Full-field Strain Measurements for Microstructurally Small Fatigue Crack Propagation Using Digital Image Correlation Method

Full-field strain measurements for micro-structurally small fatigue crack propagation using Digital Image Correlation method. New lightweight solutions are required to improve the energy efficiency of vehicles such as ships, weight reduction of large steel structures, is possible using advanced steel materials. The efficient utilization requires high manufacturing quality and robust design methods, robust design methods mean structural analysis under realistic loading conditions, such as under wave induced loading in the case of a cruise ship, structural strength analysis of structures includes response calculations to define deformation and stresses, the allowed stress level is defined based on the strength of critical structural details, in the case of large structures it is typically welded joints within homogeneous micro-structure, one of the key design challenges is fatigue due to its cumulative and localized nature, for instance at the weld notch, for high manufacturing quality the most important issue is small fatigue crack initiation and propagation, since crack like manufacturing defects are neglected.

This research studies small fatigue crack and introduces a new, experimental approach, the novelty of the approach consists of in-situ full field strain measurement using unique pattern technique, combined with crank growth rate measurement at the same time the micro-structural analysis reveals the impact of shear stress concentrations and grain boundaries on small fatigue crack retardation. We explain the main steps of the measurement procedure and provide a summary discussion of the main finding. Step one, specimen preparation and annealing, the steel plate is annealed in nitrogen atmosphere at a temperature of 1200 degrees celsius for one hour and quenched in water, the annealing procedure results in an increase of the average grain size of the studied steel up to 349 micrometers, without extension formation of chromium carbide particles, the notched specimens with thickness of one millimeter are cut from the annealed plate of the studied ferritic steel using electrical discharge machining, the scheme of the specimen is shown here.

Specimen surfaces are polished, finishing with naught point naught two micrometer colloidal silica vibratory polishing that is required for electron back scatter diffraction analysis. Step two, fatigue pre-cracking, specimen is subjected to uniaxial cyclic loading and fatigue frequency 10 hertz, initial cracks with length from one micrometer to 20 micrometers are produced at the notch tip. Optical monitoring of the initial crack formation after 10, 000 cycles of the cycle loading, repeat the cycle loading testing if initial crack was not produced.

Step three, micro-structural characterization, vickers micro-indentation marks are used to mark the area of interest, micro-structure of the steel is studied from the side surface of the specimen in the vicinity of the notch using electron backscatter diffraction analysis. Schmid factor and grain boundary misorientation analysis is carried out is shown here. Step four, decoration with a pattern, clean the specimen surface with ethanol, deposit thin layer of ink on the glass surface, press down the silicone stamp with a pattern on the glass to move a layer of ink to the stamp's surface, we use a custom made pneumatic tool for fast and precise operation with the stamp, press down the silicone stamp covered with the ink on the specimen surface, check the speckle pattern quality using optical microscopy, an example of the speckled pattern is shown here.

Step five, fatigue testing with digital image correlation, run the fatigue testing and synchronization with the image recording system, the fatigue testing continues while the crack length approaches a critical value or plastic deformation starts to dominate. Step 6, results analysis, the obtained images are analyzed using a commercial software to perform the crack growth rate calculation and digital image correlation analysis, analysis of the shear strain deformation is performed for the studied area, cumulative analysis of the obtained results, use of the vickers micro-indentation marks for the proper alignment of the shear strain deformation field with electron backscatter diffraction mapping data, grain boundaries, grain orientation map. Representative results, shear strain field accumulation at sub-grain size during short fatigue crack propagation, combine view of the shear strain field accumulation and micro-structure of the studied steel, combination of the crack growth rate and shear strain accumulation analysis give a possible mechanism of the small fatigue crack growth, small fatigue crack propagates starting from the initial crack produced by pre-cracking procedure, shear strain zone localizes ahead of the crack tip and size of the shear strain zone grows while the crack propagates towards the localization, when crack approaches the strain localization zone, the crack growth rate decreases significantly due to change of the crack propagation mode, the crack growth rate increases continuously after the crack crosses the center of the strain localization zone, crack growth rate starts to decrease again as soon as the next strain localization zone has formed ahead of the crack tip.

Conclusion, novel research provides deeper understanding of small fatigue crack growth behavior, combination of crack growth rate measurement and strain-field analysis at sub-grain level helps to reveal the mechanism responsible for anomalous growth of the small fatigue cracks, this deeper understanding of small fatigue crack growth behavior makes it possible to develop new theoretical approaches and thus enable the design of more lighter and energy efficient structures in the future.