The Role of Surface Topography on Deformation-Induced Magnetization under Inhomogeneous Elastic-Plastic Deformation

Abstract

It is widely accepted that the magnetic state of a ferromagnetic material may be irreversibly altered by mechanical loading due to magnetoelastic effects. A novel standardized nondestructive testing (NDT) technique uses weak magnetic stray fields, which are assumed to arise from inhomogeneous deformation, for structural health monitoring (i.e., for detection and assessment of damage). However, the mechanical and microstructural complexity of damage has hitherto only been insufficiently considered. The aim of this study is to discuss the phenomenon of inhomogeneous “self-magnetization” of a polycrystalline ferromagnetic material under inhomogeneous deformation experimentally and with stronger material-mechanical focus. To this end, notched specimens were elastically and plastically deformed. Surface magnetic states were measured by a three-axis giant magnetoresistant (GMR) sensor and were compared with strain field (digital image correlation) and optical topography measurements. It is demonstrated that the stray fields do not solely form due to magnetoelastic effects. Instead, inhomogeneous plastic deformation causes topography, which is one of the main origins for the magnetic stray field formation. Additionally, if not considered, topography may falsify the magnetic signals due to variable lift-off values. The correlation of magnetic vector components with mechanical tensors, particularly for multiaxial stress/strain states and inhomogeneous elastic-plastic deformations remains an issue.

Evaluation of Ferromagnetic Steel Hardness Based on an Analysis of the Barkhausen Noise Number of Events

“…The authors, as well as other scientists [37], are aware of the deficit in basic research on the relations between changes in a broadly understood state of the material and changes in electromagnetic properties, both at the micro and macroscopic scale. Another important research issue that emerged while analyzing the testing results is the impact of the plastic strain process (whether it proceeds continuously or in stages) on changes in parameters of the Barkhausen noise quantitative description.…”

Demonstration of an OPM flux guide scanner for high-resolution magnetic non-destructive testing

“…While OPMs are commonly used in the medical field for imaging of brain activity [7], one promising application is magnetic imaging for non-destructive testing (NDT) according to the metal magnetic memory (MMM) technique [8,9]. This technique measures local stress concentrations caused by stress sites such as weld seams or notches by imaging magnetic stray fields 𝐵 ⃑ at the surface of ferromagnetic samples (see Figure 1) [10]. The physical background is that the specific magnetization per volume, 𝑚 ⃑⃑ , depends on the mechanical stress in ferromagnetic materials like ferritic steel [11].…”

Quantum magnetometry with OPM: Novel applications in non-destructive testing?

“…They achieve similar sensitivities as superconducting quantum interference devices (SQUIDs), but without complex and expensive cryogenic cooling. As passive magnetic NDT methods like "metal magnetic memory" (MMM) measure local variations of magnetization due to stress or microscopic structure within a volume in ferromagnetic materials, OPM are beneficial to decrease that measurement volume [11][12][13], i.e., to increase either lateral resolution for stress concentration measurement or depth resolution for structural changes like martensite-austenite transitions in hardening layers. The major challenges for using these systems are to control the measurement volume and to suppress ambient perturbations from microtesla to picotesla scale.…”