20 September 2025

Non-Destructive Testing (NDT) is a critical set of inspection techniques used across countless industries to evaluate materials, components, and systems without causing any damage. Its core purpose is to identify flaws, discontinuities, or material characteristics that could compromise performance, lead to fracture, or fail to meet quality standards. NDT involves sophisticated technologies that examine beyond visible surfaces, often delving into a material's internal structure. In this article, we'll delve into the indispensable role NDT plays in ensuring safety and quality, explore its underlying mechanics, introduce key NDT technologies, and illustrate its impact with real-world examples, including a major engineering failure where NDT could have made a critical difference.

Figure 1:  A diver performs ultrasonic inspections underwater.  Subsea structures need to be checked periodically to monitor for flaws and/or flaw growth to determine if they are still safe for service.  Inspections could be of wrought material or even welded joints.  Photo credit: https://www.asnt.org/standards-publications/blog/applications-for-underwater-ultrasonic-testing.

The Indispensable Role of NDT

The significance of NDT is profound, particularly in sectors where precision, safety, and unwavering reliability are paramount. To understand NDT's core, consider something familiar: checking your bicycle tires. You don't cut the tire open to see if there's enough air; instead, you perform non-destructive assessments. You look for cuts or bulges (visual inspection), squeeze to feel for firmness (a tactile NDT), or listen for leaks while pumping. If it's firm, it's "fit for service." If flat, you've found a "flaw" without ruining the tire. This simple act embodies NDT's fundamental principle: evaluating integrity without damage.

Industries like aerospace, automotive, manufacturing, and critical infrastructure apply this principle on a grand scale. Billions are invested in design and production, demanding that hardware meets exact specifications, even for unseen features. For instance, tiny tungsten inclusions in a weld might be harmless in a simple bracket, but in a high-pressure vessel, they could lead to catastrophic failure. NDT acts as a crucial safeguard, preventing such disasters during initial use and enabling periodic monitoring of flaws under dynamic stress throughout a component's life.  See Table 1 for more examples of what can be found with NDT.

Beyond disaster prevention, NDT offers substantial advantages:

1. Ensures Safety & Reliability: Confirms components are fit for service, verifying flaws are below critical sizes, allowing safe usage.

2. Cost Savings: Identifies defects early in manufacturing, drastically reducing rework, scrap, and warranty claims.

3. Product Quality: Screens for imperfections, ensures uniformity, and maintains mininum standards.

4. Regulatory Compliance: Many industries mandate specific NDT procedures to meet stringent regulations and international standards.

Table 1: Example flaws that can be found by NDT.

The Mechanics of NDT

The principle of NDT is simple: detect flaws without altering the material. Common flaws include cracks, voids, corrosion, delaminations, and inclusions, found on surfaces, subsurface, or deep within. 

The typical NDT process involves methodical steps:

• Preparation and Set Up: Preparing the object for inspection, such as smoothing a surface for ultrasonic testing or cleaning and etching a part for penetrant inspection.

• Method Application: Directing energy (sound waves, X-rays, visible light, UV light, etc.) onto, into, or through the material. The energy's interaction with the material are recorded by a receiver and any internal and detectable discontinuities reveal indications.

• Data Interpretation: Trained technicians analyze distinct signals or patterns in the returned energy to characterize the nature, size, and location of indications.

• Reporting & Documentation: All findings are meticulously documented, reviewed by engineers, and assessed against pre-defined acceptance criteria to determine a part's fitness for service. Ambiguous cases undergo further expert review.

Note the vocabulary used above is quite particular.  NDT can detect flaws, but not all indications are considered flaws in the engineering practice.  Some indications may be acceptable depending on what the nature of it, especially in the manufacturing environment where a delicate balance between business and engineering interest is at play.  For example, while a rounded indication may be acceptable in a specific use case, an indication with a sharp tail may not be.  

While the steps read to be simple, each step is rooted by engineers, inspectors, and technicians with extensive experience and training, lending to the reliability of the practice of NDT. NDT Level 3s are known to be subject matter experts in their field of NDT and are the best kind of people to have around when facing tricky interpretation presented by NDT results or where guidance is needed in NDT selection which is covered in the next section.

NDT Modalities

NDT encompasses a variety of techniques, each suited for different types of flaws and materials.  Let's review common techniques that are used across the engineering practice based on typical flaws to target.

Surface Flaws
Perhaps the simplest is Visual Testing (VT), which primarily relies on the human eye, often enhanced with controlled lighting, magnifiers, or borescopes, to detect visible surface flaws. This method is particularly effective for visible surface flaws, with systematic scanning and lens corrections ensuring thoroughness.

For detecting surface-breaking flaws, Liquid Penetrant Testing (PT) is a widely used method. It leverages capillary action: a liquid penetrant (often a dye, either visible or fluorescent under UV light) is applied to the surface. Capillary forces draw this liquid into any surface-breaking discontinuities. After a dwell time, excess penetrant is removed from the surface, and a developer is applied. The developer draws the penetrant out of the flaws, creating a visible indication that can be characterized for size and shape. The sensitivity of PT can be tailored by selecting different types of penetrants.

A closely related technique is Magnetic Particle Testing (MT), which is used specifically for ferromagnetic materials. This method involves magnetizing the part, either with a portable electromagnet or a stationary unit. If a surface or slightly subsurface flaw is present, it will disrupt the magnetic field, creating a magnetic flux leakage. Fine ferromagnetic particles, typically suspended in a liquid or applied as a dry powder, are then applied to the surface. These particles are attracted to the areas of flux leakage and accumulate there, forming a visible indication of the flaw. MT is highly effective for detecting cracks, seams, and laps.

Surface and Subsurface Flaws
Eddy Current Testing (ET) is an electromagnetic NDT method primarily used on conductive materials. This technique introduces an alternating electromagnetic field into the test object, which in turn induces eddy currents within the material. Any disruptions to these eddy currents—caused by flaws, changes in material properties, or variations in thickness—create a counter-field that interferes with the source field. This disturbance is then detected and analyzed. ET is highly effective for detecting surface flaws and has some limited penetration into the material, making it valuable for inspecting metals but generally unsuitable for non-conductive materials like composites.

Volumetric Internal Flaws
Moving beyond the surface, Ultrasonic Testing (UT) utilizes high-frequency sound waves. A transducer sends these waves into the material, and reflections from discontinuities or the back surface are detected. A signal returning sooner than expected indicates a flaw, making UT highly effective for detecting volumetric defects. It's important to note that near-surface resolution can sometimes limit the detectability of flaws close to the entry point of the sound waves. Common UT approaches include Pulse-Echo, where the same transducer both sends and receives the waves; Through-Transmission, where waves are sent from one side and detected on the other; and Phased Array, which uses multiple transducers to electronically steer and shape the sound beam for more complex and precise scanning. UT finds widespread application in weld inspection, crack detection, thickness measurement, and material characterization across industries like aerospace and power generation.

Another powerful technique for internal inspection is Radiographic Testing (RT), which employs X-rays or gamma rays. These forms of radiation pass through the material, and their attenuation—which varies based on material density and the presence of flaws—is captured on a detector, such as film or a digital sensor. This creates an image revealing internal discontinuities, making RT excellent for volumetric flaws. RT methods include traditional Film Radiography, Computed Radiography (CR) (using phosphor-coated imaging plates), and Digital Radiography (DR) (offering immediate digital images). Due to the use of ionizing radiation, RT necessitates strict safety protocols, including controlled access areas and lead shielding, to prevent exposure, which can sometimes impact other co-located operations.

NDT Selection
The selection of an NDT modality depends on many factors including the type of flaw being sought, the materials and geometry of the part, and the environmental conditions of the inspection. For instance, while both UT and RT can detect internal voids, an RT inspection of a pressurized pipe in an active plant may be impractical due to the safety protocols required for X-rays. In this case, UT would be the preferred choice. However, RT might be selected for its ability to reveal flaw shapes in complex geometries, even if it requires more effort.   Check out Figure 3 for an interactive overview of these methods to learn more.

As with any kind of engineering inspection requirement, a call out of NDT must be accompanied with acceptance criteria.  NASA-STD-5009, NASA-STD-5019, MIL-STD-1907, and ASTM Standards are great tools I use in my practice and for others to familiarize themselves with the landscape of NDT.  These references are also useful vehicles to call NDT when specifying requirements on engineering drawings and other contractual documentation.

When Skipping NDT Leads to Catastrophe: The Aloha Airlines Flight 243 Incident

The critical importance of NDT is highlighted by major engineering failures. The Aloha Airlines Flight 243 incident on April 28, 1988, is a famous, chilling example.

During a routine flight, a Boeing 737's fuselage roof tore off at 24,000 feet due to widespread fatigue damage and corrosion that went undetected. Despite the terrifying decompression, pilots miraculously landed the plane, though one flight attendant was tragically lost.

The investigation revealed that the older aircraft, with many flight cycles, had developed microscopic cracks at rivet joints, which propagated and linked up. Standard visual inspections were insufficient to detect these subtle, pervasive fatigue cracks hidden beneath paint and within the complex structure. NDT methods, like ET for surface cracks or thorough UT in critical joint areas, would likely have revealed the damage. The failure to apply effective NDT allowed the damage to escalate, leading to catastrophic structural failure.

This disaster significantly reshaped aviation maintenance, emphasizing enhanced NDT procedures and "aging aircraft" programs globally to prevent similar fatigue-related failures. It's a powerful reminder of NDT's vital role in ensuring safety in highly stressed engineering applications.

Figure 4: Aloha Airlines Flight 243 made headlines due to a structural failure of its fuselage during flight.  Poor maintenance programs of the company's fleet was determined to be a contributing factor to the catastrophe, with flaws being left undetected.  Photo credit: Honolulu Star-Advertiser.

Final Thoughts

This excursion into the world of Non-Destructive Testing has truly underscored the universality of sound quality and safety principles. Despite the apparent differences between inspecting critical aerospace components and, say, the structural integrity of a bridge, the fundamental tenets of identifying flaws and ensuring reliability without causing damage remain paramount. The power of NDT to literally "look inside" materials, from microscopic cracks to large internal voids, serves as a compelling example of how robust control over material integrity can drive success and, most importantly, prevent tragic failures across diverse industries. It was a privilege to delve into these practices and to appreciate the intricate dance between sophisticated technology and the human expertise dedicated to excellence.

For those curious about the unseen world of material safety and quality, understanding NDT truly warrants consideration. This overview, coupled with the insights into both common and catastrophic applications, provides a holistic understanding of NDT's operation—it's got something good going! If you ever think about the safety of the products and systems around you, remember the silent guardians of NDT—you won't be disappointed by the peace of mind they provide.

Thx, m