What is DAC?
DAC stands for Distance Amplitude Correction Curve in Ultrasonic Testing
Where do we use DAC?
DAC is used for evaluating the equivalent flaw size in Ultrasonic Testing
What does it mean by Equivalent Flaw Size (EFS) in Ultrasonic testing?
Equivalent flaw size in Ultrasonic testing means, size of the flaw in your material that gives equal response as the flaw for evaluation in the reference block. For example, if you have a flaw of 3mm Flat Bottom Hole FBH in the reference block and your indication is equivalent in amplitude on DAC curve, it indicates that the size of flaw in your material is equivalent to 3mm but actually many be more than the size of the FBH in reference block. Because actual flaw may not be an ideal reflector like the FBH.
How to select the Reference block materials to draw DAC?
To draw DAC in Ultrasonic UT testing, you need to select a material from the same attenuation characteristics. In the sense, you need to take a block of materials from the same lot of materials under testing. It shall be of same material composition, processing conditions and metallurgical properties as the materials under ultrasonic testing. In case if such material is not available, you can choose any other material having same attenuation and Ultrasonic properties.
What is the shape of reflectors to draw DAC in Ultrasonic testing?
For normal beam testing, Flat Bottom Holes (FBH) and for Angle beam testing Side Drilled Holes or Notches
To draw DAC, What is the size of FBH or SDH or Notch ?
To draw DAC, the size of Flat Bottom Hole FBH or Side Drilled Hole SDH or Notch Dimensions are available in Code, Standard or specification. Or else, check the acceptance criteria for the job and choose accordingly. For example: FBH can be 2mm or 3 mm or 4mm or even 0.8mm for aerospace structures. Choose as per criticality of application
To draw DAC, how many reference blocks are required ? What is depth of FBH for each block?
To draw DAC you need to have at least 3 reference reflectors ( artificial flaws) either it can be on the same block or one flaw in each block. Both are acceptable as long a the indication from the FBH do not interfere with each other while using. Of course, Flaws should be for a depth of 1/4, 1/2, 3/4 of height of the block. Distance between the holes shall be more than 50mm. Finally, Duly check the blocks for dimensions after making the blocks for accuracy.
How do you manufacture artificial flaws in the DAC Blocks?
You can drill the holes if it is FBH using a flat drill bit or a better and more accurate way is to use EDM ( Electro discharge machining) process. EDM process will give you the best results as the flaw sizes are more accurate.
How to Draw DAC for Normal Beam Ultrasonic testing using FBH?
It is quite common that casting and forgings undergo Ultrasonic testing to find internal flaws. To evaluate the size of flaw DAC is quite helpful.
Steps to draw DAC in Ultrasonic Testing
- Select the block with lowest metal distance i.e. more depth containing hole.
- Firstly, Place the Ultrasonic probe to get maximum reflection
- Secondly, Adjust the gain to 80% FSH ( Full Screen Height)
- Mark the Peak of 1st echo using a marker or software
- Freeze the dB and place the probe on 2nd block
- Mark the peak without adjusting the dB
- Similarly do the same for 3rd flaw
- Join all the 3 points using smooth curve
- Finally, You will get the DAC curve for the size of flaw in block
The above procedure is for Normal Straight beam using FBH reference block. For angle beam testing follow the same procedure using SDH or Notches.
Learn more about in depth Ultrasonic testing at Trinity NDT Level 2 Ultrasonic testing certification Courses. Also, find more about Ultrasonic testing and other NABL accredited NDT testing services.