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NATIONAL UNIVERSITY OF SINGAPORE

FACULTY OF ENGINEERING

EXAMINATION FOR

(Semester II: 2010-2011)

CE5887 - Topics in Offshore Engineering: Fatigue and Fracture

April/ May 2011 - Time allowed: 2.5 hours

INSTRUCTIONS TO CANDIDATES

1. This examination paper contains FOUR(4)questions and comprises THREE(3)printed

pages.

2. Answer ALLFOUR(4)questions.

3. All questions carry equal marks.

4. This is an OPEN NOTES examination. Only materials given by the examiners are

allowed during the examination.

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Question 1

Write a short essay to critique the design, construction and fracture control for Bullwinkle in

light of what you learned in the rest of the course.

[25 marks]

Question 2

An engineer tested three 1T SE(T) specimens, each with a different initial crack length and

recorded the load-deformation relationships before significant crack extensions take place in

each specimen, as shown in Figure Q2a. The material of specimens utilizes the X100 pipeline

material ( 690 MPays = , 200 GPaE = ). Determine:

1) the energy release rate at a= 2 mm and a= 4 mm.

2) the maximum uniformly distributed load (caused by the current) on a segment of a pipelinewith an internal circumferential crack shown in Figure Q2b, based on the fracture toughness

at a= 2 mm. The crack is located at the mid-span of the simply supported pipeline segment.

The pipeline also experiences an axial tensile stress of 200 MPa in addition to the lateral

current force. Assume that the pipeline remains elastic. The moment of inertia for a solid

circle is 4 / 4r , where rdenotes the radius of the circle.

[25 marks]

Figure Q2. Material testing and a pipeline under lateral loads

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12 m

w= ?

P(kN)

mm

35

3 3.5 4

a= 12 mma= 14 mm

a= 16 mm

3330

ri

ro

a

ro= 254 mm

ri= 228.6 mm

a= 5 mm

b A defected i eline under lateral loads

a Load-deformation curves for three SE T s ecimens

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Question 3

TheJ-Rtest on a 1T SE(B) steel specimen indicates that the load-displacement curve follows

approximately the power-law relationship, 5 16 46 10 6 10LLD

P P

= + ( LLD is

measured in millimetrewhile P in Newton). The crack initiation takes place at P= 20 kN.

The specimen has S/W= 4, a/W= 0.5 and W/B= 2.

1) Estimate theJIcvalue.

2) Based on the failure assessment diagram, estimate the maximum brace axial load that a

cracked tubular T-joint can resist if the maximum crack driving force is to be capped at 85%

of the JIcvalue calculated in 1). Use API equations with Qf= 1.0. Assume that the T-joint

experiences brace axial load only. The joint dimensions are shown in Figure Q3. The chord

yield strength equals 355 MPa.

[25 marks]

Figure Q3. A T-joint under brace axial tension.

Question 4

A wide steel plate ( 150 MPa mIc

K = ) contains a hole of diameter D (= 100 mm) and a

crack length on one side only. Assume that the stress intensity factor can be approximated

by, / 2I

K D a = + , where refers to the remote applied stress. Determine the number

of cycles to failure for the remote stress varying between 200 MPa to 250 MPa. The Paris law

constants are C= 310-12and m= 4 for KImeasured in MPa m and da/dNmeasured in

m/cycle. Assume the detail category to be 30, determine the number of cycles corresponding

to crack initiation using EC-3 S-N approaches.[25 marks]

- END OF PAPER -

D= 406 mmd= 254 mm

T = t=25 mm

t

T

Chord

Brace

D

d

crack