A Novel Specimen Produced by Additive Manufacturing for Pure Plane Strain Fatigue Crack Growth Studies
Abstract
:1. Introduction
2. Production of Specimens by Selective Laser Melting (SLM)
3. Experimental Determination of da/dN-ΔK Curves
4. Numerical Analysis
5. Discussion
- the use of synchrotron tomography and in situ testing [31]. However, this needs complex equipment that is not easily available. Additionally, it would not be easy to place the mechanical device needed to apply the load, inside the tomography equipment.
- use of vibration analysis to measure crack length. The resonant frequency depends on material properties and geometry. The presence of defects may change the resonant frequency. Macek et al. [48] proposed the use of the changes in the dynamic response, using uniaxial acceleration sensors, enabling the correlation of the stiffness variations with the crack length.
- use of potential drop. A similar problem exists in tests at elevated temperature inside a furnace even using standard specimens [49]. Therefore, crack length was measured using a potential drop. Small wires made of titanium were welded to opposite flanks of the notch, and the crack growth produces an increase of potential drop. A calibration curve was used to obtain crack length from potential drop measurements. Campagnolo et al. [50] studied fatigue crack initiation and propagation phases of cylindrical circumferentially notched specimens characterized by different values of the notch tip radius. Crack length was monitored by the electrical potential drop method, and the calibration curves were derived by means of electrical finite element analyses.
- The variation of compliance is an alternative solution to measure the crack length. This is also proposed in ASTM standard for the measurement of crack length in C(T) and MT specimens.
6. Conclusions
- A pure plane strain state exists along all crack front.
- There is no crack closure level under plane strain state for the titanium alloy studied. Under plane stress state, a crack closure level of 10% was predicted.
- The crack grows, keeping a circular shape, without corner points. This avoids different issues, namely the effect of crack shape on compliance changes. Besides, any departure from pure circular shape is an indication of incorrect loading alignment.
- Delays of 1250 cycles and 5400 cycles were found under plane strain and plane stress states, respectively, for an overload ratio of 1.75. Therefore, under plane stress state there is a much higher effect of overloads on fatigue life. Although there is a plane strain state in the central cracked specimens, overloads can affect the da/dN-ΔK curves. Thus, alternative approaches not involving the application of overloads are recommended for crack length measurement, which will be exploited in the near future.
- Since the atmosphere does not have access to crack front, the specimens are also interesting to develop studies in vacuum. Similar specimens with channels to conduct air to the crack front may be produced for comparative studies of the effect of atmosphere. These comparative studies are more relevant at relatively low crack growth rates because the diffusion is a time-dependent mechanism.
Author Contributions
Funding
Conflicts of Interest
Nomenclature
a | Crack length |
aOL | Crack length at which the overload was applied |
C(T) | Compact-Tension (specimen) |
CTOD | Crack Tip Opening Displacement |
Cx | Material constant of kinematic hardening |
da/dN | Fatigue crack growth rate |
K | Stress intensity factor |
Kmax | Maximum stress intensity factor |
M(T) | Middle-Tension (specimen) |
N | Number of load cycles |
OLR | Overload ratio |
PICC | Plasticity Induced Crack Closure |
r | Radius of the specimen with central crack |
R | Stress ratio |
SLM | Selective Laser Melting |
U* | Crack closure level (in percentage) |
Xsat | Material constant of kinematic hardening |
Y0 | Initial yield stress |
β | Angle of crack front with surface at corner point |
ν | Poisson’s ratio |
λ | Ratio between the crack length and the radius of the specimen |
λ* | Order of singularity at the crack tip |
σ | Remote stress |
σmax | Maximum remote stress |
σmin | Minimum remote stress |
ΔK | Stress intensity factor range |
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Material | Y0 [MPa] | K [MPa] | n | CX | XSat [MPa] |
---|---|---|---|---|---|
Ti-6Al-4V + HT | 700.0 | 738.6 | −0.013 | 88.1 | 585.2 |
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Jesus, J.d.; Borges, M.; Antunes, F.; Ferreira, J.; Reis, L.; Capela, C. A Novel Specimen Produced by Additive Manufacturing for Pure Plane Strain Fatigue Crack Growth Studies. Metals 2021, 11, 433. https://doi.org/10.3390/met11030433
Jesus Jd, Borges M, Antunes F, Ferreira J, Reis L, Capela C. A Novel Specimen Produced by Additive Manufacturing for Pure Plane Strain Fatigue Crack Growth Studies. Metals. 2021; 11(3):433. https://doi.org/10.3390/met11030433
Chicago/Turabian StyleJesus, Joel de, Micael Borges, Fernando Antunes, José Ferreira, Luis Reis, and Carlos Capela. 2021. "A Novel Specimen Produced by Additive Manufacturing for Pure Plane Strain Fatigue Crack Growth Studies" Metals 11, no. 3: 433. https://doi.org/10.3390/met11030433