CLADDING TECHNOLOGY SHANXI CO., LTDPropagation Characteristics of Ultrasonic Interface Waves in Liquid-Filled Bimetallic Composite Pipes
Overview and Motivation
Liquid-filled bimetallic composite pipes are increasingly employed in high-pressure hydrogenation reactors, acid service heat exchangers, and cryogenic transfer lines where the combination of a carbon steel or low-alloy steel base layer and a stainless steel or nickel alloy cladding layer provides both structural strength and corrosion resistance. The integrity of the bond interface between these two layers is critical to long-term service reliability. Conventional NDT methods such as conventional pulse-echo ultrasonic testing often struggle to detect debonding defects at the cladding interface, particularly when the pipe is filled with process fluid. This literature addresses a fundamental question in the field: how do ultrasonic interface waves (specifically Stoneley waves and shear horizontal waves) propagate along the cladding-bondline interface of a liquid-filled composite pipe, and can their attenuation or mode conversion be used as a reliable indicator of bond quality?
The study examines a pipe geometry consisting of a carbon steel base tube (e.g., 20# steel, inner diameter 57 mm, wall thickness 3.5 mm) with a stainless steel overlay layer (e.g., 304L, thickness 1.5–2.0 mm) applied via electroslag welding or explosive cladding. The pipe is filled with water or a process simulant to replicate in-service conditions. Ultrasonic transducers are excited at frequencies ranging from 1 MHz to 5 MHz to generate interface-guided waves.
Core Technical Points
Wave Mode Generation and Dispersion
Interface waves propagate along the boundary between the cladding layer and the base metal. In a liquid-filled pipe, the presence of the fluid load modifies the dispersion characteristics significantly compared to dry conditions. The key wave modes identified include:
| Wave Mode | Typical Frequency Range | Phase Velocity Range | Sensitivity to Debonding |
|---|---|---|---|
| Stoneley wave | 0.5–3 MHz | 800–2200 m/s | High |
| SH-1 mode | 1–5 MHz | 1800–3100 m/s | Moderate |
| Lamb-like mode in cladding | 2–5 MHz | 2000–5800 m/s | Moderate |
The critical finding is that the Stoneley wave, which is inherently sensitive to the mechanical continuity of the interface, experiences measurable amplitude attenuation and phase velocity shifts when partial debonding or intermetallic compound formation is present. In liquid-filled conditions, the fluid provides additional damping that must be accounted for in signal interpretation.
Effect of Interface Condition on Wave Propagation
The study systematically introduces artificial defects at the cladding interface, including:
- Partial debonding with gap widths of 0.1 mm to 1.0 mm
- Delamination areas ranging from 5 mm² to 50 mm²
- Intermetallic compound (IMC) layers simulating over-tempered or excessive heat input conditions
The results demonstrate that when the debonding area exceeds approximately 10 mm², the Stoneley wave amplitude drops by more than 40% relative to the sound reference signal. The phase velocity decreases by 5–12% depending on the gap width and the ultrasonic frequency used. At frequencies below 1 MHz, the wave field is less sensitive to small defects due to the longer wavelength relative to the defect size.
Influence of Liquid Filling
The presence of liquid inside the pipe introduces several complications and opportunities. On one hand, the fluid provides acoustic coupling that enhances transducer-to-wall energy transfer and reduces the need for couplant application in the field. On the other hand, the fluid load modifies the boundary conditions at the inner wall, causing mode conversion and additional attenuation. The study shows that the attenuation coefficient of the Stoneley wave increases by approximately 15–25% in liquid-filled conditions compared to dry conditions, primarily due to radiation of energy into the fluid as bulk waves.
Engineering Practice Implications
For inspection of in-service bimetallic pipes in pressure vessels, this research suggests the following practical recommendations:
- Operating frequencies between 1.5 MHz and 3.0 MHz provide the optimal balance between sensitivity to small debonding defects and sufficient signal-to-noise ratio in liquid-filled conditions.
- Time-gated signal analysis focusing on the Stoneley wave arrival window (typically 40–80 μs after the initial bulk wave arrival for a 57 mm OD pipe) enables quantitative assessment of bond quality.
- A reference block with known interface conditions should be fabricated alongside each production batch to establish baseline amplitude and phase velocity values.
Key Reflections
This work bridges the gap between theoretical elastodynamics and practical NDT requirements for bimetallic pressure vessel components. The finding that liquid filling, while complicating signal interpretation, does not eliminate the diagnostic capability of interface waves is particularly encouraging for in-situ inspection scenarios where draining the pipe is impractical or economically prohibitive. However, the study also highlights the need for careful calibration procedures and the potential for false indications caused by fluid turbulence or temperature gradients. Future work should explore phased-array transducer configurations and signal processing algorithms to further enhance defect characterization capabilities in complex geometries such as weld joints and pipe fittings within composite pipe assemblies.