Visualization of Stress-Strain Distribution on Fatigue Crack With Grain-Level Spatial Resolution via Self-Magnetic Flux Density

Fatigue crack—known for its unpredictability and rapid propagation—is influenced by various physical and chemical factors. A promising approach to understanding crack nucleation and growth involves visualizing local stresses and phase changes around defects. To achieve high-resolution mapping, ideally at the grain level, we propose a method to detect stress-strain maps through self-magnetic flux density (MFD) distribution using a linear three-axis tunnel magnetoresistive (TMR) sensor that measures in all orthogonal directions. Theoretical and experimental bases for the reciprocal relationship between stress components and MFD distribution are discussed. Experiments are conducted on three specimens with artificially induced fatigue cracks generated using a compact test (CT). The defect depths vary from 3 to 7 mm, at depth angles of 0°, 30°, and 60°. These experiments successfully map grain-level stress concentrations near the crack tip, identifying the stress concentration zone (SCZ). This analysis should enhance our understanding of stress-strain behavior associated with fatigue cracks, enabling early detection.