Analyzing the Differences Between TOFD and Phased Array

Release Date:

2021/12/04

At present, TOFD technology is the most widely used nondestructive testing method in China, and it has already yielded countless successful applications across various industrial sectors. As an ultrasonic imaging technique, TOFD employs a single-transmit–single-receive probe configuration with a specified interprobe spacing, giving it significant advantages in the inspection of thick plates for butt joints, circumferential welds, and longitudinal seams with diameters greater than 500 mm. However, this technique also has certain limitations—for instance, it struggles to effectively inspect structural components with complex geometries and to eliminate detection blind spots in welds. To date, ultrasonic phased-array technology has been developed in China for 20 years, and in its early stages

Currently, TOFD technology is the most widely used nondestructive testing method in China, with countless successful applications across various industrial sectors. As an ultrasonic imaging technique, TOFD employs a single-transmit–single-receive probe configuration and requires a specific interprobe spacing; it offers significant advantages in inspecting flat butt welds, circumferential welds, and thick plates with longitudinal seams exceeding 500 mm in diameter. However, this method also has certain limitations, such as its inability to effectively address complex geometries and detection blind spots in welds. To date, phased-array ultrasonic technology has been developed in China for nearly two decades. In its early stages, it was primarily applied in the medical field, where it enabled high-quality imaging of target organs in clinical ultrasound examinations, thereby contributing to the ongoing advancement of medicine. Nevertheless, due to numerous objective constraints—such as system complexity, the intricate nature of wave propagation in solid materials, and high costs—the scope of its application has remained limited. Under these circumstances, the integration of advanced technologies—including piezoelectric composite materials and sophisticated data processing and analysis—into phased-array ultrasonic imaging has become an inevitable trend, and phased-array ultrasonic inspection is poised for even broader adoption in the future. Phased-array ultrasonic technology utilizes multi-element transducer arrays to electronically steer and focus sound beams, enabling electronic scanning over a wide range of beam angles and coverage from a single probe position, making it particularly well suited for inspecting components with complex geometries.

Next, let’s make a brief comparison of TOFD and phased-array inspection techniques:

I. Technical Features of TOFD

Advantages of TOFD TOFD technology not only boasts strong defect detection capability and high defect quantification accuracy, but also offers excellent timeliness and safety, with the added advantage of permanent data storage.

1. High efficiency This technique requires only linear scanning to complete the inspection of welds, significantly expanding the coverage area of single-probe inspections and far surpassing that of conventional inspection methods.

　　 2. High sensitivity : Due to the high sensitivity of the diffraction wave signals generated by this technology, the detection rate is largely assured.

　　 3. High precision : The height of the defect can be accurately calculated using the diffraction time-of-flight method.

　　 4. Minimal impact This technique is unaffected by the orientation of weld geometry or defects, resulting in highly stable inspection outcomes that are virtually impervious to other influencing factors.

　　 5. Fewer missed detections : Diffraction waves exhibit high sensitivity, enabling comprehensive data acquisition via image recording and excellent repeatability.

　　 6. Complete data : The test results are highly time-sensitive, and the relevant data and documentation are permanently archived—either electronically stored or printed out—for analysis and processing at any time.

　　 7. More Secure : The use of this technology will not cause any physical harm to relevant personnel.

　　 8. More flexible : On-site testing is very convenient, allowing you to freely choose between manual and automatic modes based on the actual situation.

9. Low cost : This technology not only eliminates the need for additional consumables, but also minimizes wear because it does not require direct contact with the workpiece. Moreover, it can withstand high-temperature contact surfaces (up to and exceeding 200°C), allowing for the online application of relevant engineering evaluation standards during defect assessment and enabling the minimization of unnecessary weld gouging during defect repair. As a result, inspection and production cycles are significantly shortened, other adverse issues are effectively avoided, and the overall cost is substantially lower than that of conventional ultrasonic testing.

Limitations of tofd ：

1. Excessive sensitivity can sometimes exaggerate benign defects in welds.

2. There must be sufficient space on both sides of the weld for probe placement.

3. Detection blind spots exist, specifically near the surface of the workpiece (typically 2–10 mm).

4. High standards are required for the qualifications and competencies of relevant personnel.

5. Low detection efficiency of TOFD: TOFD inspection can be performed in two modes: non-parallel scanning and parallel scanning. Parallel scanning is highly inefficient but offers superior quantitative accuracy; therefore, it is rarely used in practice and is typically reserved for laboratory settings or for the precise quantification and localization of specific defects. Non-parallel scanning, on the other hand, is faster but cannot determine which side of the weld the defect lies on, thereby complicating localization. In some cases, multiple scans are required to pinpoint the defect’s location, which reduces overall inspection efficiency. Consequently, non-parallel scanning is the more commonly employed method in routine inspections.

6. Evaluation of TOFD Defects It is well known that TOFD technology does not rely on amplitude-based detection, and its detection rate is significantly higher than that of other inspection methods, such as manual ultrasonic testing, automated ultrasonic testing, and radiographic testing. This is an established fact. TOFD can detect both large and small defects, which complicates defect evaluation; if not properly controlled, the rework rate can be very high, and in some cases, defects become invisible after rework.

II. Characteristics and Advantages of Ultrasonic Phased Array Testing Technology

Ultrasonic phased array inspection, when compared with other nondestructive testing methods, exhibits the following characteristics:

1. Electronic methods are used to control beam focusing and scanning, resulting in a dramatic increase in inspection speed.

① The direction of the ultrasonic beam can be freely adjusted.

② The focal point can be adjusted, and dynamic focusing can even be achieved.

③ The probe can perform ultrasonic sector scanning or linear scanning while remaining stationary.

④ Phased-array technology enables electronic scanning, which is one order of magnitude faster than conventional raster scanning.

2. Exhibits excellent beam accessibility, enabling inspection of workpieces with complex assembled geometries.

① Traditional inspection methods are limited in their applications, whereas ultrasonic phased-array inspection technology can address a wide range of applications using a single phased-array probe; b Traditional inspection methods are often ineffective for many specialized inspections, but ultrasonic phased-array inspection technology can readily handle various inspection tasks with just a compact array probe.

3. Regular optimization of the focal spot size, focal depth, and sound beam direction can enhance detection performance metrics such as resolution, signal-to-noise ratio, and sensitivity.

4. Auxiliary scanning equipment is generally not required; the probe does not make direct contact with the workpiece, and data are stored in electronic file format, resulting in flexible, simple operation and low cost.

5. Simulation-based imaging technology: addresses the challenge of inspecting geometrically complex structures; generates real-time, on-site images of geometric shapes; easily identifies the true location and characteristics of defects; imaging is derived from A-scan data acquired by multiple ultrasonic beams; integrates with process trajectory tracking during actual inspections; applicable to all types of weld inspection, with simultaneous display of A-, B-, S-, C-, D-, P-, and 3D-scan data.

Traditional scanning imaging methods require probe movement to perform inspections, whereas phased-array ultrasonic imaging can achieve computer-controlled focused scanning across a specified acoustic field of the test specimen simply by using an array transducer (probe). In addition, in scan-imaging modes such as B-scan and C-scan, the sound beam is typically emitted from a single probe; since all its parameters are fixed, it is difficult to obtain clear, consistent images across the entire field of view without repositioning the probe. By contrast, because the parameters in phased-array ultrasonic imaging are highly flexible and adjustable, uniform, consistent, high-resolution images can be obtained even without moving the probe.

Although ultrasonic holography can produce a three-dimensional image of the target, its limited sensitivity and resolution, coupled with the complexity and high cost of the required equipment, have thus far prevented its widespread adoption. In contrast, phased-array ultrasonic imaging offers significantly higher detection sensitivity and resolution than ultrasonic holography, and by computationally reconstructing the acquired data from beam scans in multiple directions, it can also generate a three-dimensional image of the inspected object. When a two-dimensional array probe is used, real-time three-dimensional imaging can be achieved.

Due to the high frequency of the transducers used in ultrasonic microscopy, it offers high resolution; however, its limitation is that it is suitable only for examining the surface and near-surface microstructures of objects. In contrast, phased-array ultrasonics does not require such high frequencies, resulting in relatively lower resolution, but it can perform internal imaging inspections of thicker, larger workpieces.

III. Conclusion

Ultrasonic phased-array inspection technology is increasingly attracting attention due to its flexible beam steering and focusing capabilities. In the near future, this technology is poised to replace other imaging-based inspection methods thanks to its unparalleled advantages, thereby making nondestructive testing more convenient and widely applicable in industrial settings.

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To further carry forward the spirit of Lei Feng and promote the volunteer ethos of “dedication, friendship, mutual assistance, and progress,” in accordance with the spirit of the Henan Provincial Association for Science and Technology’s “Notice on Launching a Centralized ‘i-Science Popularization’ Science and Technology Volunteer Service Campaign in the Spirit of Lei Feng,” Luoyang Weicheng Engineering Testing Co., Ltd. responded enthusiastically to the call and organized a science popularization event on the safe operation of forklifts. On March 4, our company meticulously planned and conducted an emergency response drill for special equipment—forklift accidents—and invited several nearby companies that use forklifts to observe the exercise. The purpose of this drill was to simulate real-world working conditions, specifically the sudden occurrence of an accident during the process of using a forklift to handle goods.

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The 10th Anniversary Gratitude Celebration of Luoyang Weicheng Engineering Testing Co., Ltd. Concludes Successfully

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In 2023, Luoyang Weicheng Engineering Testing Co., Ltd. Successfully Held the “Intensive Training and Competition” — Gamma-Ray Radiographic Inspection Emergency Drill

On October 11, Luoyang Weicheng Engineering Testing Co., Ltd. held a large-scale training and competition—specifically, a γ-ray radiographic inspection radiation emergency response drill—under the theme “Everyone Talks About Safety, Everyone Knows How to Respond to Emergencies.” The event invited leaders from the provincial, municipal, and district levels to attend and observe. Prior to the start of the emergency response drill, Cui Songbiao, General Manager of Luoyang Weicheng Engineering Testing Co., Ltd., and Gong Aifeng, Deputy General Manager, extended a warm welcome to the visiting provincial, municipal, and district leaders and pledged that Weicheng Testing would consistently focus on enhancing its professional technical expertise and emergency response capabilities as key drivers for corporate safety management.

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