How to choose the right frequency Ultrasonic probe in non-destructive testing
Ultrasonic Flaw detector is widely used in the Non-destructive testing(NDT), their ability to peer inside solid materials without causing any harm and quality control, from aerospace to power generation. As the key component of the Ultrasonic flaw detector, the Ultrasonic probe is very important because it's Ultrasonic Frequency.
We can treat the The Ultrasonic frequency as the lens of the Ultrasonic probe, choosing the right ultrasonic frequency makes what you can see and how clearly you see. Let's see the crucial factors governing Ultrasonic frequency selection and how it impacts the NDT results.
Material Properties & Attenuation:
Higher frequencies suffer greater attenuation (signal loss) as they travel through a material.
Higher frequency will suffer greater attenuation(signal loss) travel though the same material comparing to the lower frequency.
This attenuation is more pronounced in coarse-grained, porous, or structurally inhomogeneous materials.
Rule:Use lower frequencies (0.5 - 2 MHz) for large, coarse-grained (castings, large forgings), porous, or thick sections requiring deep penetration. Use higher frequencies (2.5 - 15 MHz+) for fine-grained materials (rolled metals, fine forgings, welds) where attenuation is low. you can refer to our 2 MHz dual element contact UT probe , Dual crystal straight probe , Single crystal straight probe, you will see what kind of material can be detected in this Frequency.
Defect Detection Capability:
Small Defects & Resolution: Higher frequencies produce shorter wavelengths. This directly translates to a better ability to detect very small flaws and to resolve closely spaced defects (improved resolution). This is critical for finding minute cracks, inclusions, or porosity clusters near the surface.
Rule: Opt for higher frequencies (5 MHz and above) when detecting small flaws, defects near the surface, or needing to distinguish between two closely positioned reflectors.
Beam Characteristics:
Directionality: Higher frequencies generate a narrower, more focused beam (smaller divergence angle). This improves sensitivity to flat defects and provides more precise defect location. Lower frequencies produce a wider beam, potentially finding defects outside the central path but with lower energy concentration and location accuracy.
Rule: Choose higher frequencies for precise defect sizing and location, especially with known orientation. Lower frequencies offer better coverage for unknown defect locations in complex geometries.
Surface Condition & Geometry:
Rough surfaces scatter high-frequency sound waves significantly, reducing signal effectiveness.
Small diameters or thin sections can cause confusing reflections (multiple echoes) with lower frequencies due to longer pulse lengths.
Rule: Select lower frequencies for rough surfaces. Use higher frequencies for inspecting small diameters or thin materials to maintain sensitivity and avoid pulse-width interference.
Defect Type Specifics:
Cracks: Often favorably oriented for detection with shear waves. Lower frequencies can be advantageous due to better penetration around crack tips and less sensitivity to unfavorable crack-face roughness.
Inclusions/Porosity: These volumetric defects are often best detected with higher frequencies, leveraging the improved resolution to identify smaller clusters and define their boundaries clearly.
Practical Frequency Guidance (Based on Common Scenarios)
|
Material / Component |
Typical Frequency Range (MHz) |
Why This Range? |
|
Large Steel Forgings |
1 - 3 MHz |
Penetration depth crucial; coarse grains cause attenuation. |
|
Small Steel Forgings |
2 - 5 MHz |
Balance penetration with ability to detect moderate-sized flaws. |
|
Large Steel Castings |
0.4 - 3 MHz |
Very coarse structure; high attenuation demands very low frequencies. |
|
Small Steel Castings |
1.5 - 5 MHz |
Moderately coarse structure; needs some penetration and resolution. |
|
Steel Welds (Manual UT) |
1 - 4 MHz |
Common compromise; good penetration into weld metal & HAZ. |
|
Aluminum & Alloys |
3 - 10 MHz |
Fine grain structure allows higher frequencies for better resolution. |
|
Steel Plate (Immersion) |
2 - 5 MHz |
Consistent material; balance penetration and flaw detection. |
|
Steel Tube/Pipe |
5 - 15 MHz |
Often thin-walled; requires high frequency for resolution/sensitivity. |
|
Small Diameters (Shafts) |
5 MHz+ |
Avoids pulse-width issues and maintains sensitivity. |
So when you are design what kind of Ultrasonic frequency should be choosen? We should think of following factors:
Material Thickness & Structure: What depth must you reach? What's the inherent grain size?
Defect Type & Size: Are you hunting for micro-porosity, deep-lying cracks, or subtle laminations?
Component Geometry: Simple plate or a complex forging with contours and changes in section?
Surface Finish: Is it machined smooth or as-cast and rough?
Sensitivity Needs: How critical is detecting the absolute smallest flaw?
Modern ultrasonic flaw detectors often feature probes with selectable frequency ranges or multi-frequency capabilities, providing greater flexibility. Understanding the principles behind frequency selection allows you to leverage your equipment effectively.
We are the professional manufacturer of Ultrasonic probe for NDT device, if you want more information the Ultrasonic sensor, so please contact us:
Email: info@sonerc.com
Website: www.sonerc.com
