mp4a0610112

MP4A06

1

NANYANG TECHNOLOGICAL UNIVERSITY

SEMESTER 2 EXAMINATION 2010-2011

MP4A06 – NON-DESTRUCTIVE TESTING

May 2011 Time Allowed: 2 ½ hours

INSTRUCTIONS

1. This paper contains FOUR (4) questions and comprises SIX (6) pages.

2. Answer ALL questions.

3. All questions carry equal marks.

4. This is a CLOSED-BOOK Examination.

1(a) Describe and explain the different components in an operator assisted vision system.

Discuss the factors which limit the resolution of a vision system. State one advantage

and one disadvantage of a machine vision system over the operator assisted system.

(10 marks)

(b) A vision system is used to check a printed circuit board of a sound card as shown in

Figure 1. The size of the board is 120 x 90 mm and the smallest component has a size

of 1.2 x 0.9 mm. The photosensitive area of the CCD camera is 6 x 4.5 mm and it has

600 x 450 pixels. The camera is located at a distance of 300 mm above the board. If

the lens has a focal length of 30 mm, (i) can the whole of the object be viewed at any

orientation and (ii) can the smallest component be resolved?

(10 marks)

Figure 1 Image of PCB of a Sound Card

Note: Question 1 continues on page 2.

ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library

MP4A06

2

(c) The solders (small bright spots) and the capacitors (circular shaped objects) are

reflective and hence their intensity is very bright. Other objects like the chips are dark

and the characters on the chip cannot be seen. How would you compensate for this

difference in brightness.

(5 marks)

2(a) Explain the method of Shearography for NDT. Highlight the recording process using

a schematic diagram, the type of materials that can be tested, what information we can

get from the fringe pattern and how it is used in NDT.

(10 marks)

(b) State 3 differences between Shearography and Electronic Speckle Pattern

Interferometry (ESPI).

(6 marks)

(c) For the shearographic fringe pattern shown in Figure 2, how has the surface

deformed? Hence what is the possible type and shape of defect? Is the defect on the

surface or below the surface? What would be the pattern we would get if we used

ESPI to do the same experiment?

Figure 2 Shearographic Fringe Pattern of a defect

(9 marks)


MP4A06

3

3(a) A liquid penetrant test is conducted from beginning to end in a total of 20 seconds.

Describe all the five procedures and give reasons why each procedure (because they

are done quickly) may cause the complete process to be ineffective.

(8 marks)

(b) Write a Magnetic Particle Inspection procedure for a steel tube 100 mm long with an

outer diameter which varies in size from 45 to 50 mm. The inner diameter is constant

with a value of 30 mm. State the locations and directions of all the defects which are

detectable with each magnetization shot and give reasons why they are detectable.

Use 6 amps AC RMS/mm diameter. If the outer diameter varies by a ratio of 2 to 1 or

more then 2 shots are required.

(8 marks)

(c) Two large aluminum plates “A” and “B” are joined together. Plate “A” is 1 mm thick

and Plate “B” is 3 mm thick. An air gap exists between the plates. An eddy current

probe is used to test for defects in both plates by testing only from the surface of plate

“A”. Calculate the eddy current frequencies needed (i) to test for defects only in plate

“A” and not in plate “B” (ii) to test for defects in both plates “A” and “B”. For case

(ii) what differences would you expect between the phase display signals from defects

in plate “A” and defects in plate “B” and give reasons.

Describe any differences you would expect if testing were conducted from the surface

of plate “B”.

Given:

)(

1,

.3.5,

50

Hzfrequencyf

typermeabilirelativer

cmiumminaluofyresistivit

mmrf

DepthSkin

(9 marks)


MP4A06

4

4(a) A thin aluminium plate has a 2 mm long crack in it which extends through the plate

i.e. it is open to both surfaces of the plate. One of the surfaces of the plate is covered

entirely with a thin piece of sellotape. The plate is tested using liquid penetrant,

magnetic particle inspection, eddy current, ultrasonics and x-ray. For the first four of

these techniques, testing is conducted from each surface. Assume that the best

possible procedure has been selected for each test. Give the results you would expect

and the reasons.

(8 marks)

(b) The geometric un-sharpness for a radiographic technique is 0.2 mm. An aluminum

plate has a thickness of 145 mm. Calculate a radiographic technique for the plate

using an x-ray instrument kilo-voltage of 180 kV which produces a film density of 2

using film “A”. Use Figures 3 and 4.

Given:

The linear radiographic absorption coefficient of Iron (Fe) is 5 times greater than

aluminium.

Instrument source size = 3 mm

Instrument maximum tube current = 4 mA

(8 marks)

(c) A specimen consists of two horizontal metal plates, each 20 mm thick, fused together

with no air gap between them. The top plate is designated “A” and the bottom plate

“B”. An ultrasonic instrument is calibrated for 0 to 100 mm for shear wave testing in

plate “A”. This instrument is used to drive a shear wave probe, which when placed on

top of plate “A”, produces an angle of 45 degrees to the normal in the plate. A small

spherical defect is detected in plate “B” which produces a maximised signal which

appears at a range of 50 mm on the ultrasonic screen. What is the horizontal and

vertical distance of the defect from the probe index (or emission) point? The velocity

of a shear wave in plate “A” is 3,230 m/s and in plate “B” it is 3,000 m/s.

(9 marks)

Note: Figure 3 appears on page 5, and

Figure 4 appears on page 6.


MP4A06

5

Figure 3 Exposure chart for Iron (Fe) for a density of 2 using MX film and lead screens


MP4A06

6

Figure 4 Film curves

END OF PAPER


mp4a0610112

Documents