NANYANG TECHNOLOGICAL UNIVERSITY

SCHOOL OF MECHANICAL AND AEROSPACE ENGINEERING

E2.8 NON-DESTRUCTIVE TESTING Technical Report

Name: DHARSHANAA

MATRICULATION NO: U1221834A

Lab Group: LG04

Date: 15 February 2014

Tutor: Tao Kai

1) Introduction

Non-destructive testing (NDT) techniques are used in inspecting, testing and evaluating materials and components, or differences in the characteristics of these parts without impairing its future usefulness. Some of the commonly used NDT techniques include visual inspection, dye penetration, radiography, optical techniques, ultrasonic or acoustic energy and magnetic particle inspection.

Two types of testing are used in this experiment, ultrasonic testing and eddy-current testing. Ultrasonic testing requires equipment that produces an electric pulse which sends a high frequency sound pulse into the structure. It is usually used to detect defects that occur below the surface but it is also used to detect surface opening defects.

Eddy current testing uses electromagnetic radiation to detect flaws in conductors. The flow of the eddy current will be affected when it encounters a discontinuity in the test piece and the change in the eddy current density can be detected and used to characterize the discontinuity causing that change.

2) Objectives The objective of this experiment is to calibrate ultrasonic instruments and determine the size and locations of internal defects in metal specimens.

3) Theory Sound waves are simply organized mechanical vibrations traveling through a medium, which may be a solid, a liquid, or a gas. These waves travel through a given medium at a specific velocity or speed, in a particular direction. When they encounter a boundary with a different medium, they will be reflected or transmitted according to simple rules. This is the principle of physics that underlies ultrasonic flaw detection.

Ultrasonic waves of different types in a frequency range between 0.5MHz and 25MHz are used and the resulting wave length is in micrometres. With lower frequencies, the interaction effect of the waves with internal flaws would be so small that detection becomes problematic.

When sound energy traveling through a material encounters a boundary with another material, a portion of the energy will be reflected back and a portion will be transmitted through. The amount of energy reflected, or reflection coefficient, is related to the relative acoustic impedance of the two materials. Acoustic impedance in turn is a material property defined as density multiplied by the speed of sound in a given material (Z= ρv) . Acoustic transmission and reflection at boundary of two materials (at right angle) is given by:

Reflection, Er = ((Z1- Z2) / (Z1+Z2))2

Transmission, Et = (( 2Z1Z2) / (Z1 + Z2))2

Er and Et can be used to determine the percentage of the incident sound energy in reflected and transmitted waves.

4) Experiment No.1 –Ultrasonic “A” Scan

Apparatus used:

1) 2)

Couplant USD10 Ultrasonic Instrument Screen

3) 4)

Test block with 5 different thicknesses Probe used to measure thickness

Procedure for experiment 1

1) Place some couplant on the test block and ensure there is adequate amounts between the probe and the specimen. The beginning of signal pulses on the ultrasonic instrument screen should be used for measurement purposes. This can be identified as where the left hand almost vertical line of the two lines which form an ultrasonic signal meets the bottom of the screen.

2) It should be noted that in “A” scan tests, the travel distance of a wave to a defect such as the back of the specimen) and back to the probe is always divided by 2 to obtain the distance of the defect from the probe. This latter distance is always used on the screen of the ultrasonic instrument. For example for a defect 25 mm from the probe the travel distance is 50mm.

3) Calibrate instrument: Use the 25 mm thick test block no. 1 to produce a screen calibration of 0 to 50mm in steel using the USD10 delay and expand contract controls.

4) Measure specimen thickness: Use the compression wave probe to measure the 5 thickness of the stepped steel specimen (no. 2) supplied. Measure the actual thickness of the specimen using a micrometer. Calculate the errors.

Thickness using micrometer(m) in (mm)

Thickness using ultrasonic (u) in (mm)

Error((u-m)/m) x 100%

24.6825.00 1.30

24.17 24.92 3.10

23.90 24.02 0.50

19.85 20.23 1.91

15.82 15.19 3.98

Table 1: Thickness measuring results

Calibrate ultrasonic instrument for shear wave testing

Direct the ultrasonic beam towards the 25 mm radius of the test block (no. 3) shown in Fig. U1. Position “A” is a 25mm radius with centre “O”. Position “B” is a 50 mm radius with centre “O”. Two peaks are observed. The first peak is from the reflection at point A (total travel distance 50 mm). The second peak is due to multiple reflection at points A, O, B, O and then finally back to A again (total distance = 50 + 100 + 50 = 200 mm). Position the two peaks at the 25 and 100 mm positions respectively on the timebase.

Sizing a crack using the maximum amplitude technique.

The side drilled holes in the test block no. 4 simulate a crack. Move the 60 degree shear wave probe until the maximum signal from the top hole is obtained. Record the distance from the probe index to the top hole from the ultrasonic screen, “OC”, and measure the distance of the index point from the end of the test block using a ruler, “OD”. Repeat the procedure for the bottom hole. Using this information and the fact that the probe has an angle of 60 degrees, plot the positions of these 2 holes (the crack extremities) found using “ultrasonics” on the 1 to 1 scale drawing shown in Fig. U2. Record the “x” (horizontal) and “y” (vertical) locations errors of the holes. Measure from your drawing using a ruler, the size of the defect determined using ultrasonic and compares this with the actual size of the defect.

Top hole(location errors) Bottom hole(location errors) Total defect sizing error“x” error (mm)

“y” error“mm”

“x” error“mm”

“y” error“mm”

((u-m)/m)x 100%

-0.58 4.5 -1.39 0.5Where m= actual size of hole, u= ultrasonic determined size

For top hole:

X1- X1’=119.77-120.0 = -0.58

Y1- Y1’=19.50 -15.0 = 4.5

For bottom hole:

X2 – X2’ =110.615 -112 = -1.39

Y2 – Y2’ =30.5 – 30 = 0.5

5) Results and Discussions

a) Sources of errors in all the experiments

1) Firstly the probe used to measure the surface defects might not be flat enough to produce very accurate results. Therefore, some ultrasonic waves might not pass through the couplant to produce precise wave peaks on the monitor screen. This can cause probe to emit the waves out in different directions than intended .Hence better calibration is required. Also, we can use a different medium like water or oil to find out if the results obtained are more accurate.

2) The test block used in this experiment is made of metal which might expand or contract based on the surrounding temperatures. This can cause discrepancies in the results obtained .To minimise this error; we should set the surrounding room temperature to standardized quality controlled temperature.

3) Also, there might be insufficient usage of the couplant between the surfaces of the probe and hence inefficient

4) The ultrasonic devices can become faulty overtime and this might lead to inaccurate readings taken while doing the experiment.

5)

How does “A” scan thickness measuring accuracy compare with the maximum amplitude

crack sizing accuracy?

In comparison, the “A” scan used to measure the thickness of the specimen is more accurate than the maximum amplitude crack sizing accuracy. This is because , for the “A” scan, the ultrasonic beam is projected vertically down to measure the thickness ,whereas for the maximum amplitude crack sizing accuracy, the ultrasonic beam is projected at an angle of 60 degrees .This may not be very accurate as the defect size may not reflect well in the second method.

6) Conclusion

References