The failure mode features which depended on the degradation conditions or loading conditions were obtained from the tests.

The stiffness of the pipe was affected by the existence wall thinning, but not bycracks.

The failure modes for thinned wall pipes were affected by the occurrence ofthe ratchet phenomena.

The configuration of wall thinning and the type of applied bending load affects the degree of occurrence of the ratchet phenomena.

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Piping system testsPurpose :To clarify the effects of degradation on the

dynamic characteristics and failure mode of the piping system

Thinned wall part of B01

Thinned wall part of C01

Weight (200kg)

760

2900

Shaking Direction

4210 1410

100A, Sch120100A, Sch80

4270

Part A

Part B

(Elbow1)

Elbow2

Elbow3

Thinned wall part of 2D_C01

Thinned wall part of 2D_B01

3000

2600

3000 2600

Elbow1

Elbow2

Elbow3

Crack induced position(for 3D_D01 & 3D_D02)

Shaking Direction

Weight (200kg + 60kg)

Degradation condition:Wall thinning at Elbow1 and/or Elbow2EDM notch at straight pipe next to Elbow1

2-D piping system model 3-D piping system model

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Input conditionPiping system test – Excitation test using a shake tableNarrow band random waves were used for the tests

0

5

10

15

20

25

0 1 2 3 4 5

h=0.005h=0.01h=0.02h=0.05h=0.10h=0.15h=0.20h=0.25

Mag

nific

atio

n of

Res

pons

e A

cc.[G

al/G

al]

Frequency [Hz]

0

5

10

15

20

25

0 1 2 3 4 5

h=0.005h=0.01h=0.02h=0.05h=0.10h=0.15h=0.20h=0.25

Mag

nific

atio

n of

Res

pons

e A

cc.[G

al/G

al]

Frequency [Hz]

-1000

-500

0

500

1000

0 10 20 30 40 50 60 70

Acc

. [G

al]

Time[s]-1000

-500

0

500

1000

0 5 10 15 20 25 30

Acc

. [G

al]

Time[s]

(b) Response spectrum

(a) Time history of acceleration (a) Time history of acceleration

(b) Response spectrumInput wave for 3-D piping system testInput wave for 2-D piping system test

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Reasons for using narrow band random waves

1.5

2

2.5

3

0 500 1000 1500 2000

3D_A013D_C013D_C023D_C03

Dom

inan

t fre

quen

cy [H

z]

Input acceleration [Gal]

decrease as the wall thinning ratio increase

decrease as the elastic-plastic behavior proceed

The dominant frequency of piping system models …

Narrow band random waves were used for the shake table testTo compare the elastic-plastic behavior of the several piping system models which have a little different dynamic characteristics by the same input acceleration.

To produce a large elastic-plastic deformation and cause failure even though the dominant frequency of the model changed due to theelastic-plastic deformation or the existence of degradation

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Result of piping system test

0

2

4

6

8

10

0 50 100 150 200 250 300 350

3D_A01 Elbow13D_A01 Elbow23D_C01 Elbow13D_C01 Elbow23D_C02 Elbow13D_C02 Elbow23D_C03 Elbow13D_C03 Elbow2

Elbo

w d

efor

mat

ion

angl

e [d

eg]

Response displacement[mm]

Normal thickness elbows

Thinned wall elbows

Relation between max. response disp. and range of elbow deformation angleFailure modes of a model with wall thinning

The effects of degradation on the vibration characteristics and the deformation of the piping system were obtained through the excitation tests.

The dominant frequency of the piping model was affected by the existence of wall thinning, but not by crack.

The failure mode of the degraded piping system was mainly due to fatigue failure with ratchet deformation at the weakest elbow, or fatigue crack propagation from the initial crack. Unstable failure did not occur except for the model with a full-circumferential deep crack.

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Failure mode of piping systemApplied load level in the 3-D piping system tests: about 16.5Sm

(calculated as the model has no defect)Allowable load level by current Japanese code: 3Sm

(caused to the model at 320Gal input)• Piping system without degradation and with wall thinning withstood several times input over 3Sm input level

• First excursion failure did not occur for all test models though the input acceleration was risen up to 5.5 times larger than the allowable load level

Fatigue failure accompanied with ratchet deformationIn the case of wall thinning:

Fatigue crack propagation from the initial crackIn the case of crack:

• The maximum applied moment and the histogram of the moment should be estimated.

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Effect of degradation on strength and dynamic behavior of the piping system

Crack Wall thinning

Dominant frequency at elastic level ○

Elastic-plastic response ○

Strength at locally degraded part ○ ○

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Estimation of seismic safety of degraded piping system (in the case of wall thinning)

Designed Piping system

Aging

Piping system with wall thinning

Influence of wall thinning which should be considered to estimate the failure by seismic motion

Increasing the stress by internal pressure

Increasing the stress by the stress concentration

Stress caused by the seismic load

Number of cycles of the applied load

Input seismic motion to the piping system

Decreasing the natural frequency of the piping system

Seismic response of the thinned wall piping system

Fatigue failure Ratchet deformation

Excessive deformation??

(not observed at the test)

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SummaryThe failure mode of piping systems with degradation is considered to be mainly fatigue. First excursion failure did not observed in the tests.The maximum applied moment and the histogram of the moment should be estimated.For the piping system with crack, the estimation of response of piping system and the integrity at cracked part can be done independently.For the piping system with wall thinning, the effect of degradation should be considered to estimate the response of the piping systems with wall thinning.