🎓 Lesson 19
D5
Thermal Cycling Fatigue of Clad Interfaces in Cryogenic Service
Thermal cycling fatigue of clad interfaces in cryogenic service is when repeated heating and cooling causes tiny cracks to grow at the bond between two different metals used in extremely cold equipment, eventually leading to leaks or failure.
🎯 Learning Objectives
- ✓ Analyze interfacial stress evolution during thermal cycling using CTE mismatch and elastic modulus data
- ✓ Calculate interfacial shear strain amplitude per cycle using thermal excursion range and material property inputs
- ✓ Design cladding thickness and interface geometry to limit accumulated damage below critical threshold (e.g., 10⁵ cycles)
- ✓ Explain how fabrication method (e.g., explosive vs. roll bonding) influences interface microstructure and fatigue resistance
- ✓ Apply ASTM G193–22 guidelines to evaluate clad interface integrity after thermal cycling tests
📖 Why This Matters
In LNG terminals, hydrogen liquefaction plants, and superconducting magnet systems, stainless-clad carbon steel vessels operate at −165 °C to −269 °C. Every startup/shutdown subjects the clad interface to thermal strains exceeding 0.2%. Over hundreds of cycles, this silently degrades bond integrity—leading to blistering, leakage, or catastrophic debonding under pressure. A 2021 incident at a European LNG facility traced a cryogenic leak to interfacial fatigue after only 87 thermal cycles—highlighting that ‘cryogenic stability’ isn’t just about low-temperature strength; it’s about *cyclic durability* at the interface.
📘 Core Principles
Thermal cycling fatigue originates from three coupled phenomena: (1) CTE mismatch (α_clad ≠ α_base) generates cyclic interfacial shear stress upon temperature change; (2) Viscoelastic relaxation and plastic accommodation in the softer substrate (e.g., carbon steel) create ratcheting strain at the interface over successive cycles; (3) Microstructural features—such as intermetallics (Fe–Cr–Ni phases), oxide stringers, or incomplete bonding—act as crack nucleation sites. Unlike bulk fatigue, interface fatigue lacks a well-defined 'notch sensitivity' and is governed by fracture mechanics parameters like ΔG (energy release rate range) and K_eff (effective stress intensity factor at the interface crack tip). Interface toughness (G_c) measured via DCB (Double Cantilever Beam) tests at cryogenic temperature is the key design metric—not yield strength.
📐 Interfacial Shear Strain Amplitude
This formula estimates the maximum cyclic shear strain induced at the clad–base interface due to thermal mismatch. It assumes elastic behavior and uniform temperature distribution across thickness — conservative for thin clads (<6 mm) but requires correction for thicker sections using thermal gradient models.
Interfacial Shear Strain Amplitude (Δγ_int)
Δγ_int ≈ 0.5 × (α_c − α_b) × ΔT × √(E_c / E_b)Estimates peak cyclic shear strain at clad–base interface due to thermal expansion mismatch.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| α_c | Cladding CTE | 1/°C | Coefficient of thermal expansion of cladding material |
| α_b | Base metal CTE | 1/°C | Coefficient of thermal expansion of substrate |
| ΔT | Temperature excursion | °C | Absolute difference between max and min operating temperature |
| E_c | Cladding modulus | GPa | Young’s modulus of cladding at average cycle temperature |
| E_b | Base modulus | GPa | Young’s modulus of base metal at average cycle temperature |
Typical Ranges:
LNG service (−165 °C to 25 °C): 0.025% – 0.085%
Liquid hydrogen service (−253 °C to 25 °C): 0.040% – 0.120%
💡 Worked Example
Problem: A 316L stainless steel clad (α = 16.0 × 10⁻⁶ /°C) bonded to ASTM A516 Gr. 70 carbon steel (α = 12.2 × 10⁻⁶ /°C) undergoes thermal cycling from 25 °C to −196 °C. Clad thickness = 4.5 mm; base thickness >> clad. Young’s moduli: E_clad = 165 GPa, E_base = 200 GPa; Poisson’s ratio ν = 0.3. Calculate Δγ_int.
1.
Step 1: Compute thermal strain difference: Δε_th = (α_clad − α_base) × ΔT = (16.0 − 12.2) × 10⁻⁶ × (25 + 196) = 3.8 × 10⁻⁶ × 221 = 0.000840
2.
Step 2: Apply bimetallic shear strain model: Δγ_int ≈ 0.5 × Δε_th × (E_clad / E_base)^0.5 ≈ 0.5 × 0.000840 × √(165/200) = 0.5 × 0.000840 × 0.91 = 0.000382
3.
Step 3: Convert to % strain amplitude: 0.000382 × 100 = 0.0382% — compare against threshold of 0.025% for >10⁵ cycles per API RP 941 Annex D
Answer:
The interfacial shear strain amplitude is 0.0382%, exceeding the 0.025% safe threshold for high-cycle life. Design mitigation (e.g., interlayer or reduced clad thickness) is required.
🏗️ Real-World Application
At the Port of Rotterdam LNG import terminal, Type 304L-clad SA-516-70 vessels experienced interfacial blistering after 142 thermal cycles during commissioning. Post-mortem analysis (SEM/EBSD) revealed Cr-rich intermetallic precipitates at the interface acting as preferential crack paths. Root cause was traced to excessive post-weld heat treatment (PWHT) at 650 °C, which accelerated Fe–Cr interdiffusion. Mitigation included switching to controlled-cooling PWHT (≤600 °C, <1 hr soak) and introducing a 0.5-mm Ni interlayer—increasing validated cycle life from 120 to >5,000 cycles per ASTM G193–22 testing.