Phased Array Testing for Bolt in Wind Power Industry

1. Background

Wind power, as a renewable and clean energy source, has become a highly competitive new energy product with the development of large-scale wind turbine power generation and the continuous reduction of generation costs. Wind turbines are usually installed in outdoor or offshore environments and are exposed to harsh conditions such as extreme temperatures and corrosion. Therefore, regular inspections are required to eliminate safety hazards. Among them, threaded connections are important for ensuring the safety and reliability of wind turbine components.

2. Testing Objects

The bolts used in wind turbines are of high strength grade and mainly include tower foundation bolts, tower flange bolts, main shaft bolts, and blade bolts. They are usually made of carbon steel, have an internal hexagonal shape, and have specifications ranging from M20 to M60 with lengths of 150 to 500mm.

In addition to the axial pre-tensioning load, bolts in operation are also subjected to additional alternating tensile loads, transverse alternating shear loads, or combined bending loads, and sometimes even impact loads. These conditions can lead to the occurrence of crack-like defects, and in severe cases, can develop into fracture failures, resulting in serious accidents such as blade detachment and wind tower collapse.

3. Testing Methods

Due to the presence of internal hexagonal holes on the tested workpiece, conventional phased array linear array probes may have blind zones. In this case, using a Phased Array ring probe is an ideal solution. By placing the probe on the bolt end face and performing a 360-degree circular linear electronic scan, full coverage testing of the threaded areas at both ends of the workpiece and the middle bolt shank can be achieved.

4. Testing System

Equipment: SyncScan 3 (64:128PR PA flaw detector)

Phased Array ring probe: 5.0AS64-26-12 (5.0MHz, 64 elements)

Tested Workpieces

1) M34 bolt, length 575mm, with 2 artificial grooves of 1mm depth and 0.25mm width. The distances from the end face are 45mm and 76mm. The end face has an internal hexagonal hole.

2) M38 bolt, length 480mm, with 1 artificial groove of 1mm depth and 0.25mm width. The distance from the end face is 381mm.

5. Testing Results

For the M34 bolt, 2 grooves can be clearly identified.

The measured depths are 76.61mm and 45.97mm, which match the actual groove depths of the workpiece.

For the M38 bolt, the groove can be clearly identified.

The measured depth is 381.05mm, which matches the actual groove depth of the workpiece.

6. Conclusion

Based on the above experiments, it is evident that phased array testing can effectively detect cracks on wind turbine bolts, thereby preventing serious accidents caused by bolt failures. In particular, the Phased Array ring probe, based on the special distribution of elements, can effectively detect bolts with internal hexagonal holes through a 360-degree circular linear electronic scan. This method is an efficient and fast means of in-service bolt inspection.