What's the Advantage and Limitation of Ultrasonic Testing?

Ultrasonic testing, or UT as it is often called, has many uses in the world of manufacturing and quality control. It involves the use of ultrasound to detect defects in products such as metal components and other structures, even when they are hidden under paint or another surface layer.

If you are looking for more details, kindly visit our website.

Ultrasonic testing can be used to find ut flaws that may have been overlooked with more traditional quality control techniques, improving product safety and reliability while reducing costs over time.

However, there are several limitations to ultrasonic testing that should be kept in mind when considering this technology as part of your QA process. Continue reading below to learn more about the advantages and limitations of ultrasonic testing.

 

 

I. What is Ultrasonic Testing? 

 

 

 

Ultrasonic testing, also known as UT, or ultrasonic examination, uses high-frequency sound waves to inspect an object’s surface and detect flaws.

UT was originally developed in the 1940s to determine the thickness of welded seams in aircraft carriers and battleships, but since then, it has evolved into an indispensable tool that quality control technicians use every day to ensure consumer safety and monitor manufacturing quality.

It works by emitting high-frequency sounds and detecting how they bounce off the subject under inspection—subtly different sounds will mean there are surface defects on the object being tested.

The testing involves placing a transducer over a material and transmitting sound waves into it. When these waves hit an imperfection in a structure, they will reflect back to their source at an altered frequency; that’s how you can measure flaws in real-time with no contact.

An expert ultrasonics technician may be able to pinpoint problems that traditional methods miss—that’s why ultrasonic testing is so commonly used for detecting cracks in structures like aircraft wings or bridges.

The test results in images are called sonograms. Like any type of nondestructive testing (NDT), ultrasonics is a valuable technique because it allows engineers to evaluate a product without causing physical damage.

 

➤ Related Article: Everything you Want to Know About Ultrasonic Testing

 

 

II. Why is Ultrasonic Testing Important?

 

Ultrasonic testing (UT) is an important part of many manufacturing processes. It is used to detect defects or check for flaws in an object’s construction.

Ultrasonic testing gives manufacturers better insight into how their products age over time; if a manufacturer can find out why some products fail faster than others after shipping—whether it’s environmental factors or incorrect manufacturing processes—they can reduce those risks for future shipments.

This can help a manufacturer identify issues that may cause problems in later stages and be addressed before they affect quality or functionality.

 

 

III. How Ultrasonic Testing Works?

 

The most common type of ultrasonic testing works by producing a sound that’s above what humans can hear. It then analyzes how long it takes for that sound to bounce back after colliding with something like a surface or object.

The time taken for reflection, along with properties like amplitude, is used to determine information about whatever is being tested. These test results can be used to evaluate things like weld quality on car doors or weld integrity on bridges.

They also allow technicians to detect problems early before they become major issues. Small variations in sound waves could indicate cracks or other structural defects. In some cases, when combined with other testing methods, it allows engineers to test components during operation without stopping any processes or interrupting anything happening at all.

 

 

IV. What is the Basic Principle of Ultrasonic Testing?

 

Often referred to as ultrasonic inspection, ultrasonic testing uses sound waves to examine a material’s internal and external structure. The basic principle underlying ultrasonic testing is that materials have different acoustic properties.

A pulse of high-frequency sound is produced in an emitter or piezoelectric transducer, transmitted into a receiving transducer and recorded by a receiver or strain gauge. Sound travels at approximately 3.1 miles per second in the air (or 5 miles per second through water). The speed of sound varies depending on temperature, however.

 

V. What are Advantages and Disadvantages of Ultrasonic Testing?

 

Here are the advantages and limitations of ultrasonic testing are so that you can make an informed decision about using this technology during your production line inspections.

 

Advantages Of Ultrasonic Testing

• Portability

The equipment can be portable. It is also one of several methods that can be used to detect flaws in metal, plastic, wood, concrete, etc.

• It’s reasonably simple to use

The most common types of ultrasonic testing equipment are sound source generators and electronic receivers.

Both must have a specific frequency for optimal results—typically 40 kilohertz for ferrous materials (metal), 50 kilohertz for nonferrous material (plastic or wood), and 20 kilohertz for concrete structures or structures made from composite metals.

A typical test may last only two or three minutes, but tests with more sensitive equipment may take longer.

If you want to learn more, please visit our website AJR.

• Little to no damage

An ultrasonic inspection does not damage items being tested because it uses sound waves at frequencies too high to cause pain when humans hear them.

• Lower energy consumption

Energy consumption is low compared with other physical non-destructive testing processes such as magnetic particle inspection and radiography, which use ionizing radiation to evaluate objects (although they do not involve sound waves).

Moreover, ultrasound is unaffected by magnetism and radioactivity, so it can be used on these types of objects without damaging them.

• Safe for testers

The devices that create and receive ultrasonic signals are designed to protect against possible harm caused by exposure to loud noises. This can help prevent temporary hearing loss caused by continuous loud sounds or permanent hearing loss caused by repeated exposure over time.

 

Disadvantages Of Ultrasonic Testing

• Cost

The testing equipment is expensive. Unless you can afford to own a device, you have to pay per test. Per-test fees can run $1,000 or more if you test often or with small parts. You might need special adapters for some parts and surface finishes. Test points tend to be concentrated on larger, more complex parts.

• Convenience

It’s hard to get a lot of test points when ultrasonic testing involves heating samples in an oven, so it takes time to go back and forth between tests on different surfaces, especially with small samples that heat up quickly.

• Reliability

This type of testing uses visual methods rather than instruments to see flaws, making them subject to operator interpretation errors.

• Repeatability

Visual methods are less accurate than destructive analytical techniques like magnetic particle inspection (MPI). They lack sensitivity, too: even top technicians find it hard to diagnose fine cracks at one micron or smaller.

• Accuracy

An ultrasonic flaw inspection will miss some tiny defects and likely overestimate others, which creates more significant uncertainty in your results.

• Simplicity

Ultrasonic inspections require experience for anyone outside a lab setting to interpret images correctly—even techs who do nothing but interpret images every day struggle sometimes.

 

➤ Related Article: Phased Array Ultrasonic Testing: Definition and Working Principle

 

 

Conclusion

 

In conclusion, you should use ultrasonic testing in order to inspect your product. Its advantages are long-term usage, precision, and detection.

Its limitation is if there is noise interference while testing. Therefore, we suggest using ultrasonic testing for inspection of your products on various scales such as pipes or plates. Also, ultrasonic tests can be used not only for plastic pipes but also for metal materials like plates due to their versatility despite different test surfaces.

4.9/5 - (63 votes)

Quality ultrasonic thickness gauges can offer highly accurate testing on metals, plastics, and other materials. However, several factors related to the test material, equipment, part geometry, and user skill and care can affect the degree of accuracy achieved in an application. Read on to learn factors that can impact ultrasonic testing results.

Material-Related Factors

The physical properties of the test material is one factor that affects an ultrasonic thickness gauge’s measurement range and accuracy. This includes both acoustic and geometrical factors.

1. Acoustic Properties of the Test Material

Several conditions found in some engineering materials can limit the accuracy and range of ultrasonic thickness measurements:

• Sound scattering: In cast stainless steel, cast iron, fiberglass, and composite materials, sound energy will scatter from individual grain boundaries in castings or boundaries between fibers and matrix in the fiberglass or composite. Porosity in any material can have the same effect. Make sure to adjust the gauge sensitivity to prevent the detection of these spurious scatter echoes. This compensation can limit the gauge’s ability to detect a valid return echo from the back wall of the material, restricting the measurement range.
• Sound attenuation or absorption: In many polymers like low density plastics and in most types of rubber, sound energy is attenuated very rapidly at the frequencies used for ultrasonic gauging. This attenuation typically increases with temperature. The maximum thickness that can be measured in these materials is often limited by attenuation.
• Velocity variations: An ultrasonic thickness measurement is accurate only to the degree that the material sound velocity is consistent with the gauge's velocity calibration. Some materials exhibit major variations in sound velocity from point to point. This happens in some cast metals due to the changes in grain structure that result from varied cooling rates and the anisotropy of sound velocity with grain structure. Fiberglass can show localized velocity variations due to changes in the resin/fiber ratio. Many plastics and rubbers show a rapid change in sound velocity with temperature, requiring that operators perform the velocity calibration at the same temperature as the measurement.
• Phase reversal or phase distortion: The phase or polarity of a returning echo is determined by the relative acoustic impedance (density × velocity) of the boundary materials. Ultrasonic gauges assume the customary situation where the test piece is backed by air or a liquid, both of which have a lower acoustic impedance than metals, ceramics, or plastics. However, in some specialized cases, such as measurement of glass or plastic liners over metal, or copper cladding over steel, this impedance relationship is reversed and the echo appears phase reversed. To maintain accuracy in this situation, make sure to change the appropriate echo detection polarity. An even more complex situation can occur in anisotropic or in homogeneous materials, such as coarse-grain metal castings or certain composites, where material conditions result in multiple sound paths in the beam area. In these cases, phase distortion can create an echo that is neither positive nor negative. Carefully experiment with reference standards in these cases to determine the effects on measurement accuracy.

2. Physical Properties of the Test Material

The size, shape, and surface finish of the test piece must also be considered to establish the limits of measurement range and accuracy.

• Surface roughness of the test piece: The best measurement accuracy is obtained when both the front and back surfaces of the test piece are smooth. If the contact surface is rough, then the minimum thickness that can be measured will be increased because of sound reverberating in the increased thickness of the couplant layer. Inefficient coupling may reduce the echo amplitude. Also, if either the top or bottom surface of the test piece is rough, it can cause distortion in the returning echo due to the slightly different multiple sound paths seen by the transducer, resulting in measurement inaccuracies.

  For corrosion measurements, loose or flaking scale, rust, corrosion, or dirt on the outside surface of a test piece will interfere with the coupling of sound energy from the transducer into the test material. For this reason, clean any loose debris from the sample with a wire brush or file before measuring. Generally, performing corrosion measurements through thin layers of rust is possible as long as the rust is smooth and well bonded to the metal below. Keep in mind that some very roughcast or corroded surfaces might need to be filed or sanded smooth to ensure proper sound coupling. You may also need to remove paint if it is flaking off the metal.

• Curvature of the test piece: A related issue is the transducer alignment with the test piece. When measuring on curved surfaces, it is important to place the transducer around the centerline of the part and hold it as steadily as possible on the surface. A spring-loaded V-block holder can be helpful for maintaining this alignment. In general, as the radius of curvature decreases, the size of the transducer should be reduced, and transducer alignment becomes progressively more critical. For very small radii, an immersion approach with a focused transducer is required. In some cases, it helps to use a waveform display as an aid to maintain the best alignment. Also, on curved surfaces it is important to use only enough couplant to obtain a reading. Excess couplant will form a fillet between the transducer and the test surface where sound will reverberate and might create spurious signals that can trigger false readings.
• Taper or eccentricity: If the contact surface and back surfaces of the test piece are tapered, eccentric, or otherwise angled or misaligned with each other, the return echo will be reduced in amplitude and might be distorted due to the variation in sound path across the width of the beam, reducing the measurement accuracy. Typically, the measured thickness will represent an approximate integrated average of the changing thicknesses in the beam diameter. In cases of significant misalignment, measurement is impossible because the reflected beam will form a V-path away from the transducer and cannot be received. This effect becomes greater as the material thickness increases.

Operator-Related Factors

Calibration: The accuracy of any ultrasonic measurement is only as good as the accuracy and care taken during calibration. Make sure to perform the velocity and zero calibrations described in Section 4 when the test material or transducer is changed. We also recommend periodic checks with samples of known thickness to verify that the gauge is operating properly.

• Beam alignment: Always hold the transducer flat when testing on flat surfaces and normal to the radius of curvature when testing on curved surfaces. When testing on curved surfaces, always center the transducer on the curve. Misalignment will cause echo distortion, which will negatively affect the accuracy.
• Coupling technique: In Mode 1 measurements with contact transducers, the couplant layer thickness is part of the measurement and is compensated by a portion of the zero offset. To achieve maximum accuracy, the coupling technique must be consistent. For consistent measurements, use only enough couplant to achieve a stable reading and apply the transducer with uniform pressure. Practice will show the degree of moderate to firm pressure that produces repeatable readings. Also, never scrape or drag transducers across rough surfaces. In general, smaller-diameter transducers require less coupling force to squeeze out the excess couplant than larger diameter transducers. In all modes, tilting the transducer distorts echoes and causes inaccurate readings.

  For corrosion gauging on small-diameter pipes and tubes, hold the transducer so that the sound barrier material visible on the probe face is aligned perpendicular to the center axis of the pipe, as shown below.

Equipment-Related Factors

While instrument design factors like digital sampling rate will set the limits of range and accuracy for an ultrasonic gauge, the range and accuracy in an application is ultimately determined by the combination of gauge, transducer, and setup, as well as material-related factors. For information on the typical materials and thickness ranges that can be measured with ultrasonic gauges using specific transducers and appropriate instrument setups, visit Section 9.0 Appendices – Transducer Range Charts.

Note that precision gauges using single element transducers typically have higher inherent accuracy than corrosion gauges using dual element transducers.

If you want to learn more, please visit our website Ultrasonic Thickness Gauge Manufacturers.