|NDT.net - April 2000, Vol. 5 No. 04|
|TABLE OF CONTENTS|
However, there are many limitations in this new TOFD technique which is discussed in the next chapter of this report. And there are also positive and negative comments from the panel discussion of " Ultrasonic Session III " . This report also mentioned the recent applications of the TOFD technique and showed a number of companies are already implementing the TOFD method instead of Radiography method [13,14,15,16]. It explains detail of the TOFD mathematical model in Fig (2) and sets up the necessaries formulas to calculate the size and depth of the defects, thickness of the steel plates and their arrival times. End of this report compares the Automated and Manual TOFD's experimental results which are achieved in the Centre of Automated and Robotic NDT at South Bank University.
Mr. Udo Schlengermann was one of the creators of the TOFD pre-standard ENV583 and was also involved in the development of the British standard BS7709.The principle of the TOFD techniques according to that report  mention in the below:
|Fig 1: The basic setup of TOFD testing is shown on Fig.1 a-c.|
The requirements of TOFD experiment:
The two ultrasonic angle probes are used in the TOFD experiment. One is transmitter (T) and another one is receiver(R). They are placed on the same surface of the test object Fig (1-a). The distance of the probes is calculated according to the wall thickness. See equation (8). The lateral wave (1) runs along the surface, the back wall echo (4) reflects the bottom surface of the test object and reach to the receiver. The other two signals, upper flaw tip diffracted signal (2) and lower flaw tip diffracted signal (3) appear due to inhomogeneity.
Fig (1-b) shows the A-scan and the time delay for each signal. The horizontal axis measures the time of flight and the vertical axis measures the amplitude. The signal (1) is the lateral wave, signal (2) & (3) are the diffracted signals top and bottom respectively and the final signal (4) is from the back wall echo. Fig 1-c shows the B-scan image generated by horizontal probe movement and time of flight in a vertical direction . The echo amplitude is displayed as gray scale, usually zero amplitude light gray (negative maximum amplitude black, positive maximum amplitude white). For defect testing it is important to notice that the probes are aligned transversal to the defect, while the image is generated in the direction of the defect. That means the image projection of Fig 1-c stands perpendicular to the probe projection shown in Fig 1(b).
|Fig 2: The two probe basis mathematical model of the TOFD technique|
The TOFD technique is based on timing measurements made on the signals diffracted by the crack. The general situation is depicted in Fig 2. The transmitting transducer T emits a short burst of ultrasound into the steel plate which thickness is H mm. This energy spreads out as it propagates into a beam with some definite angular variation. Some of the energy is incident on the crack tip (O & O') and is scattered by it. Scattering from the edge of the cracks, called diffraction, causes some fraction of the incident energy to travel towards the receiving transducer R.
If the crack is big enough then the signals from the two extremities of the crack will be time-resolved. As well as these two signals, there will be some energy which arrives at the receiver directly from the transducer by the shortest possible path (L1+L2) and (L3+ L4)- just below the surface of the component and an echo from the back wall. Such a set of actual signals is displayed in the lower part of Fig. 2. In the example, the transducers were moved, at constant separation, in the vertical plane, over a defect perpendicular to that plane. The signals appearing are, from the top of the figure to the bottom, the lateral wave, signals from the top crack (O) and bottom crack (O') of the defect and finally the back wall echo.
Calculation Size and Depth of defects
To calculate the defect size and depth from the inspection surface uses Pythagoras's theorem. Suppose, that the defect is oriented in a plane perpendicular to both the inspection surface and the line joining transmitter and receiver along the inspection surface. Suppose also that the defect is midway between the transmitter and receiver ( a position that can be found by minimizing the time of flight by probe scanning), its position below the inspection surface at a depth D mm. The distance of the two probes separation is taken S mm, the length of the defect is L mm , the thickness of the steel plate is H mm and the speed of propagation of sound waves is C , then the arrival times of the various signals are:
The first arrival time from the lateral wave signal to the receiver:
The second arrival time from the top tip diffracted signal to the receiver:
AOB is right-angled triangle and OA perpendicular to the surface AB.
According to Pythagoras's theorem
OB2 = OA2 + AB2
OB = L2, OA = D and AB = S/2 (half of the probes separation).
L22 = D2 + (S/2)2 and L2 = C*t1 / 2
C - speed for the longitudinal wave in steel
L2 - Half of the path of the diffracted signal so it takes time t1/2.
S/2 - Half distance of the probes separation.
t1 - The arrival time of the top tip diffracted signal.
Substitute L2, the equation is form as below:
(C*t1/2)2 = D2 + (S/2)2
Then the arrival time from the top tip diffracted signal to the receiver is :
The third arrival time from the bottom tip diffracted signal to the receiver.
The basic principle is the same as described above, the only change need to be done O`A instead of OA.
O`A = D + L ; L - defect size
The fourth arrival time from the back wall echo to the receiver.
The time for the back wall echo is:
Rearranging the above equations, we are able to calculate depth (D), size (L), Thickness (H) and the probe separation (S). They are shown in the below:
The value of the depth D is from the equation 2:
The value of the defect size L is from the equation 3:
The value of the thickness of the plate H is from the equation 4:
The value of the separation of the probes S is from the equation 4:
Where C is the speed of the lateral wave. On a flat plate this speed is identical to longitudinal wave.
An experiment was carried out on a steel plate with introducing 4mm defects from the bottom flat of the test specimen (Table 2) in the Centre for Automated and Robotic NDT at South Bank University. The specification for the experiment are shown in the below:
|Type of probes||Krautkraemer TOFD probes|
|Number of probes||2|
|Type of wave||Longitudinal|
|Angle of the wedges||60 degree|
|Type of Metal||steel|
|Size of the plate||350 x 340 mm|
|Thickness of the plate||13 mm|
|Speed of Longitudinal wave||5920 m/s|
|TABLE 1: UT equipment characteristics|
|Probe separation||96 mm|
|Delay time||7.5 ms|
|Digitizing Sampling Rate||100 MHz|
|Pulse Repetition frequency||1kHz|
|TABLE 2: Parameter setting of UT equipment|
The result of the experiment obtained using the 350x340 x13 mm steel plate and 4 mm defect from the bottom surface . The lateral and diffracted waves arrival time are shown in the below:
|Actual depth of the defects||Wave||Experimental Depth|
|TABLE 3: TOFD results|
From the experiment result of the A-scan in Fig (3), can easily be measured the delay time of the lateral and diffracted signals. Each division is divided in to 10mm and each millimeter is 0.75micro-second.The time of the diffracted and lateral signals are calculated in table (3).
|Fig 3: A scan for the 9mm depth defect steel plate|
As we know the arrival time of the diffracted signal (t1), probe separation and speed of the sound from the table (1). We easily can calculate the size of the depth by using the equation (5). The calculation shows the depth D = 9.1 mm which is almost the same value of the actual depth. The result obtained the 90% of its accuracy. Due to lack of test specimen we are unable to do more experiment at the moment. But in future we are expecting to do more TOFD experiments using different thickness of the metal with various defect sizes and orientations. As we requested SOFRATEST Company to provide us more test specimens, hopefully, we will be able to prove that TOFD method has the higher accuracy than any other NDT methods.
Parameters that might affect the application of the TOFD technique are below :
The TOFD technique can be used in the following fields:
The recent examples of TOFD application:
The purpose of the experiment is to compare the results between Automated and Manual TOFD scan on the non-smooth surfaces and their effects on B scan images. The test specimen is two layers (steel and rubber) with no defects. The surface was rug (non-smooth). The size of the test specimen is 300 x 300 mm and the height is 10 mm.
|Fig 3a: Automated TOFD scan by IBM 7545 Robot arm|
|Fig 3b: Manual TOFD scan using SOFRA Xscanner|
The Xscanner is connected with the encoder Board PCCDO2. The PCCDO2 board can read 2 axis encoders. The display shows individual bytes and total pulses count. The total count displayed for one revolution will be 4 times the number of cycles set in configuration function, due to quadrate decoding. The position in millimeters is then displayed, according to the developed length set in configuration.
Manual system is carried out on the same rug (non-smooth) surface of the test specimen. The experimental set up was almost same the automated system. The only different was moved the Xscanner manually instead of the robot arm. During the experiment the speed alarm was warned many times due to variable hand speed. These variable speeds have been effected on the data acquisition system. The experimental result is shown in Fig 3(b).
As we can see from the above Fig 3(a) and Fig 3(b) that the automated system can be used for the better scan than the manual system. The poor resolution in the manual system, most probably due to shaking hand while moving, override the Xscanner and non-constant force on the rug surface.
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