What is the cheapest NDT process?

Another important metric not explicitly mentioned in this text is probability of detection (POD) for a given type and size of defect. Depending on the consequences of not finding a defect you can set what POD you find acceptable for a given defect. POD can be found experimentally, or perhaps from literature. POD will depend on the material and defect compared to the technique used, but also the skill and experience of the inspector. Also, don't forget about the operational context! POD in a well lit lab when the part is lying on a table is probably better than when the inspector is outside at night in the cold and rain, or when the inspector is crawling around inside a narrow structure.

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Below you will find a general outline of the capabilities of common nondestructive testing (NDT) methods. This is intended as brief summary of each discipline's capabilities.

 Excellent Fair Poor RAM
Resonant
AcousticET
Eddy
CurrentMT/PT
Magnetic/
PenetrantUT
UltrasonicRT
RadiographyDefect/IssueCracks/Chips/VoidsMaterial PropertiesMissed OperationsStructural IntegrityProduct Lot VariationDefect LocationSurface (External)InternalBrazing/Bonding/WeldingSpeed/Training/CostThroughputTraining/Certification Total Inspection CostsAutomation CapacityQuantitative ResultsEase of AutomationCost of Automation   

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Resonant Inspection is a whole-body test method which detects resonant frequency shifts resulting from changes in mass, stiffness or damping of a part. Resonance can detect defects such as cracks, voids, chips, brazing problems, nodularity, variations in hardness, missed manufacturing processes, delamination, and more.

Applications include, but are not limited to, powder metal parts, castings, ductile iron parts, brazed assemblies, forgings, stampings and ceramic parts.

Non-Destructive Testing Resonant Acoustic Method (NDT-RAM) is a form of resonant inspection offered by The Modal Shop. NDT-RAM is available in automated, semi-automated, manual, and small-part drop-test systems. Learn more about  which system is right for you. 

AdvantagesDisadvantages

• Whole body test for internal and external flaws
• Fast testing time: 1 to 3 seconds per part (typical)
• No part preparation or cleaning required
• Objective pass/fail result
• No consumables expenses
• Easily automated
• Easily integrated into manufacturing process
• Permanent record capability
• Easily finds first n number of natural frequencies for NVH applications
• Best for high volume quality inspection
• Designed to be on the plant floor
• PCRT (Process Compensated Resonance Testing) techniques account for normal process variation

• Not diagnostic - does not indicate where flaw is, just that there is one
• Materials that resonate only - metal, composites and ceramic parts
• Large parts (> 100 lbs.) difficult to test
• Significant manufacturing process variations can mask defect detection

 

ULTRASONIC TESTING (UT)

Measures thickness, velocity or detects internal defects and variations, such as cracks, lack of fusion, delaminations and lack of bond.

Applications include wrought metals, welds, brazed joints, adhesive or bonded joints, non-metallic materials, in-service parts.

AdvantagesDisadvantages

• Most sensitive to cracks
• Immediate results
• Automation possible
• Permanent record capability
• Portable
• High penetration capability

• Couplant required
• Complex, or small parts may be difficult to check
• Defect may be missed if not in the path of the ultrasonic signal
• Reference standards required
• Trained operators for manual inspections
• Special probes
• Surface condition

 

RADIOGRAPHY (RT)

Measures or detects, internal defects and variations, porosity, inclusions, cracks, lack of fusion, corrosion, geometry variation, density changes, misassembled and misaligned parts. 

AdvantagesDisadvantages

• Permanent records
• Portable
• Geometry variation does not affect direction of radiation beam

• Radiation hazard
• Expensive
• Trained operators needed
• Linear defect may be missed
• Depth of defect not indicated
• Access to at least two sides of the part required

 

EDDY CURRENT (ET)

Measures or detects surface and subsurface cracks and seams, alloy content, heat treatment variations, wall and coating thickness, crack depth, conductivity and permeability.

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AdvantagesDisadvantages

• High speed
• Low cost
• Permanent record capability
• No couplant required
• No probe contact required

• Conductive material only
• Shallow depth of penetration
• Surface roughness may affect test quality

 

LIQUID PENETRANT (LPI / PT)

Measures or detects defects open to the surface of parts such as cracks, porosity, seams, laps and through wall leaks.

AdvantagesDisadvantages

• Low cost
• Portable
• Indications may be further examined

• Defect must be open to the surface
• Parts must be cleaned before and after testing
• Surface films, such as coatings, scale, and smeared metal may visually mask defects

 

MAGNETIC PARTICLE INSPECTION (MT/ MPI)

Measures or detects surface and qualified subsurface defects, cracks, seams, porosity, inclusions, and very sensitive for locating small tight cracks.

AdvantagesDisadvantages

• Low cost
• Portable
• Subsurface defects

• Ferromagnetic materials only
• Alignment of magnetic field is critical
• Demagnetization required after the test
• Surface coatings can mask defects
• Pre and post cleaning necessary
• Messy
• Subjective: dependent on operator interpretation
• Difficult to automate

 

INFRARED THERMOGRAPHY

Measures or detects hot spots, heat transfer, temperature ranges, temperature monitoring and electrical assemblies.

AdvantagesDisadvantages

• Permanent record or thermal picture
• Remote sensing
• Portable

• Expensive
• Reference standards required
• Poor resolution on thick sections

 

 

VISUAL TESTING (VT)

Measures or detects surface flaws, blemishes, missing features, and dimensional flaws.

AdvantagesDisadvantages

• Generally fast test time
• Can be automated
• Good at finding dimensional flaws or surface blemishes
• Portable

• Reference standards required
• May require multiple cameras and viewing angles for automated systems
• Subject to operator interpretation if not automated

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