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Ultrasonic Testing (UT)
This method of testing involves introduction of sound, with the frequencies in the ultrasonic range, into the material using a sensor (transducer) and listening to the change in different parameters of the introduced sound using the same or a different sensor.
There are many different techniques within this method with the conventional classification generally based on how the sound wave is introduced, the predominant mode of travel of the sound waves, whether the receiving transducer is listening to reflected sound or transmitted sound and how the received information is displayed. Another set of classification is based on the complexity of the instrumentation, whether the waves are steered inside the object electronically or mechanically and what measurements are taken and how they are taken.
Predominantly used technique in ultrasonic testing is called Pulse-Echo technique, where the sound is introduced into a material and its reflections are received and analyzed. This technique relies on difference in acoustic impedance between different materials and during the sound travel whenever the sound encounters a change in this material property a reflection of sound waves is expected. The interface between the two materials somewhat acts like a mirror. It is important here to understand that a crack that is within a material is actually a different material, usually a gas.
Not all of the sound is reflected. Some of it is transmitted to the other material. The larger the difference in the value of acoustic impedance between 2 materials in the path of the sound, the greater is the amount of reflection expected.
In inspections, the most common modes of sound waves are longitudinal & shear. Longitudinal mode is usually introduced perpendicular to the thickness of the material. Sound waves travel across the thickness and reach the other surface and encounter air which usually has a drastically different acoustic impedance from the material being inspected. This results in the sound bouncing back to the sensor that introduced the sound. The time taken for this to & fro travel across the wall thickness is noted by the instrumentation usually in microseconds. The velocity (speed) of sound travel in a particular mode is constant for a given material. So the sound cannot travel any faster or slower. Using a simple formula we can find the total distance travelled by the sound wave. Now since the sound has travelled from one side to the other and back, the distance travelled will be 2 times the thickness.
Thickness = Distance Travelled / 2 = (Speed of travel x Time for travel) / 2.
Since reflections occur wherever there is a change in acoustic impedance, the same inspection technique can be used to locate, identify & size of other flaws like laminations, voids, inclusions corrosion and many others.
One important point to remember is that the laws of reflection of light are applicable to sound too. SO it is good to imagine a mirror at the place where the 2 different materials meet.
When sound is introduced at an angle, instead of being perpendicular as described above, the sound reflects but instead of coming back to the sensor, it bounces away, just as it would happen with the mirror. The same is true if the other surface is not parallel to the surface from where the sound is introduced. So the key point here to understand is that if the sound doesn’t meet an interface (where 2 materials meet) perpendicularly, the reflection is not going to be captured by the sensor. This means that for a flaw to be detected, the flaw needs to be perpendicular to the sound path in this technique.
This bouncing of sound waves at the interface is used extensively to examine materials for flaws that are not parallel to the surface. This is usually done by introducing a shear mode of sound at an angle into the material being examined.
Irrespective of the mode or the technique used, knowledge of the part being examined, understanding of the flaw being sought and the ability to visualize the path of sound are very important along with a clear understanding of the capabilities and more importantly the limitations of the method & technique for a meaningful examination. While a hypothetical crack with a particular set of dimensions can be calculated as detrimental using fracture mechanics and the like, the test system along with all its variables may not be able to size such dimensions within reasonable level of accuracy or worse even identify the flaw.
Two important aspects are very important to understand properly when it comes to examination of fine flaws. One is repeatability with reasonable accuracy and the other is operator confidence when it comes to identifying, characterizing and sizing a flaw.
At CMW seamless pressure vessels are routinely examined using both longitudinal and shear modes. All vessels go through a wall thickness survey on areas in question and are used in determining remaining wall thickness at areas having pitting or general corrosion. Such an inspection is usually conducted with hand held meters. When the operator confidence is less due to signal strength which usually stems from internal flaws such as lamination and inclusion, a more advanced flaw detector is employed. During Acoustic Emission Testing, the vessels under examination may be identified with areas of activity from emission sources similar to flaws being sought, ultrasonic testing of those areas are performed using a flaw detector. Since a fatigue crack is the most sought after flaw, a circumferential scan of the area is required. Since a rejectable fatigue crack is likely to be less than 1” in length and 1/10th of an inch in depth and the circumference of the vessels are usually over 60 inches, evaluation of such a small flaw should not be done with the sound that bounces many times. Ideally they need to be evaluated before the second bounce.
Though ultrasonic inspection is primarily employed for examination of pressure vessels, structural inspections are also carried out often.
Non Destructive Testing methods are indirect way of measuring certain properties and characteristics of a material. Each one focuses on a small set of variables and the results are based on the changes to those variables. When it comes to testing, there is no one method that is complete or fool proof, even if they are the touted to be the latest and the greatest.
Though the ocean of rules and regulations may permit multiple methods to certify a pressure vessel’s fitness, it is essential to understand the method, the assumptions, capabilities and more importantly their limitations prior to deciding on a particular test method for your pressure vessels. Users are encouraged to gain knowledge from the suggested reference material. If you have questions or require further clarification, please feel free to contact us.
Ultrasonic Testing of Materials by J. Krautkramer & H. Krautkramer
American Society for Non Destructive Testing (ASNT) Handbook Volume -7 Ultrasonics