In this dissertation, the fatigue characteristics of three newly developed experimental compositions for aluminum 7175, with improved mechanical strength, that uses magnesium-calcium alloy instead of pure magnesium are studied.
Specimens of each variant were fabricated and subjected to fatigue life testing, fatigue life data analysis, and observation of their fracture characteristics through optical microscopy and scanning electron microscopy (SEM), and metallography to study their grains and surface characteristics.
Fatigue life testing shows all three variants have a fatigue strength that is approaching approximately 200 MPa. ECO7175v3 is shown to have the highest fatigue strength of approximately 220 MPa at 5x107 cycles, approximately 40% of its tensile strength of 550 MPa. This is shown by its considerably higher fatigue strength coefficient determined by Basquin's equation compared to the other two variants. ECO7175v1 is shown to generally have large scatter in its fatigue life at higher stress levels (65% or higher of their tensile strength) with coefficient of variations typically twice or more to those of ECO7175v2 and ECO7175v3.
The results of the SEM analysis shows that irrespective of the stress levels, ECO7175v1 and ECO7175v3 all have crack initiation points at the surface with no inclusions to act as stress concentrators. The lack of inclusions are supported by the reliability analysis which shows the hazard rates for all variants remains relatively constant the majority of the time before increasing towards the end. These trends for all variants indicates failures are due to wear-outs instead of defects, which were not seen. Reliability analysis also shows that at any given fatigue life cycle and stress level, ECO7175v3 has a lower probability of failure when compared to ECO7175v1 and ECO7175v2. On the other hand, at any given fatigue life cycle and stress level, ECO7175v1 is shown to have a higher probability of failure when compared to ECO7175v2 and ECO7175v3.
| Identifer | oai:union.ndltd.org:pdx.edu/oai:pdxscholar.library.pdx.edu:open_access_etds-6057 |
| Date | 01 May 2019 |
| Creators | Vu, Chinh Q.L. |
| Publisher | PDXScholar |
| Source Sets | Portland State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Dissertations and Theses |
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