Thickness Measurement at High Lift-Off for Underwater Corroded Iron-Steel Structures Using a Magnetic Sensor Probe

Area of Science:

• Materials Science and Engineering
• Non-Destructive Testing (NDT)
• Civil Engineering

Background:

• Deterioration of aging infrastructure, specifically steel sheet piles in wharfs, leads to significant structural damage.
• Existing non-destructive inspection techniques face limitations due to marine biofouling, causing large lift-off distances and signal interference.
• Accurate assessment of steel sheet pile thickness is crucial for maintaining the integrity of maritime structures.

Purpose of the Study:

• To investigate the feasibility of extremely low-frequency eddy current testing (ELF-ECT) for measuring steel sheet pile thickness under challenging conditions.
• To optimize sensor probe design to mitigate the effects of large lift-off distances caused by marine organisms.
• To evaluate the performance of developed probes on actual steel sheet piles in situ.

Main Methods:

• Prototyping of sensor probes with varying coil diameters for ELF-ECT.
• Application of magnetic fields with controlled lift-off ranging from 0 to 80 mm.
• Analysis of magnetic field intensity and phase to determine thickness sensitivity.
• Testing of optimized probes on actual corroded steel sheet piles.

Main Results:

• Increased probe diameter significantly improved sensitivity for thickness estimation at lift-off distances up to 60 mm.
• The developed sensor probes successfully measured the thickness of steel sheet piles despite substantial lift-off.
• Magnetic signal fluctuations caused by large lift-off were effectively managed through optimized probe design.

Conclusions:

• Optimized sensor probes for ELF-ECT are effective for non-destructive thickness measurement of steel sheet piles in marine environments.
• The developed technique overcomes the limitations imposed by marine biofouling and large lift-off distances.
• This advancement provides a viable solution for the inspection and maintenance of aging maritime infrastructure.