Buckling Failure Mechanism of Lining Layer in Bimetal Mechanical Composite Pipes under Bending Load

Literature Overview and Problem Statement

Bimetal mechanical composite pipes—fabricated by the hydraulic expansion process or explosive cladding—find extensive application in the oil, gas, chemical, and nuclear industries where a corrosion-resistant inner lining is bonded to a high-strength structural outer shell. Typical configurations include stainless steel or nickel-based alloy linings bonded to carbon steel or low-alloy steel outer pipes, providing resistance to sour service, high-temperature oxidation, or chemical corrosion while maintaining structural integrity.

A critical failure mode that receives insufficient attention in design practice is the buckling of the inner lining layer under bending loads. During field installation, piping systems are routinely bent to accommodate routing constraints, and the bending process imposes significant interfacial stresses that can initiate delamination or buckling of the thinner, lower-strength lining layer. The literature under study provides a systematic investigation of this buckling failure mechanism, combining theoretical analysis, finite element simulation, and experimental validation to establish the critical conditions under which lining buckling occurs.

Core Failure Mechanism Analysis

Stress State in Bimetal Pipes under Bending

When a bimetal pipe is subjected to bending, the composite cross-section develops a non-uniform stress distribution. The outer shell, being stiffer and stronger, carries the majority of the bending moment, while the inner lining experiences a combination of hoop stress, longitudinal stress, and radial interfacial pressure. The key insight from the literature is that the interfacial radial pressure is not uniform—it varies along the pipe circumference, with maximum compressive pressure on the inner compression side of the bend and potential tensile separation on the outer tension side.

Buckling Initiation and Propagation

The buckling of the lining layer occurs when the interfacial compressive stress exceeds the critical buckling pressure of the lining. This is analogous to the buckling of a thin cylindrical shell under external pressure, but with the important distinction that the lining is bonded to the outer shell and experiences a non-uniform pressure distribution. The literature identifies three primary buckling modes:

1. Axisymmetric buckling: The lining deforms uniformly in the circumferential direction, forming a diamond-shaped deformation pattern. This mode is typically associated with high interfacial pressure and low lining thickness.
2. Non-axisymmetric buckling: The lining deforms in a localized manner, forming localized dimples or wrinkles. This mode is more common in practice and is driven by geometric imperfections and material non-uniformities.
3. Interfacial delamination: At the tension side of the bend, the lining may separate from the shell, creating a void that can propagate under cyclic loading. This is particularly dangerous in sour service where hydrogen ingress through the delamination can cause hydrogen-induced cracking.

Critical Buckling Pressure Derivation

The literature derives the critical buckling pressure using classical shell buckling theory modified for the bimetal configuration. The critical pressure is given by:

P_cr = (E_l t_l^3) / (4 R^3 (1 - ν_l^2)) f(θ, L/R)

Where E_l is the Young's modulus of the lining, t_l is the lining thickness, R is the inner radius, ν_l is the Poisson's ratio, and f(θ, L/R) is a function of the angular position and the length-to-radius ratio. The analysis shows that the critical pressure is highly sensitive to the lining thickness (cubic relationship) and relatively insensitive to the lining modulus, which has important implications for material selection.

Experimental Validation and Key Parameters

Test Configuration and Results

The experimental program involved bending bimetal pipes with various configurations and measuring the interfacial pressure using embedded pressure sensors and strain gauges. The key findings are summarized below:

Influence of Process Parameters

The hydraulic expansion process parameters directly affect the interfacial bond quality and residual stress state, which in turn influence buckling resistance. The literature identifies the following critical process parameters:

• Expansion ratio: An expansion ratio of 0.5-1.5% (relative to the outer shell inner diameter) provides optimal interfacial contact without causing plastic deformation of the lining. Excessive expansion (>2%) introduces high residual stresses that reduce buckling resistance.
• Lubricant type: The lubricant used during hydraulic expansion affects the friction coefficient at the interface, which influences the residual stress distribution. Low-friction lubricants (such as molybdenum disulfide) reduce tangential residual stresses and improve buckling resistance.
• Temperature: Cold expansion at room temperature produces higher residual stresses than warm expansion at 150-200°C. Warm expansion is preferred for applications where bending is expected during installation.

Engineering Practice and Design Recommendations

Minimum Bending Radius Guidelines

Based on the analysis and experimental results, the following guidelines are recommended for the design of bimetal mechanical composite pipe systems:

These values represent the minimum safe bending radius under cold bending conditions. For hot bending (with preheating to 200-300°C), the minimum radius can be reduced by approximately 20-30% due to stress relief.

Inspection and Quality Control

Post-bending inspection is critical to detect any interfacial damage. The recommended inspection sequence is:

1. Visual inspection: Check for surface deformation, wrinkles, or visible separation at the bend region.
2. Ultrasonic testing (UT): Use dual-element probes to detect interfacial delamination. The inspection should cover the entire bend circumference with a scan rate of 200 mm/s or less.
3. Eddy current testing (ET): For thin linings (<2 mm), eddy current testing provides high sensitivity to interfacial defects. The probe frequency should be selected based on the lining thickness to optimize skin depth.
4. Dye penetrant testing (PT): Applied to the cut ends of bent pipes to detect any interfacial separation that may have propagated from the bend region.

FMEA Analysis for Bending Operations

A Failure Mode and Effects Analysis (FMEA) for the bending operation of bimetal mechanical composite pipes identifies the following critical failure modes:

The highest RPN values are associated with lining buckling and ovalization, confirming that bending radius control and process parameter optimization are the most critical quality factors.

Key Questions and Reflections

The study highlights an important gap in current design practice: most piping codes (such as ASME B31.3 or GB/T 20801) specify minimum bending radii for homogeneous pipes but do not address the specific requirements for bimetal mechanical composite pipes. The literature's findings suggest that the minimum bending radius for bimetal pipes should be significantly larger than for homogeneous pipes of the same outer shell material, particularly when the lining is thin and the outer shell is thick.

Another important reflection is the role of residual stresses from the manufacturing process. The hydraulic expansion process introduces significant residual stresses in both the lining and the outer shell, and these stresses interact with the bending stresses to either promote or resist buckling. A comprehensive design methodology should therefore include a residual stress assessment as part of the bending analysis, which is currently not addressed in any of the major piping codes.

Study Insights and Implications

The investigation of lining buckling under bending loads provides valuable insights for the design and fabrication of bimetal mechanical composite pipe systems. The cubic dependence of critical buckling pressure on lining thickness emphasizes the importance of maintaining adequate lining thickness throughout the bending operation, and suggests that thicker linings should be specified for applications where significant bending is anticipated.

For engineering practice, the key recommendations are: (1) always verify the minimum bending radius against the specific bimetal configuration, (2) perform post-bending inspection using appropriate NDT methods, (3) consider warm bending for thin linings to reduce residual stress effects, and (4) document the bending process parameters for traceability and future analysis. These practices, combined with the analytical tools presented in the literature, can significantly reduce the risk of lining buckling failures in field installations.