Effects of MEF Defects

Effects of Macroscopically Elastic Fatigue (MEF) Defects on Subsequent Material Properties

Mechanical properties, e.g. strength and ductility, of a material that has undergone fatigue loading varies greatly depending on the material, fatigue loading parameters, and subsequent loading strain rate. Therefore, the objective of this research area is to investigate how prior fatigue loading below the yield stress, i.e. macroscopically elastic fatigue (MEF), influences the subsequent mechanical behavior under quasi-static, dynamic, thermo-mechanical, or shock loading. This research is important since specific microstructural features and dislocation arrangements are formed during MEF. These new substructures and associated defects will a affect plasticity and deformation, altering the material response compared to when deformation is initiated in pristine material. Accordingly, quantification and evolution of pre-existing microstructural changes due to MEF is critical since the subsequent loading behavior is dependent on the prior loading history.

Research Objectives

The specific objectives of this research are to investigate the effect of the pre-existing MEF-induced defects on the subsequent mechanical behavior. Once the effect of MEF on the subsequent properties in pure wrought metals or as-built AM parts are understood, we will investigate the effect of different controllable parameters such as grain size and alloying elements. Alloying elements can change, both increase and decrease, subsequent mechanical behavior due to changes in the microstructure evolution. Figure 1 overviews the developed methodology to quantify the evolution of MEF defect characteristics (e.g. size, distribution, and alignment) and track their influence on subsequent mechanical properties and failure mechanisms.

Key Findings

The following findings have been reported for pure iron:

Loading to approximately half of the fatigue life, 31% - 73%, there is a tendency of void growth over void formation. Further fatigue loading to near failure, (N=Nf) = 94%, results in an increase of both the total volume and average size of bulk voids.
Decreases in the quasi-static yield stress and ultimate tensile stress between 31%{94% of the fatigue life are due to increases in the average void size, total percentage of voids, and grouping of voids along the loading axis. Steady decreases in the yield stress are due to the continually changing void sizes, whereas a sharp decline in the ultimate tensile stress is related to the increase of void volume and grouping of voids along the same axis.
The effect of MEF induced bulk voids on the material's mechanical response was found to be strain rate dependent. There was a decrease of strength during quasi-static loading, but an increase of strength during dynamic loading. Strength increases during dynamic loading is thought to be due to an increased degree of twinning originating from the MEF induced voids.

Active Research

Studies on various material systems are currently in progress including several on aluminum. Figure 2 shows the post-fatigue mechanical response of aluminum and aluminum alloys. Of note are the lack of results on pure aluminum. Only a single study (on ultra-fine grained (UFG) aluminum) exists looking at the evolution of mechanical properties past 5% of the fatigue life. Recently obtained results, on more common grain sized aluminum, display a different behavior than the UFG aluminum. Microstructure evolution of both grains and dislocations are being inspected to explain the trends in mechanical behavior. Results from the pure aluminum and pure iron studies can be compared to study the differences in MEF defect accumulation and sensitivity between an FCC and BCC material. Aluminum 7075, manufactured by two different means, is also being researched. Wrought material alongside additively manufactured (AM) material are being tested to see how the modern process of AM compares to traditional manufacturing methods. Furthermore, this study expands from previously completed research by incorporating different fatigue stress ratios, tension-tension, tension-compression, and compression-compression.

References

[1] Joseph Indeck, Jefferson Cuadra, Cyril Williams, and Kavan Hazeli. Accumulation and evolution of elastically induced defects under cyclic loading: quantification and subsequent properties. International Journal of Fatigue, 2019.

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