The Advantages and Limitations of Ultrasonic Testing

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Do you know that ultrasonic testing can be used in more industries than health? Yes, its uses are cut across many different industries. In this article, we&#;ll look at its utilization in the metal and construction industries. First, what is ultrasonic testing all about?

Ultrasonic testing is one of the non-destructive testing methods available in industries today. It is a versatile and very useful technique used in testing for discontinuities in materials. Just like in healthcare and underwater navigation, ultrasonic testing is a very useful method of testing and discovering abnormalities. In healthcare, OB/GYN uses it to test for anomalies in a pregnant woman while it is also used in ships to detect objects under the surface of the water.

This form of testing send high frequency sounds also known as ultrasonic waves through a material or object to check for flaws or to characterise the material. Most inspection applications that use ultrasonic testing makes use of short waves with a frequency that ranges from 0.1&#;15 Mhz, even though a high frequency like 50 Mhz can also be used.

What is Ultrasonic Testing?

Ultrasonic testing is a type of testing that involves the use of high-frequency sound waves that travel through a part to detect continuities. If the sound part hits a material with a different acoustical impedance, some of the sounds will reflect back. This reflected sound is caught by the sending unit, and it will be represented as an indication on the screen. The technician will use this information to determine the distance to the indicated material with a change in acoustic impedance since he already knows the velocity at which sound travels through a part, along with the time of travel, will al.

To perform ultrasonic testing on the arts, the technician introduces the sound waves through a transducer made up of a piezoelectric crystal. The work of this crystal is to convert electrical current to sound waves. The two common types of sound waves that are used for this are are compression also called the longitudinal or straight beam and the shear wave is also known as Transverse or Angle Beam.

Why is Ultrasonic Testing Important?

Ultrasonic testing technique is important because it is used in many applications. A good one is the measurement of ultrasonic thickness used to ascertain the thickness of an object example in the assessment of pipework corrosion. Ultrasonic testing is a non-destructive testing technique and this makes it perfect for detecting defects and flaws without causing damage to the material.

When you use ultrasonic testing to periodically check for the growth of flaws or corrosions, you&#;ll be able to prevent future problems. This is because you can see the damage on time and therefore stop the failure of a machine part, a component or the whole object. Ultrasonic testing is used in an extensive variety of industries such as construction, aerospace, medical, automotive, metallurgy, and manufacturing.

How does Ultrasonic Testing work?

Ultrasonic testing works by using a piezoelectric transducer connected to a pulse-receiver and oscilloscope display which is known as a flaw detector. This probe is passed over the test material which is usually attached to the test object by oil, water or gel. The couplant efficiently transmits the sound energy from the transducer into the material.

There are two basic methods of performing ultrasonic testing and they are through-transmission and pulse-echo. Testing by through-transmission uses an emitter to send the ultrasound waves from a part of the material. This method then catches the sound energy that has reached the opposite side of the material using a separate receiver.

Testing with pulse-echo uses the same transducer to emit and receive the sound wave energy. Here, echo signals like the back of the object or an imperfection, are used at an interface to reflect the waves back to the probe. The results from this method are shown as a line plot. On the y-axis is the amplitude which -axis represents the reflection&#;s intensity while the x-axis shows the distance (time) which represents the depth of the signal through the material.

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What is the Basic Principle of Ultrasonic Testing?

The basic principle of ultrasonic testing is very simple, as it uses a process called ultrasonography which is simply the propagation and reflection of sound waves. In this process, sound waves called ultrasonic waves are passed through the material to be tested and then reflected back into a receiver. This receiver then processes the signals to produce a three-dimensional image of the materials so any flaw can be seen.

Advantages of ultrasonic testing

Ultrasonic testing has many advantages in the manufacturing world. Here are some of them.

• It can be used to produce images with automated systems as it is highly portable.
• It has very high sensitivity which helps to detect extremely small flaws.
• It has a sensitivity for both surface and subsurface continuities.
• With ultrasonic testing, only single side access is required when the pulse-echo is used.
• It is not dangerous to people nearby the testing centre and has no effect on equipment and materials in the surrounding.
• It can be used for thick measurements and flaw detection
• Ultrasonic testing depth of penetration is superior to other NDT methods for flaw detection and measurement.
• There is minimal preparation required for ultrasonic testing.
• Ultrasonic testing is very accurate in getting the reflector position and estimating the sizes and shapes of materials.
• It can be used for on the spot assessment as the results are immediate.
• It has some capacity in estimating the structure of alloys of components with different acoustic properties.

Limitations of ultrasonic testing

Even though there are many advantages to ultrasonic testing, it has some limitations. Here are some of the disadvantages associated with ultrasonic testing.

• Small, rough, exceptionally thin, and irregular-shaped materials are difficult to inspect.
• You need to have an accessible surface so that you can use ultrasound.
• The dead zone in ultrasonic testing might make you miss the near-surface in thin steel plates.
• With cast iron and other coarse materials, the low sound transmission and high signal noise of ultrasonic testing make it ti difficult to inspect.
• The training and skill required for it are more extensive than other non-destructive methods.
• The sound beam that is parallel to the linear defects oriented can go undetected with ultrasonic testing.
• There are reference standards needed for both equipment calibration and the characterization of flaws.
• To promote the transfer of sound energy into the test specimen usually require a coupling medium.
• Ultrasonic testing is also more expensive than other methods.

Conclusion

Ultrasonic testing is one of the best non-destructive testing methods available today. It is based on the propagation of sound waves in the material tested which gives it many advantages like portability and showing immediate results.

Ultrasonic testing (UT) comprises a range of non-destructive testing (NDT) techniques that send ultrasonic waves through an object or material. These high frequency sound waves are transmitted into materials to characterise the material or for flaw detecting. Most UT inspection applications use short pulse waves with frequencies ranging from 0.1-15 MHz, although frequencies up to 50 MHz can be used. One common application for this test method is ultrasonic thickness measurement, which is used to ascertain the thickness of an object such as when assessing pipework corrosion.

How does it Work?

Ultrasonic inspection uses a piezoelectrictransducer connected to a flaw detector, which in its most basic form is a pulser-receiver and oscilloscope display. The transducer is passed over the object being inspected, which is typically coupled to the test object by gel, oil or water. This couplant is required to efficiently transmit the sound energy from the transducer into the part, however This couplant is not required when performing tests with non-contact techniques such as electromagnetic acoustic transducer (EMAT) or by laser excitation.

Pulse-echo and Through-transmission

Ultrasonic testing can be performed using two basic methods &#; pulse-echo and through-transmission.

With pulse echo testing, the same transducer emits and receives the sound wave energy. This method uses echo signals at an interface, such as the back of the object or an imperfection, to reflect the waves back to the probe. Results are shown as a line plot, with an amplitude on the y-axis representing the reflection&#;s intensity and distance or time on the x-axis, showing the depth of the signal through the material.

Through-transmission testing uses an emitter to send the ultrasound waves from one surface and a separate receiver to receive the sound energy that has reached the opposite side of the object. Imperfections in the material reduce the amount of sound that is received, allowing the location of flaws to be detected.

Contact and Immersion Testing

Ultrasonic testing can also be split into two main types: contact or immersion testing.

Contact ultrasonic testing is typically used for on-site inspections accessibility or portability. Contact ultrasonic inspection can be performed where only one side of a test specimen as reachable, or where the parts to be tested are large, irregular in shape or difficult to transport.

Immersion ultrasonic testing is a laboratory-based or factory-based non-destructive test that is best suited to curved components, complex geometries and for ultrasonic technique development. In this method, the component or material is submerged in a water, which acts as a couplant in place of the gels used for contact ultrasound. Immersion UT generally uses pulse-echo method, and robotic probe trajectories can be used to inspect complex surfaces which would be hard to cover with contact probes. Immersion UT can be used for a wide range of wall thickness and material types, making it a suitable testing method for a variety of applications and industries.

Why is it Used?

As a non-destructive testing method, ultrasonic testing is ideal for detecting flaws and defects without damaging the object or material being tested. Periodic ultrasonic inspections can also be used to check for corrosion or for growth of known flaws, and thus potentially prevent to a failure of a part, component or entire asset. It is used in a wide range of industries including aerospace, automotive, construction, medical, metallurgy, and manufacturing. 

What Materials Can Be Tested?

Ultrasonic testing is used in a wide range of industries due to its suitability for many different materials. UT is ideally used for inspection of dense, crystalline structures such as metals. Ceramics, plastics, composites and concrete can also be successfully inspected but with reduced resolution, since the attenuation in these materials is higher.

Ultrasonic technology has been successfully employed in the medical sector for many decades, and is increasingly the preferred option for both routine diagnostic imaging and medical research because of the absence of ionising radiation.

Advantages

The advantages of ultrasonic testing include:

• High penetration power, allowing for flaw detection deep within a part
• High sensitivity, allowing for the detection of very small flaws
• Can be used to test when only one side of an object is accessible
• Greater accuracy, when compared to other non-destructive testing methods, for determining depth of internal flaws and the thickness of parts with parallel surfaces
• Able to estimate size, shape, orientation and nature of defects
• Able to estimate alloy structures of components with differing acoustic properties
• Non-hazardous to nearby personnel, equipment or materials
• Highly automated and portable operations possible
• Immediate results can be obtained, allowing for immediate decisions to be made

Limitations

There are, however, a few limitations to ultrasonic testing, as follows:

• Requires experienced technicians for inspection and for data interpretation
• False positive results, also known as spurious signals, may result from tolerable anomalies as well as the component geometry itself
• Objects that are rough, irregularly shaped, very small or thin, or not homogeneous are difficult to inspect
• Loose scale or paint will need to be removed before testing can commence, although clean, properly bonded paint can be left in place
• Couplants required for tests that use conventional UT
• UT may have reduced sensitivity for volumetric flaws, particularly metal inclusions, than radiographic testing

Applications

Ultrasonic testing has a variety of applications across industry, including testing the integrity of a material or component. This can include testing of welds to determine if there are any discontinuities present. This testing can be performed on both ferrous and non-ferrous materials as well as for thicker sections and those that are reachable from one side only. UT is also capable of detecting finer defects and planar flaws which may not be assessed as readily with radiographic testing.

Applications for UT include those within the aerospace, automotive, construction, rail, medical and oil and gas industries.

TWI Services and Courses

TWI provides a number of ultrasonic testing services to our Industrial Members as well as a range of non-destructive testing training courses for those wishing to learn about the techniques involved.

We can provide a full range of testing services and expertise, including in methods such as phased array ultrasonic testing (PAUT), laser ultrasonic testing and manual ultrasonic testing.

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