Defect Detection Methods
- Introduction
Defect Detection Methods involve identifying flaws in products or systems to ensure quality and reliability. Common techniques include visual inspection, ultrasonic testing, X-ray imaging, and magnetic particle testing. Automated methods like machine vision and AI-based anomaly detection improve speed and accuracy. Selection depends on material type, defect size, and environment. These methods are essential in manufacturing, aerospace, and construction to prevent failures, reduce costs, and maintain safety standards.
Stationary Coil
Encircling Coil
This method is used to detect the “Transverse Type, short or abrupt” defects in Metal- tubes, bars, & wire rods.
The test object (tube or bar) passes through a coil that generates an alternating magnetic field. This induces eddy currents in the material, which are influenced by its conductivity, permeability, and defects. Any discontinuity (crack, inclusion, or variation in material properties) disrupts the eddy current flow the system detects these disturbances and generates a signal corresponding to the defect or material change.
1. ERW tubes
2. Seamless tubes
3. Wire Rods
4. Bars
5. Oil and gas
6. Nuclear power generation
7. Boilers
8. Heat exchangers
Test procedure for Encircling type:
1. Material & Dimensions of Standard Sample
2. The standard sample must match the OD (outer diameter), material, and wall thickness of the test material.
3. The encircling coil must be designed for the same OD as the standard sample.
4. The sample should be long enough to accommodate multiple defect types while allowing proper coil interaction. - Artificial Defects in Standard Sample
The defects introduced in the standard sample must be precisely machined to simulate real-world flaws. Common defects include
Through Drill Holes (TDH) – Mandatory
Used for system calibration due to their repeatability and ease of access.
Placement Configurations
1. 3 holes spaced 1 meter apart, positioned 120° apart around the circumference.
2. 4 holes spaced 1 meter apart, positioned 90° apart around the circumference.
3. Ensures full 360° defect detection.
Typical Sizes
Diameter: 8 mm to 3 mm.
Electro-Discharge Machined (EDM) Notches
1. Simulate cracks and stress-related defects.
2. Circumferential Notches (Perpendicular to Tube Axis) – Simulate hoop stress cracks. - Typical Dimensions
Width:2 mm to 0.5 mm.
Depth: 5–10% of the wall thickness.
Length: 5 mm to 10 mm.
Flat Bottom Drill Holes (FBDH) – Optional
1. Simulates pitting corrosion and subsurface defects.
2. Typical Sizes
3. Diameter:8 mm to 3 mm.
4. Depth: 10–30% of wall thickness.
Frequency & Gain Settings Based on Material
| Material Type | Frequency Range | Gain Range |
| Ferrous (Steel, Iron, etc.) | 5–15 kHz | 6–45 dB |
| Non-Ferrous (Aluminium, Copper, SS, etc.) | 30–100 kHz | 6–45 dB |
- Testing Configuration
Online Testing (Continuous Production Testing)
Only through drill holes (TDH) are introduced on strips.
Used for real-time defect detection during manufacturing.
Offline Testing (Post-Production Testing)
Includes TDH, notches, and FBDH for full defect evaluation.
Used for batch quality control before dispatch.
Inline Testing (During Processing)
Uses TDH, notches, and FBDH.
Performed after forming but before final machining/processing. - Calibration & Threshold Setting
Low-Pass & High-Pass Filters: Adjusted based on inspection speed to optimize defect detection.
Threshold Setting for Crack Rejection:
Set based on the amplitude of drill holes in the ECT signal.
Minimum signal-to-noise ratio (SNR) of 3:1 is required for defect classification.
Gain is increased so that drill hole amplitude rises by 20% above the threshold to ensure no similar-sized cracks pass undetected.
Signal Equalization:
The amplitude of all drill holes should be equal, confirming that the tube/bar/rod is passing straightly through the coil.
Final Validation Before Production
Ensure equal signal amplitude for all TDHs.
Confirm threshold and gain settings for optimal defect detection.
Verify 360° detection capability.
Proceed with production only after successful calibration.
Segment Coils
This method is used specifically at particular angle or sector of metal tube most commonly used for seam Transverse Crack detection.
The coil is divided into multiple segments (typically 2, 4, or more). Each segment functions as an individual eddy current sensor. A high-frequency AC signal generates a magnetic field in each coil segment. Eddy currents are induced in the material. If a defect is present, the eddy current flow is disrupted. Each segment provides separate signals, allowing precise location of defects around the circumference. Unlike encircling coils, which only indicate a defect’s presence, segment coils can determine where the defect is located.
1. ERW tubes
2. Oil and gas
3. Nuclear power generation
Test procedure for Segment Coil Testing
Same as Encircling coil method only electronic apparatus has multiple channels.
Bobbin Type
This method is used to detect Transverse crack detection, Bobbin coil testing is a widely used eddy current testing (ECT) method for inspecting heat exchanger tubes, boiler tubes, and condenser tubes in industries such as power plants, petrochemicals, and aerospace. It is an internal probe-based method that detects corrosion, pitting, wall thinning, and cracks in non-ferromagnetic and some ferromagnetic tubes.
1. ERW Tubes
2. Seamless Tubes
3. Oil and Gas
4. Nuclear power generation
5. Boilers
6. Heat Exchangers
- Test procedure for Bobbin type testing:
System Components
Electronic Equipment – Processes signals from the probe.
Probe Length: 5–10 meters to accommodate long tubes.
Type: Flexible or rigid, depending on tube geometry.
Standard Sample: Used for system calibration before testing actual tubes
2. Standard Sample Composition
The standard sample contains artificial defects positioned at known locations to ensure system accuracy. These include
Through Drill Holes (TDH) – For calibration and sensitivity verification.
Flat Bottom Hole (FBH) – Simulates pitting and localized thinning.
EDM Notches – Represents surface cracks (axial & circumferential).
Wall Thinning Areas – Mimics erosion and general corrosion
Grooves – Simulate baffle cuts, mechanical wear, or stress-related damage.
Each defect type is strategically placed apart to verify detection capability and system balancing.
3. Calibration & System Setup
Probe is inserted into the standard sample.
Each defect type is detected and analysed on the electronics system.
Gain, frequency, and filter settings are adjusted for optimal defect recognition.
Once parameters are set, the system is ready for in-service inspection
1. In-Service Inspection Procedure
Insert Probe into Tube
The probe is fully inserted into the tube from one end.
2. Start Recording:
The electronics system begins data recording once the probe is in place.
3. Uniform Extraction of Probe:
The probe is pulled out at a constant speed to ensure consistent data acquisition.
4. Stop Recording & Data Analysis:
Once the probe exits the tube, recording is stopped.
A qualified technician reviews the signal data to identify any defects.
5. Data Storage & Further Evaluation:
The recorded data is saved for future reference and further analysis
Key Advantages of This Method
✅ Non – destructive– No Damage to the tube during inspection.
✅ Detects multiple defect types – Cracks, thinning, pitting, and wear.
✅ Real – time recording – Allows post-analysis and comparison.
✅ Applicable to various Industries – Power plants, refineries, aerospace, and more.
Rotating Type
Part Rotating
Test specimen rotates while the probe remains stationary. A coil in the probe induces eddy currents in the conductive test material. Changes in material properties, cracks, or surface irregularities alter the eddy current flow, which is detected by variations in impedance. The impedance variations are processed and analsed to determine the presence, size, and nature of defects.
1. Billets
2. Shaft
3. Washers
4.Fillers
5. Piston pin
Testing procedure Rotating type testing:
1. Material & Dimensions of Standard Sample
Standard sample material and OD must same as Test sample.
2. Artificial Defects in Standard Sample
The defects introduced in the standard sample must be precisely machined to simulate real-world flaws. Common defects include:
1. Electro-Discharge Machined (EDM) Notches
Simulate cracks and stress- related defects.
Longitudinal Notches – Simulate pit and stress cracks.
Typical Dimensions:
1. Width:2 mm to 0.5 mm.
2. Depth: 5–10% of the wall thickness.
3. Length: 5 mm to 10 mm.
1. Flat Bottom Drill Holes (FBDH) – Optional
Simulates pitting corrosion and subsurface defects.
Typical Sizes
Diameter: 8 mm to 3 mm.
Depth: 10–30% of wall thickness.
Frequency & Gain Settings Based on Material
| Material Type | Frequency Range | Gain Range |
| Ferrous (Steel, Iron, etc.) | 400Khz | 6–50 dB |
| Non-Ferrous (Aluminium, Copper, SS, etc.) | 800Khz | 6–50 dB |
- Testing Configurations
1. Offline Testing (Post-Production Testing)
2. Includes TDH, notches, and FBDH for full defect evaluation.
Used for batch quality control before dispatch.
Inline Testing (During Processing)
Uses TDH, notches, and FBDH.
Performed after forming but before final machining/processing.
Calibration & Threshold Setting
Low-Pass & High-Pass Filters: Adjusted based on inspection speed to optimize defect detection.
Threshold Setting for Crack Rejection:
1. Set based on the amplitude of notch signal in the ECT signal.
2. Minimum signal-to-noise ratio (SNR) of 3:1 is required for defect classification.
3. Gain is increased so that drill hole amplitude rises by 20% above the threshold to ensure no similar-sized cracks pass undetected.
Signal Equalization:
The amplitude of notches must remain same at any given time.
- Final Validation before Production
✅ Ensure equal signal amplitude for Notches.
✅ Confirm threshold and gain settings for optimal defects detection.
✅ Verify 360° detection capability
✅ Proceed with production only after successful calibration
Probe Rotating
Unlike encircling coil testing, which inspects the entire circumference simultaneously, rotating probes inspect the surface point by point as they rotate and move axially. The probe generates eddy currents in a small localized region as it moves. Any defect interrupts the eddy current flow, creating a signal variation detected by the system. This method provides higher sensitivity for small cracks and defects.
1. Bars
2. Seamless tubes
3. Oil and gas
4. Nuclear power generation
5. Heat Exchangers
Testing procedure for Part Rotating crack detection:
- Material & Dimensions of Standard Sample
Standard sample material and OD must same as Test sample. - Artificial Defects in Standard Sample
The defects introduced in the standard sample must be precisely machined to simulate real-world flaws. Common defects include: - Electro-Discharge Machined (EDM) Notches
Simulate cracks and stress-related defects.
Longitudinal Notches – Simulate pit and stress cracks.
B. Typical Dimensions:
1. Width:2 mm to 0.5 mm.
2. Depth: 5–10% of the wall thickness.
3. Length: 5 mm to 10 mm.
Flat Bottom Drill Holes (FBDH) – Optional
Simulates pitting corrosion and subsurface defects.
Typical Sizes
Diameter: 8 mm to 3 mm.
Depth: 10–30% of wall thickness.
Frequency & Gain Settings Based on Material
| Material Type | Frequency Range | Gain Range |
| Ferrous (Steel, Iron, etc.) | 400Khz | 6–50 dB |
| Non-Ferrous (Aluminium, Copper, SS, etc.) | 800Khz | 6–50 dB |
8. Testing Configurations
1. Offline Testing (Post-Production Testing)
2. Includes TDH, notches, and FBDH for full defect evaluation.
3. Used for batch quality control before dispatch.
Inline Testing (During Processing)
1. Uses TDH, notches, and FBDH.
2. Performed after forming but before final machining/processing.
9. Calibration & Threshold Setting
Low-Pass & High-Pass Filters: Adjusted based on inspection speed to optimize defect detection.
Threshold Setting for Crack Rejection:
1. Set based on the amplitude of notch signal in the ECT signal.
2. Minimum signal-to-noise ratio (SNR) of 3:1 is required for defect classification.
3. Gain is increased so that drill hole amplitude rises by 20% above the threshold to ensure no similar-sized cracks pass undetected.
Signal Equalization:
The amplitude of notches must remain same at any given time.
Final Validation before Production
1. Confirm threshold and gain settings for optimal defect detection.
2. Verify 360° detection capability. 3. Ensure equal signal amplitude for Notches.
3. Proceed with production only after successful calibration.
