Master Guide to DAC Curve in Ultrasonic Testing: Theory, Plotting, and Practical Application
In the world of Non-Destructive Testing (NDT), precision is the difference between a safe industrial plant and a catastrophic failure. For Production Engineers and Quality Control Inspectors, understanding the internal integrity of welds and materials is paramount. One of the most critical tools in an Ultrasonic Testing (UT) technician's arsenal is the Distance Amplitude Correction (DAC) Curve.
In this comprehensive guide, we will break down everything you need to know about the DAC curve—from the basic physics of sound attenuation to a step-by-step example of plotting a curve according to international standards like ASME Section V.
1. What is a DAC Curve? (The Fundamental Concept)
In Ultrasonic Testing, we send high-frequency sound waves into a material. However, sound doesn't travel forever with the same strength. As the sound wave moves deeper into the steel, it loses energy. This loss is called Attenuation.
Why Attenuation Matters
Imagine two identical 3mm cracks in a steel plate.
Crack A is located 10mm from the surface.
Crack B is located 60mm from the surface.
On your UT machine screen (A-Scan), the signal from Crack A will look very tall (high amplitude), while the signal from Crack B will look much smaller. Without a DAC curve, an inspector might mistakenly think Crack B is "too small to worry about," when in reality, it is just as dangerous as Crack A.
The Definition: A DAC Curve is a graphical line drawn on a UT screen that connects the peak amplitudes of identical reflectors at different depths. This "levels the playing field," allowing the inspector to evaluate defects fairly, regardless of how deep they are in the material.
2. The Physics Behind the Curve: Sound Attenuation
To write a high-quality NDT report, you must understand the two main reasons sound loses strength:
Beam Spreading: As the sound travels, the beam spreads out like a flashlight. The energy is distributed over a larger area, reducing the intensity of the reflection.
Absorption and Scattering: Real-world materials like Carbon Steel or Stainless Steel aren't perfect. Small grains in the metal scatter the sound, and the material itself absorbs some vibration.
The DAC curve compensates for these factors mathematically and visually, ensuring that a "Recordable" defect is identified correctly at any depth.
3. Necessary Equipment for DAC Plotting
Before you start, you cannot simply "guess" a DAC curve. You need standardized calibration blocks. As a production engineer, you should be familiar with:
SDH Blocks (Side Drilled Holes): These are the industry standard for DAC. The holes are usually 3mm or 1.5mm in diameter.
Calibration Standards: Reference ASME Section V, Article 4 or ISO 17640 to ensure your block matches the thickness of the production part you are testing.
The Right Probe: Usually a 2MHz or 4MHz Angle Beam probe (45°, 60°, or 70°) depending on the weld geometry.
4. Step-by-Step: How to Plot a DAC Curve
Follow these steps for a professional setup on a digital or analog UT machine:
Step 1: Surface Preparation and Coupling
Ensure the calibration block surface is clean and matches the "Roughness" of your production part. Apply a consistent layer of couplant (grease or oil).
Step 2: Finding the First Reference Point
Place the probe on the calibration block to catch the reflection from the shallowest Side Drilled Hole (SDH). Move the probe until you find the maximum signal. Adjust your Gain (dB) until this peak reaches 80% of the Full Screen Height (FSH). This is your "Primary Reference Level." Mark this point on the screen.
Step 3: Finding the Second and Third Points
Without changing the Gain, move the probe to the next deepest SDH. You will see the signal is lower (perhaps at 50% FSH). Mark the peak. Repeat this for the third (deepest) hole.
Step 4: Drawing the Curve
Connect these three (or more) peaks with a smooth, curved line. Modern digital UT machines do this automatically when you hit "Enter" at each peak. This line is your 100% DAC Curve.5. The "Three-Line" Evaluation System
In professional NDT reporting, we don't just use one line. We use a system of levels:
100% DAC (Evaluation/Reject Level): Any signal that touches or goes above this line is considered a "Defect" and must be checked against the acceptance criteria (e.g., length vs. depth).
50% DAC (Recording Level): Signals above 50% of the curve must be recorded in the NDT report, even if they aren't rejected.
20% DAC (Scanning/Disregard Level): Anything below this is considered "background noise" or minor inclusions that do not require reporting.
5. The "Three-Line" Evaluation System
In professional NDT reporting, we don't just use one line. We use a system of levels:
100% DAC (Evaluation/Reject Level): Any signal that touches or goes above this line is considered a "Defect" and must be checked against the acceptance criteria (e.g., length vs. depth).
50% DAC (Recording Level): Signals above 50% of the curve must be recorded in the NDT report, even if they aren't rejected.
20% DAC (Scanning/Disregard Level): Anything below this is considered "background noise" or minor inclusions that do not require reporting.
6. Practical Example: Inspecting a 40mm Weld
Let's look at a real-world example you might encounter on a production floor.
Scenario: You are inspecting a V-butt weld on a 40mm thick carbon steel plate using a 60° Angle Beam Probe.
Standard: ASME Section V.
Calibration: You use a block with SDHs at 1/4T, 1/2T, and 3/4T depths (10mm, 20mm, and 30mm).
The Curve: You set your 10mm hole peak at 80% FSH. The 20mm peak falls to 60%, and the 30mm peak falls to 40%. You draw your DAC curve through these points.
The Discovery: During the scan of the actual production weld, you find a signal at a depth of 28mm.
The raw amplitude of this signal is 45% FSH.
The Analysis: At a 28mm depth, your DAC curve is sitting at roughly 42% FSH.
The Result: Because your signal (45%) is higher than the DAC curve (42%) at that specific depth, this is an Indication. You must now measure its length to see if it meets the rejection criteria of the project.
7. DAC vs. TCG (Time Corrected Gain)
DAC shows a curved line on the screen, and the signals stay at their natural heights.
TCG (Time Corrected Gain) electronically increases the gain for deeper signals so that all identical reflectors appear at the same height (80% FSH) across the screen.
Which is better? DAC is often preferred for manual inspection because it shows the "real" behavior of the sound, making it easier to distinguish between types of defects based on their signal shape.
8. Common Errors to Avoid in DAC Construction
Incorrect Couplant Pressure: Pressing too hard on the first hole and too light on the third will ruin the curve.
Temperature Mismatch: If your calibration block is cold but the production plate is hot (e.g., after welding), the sound velocity changes. Always ensure they are at a similar temperature.
Transfer Correction: If the production part is rougher than the calibration block, you must add "Transfer Correction" (usually 2-4 dB) to your gain to compensate for the rough surface.
9. Conclusion: Why NDT Hub Prioritizes DAC
As a Production Engineer, your goal is Quality without Delay. Using a DAC curve ensures that you don't waste time cutting out "false" defects, and more importantly, you don't ship a product with a hidden, deep-seated crack.
Mastering the DAC curve moves you from being a basic technician to a Quality Leader. It provides the technical data needed to back up your decisions in an NCR (Non-Conformance Report) and ensures your company meets international safety standards.
Continue to Learning :
"Ready to take your career to the next level?"
Mastering the DAC Curve is just the beginning. If you want to know how these technical skills translate into better pay, check out our popular guide: NDT Technician Salary & Career Path in 2026
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