Damage Mechanisms and Anisotropy of an AA7010-T7452 Open-Die Forged Alloy: Fatigue Crack Propagation
Abstract
:1. Introduction
2. Materials and Methods
2.1. Material
- Casting a cylindrical forging stock;
- Multiple forging steps to achieve the final plate-like shape of the blanks; forging temperature ~400 °C;
- Solution heat treatment at 475 °C for 4.5 h;
- Water quenching;
- Cold deformation/cold heading to relieve residual stresses;
- Overaging in two steps: 120 °C/20 h followed by 175 °C/7 h.
2.2. Fatigue Crack Growth Experiments
- The strain and force signals were both measured with an HBM precision measuring amplifier to keep the phase shift between both signals low. In addition, the remaining phase shift was corrected by a Peak-To-Peak fit to synchronize the measurements. The strain gauge was located at the specimen’s back face (see insert in Figure 1). The strain signal was recorded every 0.2 mm increment of crack extension at a sampling rate of 9.6 kHz for a time interval of 0.5 s, i.e., each set of load-strain data contains 10 load cycles.
- A resulting Fy-εyy curve over these 10 load cycles is exemplarily shown in Figure 1. The graph shows a decrease in the slope of the Fy-εyy curve as the strain decreases (i.e., higher compression of the specimen’s back face) until a linear region is reached. The point at which this linear dependence starts indicates a fully open crack and therefore corresponds to the crack opening force Fop (Figure 1). Fop can be used to obtain the opening stress intensity factor Kop.
- The cyclic stress intensity factor (SIF) ΔK was corrected using the opening forces as ΔKeff = Kmax − Kop.
2.3. Metallography and Quantitative Fractography
- The fracture surface was imaged by SEM over a region of 0.6 × 20.0 mm².
- SEM images of an area ~117 × 117 µm² were selected randomly without knowledge of the location. This was performed to avoid human biasing during imaging analysis.
- Each of these micrographs was classified manually according to the active damage mechanisms.
- Finally, all images were stitched together and the area fraction for each damage mechanism was quantified as a function of the crack length.
2.4. X-ray Computed Tomography
3. Results
3.1. Grain Structure and Crystallographic Texture
3.2. Fatigue Crack Propagation
3.3. Damage Mechanisms
4. Discussion
4.1. Damage Mechanisms and their Transition Regime A3
4.2. Regime A2
4.3. Regime B
4.4. Regime C
5. Conclusions
- The damage mechanisms change gradually from a microstructure-sensitive slip plane driven fracture to a microstructure-insensitive multi-slip-system fracture. The FCP rate in the transition zone shows a gradual change of slope until mechanism II is dominant and the Paris regime is initiated.
- In regime A2, we found a correlation between the mean Schmid factor and FCP rate. In detail, an increased Schmid factor from m = 0.413 to m = 0.445 comes with a ~60% higher mean FCP rate.
- Regime B is independent of the grain structure for L-T-oriented specimens. On the other hand, the smaller grain boundary distances in var 3 (~60% smaller) increase the FCP rate by ~60% in the T-L orientation. This correlation is complemented by the result that a higher aspect ratio L:T correlates with a higher amount of primary phase Al7Cu2Fe on the fracture surfaces (T-L orientation).
- Regime C has similar correlations as stage B; however, Al7Cu2Fe particle clusters and weaker high angle grain boundaries perpendicular to the primary crack can enable crack branching and, consequently, promote a reduction in stress intensity at the primary crack tip.
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Title 1 | Zn | Mg | Cu | Fe | Si | Zr |
---|---|---|---|---|---|---|
reminder | 5.7–6.7 | 2.1–2.6 | 1.5–2.0 | ≤0.15 | ≤0.12 | 0.10–0.16 |
Variants | L | T | S |
---|---|---|---|
var 1 low deformation | 6.2 | 2.4 | 1 |
var 2 medium deformation | 14 | 2.4 | 1 |
var 3 large deformation | 22 | 1 | 1 |
var 4 alternative casting direction | 57 | 7.8 | 1 |
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Strohmann, T.; Breitbarth, E.; Besel, M.; Zaunschirm, S.; Witulski, T.; Requena, G. Damage Mechanisms and Anisotropy of an AA7010-T7452 Open-Die Forged Alloy: Fatigue Crack Propagation. Materials 2022, 15, 3771. https://doi.org/10.3390/ma15113771
Strohmann T, Breitbarth E, Besel M, Zaunschirm S, Witulski T, Requena G. Damage Mechanisms and Anisotropy of an AA7010-T7452 Open-Die Forged Alloy: Fatigue Crack Propagation. Materials. 2022; 15(11):3771. https://doi.org/10.3390/ma15113771
Chicago/Turabian StyleStrohmann, Tobias, Eric Breitbarth, Michael Besel, Stefan Zaunschirm, Thomas Witulski, and Guillermo Requena. 2022. "Damage Mechanisms and Anisotropy of an AA7010-T7452 Open-Die Forged Alloy: Fatigue Crack Propagation" Materials 15, no. 11: 3771. https://doi.org/10.3390/ma15113771
APA StyleStrohmann, T., Breitbarth, E., Besel, M., Zaunschirm, S., Witulski, T., & Requena, G. (2022). Damage Mechanisms and Anisotropy of an AA7010-T7452 Open-Die Forged Alloy: Fatigue Crack Propagation. Materials, 15(11), 3771. https://doi.org/10.3390/ma15113771