Determining Residual Stress Fields and Plastic Zone Sizes Surrounding Fatigue Cracks Using Nanoindentation
Additional Funding Sources
This project is supported by a 2019-2020 STEM Undergraduate Research Grant from the Higher Education Research Council.
Abstract
Crack propagation is typically observed in systems that experience cyclic loading, often resulting in critical failure (fatigue). Compressive residual stresses induced in a material contribute to stunting crack growth and can increase the fatigue life or prevent failure. The tensile loading of cracks creates a plastic zone at their tips, and growth of the crack leaves a "plastic wake" region adjacent to the newly created surface wherein the material has been strained. While some models and validation techniques to describe and measure the plastic zone size are available, the plastic zone has not previously been measured using nanoindentation. Nanoindentation is the process of inducing compressive plastic deformation at a micrometer length scale, typically using a pyramidal indenter probe. Data analysis of the force vs. displacement data can be used to determine the specimen's material properties. By coupling nanoindentation with atomic force microscopy (AFM) to measure material upheaval around the impression, the residual stress field can be determined at the tip and the wake surrounding a crack. These data can also be used to model the plastic zone around the tip of a crack. This research will provide a comparison between the nanoindentation method and a mathematical model in the literature. Ultimately, results from these experiments can help us to understand crack propagation and potentially provide a non-destructive fatigue damage prediction.
Determining Residual Stress Fields and Plastic Zone Sizes Surrounding Fatigue Cracks Using Nanoindentation
Crack propagation is typically observed in systems that experience cyclic loading, often resulting in critical failure (fatigue). Compressive residual stresses induced in a material contribute to stunting crack growth and can increase the fatigue life or prevent failure. The tensile loading of cracks creates a plastic zone at their tips, and growth of the crack leaves a "plastic wake" region adjacent to the newly created surface wherein the material has been strained. While some models and validation techniques to describe and measure the plastic zone size are available, the plastic zone has not previously been measured using nanoindentation. Nanoindentation is the process of inducing compressive plastic deformation at a micrometer length scale, typically using a pyramidal indenter probe. Data analysis of the force vs. displacement data can be used to determine the specimen's material properties. By coupling nanoindentation with atomic force microscopy (AFM) to measure material upheaval around the impression, the residual stress field can be determined at the tip and the wake surrounding a crack. These data can also be used to model the plastic zone around the tip of a crack. This research will provide a comparison between the nanoindentation method and a mathematical model in the literature. Ultimately, results from these experiments can help us to understand crack propagation and potentially provide a non-destructive fatigue damage prediction.