Non-Destructive Testing Methods: Choosing the Right Quality Test

If every quality concern were visible on the surface, inspection would be straightforward. In reality, many part conditions occur beneath the surface or require specialized testing methods to identify.

Non-destructive testing methods give manufacturers different ways to evaluate component quality without damaging a usable part. Selecting the right method can provide a closer look at the condition being evaluated and help manufacturers spot changes in the production process. .

In Episode 17 of The PM Blend, Atlas is joined again by Brian Butch of The Modal Shop and Jeff Monks of EquipCon to talk through NDT methods and how manufacturers determine which approach best fits the application.

What Non-Destructive Testing Methods Are Used in Manufacturing?

There isn't one NDT method designed to evaluate every type of condition or quality concern. Manufacturers have several options depending on what needs to be evaluated.

Several NDT methods are used across manufacturing, including:

• Resonance testing: Evaluates the structure of a component by measuring changes in its natural vibration frequencies
• Magnetic particle testing: Uses magnetic fields to identify small surface indications in ferromagnetic materials
• Penetrant testing: Uses liquid penetrants to reveal surface defects and can be used with non-ferromagnetic materials
• Eddy current testing: Uses electromagnetic induction to evaluate certain material characteristics and defects
• Ultrasonic testing: Sends sound into a component to identify internal conditions
• X-ray and CT inspection: Provides a view of internal features that cannot be seen from the surface
• Visual inspection: Identifies defects and variations visible on the component's surface

What works for one inspection may not work for another. The method needs to fit the condition being evaluated.

How Do Manufacturers Choose an NDT Method?

The condition being evaluated is often the starting point when selecting an inspection method.

As Jeff Monks points out during the episode, resonance testing evaluates the structural integrity of the entire component. Because it looks at the part as a system, it can be highly effective for identifying structural changes that affect overall part behavior.

However, extremely small surface indications may not create enough structural change to produce a measurable response.

In these cases, magnetic particle testing can provide a better solution because it is designed to identify small surface indications on ferromagnetic materials.

Material can also influence inspection strategy. For example, aluminum and other non-ferromagnetic materials cannot be evaluated using magnetic particle inspection, so penetrant testing may be used when looking for surface indications.

Rather than asking which NDT method is best overall, manufacturers must determine which method is best suited to the inspection objective.

Why Are Multiple NDT Methods Sometimes Used Together? 

Sometimes one inspection method doesn't provide enough information about a component to evaluate part quality.

Atlas uses multiple NDT methods on some parts for this reason. The episode points to powder metal gears as one application where several inspection methods may be used on the same component to evaluate different characteristics of the part.

One method may evaluate the overall structural integrity of the part, while another focuses on specific surface or material conditions requiring a different approach.

Because each NDT method has unique capabilities, combining methods can provide a more complete picture of component quality.

How Does Part Geometry Affect NDT Testing?

Part geometry plays an important role in determining which inspection methods can be used effectively.

Some NDT technologies work by sending a signal into a component and measuring what comes back. That means the signal needs to travel through the part in a way that produces useful information.

Ultrasonic testing is one case discussed during the episode. Traditional ultrasonic inspections frequently rely on parallel surfaces and consistent sound paths. As component geometry becomes more complex, obtaining reliable data can become more challenging.

Magnetic particle inspection presents a different set of considerations. The geometry of a component can change how magnetic fields flow through the part and where indications become visible.

These challenges become particularly noticeable with highly complex parts. Brian points to metal and ceramic additive manufacturing, where 3D printing can produce shapes that are difficult to inspect with methods that rely on getting a signal into and back out of the component.

In those cases, manufacturers may need to consider other non-destructive testing methods that are better suited to the part’s geometry.

Does NDT Require Certified Inspectors?

Inspector qualifications often depend on the NDT method being used and the requirements of the application.

Many established NDT methods have certification levels based on what an inspector is trained and authorized to do:

• Level I: Performs the test according to established instructions
• Level II: Performs the test and interprets the results
• Level III: Oversees areas such as procedures and training

Resonance testing is a little different. It doesn't currently have the same personnel certification requirements, although that could change as the method continues to develop.

Brian helped develop ASTM E3397-23, which establishes a standardized approach for how resonance testing is performed. That standard could eventually help pave the way for formal certification levels.

For now, requirements often come down to the application and what the customer expects. Manufacturers doing NDT in-house still need people who are properly trained to set up and run the equipment.

Why Perform NDT When the Manufacturing Process Is Already Stable? 

Even highly capable manufacturing processes benefit from effective inspection strategies.

Jeff describes how NDT was historically viewed as a requirement driven by customers, specifications, or industry standards. Today, many manufacturers view inspection as a valuable process-monitoring tool as well.

That approach has changed as manufacturers have found ways to use NDT earlier in production.

Atlas considers inspection during new-part reviews rather than waiting for a customer to request it. When the team sees an area where additional inspection could help protect the customer, NDT can become another step in the manufacturing process.

The value extends beyond identifying nonconforming parts. Inspection data can also reveal changes within the manufacturing process that may otherwise go unnoticed.

Turning NDT Data Into Better Manufacturing Decisions

Inspection results can help manufacturers better understand process performance beyond a final pass/fail decision.

NDT data can reveal smaller process trends as well.

Andy describes seeing changes in inspection data after an OD grinding operation. Those results can help evaluate earlier production steps and make adjustments before parts reach final inspection.

How Does Production Volume Affect NDT Method Selection? 

Production volume can influence the inspection setup, but it doesn't determine which method is appropriate.

The inspection requirement comes first. When a particular method is needed to find the condition in question, manufacturers have to determine how to incorporate that test into production.

Volume becomes more influential when several methods can accomplish the same inspection goal. A faster method may make more sense when thousands of parts need to move through the same process.

Automation can help bridge the gap. Many inspection technologies that were once highly manual can now be incorporated into a production line, which reduces inspection time and maintains consistency

Resonance testing is one example where that speed can be valuable. As discussed in Episode 16, an automated system can evaluate parts quickly without introducing liquids or other consumable materials into the inspection process.

Non-Destructive Testing Methods Support Better Manufacturing Decisions

The most effective non-destructive testing methods depend on the part, the material, the geometry, and the specific condition being evaluated. When matched correctly to the application, NDT helps manufacturers verify quality without damaging the component.

Episode 17 of The PM Blend also shows how that information can feed back into manufacturing. Inspection data can help identify changes in the process and give manufacturers a clearer picture of where adjustments may be needed.

Talk With Atlas About Quality Testing for Your Part

Every component presents its own inspection considerations. Talk with the Atlas team about your application and how quality testing can fit into your powder metal manufacturing process.