Detection of Accumulated Solid-Liquid State in Steel Pipe Poles Using Circumferential Lamb Waves

Literature Overview and Research Context

This study presents a non-destructive testing (NDT) methodology for detecting accumulated solid-liquid deposits within steel pipe poles using circumferential Lamb waves. Steel pipe poles—used extensively in transmission towers, communication masts, and offshore platforms—are susceptible to internal corrosion, sediment accumulation, and moisture ingress, which can compromise structural integrity without external visual indicators. Traditional inspection methods such as ultrasonic thickness measurement (UT) and radiographic testing (RT) are limited in their ability to detect distributed internal deposits and require extensive access and preparation.

The use of Lamb waves for structural health monitoring has advanced significantly in recent years, but the application of circumferential Lamb wave modes specifically for detecting accumulated solid-liquid states within steel pipe poles represents a novel and practically significant contribution. This approach leverages the dispersive nature of guided waves to achieve long-range inspection from a single measurement point.

Core Technical Points and Wave Propagation Theory

Lamb Wave Mode Selection and Characteristics

Lamb waves are elastic guided waves that propagate in thin-walled structures, characterized by symmetric (S) and antisymmetric (A) modes. For circumferential propagation in steel pipe poles, the following wave modes are of primary interest:

Mode	Frequency-Thickness Product (f·t)	Phase Velocity (Steel)	Sensitivity to Deposits	Wavelength
A0	< 0.5 MHz·mm	~1500 m/s (dispersive)	High for thickness changes	Long
S0	< 0.5 MHz·mm	~3000 m/s (dispersive)	Moderate for deposits	Long
A1	0.5-2 MHz·mm	~3200 m/s	High for interface defects	Medium
S1	0.5-2 MHz·mm	~5900 m/s	Moderate for wall thickness	Medium
A2	2-5 MHz·mm	~5900 m/s	High for localized deposits	Short

The selection of the appropriate mode depends on the pipe wall thickness, the expected deposit thickness, and the required inspection range. For typical steel pipe poles with wall thicknesses of 6-20 mm, the A0 and S0 modes at low frequency-thickness products offer the longest inspection range but with limited resolution, while higher-order modes provide better resolution at the expense of shorter range.

Sensor Configuration and Signal Processing

The study employs a circumferential sensor array configuration where transducers are arranged around the pipe circumference to excite and receive Lamb waves in the circumferential direction. This configuration enables the detection of deposits that are distributed around the pipe wall, which is a significant advantage over conventional axial Lamb wave inspection. The sensor array typically consists of piezoelectric ceramic transducers (PZT) or electromagnetic acoustic transducers (EMATs), with a center frequency of 100-500 kHz depending on the pipe diameter and wall thickness.

Signal processing techniques including time-frequency analysis (wavelet transform), spatial filtering, and damage index calculation are employed to extract deposit-related features from the measured wave signals. The presence of accumulated solid-liquid deposits modifies the local impedance and damping characteristics, causing measurable changes in the amplitude, phase, and time-of-flight of the propagating Lamb waves.

Detection Methodology and Results Interpretation

Deposit Detection Mechanism

The detection mechanism relies on the interaction between the propagating Lamb waves and the accumulated deposits. When a deposit layer exists between the steel wall and the internal fluid or air, it creates an additional impedance mismatch that causes partial reflection, scattering, and attenuation of the wave energy. The magnitude of these effects depends on the deposit thickness, density, and acoustic impedance relative to the steel and the internal medium.

The key observable parameters that indicate deposit presence include:

Observable Parameter	Physical Mechanism	Deposit Sensitivity	Typical Threshold
Amplitude attenuation	Energy absorption and scattering	High for deposits > 1 mm	> 20% attenuation
Phase shift	Wave speed modification	Moderate for all deposits	> 5° phase shift
Time-of-flight change	Effective wave speed reduction	High for thick deposits	> 2% time change
Frequency content shift	Modal conversion and filtering	Moderate for distributed deposits	> 10% energy shift

Case Study: Field Application Results

In a field application scenario, the circumferential Lamb wave method was applied to inspect steel pipe poles of a transmission tower after 15 years of service. The inspection revealed significant internal corrosion deposits in the lower 3 meters of the pipe, where moisture accumulation was most pronounced. The detected deposit thickness ranged from 0.5 mm to 3.2 mm, with the maximum thickness occurring at the bottom of the pipe where condensate drainage was restricted.

The inspection results were validated through destructive sampling at the end of the service life, confirming that the Lamb wave method accurately identified the deposit locations and provided reasonable estimates of deposit thickness. The method demonstrated an inspection range of over 5 meters from a single measurement point, significantly outperforming conventional UT methods which are limited to local thickness measurement.

Engineering Practice and Quality Control Integration

Integration with Structural Health Monitoring Systems

The circumferential Lamb wave inspection method can be integrated into structural health monitoring (SHM) systems for steel pipe poles, enabling periodic or continuous monitoring of internal deposit accumulation. The following table summarizes the integration parameters:

Integration Parameter	Specification	Purpose
Inspection interval	6-12 months	Detect progressive deposit growth
Sensor type	PZT or EMAT	Long-term stability and coupling
Data acquisition rate	1-10 MHz	Adequate for 100-500 kHz signals
Data storage	Cloud or local server	Historical trend analysis
Alarm threshold	20% amplitude drop or 2% TOF change	Early warning of significant deposits

Comparison with Alternative NDT Methods

Method	Inspection Range	Deposit Sensitivity	Accessibility	Cost	Suitability
Circumferential Lamb waves	3-10 m	High	Single point access	Moderate	Excellent for internal deposits
Axial Lamb waves	5-20 m	Moderate	Single point access	Moderate	Good for axial defects
UT (contact)	Local (mm)	High	Full access required	Low	Good for local thickness
RT	Full length	High	Full access required	High	Good for volumetric defects
Eddy current	Local (mm)	Moderate	Surface access	Low	Good for surface defects

Study Insights and Concluding Remarks

This research demonstrates the practical viability of circumferential Lamb wave inspection for detecting accumulated solid-liquid deposits within steel pipe poles, offering a significant advancement over conventional NDT methods in terms of inspection range and accessibility. The findings confirm that Lamb wave signals are sensitive to internal deposit presence, with measurable changes in amplitude, phase, and time-of-flight that can be reliably correlated with deposit thickness. Engineers involved in the maintenance and integrity assessment of steel pipe poles should consider incorporating Lamb wave-based inspection into their regular maintenance schedules, particularly for structures where internal access is difficult or where progressive corrosion is a known concern. The method's ability to provide long-range inspection from a single measurement point makes it particularly suitable for large-scale infrastructure where inspection efficiency is critical. Future work should focus on developing automated interpretation algorithms and establishing standardized acceptance criteria for deposit detection based on Lamb wave inspection results.