Strain Field Analysis in the Near Stationary Point Region of Explosive Cladding

Literature Overview and Fundamental Principles

Explosive cladding (also known as explosive welding) is a solid-state joining process that produces metallurgical bonds between dissimilar metal sheets through high-velocity impact collision. The process involves the detonation of a shaped explosive charge positioned between a flyer plate and a base plate, accelerating the flyer to velocities typically in the range of 200–800 m/s depending on the material combination and process parameters. At the collision interface, a characteristic stationary point exists where the relative velocity between the flyer and base plate approaches zero, creating a unique deformation zone that governs the quality of the metallurgical bond.

This study focuses on the strain field analysis in the near-stationary-point region, which is of paramount importance for understanding the bonding mechanism, predicting defect formation, and optimizing process parameters. The stationary point represents the transition between the jet region (where material is ejected ahead of the collision point) and the bonded region, and the strain state in this vicinity determines whether a sound metallurgical bond or a lack-of-bond defect develops. Understanding the strain distribution is essential for engineers responsible for explosive cladding process design, particularly for production of clad plates used in pressure vessels, heat exchangers, and chemical processing equipment.

Strain Field Characteristics and Analysis

The strain field in the near-stationary-point region exhibits a highly non-uniform distribution characterized by intense shear deformation, compressive straining, and complex strain gradient variations. The analysis employs both experimental techniques including Digital Image Correlation (DIC) and High-Speed Photography, combined with finite element simulation using hydrodynamic models and coupled Eulerian-Lagrangian (CEL) formulations.

The strain field analysis reveals several critical features. First, the equivalent plastic strain in the immediate vicinity of the stationary point reaches values of 3.0–8.0, far exceeding the uniform elongation of most engineering metals. This extreme deformation is necessary to fracture surface oxides and contaminants, exposing clean metal surfaces that can form metallurgical bonds through atomic diffusion and mechanical interlocking. Second, the strain rate in this region reaches 10^4–10^6 s^-1, placing the material response firmly in the viscoplastic regime where strain rate effects significantly influence the deformation behavior.

Strain Gradient Distribution

The strain gradient distribution near the stationary point follows a characteristic pattern that can be described by a power-law decay from the collision interface. The equivalent strain ε_eq at a distance d from the stationary point can be approximated as:

ε_eq(d) = ε_max × exp(-d/λ)

where ε_max represents the maximum strain at the interface (typically 5.0–8.0) and λ is a characteristic decay length that depends on the material combination and collision angle. For typical steel/stainless steel combinations, λ ranges from 0.1 to 0.3 mm, indicating that the intense deformation is confined to a very narrow band adjacent to the bond interface.

The strain state in the near-stationary-point region is predominantly compressive in the direction normal to the interface, with significant shear strain components parallel to the interface. The stress triaxiality ratio (σ_m/σ_eq) varies from approximately -0.5 to -0.8 in this region, indicating a compressive hydrostatic stress state that suppresses void formation and promotes densification. This compressive stress state is a critical factor in achieving sound bonds, as it prevents the formation of microvoids that would otherwise develop under tensile stress conditions.

Defect Formation Mechanisms and Process Optimization

The strain field analysis provides fundamental insights into the formation mechanisms of common defects in explosive cladding. Lack-of-bond defects occur when the collision velocity is insufficient to generate adequate strain at the interface, resulting in incomplete oxide fracture and poor metallurgical bonding. The minimum collision velocity required for bonding can be estimated from the strain field analysis using the criterion that the equivalent strain at the interface must exceed a critical value ε_crit, which depends on the oxide layer thickness and material properties.

The analysis also reveals the mechanism of wave pattern formation at the bond interface. The characteristic wavy interface morphology, which is a hathe writing systemark of successful explosive cladding, results from a hydrodynamic instability that develops at the collision point. The wavelength of the wave pattern is directly related to the collision velocity and angle, and can be predicted from the strain field analysis using the Rayleigh-Taylor instability criterion. Typical wavelength values range from 0.5 to 5.0 mm depending on the process parameters, with shorter wavelengths associated with higher collision velocities and larger collision angles.

Engineering Practice and Quality Assessment

For engineering practice, the strain field analysis provides a quantitative basis for establishing process windows and quality acceptance criteria for explosive cladding operations. The key process parameters that must be controlled include: explosive charge geometry and quantity, flyer plate thickness and velocity, collision angle, and base plate thickness. The optimal collision angle for most material combinations falls within the range of 10°–25°, with angles below 10° resulting in insufficient strain and angles above 25° causing excessive material loss through jet formation.

The quality assessment of explosive clad plates should incorporate strain-based criteria in addition to conventional bond strength testing. The recommended quality indicators include: wave pattern wavelength of 1.0–3.0 mm (indicating appropriate collision conditions), bond strength exceeding 25 MPa in tensile shear testing per ASTM A263, and hardness gradient across the interface showing evidence of plastic deformation without intermetallic compound formation. For critical applications such as pressure vessel cladding, additional inspection using ultrasonic testing per NB/T 47013 should be performed to detect lack-of-bond areas.

Study Insights and Practical Implications

The strain field analysis presented in this study provides a fundamental understanding of the bonding mechanism in explosive cladding that goes beyond empirical process optimization. The recognition that equivalent strains of 3.0–8.0 are required at the interface establishes a clear physical criterion for process adequacy, enabling engineers to predict bonding quality from process parameters without relying solely on trial-and-error optimization. The strain rate sensitivity of the deformation behavior highlights the importance of collision velocity control, as variations of even 10–20 m/s in flyer velocity can significantly alter the strain field and bonding quality. For production engineering, the most significant implication is the need for precise control of explosive charge geometry and placement to ensure consistent collision conditions across the entire clad plate area, particularly for large-format plates where velocity gradients can develop due to charge detonation non-uniformity. The integration of strain field analysis with modern finite element simulation provides a powerful tool for virtual process optimization, reducing the need for expensive physical trials while maintaining confidence in the bonding quality of production clad plates.