Common Types of Nondestructive Testing

Non-destructive testing (NDT) is a type of quality testing used to analyze material properties and/or structural integrity of a material without causing any damage to the material that is being tested.  This form of material testing is used in many various manufacturing and design fields including oil and gas, automotive, aerospace, and many more! They are cost-effective ways to spot defects in materials before the full manufacturing process begins, save time, money, and possibly even lives.  Having said all that, there are several different common types of nondestructive testing.

Common Types of Nondestructive Testing

Visual Testing

The most simple nondestructive testing used is a visual inspection or known as visual testing. Visual Testing involves the collection of visual information on a material’s status. This form of testing is the most basic, as you can complete it by simply looking at an asset. For more in-depth visual inspections, you can use a Remote Visual Inspection device to get a more precise look at materials.

Magnetic Particle Inspection

Magnetic particle inspection is a non-destructive inspection method that provides the detection of linear flaws located at or near the surface of ferromagnetic materials. These include cracks, pores, cold lap, lack of sidewall fusion in welds, etc. Magnetic particle inspection is suitable for all metals which can be strongly magnetized such as ferritic steels and irons.

Magnetic Particle Inspection magnetizes the material being tested, either locally or overall. If the material is sound, the magnetic flux is predominantly inside the material. If there is a surface-breaking flaw, the magnetic field is distorted, causing local magnetic flux leakage around the flaw.

Magnetization may be produced by any of these methods:

• Applying a permanent or electromagnet to the surface (magnetic flow)
• Passing a large electric current through the specimen, or locally by means of current prods (current flow)
• Putting the specimen inside a current-carrying coil, or forming a coil around the specimen
• Making the specimen the secondary loop of a transformer – (induced current) – suitable for ring-shaped specimens
• Placing a current-carrying coil or loop close to the specimen surface
• Threading a current-carrying bar through a hollow specimen.

Iron particles – dry powder or magnetic inks (particles suspended in a liquid) – are applied to the specimen while the magnetizing current is still flowing through it. Magnetic inks are used more widely than dry powders.  The ink is applied using a low-pressure spray, dipping, or brushing. You should use plenty of ink for the test to be effective, and give it enough time so the particles can flow over the surface and migrate to any cracks or pores.

Some metallurgists use fluorescent particles, which require UV-A illumination. The benefit of this method is that indications of cracks can be preserved by photography or by the use of peel-off transparent adhesive film.

Liquid Penetrant Testing

Liquid penetrant testing was one of the first types of non-destructive testing ever used and is still widely used today. Liquid penetrant testing can detect surface-breaking defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks. The objective of the test is fairly simple, change invisible defects to visible defects by using liquid dye, having the dye bleed out from the flaw. It produces a flaw indication larger than the flaw itself, allowing the inspector to detect flaws invisible to the naked eye.

The surface must be free of contaminants such as oil and grease. These contaminants could stop the penetrant from getting into the defects. Once the sample is cleaned and free of contaminants, the liquid penetrant material then is applied, which can be either by spraying, brushing, or immersing the sample.

Give the penetrant 5 – 60 minutes to seep into the defect after applying it to the specimens. The total time the penetrant is on the sample is called penetrant dwell time. The penetrant dwell time can vary depending on several factors such as the type of specimen, the brand of the penetrant being used, and the form of the material being inspected.

After sufficient penetrant dwell time has passed, the excess penetrant is removed using a rinse, solvent, or emulsifier. Penetrant removal is a delicate process. The objective is to remove the excess penetrant while keeping enough of the penetrant on the defect.

Next, a developer is applied to draw out the penetrant trapped in the flaws. There are both wet and dry developers with various forms of application processes. Let the developer stay on the sample surface for a minimum of 10 minutes to fully draw out the trapped penetrant. Then the sample is ready for inspection using appropriate lighting.  The drawn out penetrant makes it easy to review for and identify defects.

Ultrasonic Testing

Ultrasonic testing is a particularly useful and versatile NDT method. Ultrasonic Testing uses high-frequency sound energy to examine specimens and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more. Unlike other surface tests that are limited to a specimen’s surface, ultrasonic testing finds flaws inside the material. Ultrasonic testing can be used on metals, plastics, composites, and ceramics. Wood and paper specimens are not suitable for ultrasonic testing.

In ultrasonic testing, directional sound waves travel through a material until they hit a boundary of a different material or medium. An example would be sound traveling through a metal specimen until it hits air. Ultrasonic testing uses an ultrasound transducer connected to a diagnostic machine. The inspector passes the transducer over the object being inspected to make measurements.

There are two methods of receiving the ultrasound waveform: reflection and attenuation. In reflection (or pulse-echo) mode, the transducer performs both the sending and the receiving of the pulsed waves as the “sound” is reflected back to the device. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and it travels through the medium. Then a separate receiver detects the amount of sound using another surface.

Eddy Current Testing

Eddy current testing is an electromagnetic testing technique usually used to inspect non-ferromagnetic materials. An excitation coil carrying current is placed in proximity to the component to be inspected. In simpler terms,  a copper wire is excited with an alternating electrical current. The coil generates a changing magnetic field using an alternating current, which interacts with the component generating eddy currents, also known as electrical currents. The Eddy Current testing method is used for a variety of purposes, including flaw detection, material and coating thickness measurements, material identification, and establishing the heat treatment condition of certain materials.

Conclusion

A typical facility will use more than one form on NDT testing for quality control purposes. With NDT testing, companies can preserve the sample. This helps them not only lower their costs but also provide a higher quality product to their customers.

Metallurgical Supply Company (METSUCO) has been a leading supplier of NDT testing supplies for over 30 years. We serve not only industries such as oil and gas, aerospace, automotive, and heavy manufacturing, but also universities and quality testing labs. Call 713-827-0700 or email sales@metsuco.com with inquiries and quote requests.