Effect of Overlay Cladding on Structural Mechanical Properties and Stress Intensity Factor

Literature Overview

This paper by Chen Mingya, Zheng Yafang, Gao Hongbo, and colleagues from Suzhou Thermal Power Research Institute and Hangzhou Hangyang Turbine Machinery Co., Ltd., published in Chemical Engineering and Machinery in 2023, addresses a critical issue in pressure vessel and piping design: the influence of overlay cladding on the fracture mechanics behavior of the base material. Funded by the National Key R&D Program and the National Natural Science Foundation of China, the study investigates how weld residual stress, thermal expansion mismatch, strength matching, and the presence of overlay cladding affect the stress intensity factor (SIF) at the crack front in the base material.

Core Technical Analysis

The researchers conducted three-dimensional elastic-plastic finite element analysis to evaluate the SIF at the crack front interface between the base material and the overlay cladding layer. The analysis was performed on a pressure vessel internal surface with overlay cladding, simulating realistic operating conditions including residual stress states and thermal gradients.

A key finding is that the thermal expansion coefficient values for both materials increase when a stress-free reference temperature is set to account for the weld residual stress. The base material thermal expansion coefficient shows the largest discrepancy, reaching 23.26 percent compared to the nominal value. This finding has profound implications for finite element modeling practices in the industry.

Analysis Parameter	Nominal Value	Adjusted Value (with residual stress)	Deviation
Base material thermal expansion coefficient	Baseline	Increased	23.26%
Overlay cladding thermal expansion coefficient	Baseline	Increased	Smaller than base material
SIF at crack front (elastic analysis)	Baseline	Reference	0%
SIF at crack front (elastic-plastic analysis)	Reference	Increased	23.83%

The elastic-plastic analysis revealed that the SIF at the crack front interface point in the base material increased by 23.83 percent compared to the purely elastic analysis. This is a substantial increase that has direct consequences for the safety assessment of clad pressure vessels and piping components.

Implications for Design Codes and Standards

The study found that the current design codes and specifications apply a plastic correction factor for the effect of overlay cladding on the SIF in the base material, but this correction may be non-conservative. In other words, the standard correction underestimates the actual SIF increase caused by the overlay cladding, potentially leading to unsafe design margins.

This finding is particularly relevant to engineers working with ASME Section VIII Div. 2, API 579, and similar fitness-for-service standards that address cracked components with overlay cladding. The following table compares the approach taken in this study with typical code practices:

Aspect	Code Practice	This Study Finding	Engineering Implication
SIF correction for cladding	Fixed plastic correction factor	23.83% increase from elastic-plastic analysis	Code factor may be insufficient
Thermal expansion treatment	Nominal values	Adjusted for residual stress reference temperature	Model setup affects results significantly
Residual stress consideration	Often omitted or simplified	Explicitly included in coupled analysis	More realistic assessment required
Crack front SIF location	Interface point	Interface point confirmed as critical	Interface is the critical assessment location

Engineering Practice and Safety Assessment

For pressure vessel and piping engineers, the findings have immediate practical relevance. When performing fracture mechanics assessments on clad components, the SIF should be evaluated using elastic-plastic methods rather than relying solely on elastic solutions with code-specified correction factors. The 23.83 percent increase in SIF is not trivial; it can shift a component from acceptable to unacceptable under a given flaw size and loading condition.

The thermal expansion coefficient adjustment finding also has implications for thermal stress analysis of clad components. When performing thermal-hydraulic analyses or transient thermal stress evaluations, using nominal thermal expansion coefficients without accounting for the residual stress reference temperature can underestimate thermal stresses by more than 20 percent. This is particularly critical for components subjected to thermal cycling, such as reactor pressure vessels, steam generators, and heat exchangers.

Study Insights and Recommendations

The most significant insight from this study is the demonstration that the interaction between overlay cladding and the base material is more complex than current design codes assume. The non-conservative nature of existing plastic correction factors suggests that there may be a need for code revisions or supplemental assessment procedures for clad components. Engineers should adopt the following practices when evaluating clad components:

Use elastic-plastic finite element analysis rather than purely elastic methods for SIF evaluation at the cladding-base material interface.
Account for the stress-free reference temperature when defining material thermal expansion properties in the FEA model.
Consider the residual stress state from the overlay welding process as a permanent stress component in the fitness-for-service assessment.
Perform sensitivity analyses on the plastic correction factor to understand the margin of safety under different assumptions.

This research contributes valuable quantitative data to the fracture mechanics community and provides a technical basis for improving the safety assessment of overlay-clad pressure equipment. The findings should be considered in future code revisions and in the development of more rigorous assessment procedures for critical components in nuclear, petrochemical, and power generation industries.