What Is an Ultrasonic Weld Test?

An ultrasonic test (UT) for welding is a nondestructive testing (NDT) technique that helps detect discontinuities within a weld’s internal structure. Learn more about the fundamentals of testing techniques and why UTs are beneficial. 

NDT Options: Ultrasonic and Radiographic Testing

Nondestructive testing techniques help identify flaws without damaging components. There are two popular NDT options in welding — radiographic and ultrasonic.

Radiographic Testing

Radiographic testing (RT) uses X-rays or gamma rays to produce images of a component’s internal structure. The X-ray or isotope radiation is aimed at the object, and a film or digital sensor is placed behind the object. The radiation passes through the object to the sensor, and the inspector evaluates the resulting image. With RT testing, inspectors can catch discontinuities like cracks, porosity, incomplete fusion and inadequate weld reinforcement.

While effective on various materials, it exposes operators to high levels of radiation, which can damage human cells and potentially cause cancer. Ultrasonic testing emerged as a less risky alternative.

Ultrasonic Testing

Ultrasonic testing is one of the most reliable quality control and inspection procedures for welding, and it uses ultrasonic sound waves to detect discontinuities. Similar to medical ultrasound procedures, operators direct sound waves into a material and assess echoes to gather information about internal structures.

Aside from stress and damage detection, UT for welding can be used to measure the thickness of a part. This method is versatile, portable and much safer than radiographic testing.

Ultrasonic Testing Equipment and Tools

Ultrasonic testing requires four main tools:

1. Pulser/receiver: This device sends an electrical pulse to a transducer and receives the sound waves that get sent back in case of a flaw.
2. Transducer: The transducer converts the pulse into a sound wave and vice versa.
3. Coupling agent: Couplers like gel, oil or water ensure a tight seal between the test object and the transducer.
4. Display device: The display device portrays the results of the test.

Professionals may also use calibration blocks, drones, additional cables and data analysis software. Personal protective equipment like gloves is also necessary. Reach out to a Mertius Gas team expert to learn more about UT equipment and tools.

Weld Inspection and Testing: The UT Process

First, operators must clean the test surface to prevent contaminants like dust from impeding sound wave transmission. Then, they follow these steps:

1. Apply a couplant to the test surface: This eliminates air gaps. 
2. Move the transducer along the surface: Sound waves travel through the material and bounce back when they encounter internal discontinuities. 
3. Analyze results: The reflected sound waves are displayed on a screen as waveforms. Technicians analyze the time and amplitude of reflections to identify the discontinuity.

There are a few ways to employ UT: 

• Contact testing: Includes straight beam testing, angle beam testing and surface wave testing.
• Immersion testing: Involves a water tank and waterproof transducers.
• Air-coupled testing: Uses air for transmitting ultrasonic waves.

Interpreting UT Results

These are some examples of display formats:

• A-scan: Uses an x-y grid to show time (y-axis) versus amplitude (x-axis).
• B-scan: Provides a cross-sectional view of the test surface.
• C-scan: Offers a top-down view and layers data collected over time.

To interpret results, technicians compare the waveform on the display screen with the baseline or expected signal from a defect-free material. They may also use established criteria to classify defects.

Other techniques include assessing amplitude, which is the strength of the reflection, and time-of-flight, which describes the time it takes for a sound wave to travel to and from a reflector. These help technicians understand the size and location of a reflector.

Advanced Techniques and Future Trends in UT

Advanced UT techniques include:

• Time of flight diffraction (TOFD): The process above describes the pulse-echo transmission mode, where a single transducer sends and receives waves. TOFD is a two-transducer mode in which one sends ultrasonic signals and the other receives them.
• Phased array ultrasonic testing (PAUT): Probes are made up of many elements, each of which pulses individually. PAUT allows for three-dimensional displays of discontinuities.
• Full matrix capture (FMC) and total focusing method (TFM): Technicians get data from multiple elements, and synthetic focusing is done in post-processing.
• Electromagnetic acoustic transducer (EMAT): This technique does not require a coupler and instead uses magnetic fields to induce ultrasonic waves.
• Guided wave applications: UT detects flaws at the material’s surface while being away from the transducer.

The future of UT may involve making devices smaller, enhancing images, introducing automation and embracing sustainability. Operator training will also continue to be important — as the field changes, technicians will need to stay up to date. Working with a company with a legacy of excellence and a commitment to embracing future innovations helps ensure reliable testing results.

Benefits and Applications of Ultrasonic Testing

UT offers several benefits for welding:

• Precision: When operators are properly trained to interpret results, UT accuracy is high. 
• Detail: Testing can estimate the size, shape and orientation of defects. 
• Safety for operators: Personnel are not exposed to potentially hazardous levels of radiation. 
• Speed: While preparing the surface does take time, UT provides fast results. 
• Portability: Testing equipment is portable, so operators can complete tests in various locations. 
• Material versatility: UT is effective for ferrous and nonferrous materials.
• Flexible: Professionals can use ultrasonic testing even when they only have access to one side of an object.

There are dozens of UT applications across various industries:

• Aerospace: Inspecting aircraft engines for impact damage. 
• Automotive: Evaluating chassis and axles for misalignment. 
• Infrastructure: Assessing bridges for structural integrity. 
• Oil and gas: Inspecting pressure equipment for corrosion damage. 
• Power generation: Checking turbines for defects, especially in nuclear power plants. 
• Manufacturing: Assessing metal products for discontinuities. 

Challenges and Limitations of Ultrasonic Testing

Despite the advantages, there are some limitations and challenges to be aware of:

• Operator expertise: Technicians require a high level of training and must be able to interpret complex signals and images. 
• Equipment requirements: A rough or corroded surface may impact sound wave transmission. 
• External noise risks: External noise or interference can affect UT accuracy and sensitivity. 
• Material limitations: It may be challenging to assess irregularly shaped, rough or very thin parts. It may also be difficult to get results for heterogeneous materials. 

There are also NDT standards to adhere to, including those set by: 

• ANSI and ASNT: The American National Standards Institute (ANSI) and American Society for Nondestructive Testing (ASNT) provide guidelines for NDT personnel — ANSI/ASNT CP-105 and ANSI/ASNT CP-189 are particularly relevant. 
• ISO: There are many relevant International Organization for Standardization (ISO) standards, including ISO 16810:2024 and ISO 5577:2017.
• ASTM International: Key ASTM standards include E273-20, E164-24 and E2700-20. 

The American Welding Society, American Petroleum Institute, International Conference of Building Officials and National Board of Boiler and Pressure Vessel Inspectors also establish standards relating to UT for welding.

Relying on professional welding inspection services is the best option for ensuring accurate results and maintaining compliance.

Get Welding Testing and Inspection Services From Our Partner

Meritus Gas Partners is a network of independently operated gas suppliers, and our nationwide partners also offer valuable services. When it comes to welding inspection and testing services, you can rely on our partner, OXARC.

This division of OXARC has received accreditation through the Washington Association of Building Officials (WABO), and its staff consists of ASNT Level II and III technicians for ultrasonic testing, as well as magnetic particle testing, liquid penetrant testing and visual testing. 

The team follows WABO, AWS and ICO standards and maintains thorough documentation throughout the process. They also offer corporate and individualized training courses to help welders gain professional certification.

Connect with our professionals today!