Fatigue Initiation Resistance in Shape Memory Alloys-Theory and Experiments

Non-technical Summary

Understanding of how to prevent the formation of cracks at the atomic scale of materials, known as fatigue, that are used in aerospace, automotive, and defense applications is paramount to avoid catastrophic structural failure that could result in loss of human life. Fatigue of a material begins at the atomic scale under repeated physical pressure (loading) and result in nucleation (birth) of small cracks.

The cracks can grow under variable loads and produce component and structural failures. New experimental tools, such as high resolution microscopy, allows visualization of the atomic motions that are responsible for fatigue phenomenon, and these measurements provide unprecedented insight into processes that result in the beginning of microscopic crack formation.

These experiments can also provide a critical check on the models aimed at predicting when fatigue starts. With better understanding, one can develop new materials that withstand fatigue. The focus of the work is on a special class of metals, called shape memory materials, that change their shape upon load and recover their original shape upon removal of the load.

This phenomenon is similar to how rubber stretches and returns to its original shape upon release of the applied force. These shape memory materials can potentially exhibit higher fatigue resistance compared to conventional steels and aluminum alloys. The proposed work will advance understanding of the mechanism of nucleation and improve fatigue lifetimes, quantifying stochasticity (variability in the results) linked to the underlying microstructure, ultimately improving the safety and reliability of components and structures.

To enhance education in this field, senior design projects that involves building a fatigue test machine will be introduced. A new textbook on fatigue that covers current methods of measuring fatigue and the different models of fatigue will also be produced. Technical Summary

The intellectual aims of this work centers on a better understanding of fatigue initiation behavior from fundamental atomistic to representative micro-mechanical scales to enable determination of fatigue resistance and enhanced predictability of lifetime. To verify the models at various length scales and mitigate risks of any unwarranted artefacts in the predictive procedures, the proposed experiments include single-crystal mechanical tests and High Resolution Transmission Electron Microscopy along multiple judiciously chosen zone axes.

Information obtained will be of a 3D nature and will be analyzed using Template Matching (TeMA) and Geometric Phase Analysis (GPA) methods, further developing advanced algorithms for the same. The modeling efforts will incorporate Frank-Bilby concepts for defect evolution along with Molecular Statics to study the energy barriers and Anisotropic Elasticity Theory for slip under to- and fro-(cyclic) loading, as it is established that material performance enhancement for fatigue resistance can be achieved by controlling such characteristics at lower length scales.

Thus, utilizing such a unique combination of techniques provides greater insight into the processes responsible for the structural evolution under fatigue and allows for the direct observation of the underlying damage processes. Therefore, the proposal will create significant broader impacts in terms of improving fatigue design for high performance shape memory alloys by creating a new set of tools.

Early studies have not elucidated the experiments and theory addressing processes at length scales that are relevant to fatigue nucleation. This research will have the capability to examine potentially new materials that currently remain untested but promise considerable advantages. The outreach efforts include preparation of a textbook on fatigue combining theory and experiments, and a design project for fatigue initiation experiments under rotary-bending of wires subject to different mean strains

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.