MEC4801 S12015 PDF

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Monash University Final Examination MEC4801

There are 4 questions for a total of 50 marks. Attempt all questions.

1. (a) (10 marks) Figure 1 shows a steel plate with a vertical crack (90° to plate surface) in it represented by the area in black. The dotted line is drawn on the surface of the plate. The crack consists of 3 portions each of a different depth from the plate surface. The left hand portion is open to the plate surface and is 5 mm in length. The central portion is 1 mm deep below the surface and is 5 mm in length again. The final right hand portion is 5 mm deep below the surface and is 5 mm in length again. The total crack length is 15 mm and its largest height is 8 mm as shown. Describe whether Liquid Penetrant, Magnetic Particle, Eddy Current, Radiography and Ultrasonics inspection techniques can determine the 3 portions of this crack in terms of: (i) (ii) (iii)

location on the plate surface, length and height. Give total length which would be determined by each technique. depth below the surface (if the crack portion is below the surface).

Also describe whether each technique can determine whether the defect is clearly a crack or another type of defect.

Figure 1 Metal plate with crack

(b) (2 marks) The Distance Amplitude Correction (DAC) curve (see Figure 2) may be used as quality acceptance standards for ultrasonic tests. In an ultrasonic test from a weld two signals “A” and “B” appear at ranges of 50 mm and 100 mm, both of amplitude 40% full screen height (FSH). Are these defects acceptable to an acceptance standard of 50% DAC and explain why?

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Monash University Final Examination MEC4801

Figure 2 Distance Amplitude Correction (DAC) curve (c) (5 marks) Pulsed eddy current technique has been successfully applied in corrosion detection for several years. The key features of time domain pulsed eddy current transient response signal are shown in Figure 3. (i) Please explain the difference between conventional eddy current inspection technique and pulsed eddy current inspection technique; (ii) Please explain how to detect the corrosion based on typical features such as peak and zero-crossing time.

Figure 3 Pulsed eddy current transient response signal 2 (a) (4 marks) Write a Magnetic Particle Inspection procedure for a hollow shaft 500mm long with outer diameters of 120mm and 50mm and an inner diameter of 30mm (see Figure 4). 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.

Figure 4 Hollow shaft Page 3 of 6

Monash University Final Examination MEC4801

(b) (4 marks) Two large aluminum plates “A” and “B” are joined together. Plate “A” is 1mm thick and Plate “B” is 3mm thick. An air gap exists between the plates. An eddy current probe is used to test to for defects on 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 plate “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. Given

(c) (6 marks) Figure 5 shows a shear wave probe. The piezoelectric crystal (vibrating source) is attached to the sloping face of a Perspex block as shown. The longitudinal waves produced travel through the Perspex block and are refracted into the Aluminum specimen. This probe illustrates the fact that the refracted compression wave no longer exists when the incident angle α is large enough. This is important because it desirable to have only one interrogating wave from the probe. In order to only produce the shear wave in the specimen, what is the incident angle α range? Given: Velocity of compression wave in Perspex is 2700 m/s Velocity of compression wave in Aluminum = 6260 m/s Velocity of shear waves in Aluminum = 3080 m/s

Figure 5 Shear wave probe

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Monash University Final Examination MEC4801

3 (9 marks) Two metal plates A, B, each 25 mm thick, are joined together on top of each other with no air gap at the joint. Plate “A” is made of steel. The combined plate is placed on top of a table with plate “A” uppermost. An ultrasonic instrument is calibrated for a range of 0 to 100 mm using a shear wave probe which produces an angle of 45 degrees to the normal to the surface (from which testing is carried out) in steel. The probe is used to test the combined plate by placing it on top of the steel plate “A” and then moved to scan the combined plate. A single reflected single from a small round defect is found which has a range 60 mm on the ultrasonic screen. Assume that 50% of the ultrasound energy transmits through the interface between the two metal plates. What are the two possible depths of this defect from the top of the combined metal plate? Given: Velocity of shear waves in steel = 3230 m/s. Velocity of shear waves in metal “B” = 2700 m/s. 4 (a) (2 marks) The specimen in Figure 6 is x-rayed with the direction of the x-rays being directed vertically through the specimen as shown. Are the defects detectable? Give the reasons why they are or are not detectable. Direction of x-rays

0.2 mm 25 mm

5 mm 45º

A

B

Figure 6: Cross sectional view of a steel plate containing 2 rectangular shaped defects A and B, made by machined slots

(b) (8 marks) (i) Derive a practical radiographic technique for a circumferential butt weld in a steel pipe of outer diameter 300 mm and inner diameter 250 mm. Use a geometric unsharpness of 0.12mm. The x-ray set you will use has a focal spot size of 3 mm and a maximum tube current of 10 mA. A film density of 2 is required for your technique. Use film “A”. Use Figure 7 and 8. Use 300kV. (ii)Where would you position the IQI and why? (iii) How many shots are needed to cover the whole circumferential weld?

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Monash University Final Examination MEC4801

Figure 7 Exposure chart for Iron(Fe) for a density of 2 using MX film and lead screens

Figure 8 Film curves END OF PAPER Page 6 of 6...