There are many aspects of testing material, such as its structure, purity, and other properties. Phased Array Ultrasonic Testing (PAUT) and Time-of-Flight Diffraction (TOFD) are two of the popular material testing methods.
Many engineers who do not have sufficient knowledge on material testing may be confused about these two methods, and they may ask what is the difference between phased array ultrasonic testing (PAUT) and time-of-flight diffraction (TOFD)? So let us talk about the difference between these two methods briefly in this article.
Phased Array Ultrasonic Testing(PAUT)
What is PAUT technologies?
Phased array ultrasonic testing (PAUT) is a non-destructive testing technology that inspects welded seams to detect weld defects and perform quantitative measurements of defects.
Welding occurs at high temperatures with great pressure, often creating more than one type of defect, such as porosity and inclusions. The PAUT technique can identify these defects with great sensitivity, whether they are solid or liquid-filled, soft or hard, and buried or interstitial.
PAUT is used for a variety of applications that require high accuracy measurements in harsh environments, including offshore oil production rigs and platforms, shipbuilding yards, tank farms, and chemical plants, to name a few.
Generally, people use phased array ultrasonic testing to test the structural integrity of a weld by comparing it with a template or standard ultrasonic waveform. It’s used to evaluate complex weldments, such as in the automotive and aerospace industries. Here are some areas where PAUT is used:
- Weld inspections
- Thickness measurements
- Corrosion inspection
- Crack detection
- Corrosion mapping
- Inspection of rolling stock wheels and axles
Benefits/ Advantages of PAUT
It can cover both the inner and outer surfaces of an object with minimal manual intervention. This is done by using multiple probes that send acoustic energy toward the surface of an object.
The speed of PAUT is over three times faster than TOFD, and hence it takes considerably less time to take a lot of data points. Moreover, you can use less acoustic energy than with TOFD since you’re measuring only a few points at one time.
PAUT offers a distinct advantage over time of flight for complex geometry or noisy data (higher speed testing and lower noise environments). The result is higher accuracy measurement with less reliance on external calibration and post-processing.
Since there is no backscatter radiation with PAUT, technicians can operate while wearing almost any type of clothing that won’t get in their way. This technology also means much safer operations for plant workers and the environment.
PAUT is highly repeatable for a wide range of materials, including thin films and powders.
Phased array ultrasonic testing offers unparalleled flexibility when testing a variety of structures on either new or existing structures.
Weakness of PAUT
The main disadvantage of phased array ultrasonic testing is that it uses piezoelectric transducers, which have a lower bandwidth than transducers used in TOFD, so to resolve finer details in newer vehicles, it’s not likely to be as useful.
The second disadvantage is that phased array can only look at one area of interest at a time, while TOFD has multiple transmitters and receivers scattered throughout the vehicle, making it more effective for on-site quality control checks during production.
What is TOFD Inspection?
TOFD technology is one of the NDT methods which utilizes phase shift and frequency difference between scattered and reflected ultrasonic signals to find discontinuities in test specimens.
There are two basic components, an illumination array and a receiving array, each consisting of multiple sensors (called transducers), and both are typically arranged on an XY cartesian grid. The two arrays are separated by a known distance, usually a few centimeters, but it can be longer if desired or shorter depending on the depth of investigation needs.
TOFD inspection system measures time of flight from transmitter to the receiver and the amplitude of reflected ultrasonic waves from surface imperfections present at that location within the receiver array, creating TOFD images showing relative strength of sound reflection for each point on the surface area being tested.
TOFD is best used to characterize powdered solids and powders where diffraction and scattering are considered less critical than in liquids and slurries.
The top applications of TOFD technology include powdered polymer characterization, crystalline materials, catalysts and products made from them, ceramics, metals, minerals, and related materials found in devices such as batteries, automobiles, electronic components, appliances, and industrial manufacturing facilities. Nonporous, continuous solid samples can also be tested with TOFD.
When it comes to bulk solids or particulates, PAUT has a significant edge over all other ultrasonic inspection methods because it makes possible effective inspection of a wide range of solid and liquid samples.
Benefits/ Advantages of TOFD
The reflector-free operation, no need to reorient samples for different analysis regions, which saves a lot of time and enables examination of each sample over multiple regions in a single test run.
With TOFD, it is possible to measure liquid-filled containers with an air headspace without draining or even tiling them during testing.
No minimum sample size limit.
Ability to measure wet samples.
Can handle heavy or fragile products such as automotive components that cannot be transferred using conveyors.
Displays data immediately on detection, so there is no waiting for the scan to finish before knowing whether you have a good result. The entire measurement process can be completed within minutes.
Weakness of TOPD
TOFD is a time-consuming process of analyzing products, and more importantly, it can produce errors if not operated properly with poor sensors or if not calibrated well.
Another disadvantage is that TOFD does not provide 100% accuracy but instead estimates quality/consistency for products. Data from TOFD may also vary depending on whether the product is cooled or frozen during testing, making product consistency extremely hard to manage over large batch sizes.
If a product is cooled at any point after freezing or cooling, then testing must be conducted at 0 degrees Celsius (32 F).
Regardless of the cooling methods used, when product temperature falls below -10 C (14 F), thickness measurements are unreliable due to ice crystal formation in tissue samples which can cause variations in tests within batches.
Combine TOFD and Phased Array Technique
The phased array and time of flight (TOFD) technique can produce equivalent results, but using a combination of both techniques may provide an even more complete analysis of flaws in your components.
Although both methods have their strengths, it is important to understand each and how they are used for best results with specific components. According to Qualitrend, researchers have found that combining these two testing methods is more effective than either method used on its own.
The PAUT sends out multiple bursts of ultrasound, which create a picture on a screen showing all areas where flaws or weaknesses are present on your material’s surface. Combining them allows you to see a three-dimensional image that is easier to interpret than if you only had TOFD data.
You’ll also be able to quickly identify large flaws, such as voids and cracks, as well as small defects, such as pores and scratches.
While both PAUT and TOFD are commonly used to determine porosity in composite materials, some major differences between these two methods must be considered when choosing which technique to use on your samples.
While TOFD requires an intense amount of computational power and specialized hardware, PAUT can measure many more locations in a single testing run due to its higher spatial resolution and can deliver comparable results at a much lower cost.
However, as it is not strictly quantitative like TOFD, PAUT does not deliver measurements for each sample site that can be compared to one another as absolute values. Instead, it delivers relative porosity estimates across multiple sites within each sample structure.