Metal Fatigue Detection: Using Advanced NDT to Identify Failing Metals

Metal fatigue can cause a host of complications, especially in the aerospace industry. Metal fatigue can take many forms, but infinitesimal cracking is one of the most dangerous types of fatigue in the aerospace world, as these cracks often go unnoticed and slowly degrade the structure or component over time. These dangers also apply to other assets outside of the aerospace industry, such as mining equipment, ships, bridges, or cranes.  

Nondestructive testing (NDT) methods such as ultrasonic testing, eddy current testing, and magnetic particle testing can detect many stress points, most notably small cracks. They can detect early stress points that may inhibit the function of working machinery, or cause safety hazards that endanger the lives of staff members or everyday people. This article will show the best NDT methods for metal fatigue detection based on weld types and geometric patterns.

NDT Options for Detecting Metal Fatigue

NDT methods can detect signs of metal fatigue without the need to break down a tested item. It is most advantageous on parts or assets that undergo heavy stress repeatedly or for long periods at a time. Moreover, NDT can test assets as they are in operation, allowing operators to commence operations without interruption. With that, NDT only works best via high-quality equipment made by a trustworthy NDT provider that invests in technology.

Take note of the following NDT options for metal fatigue testing:

Eddy Current (ECT): This NDT method can detect cracks using alternating currents that can detect surface and near-surface indications on conductive materials. ECT uses coils and electromagnetic fields, where any disruption within the current flow gives away the presence of fatigue stress points.  However, it cannot detect defects on a deeper level, nor can it read defects that run parallel to the surface.

Ultrasonic (UT): UT is designed for inspections on a volumetric level. It works via sound waves that travel through a tested item and pick up flaws in the process. However, UT is not as proficient when applied to thinner materials.

Magnetic Particle Testing (MPT): MPT detects flaws on a surface and near-surface level using a magnetic field from a magnetized surface. MPT calls for ferromagnetic particles that are applied to the surface, drawing out any surface abnormalities. However, MPT tends to produce inferior data when dealing with painted or other non-magnetic surfaces.

Liquid Penetrant Testing (LPT): Otherwise known as dye penetrant inspection, LPT uses a penetrant to find surface-breaking defects in all non-porous items, including metal. With LPT, liquid is pulled through openings within the surface via capillary activity, exposing all deviations under UV light. However, LPT only applies to surface defects and requires chemicals that can be hazardous to human health, especially when used in confined spaces.

Regardless of the technique, NDT is a superior option when compared to destructive testing, which requires a breakdown of the tested materials, degrading the integrity of the asset while wasting resources in the process. Moreover, destructive testing can exacerbate metallic fatigue, rendering an asset or welding materials useless. Even though NDT is the ideal method, each technique has its strengths and weaknesses.

Choosing the Right NDT Approach for Metal Fatigue Testing

The proper NDT strategy for metal fatigue testing will depend on whether the testing will occur on a surface or subsurface level.

Surface Detection Strategies

If testing strictly on a surface level, for instance,  LPT, MPT, and ECT are all viable testing methods. For the best results, however, ECT tends to be the best choice. ECT provides better surface-level data than LPT and MPT due to its advanced capabilities. It can provide a digital record of the inspection, and because it does not require the use of any chemicals during inspections, ECT also allows for a safer testing regimen.

Eddy current array (ECA) capabilities further enhances inspection prowess. The array capabilities allow analysts to inspect fatigued areas with greater precision, especially on complex geometric patterns like weld toes or flush rivets. ECA probes can inspect assets more quickly and accurately than most other NDT options, with a single pass often being sufficient to collect the necessary data.

Look for portable and handheld ECT or ECA equipment that is light enough to carry throughout the field with ease to lessen the physical strain of inspections while increasing testing efficiency.

ECT offers clear advantages for surface-level testing for metal fatigue. However, it is not necessarily the best option if analysts must determine faults on a deeper level. For this, technology with greater depth penetration is needed instead.

Subsurface Flaw Detection Strategies

When it comes to subsurface inspections, UT is among the most reliable of NDT methods available. Similar to ECT, cutting-edge UT offers greater accuracy and convenience compared to most other NDT technologies, with many manufacturers offering portable, lightweight equipment that can be easily carried from one testing location to the next.

However, standard UT, for all its advantages, is somewhat restricted by the necessity of fixed beam positions. Phased array ultrasonic testing, or PAUT, overcomes this by offering multiple angles and beam customizations during testing that allow inspectors to thoroughly inspect assets simply and efficiently.

More importantly, PAUT can conform to different material types, such as composite materials or austenitic welds. For austenitic welds, standard PAUT can scan welds that have high grain levels that can mask certain fatigued spots. PAUT can combat propagation issues using a 2D matrix array, which relies on low frequencies to find weak points within assets or welds. In particular, PAUT can enhance the inspection of circumferential and axial flaws within austenitic welds.

While LPT and MPT can also be used for subsurface detection, they cannot provide the same level of customization, convenience, or inspection confidence that top-of-the-line UT equipment can offer.

Innovative NDT Solutions for Metal Fatigue Detection

ECT is an ideal method for surface and near-surface metal fatigue detection, while UT is the best fit for deeper, volumetric inspections. Both of these methods provide customized features and additional options that foster quicker inspections and better results. Advanced ECT and UT technologies can help technicians locate signs of fatigue that could lead to asset breakdowns or engine failures long before such catastrophes become imminent. NDT is a primary safeguard against preventable disasters—as such, it is critical to work with a trusted NDT provider who can offer top-of-the-line equipment to ensure quality data and analysis.

At Zetec, we’re proud to provide industry leaders around the world with state-of-the-art NDT solutions. Our PAUT and ECA equipment and software options, in particular, are ideally suited to metal fatigue detection. Contact us today to learn which of our solutions is right for you.