Loading ...

Drawing DAC in Ultrasonic Testing

What is DAC?

DAC stands for Distance Amplitude Correction Curve in Ultrasonic Testing (UT). It is used to evaluate the equivalent flaw size in a material.

What is Equivalent Flaw Size (EFS) in Ultrasonic Testing?

The Equivalent Flaw Size (EFS) refers to the size of a flaw in a material that gives the same ultrasonic response as a reference flaw in a known test block. For example, if you have a 3mm Flat Bottom Hole (FBH) in a reference block and the amplitude of your flaw response matches this on the DAC curve, it indicates that the flaw size in your material is approximately 3mm. However, the actual flaw in your material may be larger, as it may not be a perfect reflector like the FBH.

How to Select Reference Block Materials for DAC?

To create a DAC curve, the reference block material should have similar attenuation characteristics to the material being tested. Ideally, the reference block should be made from the same lot of material and have the same composition, processing conditions, and metallurgical properties. If such a block is unavailable, choose a material with similar ultrasonic properties and attenuation characteristics.

What is the Shape of Reflectors Used to Draw DAC?

• For normal beam testing, use Flat Bottom Holes (FBH).
• For angle beam testing, use Side Drilled Holes (SDH) or Notches.

What Size of FBH or SDH/Notch is Used to Draw DAC?

The size of the FBH, SDH, or Notch used to draw the DAC curve is often specified in standards or codes. Common sizes for FBH include 2mm, 3mm, or 4mm, and for critical applications, such as aerospace, sizes as small as 0.8mm may be used. Always refer to the acceptance criteria for the job or application to select the correct size.

How Many Reference Blocks Are Needed for DAC, and What Depths Should the FBH Have?

To draw the DAC curve, at least three reference reflectors are required. These can be on a single block or spread across multiple blocks, provided the indications from each FBH do not interfere with one another. The FBH should be located at depths of 1/4, 1/2, and 3/4 of the block height, with the distance between holes being at least 50mm. After creating the blocks, verify the accuracy of the dimensions.

How Are Artificial Flaws Manufactured in DAC Blocks?

Artificial flaws (like FBH) in DAC blocks are typically made by drilling with a flat drill bit or, for greater accuracy, using the Electro Discharge Machining (EDM) process. EDM provides better results by creating more precise flaw sizes.

How to Draw DAC for Normal Beam Ultrasonic Testing Using FBH?

DAC is often used to evaluate internal flaws in castings and forgings via Ultrasonic Testing. Here’s how to draw the DAC for normal beam testing using FBH:

Steps to Draw DAC in Ultrasonic Testing:

1. Select the Reference Block: Choose the block with the greatest depth of flaw, which will produce the most significant reflection.
2. Place the Ultrasonic Probe: Position the probe to get the maximum reflection from the flaw.
3. Adjust the Gain: Set the gain to 80% of Full Screen Height (FSH) for proper calibration.
4. Mark the Peak of the First Echo: Use a marker or software to mark the peak of the first flaw echo.
5. Freeze the dB Setting: Once the peak is marked, freeze the dB setting.
6. Move to the Second Block: Place the probe on the second reference block and mark the peak without changing the dB setting.
7. Repeat for Third Block: Do the same for the third flaw.
8. Plot the DAC Curve: Join the three points smoothly to draw the DAC curve, which represents the flaw sizes.

This process is for normal straight beam ultrasonic testing using an FBH reference block. For angle beam testing, follow the same procedure but use SDH or Notches as reference flaws.

By following this procedure, you can accurately assess and visualize flaw sizes in the material being tested.