Development of a Method for Evaluating Microstructurally Small Fatigue Crack Initiation and Growth by Using an Automatic System for <i>In Situ</i> Observation in Conjunction with a Digital-Image Correlation Technique

Abstract

We have constructed an automatic in situ observation system for monitoring the behavior of small fatigue cracks at the microstructural level that, when used in conjunction with a digital-image correlation (DIC) technique, permits the continuous and automatic tracking and recording of microscopic deformation behavior. To verify the effectiveness of this system, we applied it to the evaluation of small fatigue cracks in heat-treated low-carbon steel. The results confirmed that our system can be used in the automatic tracking and recording of the initiation and early growth behavior of microstructurally small fatigue cracks. By the use of DIC analysis, we also succeeded in visualizing the opening-and-closing behavior of small fatigue cracks as well as the behavior of microscopic microstructural deformations, such as inhomogeneous strain concentrations, that caused the fatigue cracks. Although the early-stage growth of fatigue cracks propagates faster than that of long cracks, it is consistent with long-crack data if the effective stress intensity factor range ΔK eff which calculated by crack opening stress measured by DIC is used.

Evaluation of Fatigue Crack Initiation and Propagation Properties of Structural Steels with Different Cyclic Softening Behavior Based on Local Strain

“…To expand the applicable range of the abovementioned method and improve the model to further improve the estimation accuracy, we used the digital image correlation (DIC) method to measure the strain field around fatigue cracks and evaluate their initiation and propagation life 32) . The application of the DIC method to fatigue cracks has progressed in recent years; additionally, researchers have extensively investigated artificial defects and micro fatigue cracks on the surface of steel materials [33][34][35][36] . However, studies that systematically discuss steel properties, local strain fields, and fatigue properties are few, and no studies have been performed to investigate cyclic softening properties.…”

Strain Distribution Analysis of Two Perpendicular Planes in SUS310S Austenitic Stainless Steel Using Digital Image Correlation

“…Deformation behavior has been computationally investigated using the finite element method (FEM) [1][2][3] and, more recently, digital image correlation (DIC). [4][5][6][7][8][9][10][11] The latter is an innovative noncontact optical technique to measure the strain and displacement in metals.…”

Analyses of 3D non‐planar fatigue crack propagation with smoothed particle hydrodynamics

“…The fatigue fracture process consists of microscopic crack initiation due to slip in the microstructure of the material 3–10 and macroscopic fracture due to crack propagation 10–32 . In particular, the latter relates to the critical fracture of fatigue damage.…”

“…According to the above industrial requirements, many theoretical, 11,14–21 experimental, 10,11,22–32 and computational 33–55 studies on fatigue crack propagation have been carried out. Notably, several important computational studies have been performed in this area using computing methods like the element‐free Galerkin method, 33,34 the free mesh method 35,36 (both are the pioneering works of the computational fatigue crack propagation), the extended finite element method (X‐FEM, the initial stage, 37–40 the development and the implementation 44–50 of crack modeling 41–43 ), the smoothed finite element method, 51,52 and phase‐field modeling 53–55 (which challenge the new approach by using the FEM framework), and so on.…”