Study of nondestructive evaluation of mechanical stresses

Abstract

Thesis: Magnetomechanical Acoustic Emission (MAE) Studies for Residual Stress Evaluation in Structural Steels Introduction Quantitative measurement of residual stresses is an important aspect of structural integrity evaluation. In particular, estimation of stresses without recourse to strain measurement, and in a nondestructive manner, has been a long-felt need. A review of the state of the art reveals that even after years of research, very few techniques have this capability.

Magnetomechanical Acoustic Emission (MAE), one of the latest entrants to the field of Nondestructive Evaluation (NDE), appears promising for ferromagnetic materials and has drawn considerable attention in the acoustic emission (AE) research community.

MAE is defined as transient stress waves generated due to energy released during rotation and motion of magnetic domain walls. The quantity and quality of MAE generated from a material are related to its stress state, apart from other parameters such as chemical composition, microstructure, and heat treatment condition. Thus, understanding MAE behaviour of a given material becomes a prerequisite for qualitative and quantitative evaluation of MAE in relation to stress.

Materials Investigated Four structural steels were chosen for the current investigations:

EN 24 – used in high-strength applications including aircraft engine mounts.

Maraging steel – used for rocket motor casings.

IS 2062 – used for marine applications.

Mild steel – general-purpose structural steel.

Objectives The investigations emphasized:

Sensitivity aspects in relation to spectral content.

Qualitative differences in MAE response between the four materials.

Correlation with respect to applied magnetic field.

Stress dependence of MAE.

Nature of MAE events obtained from the materials considered.

Experimental Methods Applied stress was used as a measurable substitute for residual stress.

Plain tensile specimens with flat grips were designed to maintain continuous magnetic flux lines.

Magnetization was achieved using an encircling coil with 50 Hz alternating current supply, circumventing the skin effect.

Acoustic isolation was ensured to minimize extraneous noise.

Magnetically shielded transducers were used to avoid interference from applied and induced magnetic fields.

Current supplied to the coils was varied in discrete steps using a variac.

MAE data was acquired at specific load levels.

Vrms was chosen as the representative MAE parameter, as it provides an integrated picture of the signal. Event-based analysis was attempted but limited by equipment constraints at 50 Hz magnetization frequency. To overcome this, a pulse magnetizer was designed and developed for event-based analysis.

Results Sensitivity studies showed that the 1500 kHz transducer with 0.125–2 MHz filter yielded maximum Vrms, while the 175 kHz transducer with 125–250 kHz filter showed a comparative percentage increase.

All four materials exhibited nonlinear magnetization characteristics even below saturation.

Signal envelopes differed among materials, indicating the need for event-based analysis.

Precipitates, inclusions, and impurities acted as pinning sites, inhibiting magnetic domain wall motion.

Differences in MAE activity correlated with carbon content, suggesting a possible quantitative correlation between Vrms and carbon content.

IS 2062 was found to be “Active”, while EN 24, maraging steel, and mild steel were comparatively “Inactive.”

The relationship between MAE and stress was generally nonlinear:

EN 24 and mild steel showed a consistent decrease in MAE level with increasing tensile stress.

Maraging steel exhibited different behaviour due to nickel content.

Pulse magnetization facilitated identification and analysis of individual events, improving the signal-to-noise ratio considerably.

Conclusions MAE shows strong potential as a nondestructive technique for residual stress estimation in ferromagnetic materials.

Material composition, especially carbon and alloying elements, significantly influences MAE behaviour.

Event-based analysis using pulse magnetization enhances signal clarity and provides deeper insights into MAE characteristics.

Further controlled experiments are needed to establish quantitative correlations between MAE parameters and material properties.