Non-Destructive Testing

The Non-destructive testing involves the examination of an object in any manner that will not impair its usefulness. Using six major methods of radiography, ultrasonics, eddy current, magnetic particle, liquid penetrant and visual testing performs this non-destructive testing.

These testing methods are used to test components in a variety of industries including aerospace, petro-chemical, automotive, metals, non-metals, nuclear, marine, electronics, buildings and aircraft.  Students learn to perform the basic testing techniques on both metals and non-metals such as composites as well as how to evaluate the results.

Non-destructive testing includes Ultrasonic (contact, immersion), radiography (x-ray, Gamma Ray, Film & Real-Time, Micro Focus), Liquid Penetrant (Wet/Dry, Fluorescent, Visible), Eddy Current (flaw detection, material sorting), Positive Material Identification (PMI), Tensile, Fatigue, Compression, Impact, Harness, (Brinell, Rockwell) Flexural & peel, and tear & drop weight.

Non-destructive testing equipment and services also use to inspect metal tube, pipe, bar, wire and other parts. MAC test systems can be seen in plants throughout the world.

We do this Non-destructive testing to test components in a variety of industries. Eddy Current, Ultrasonic and Flux Leakage tests are done to inspect bar, rod, tubing, wire and metal parts on- or off-line. These tests are detects surface, subsurface, ID, OD and internal defects. Eddy current Comparators detect variations in alloy, heat treatment, hardness, structure and other physical and metallurgical conditions. It is designed for production-line use.

Non-destructive testing experts based throughout the US, Mexico, Europe, China, and Australia, with representatives in India, Korea and South America.

Ultrasonic (high frequency) waves are emitted from a transducer into an object and the returning waves are analyzed. If an impurity or a crack is present, the sound will bounce off of them and be seen in the returned signal. In order to create ultrasonic waves, the transducer contains a thin disk made of a crystalline material with piezoelectric properties, such as quartz. When electricity is applied to piezoelectric materials, they begin to vibrate, using the electrical energy to create movement. Since waves travel in every direction from the source, an absorptive material is layered behind the crystal to keep the waves from going backwards into the transducer and interfering with its reception of returning waves. There are two types of Ultrasonic testing:

1- Contact

2- Immersion

One type of ultrasonic testing places the transducer in contact with the test object. If the transducer is placed flat on a surface to locate defects, the waves will go straight into the material, bounce off a flat back wall and return straight to the transducer. The animation on the right, developed by NDTA, Wellington, New Zealand, illustrates that sound waves propagate into an object being tested and reflected waves return from discontinuities along the sonic path. The material will absorb some of the energy, but some of it will return to the transducer. When the mechanical sound energy comes back to the transducer, it is converted into electrical energy. Just as the piezoelectric crystal converted electrical energy into sound energy, it can also do the reverse. The mechanical vibrations in the material couple to the piezoelectric crystal which in turn, generates electrical current.

Ultrasonic can be used to characterize flaws, cracks, delaminations, voids and inclusions. We can inspect welds for voids, cracks, porosity, missed seams or lack of penetration. Epoxy curing or ceramic sintering can be monitored. Cracks, inclusions and voids can be detected in various materials. We can measure the thickness of metal, ceramic, or polymer parts. This is use to find, hidden defects, voids, cracks and inclusions.

Radiography is one of the most versatile non-destructive testing methods. Radiography can determine the internal soundness of a material (for example cracks, inclusions, voids) without destroying it. Radiography records the amount of radiation that penetrates a sample. The magnitude (intensity) of the radiation that penetrates the sample indicates the attenuation of the radiation. Areas of differential attenuation can be identified in the images generated in either film or real-time radiography. These variations are caused by differences in density, material thickness, and material composition. Since flaws, such as voids, inclusions, cracks, etc., constitute density variations, they can be identified in the image.

There are five types of radiographic testing, those are:

1- X-Ray Testing

2- Gamma Ray Testing

3- Film X-Ray Testing

4- Real-Time X-ray Testing

5- Micro Focus

Radiographic test is a test method utilizing X-rays or gamma radiation to examine the interior of a component or a weld. This technique is applied to the inspection and testing of systems or welded structures where a high degree of safety and reliability is called for.

Radiography produces an image, a radiograph, using x-rays and Gamma rays. X-rays and Gamma rays penetrate solid objects and provide a detailed view of the internal structure. The image produced is a shadow image since the rays pass through some parts of the solid object easily and some parts less easily. Differences in density, material thickness, attenuation characteristics, and material composition result in the shadow image. Flaws, voids, inclusions, cracks, etc., are variations that can be identified. Constellation has the two methods of radiographic evaluation, film and real-time.

Film radiography produces a still shadow-image of the test sample representing time-integrated radiation penetration. Real-time radiography produces a live shadow-image on a video monitor. When is film radiography advantageous over real-time radiography? Film Radiography is advantageous for very large items and very thick items (because more powerful x-ray generators can be used).

Real-time radiography is especially useful for a quick product inspection when there are many parts that have to be inspected. Evaluation of a component can take just a few minutes. Real-time radiography produces live shadow-images so video records can be made of moving parts. In real-time, components within the object under inspection can be moved, and the result can be viewed on the system’s video monitor. Such manipulations can point out problem areas in design, component flaws, and assembly errors.

Shadow-image records of components can be viewed in real-time. Parts can be rotated or moved to optimal orientation to see internal features, flaws, or component assembly. Real-time radiography can be used to record movement or events.

In real-time radiography, the x-ray radiation passes through the object under inspection and then interacts with a fluorescent screen. The fluorescent screen, coupled to a device called an image intensifier, allows recording of the image in video or regular print film. Micro focusing of the x-ray beam allows particular areas of interest to be enlarged with an image intensifier field-of-view selection, geometric enlargement, or through the zoom lens capability of the CCD imaging camera.

Eddy current testing done for flaw detection and material sorting. The eddy current method is based on inducing electrical current (eddy currents) in electrically conductive material. Any change in the material (such as cracking, corrosion, pitting, thinning, or other discontinuities) disrupts the flow of the eddy currents. The highly sensitive MIZ-21B and MIZ-21SR reliably detects these disruptions and displays the resulting signal. Higher frequencies (up to 8MHz) are used to detect surface flaws. Lower frequencies (down to 50Hz) are used when deeper, subsurface penetration is required.

Magnetic Particle Testing is also a non-destructive testing. It is an economical and expedient test method to determine if cracks or other surface breaking discontinuity are present. This method however, can only be applied to materials that can be magnetized.

Similar to magnetic particle testing, this Dye method utilizes colored or fluorescent dyes to detect surface breaking defects such as cracks. The process of wetting the surface of the item under test with a penetrating material (fluorescent or contrast), then removing this material and applying a developing agent that draws the penetrant trapped in surface depressions or void to the surface, where it may be viewed by the inspector.

Accomplishing dye penetration inspections are wide and varied, depending on the material, type of defects suspected, surface conditions, as well as the applicable specifications. All these points must be taken into account when determining penetrant materials, dwell times, wash methods, drying time and temperature, development materials and times.

In products such as a casting, cracks are made visible with contrast dye penetrant procedures. Not easily seen by the eye, the contrast dye (and developer) makes the piece and its anomaly visible enough to be photographed under harsh light.

Visual Testing is included in Non-destructive testing. It is an economical and expedient test method to verify that a component or structure has been fabricated or assembled in accordance with a written set of instructions.

There are many such tests exists, those are not strictly a Non-destructive testing. Those tests are: Tensile, Fatigue, Compression, Impact, Harness, (Brinell, Rockwell) Flexural & peel, and tear & drop weight and Hardness testing. Although these tests are not strictly a non-destructive method, this simple yet effective method is helpful in determining the mechanical properties of a material.

Author: Anuradha Panda