Study on the Overload and Dwell-Fatigue Property of Titanium Alloy in Manned Deep Submersible
doi: 10.1007/s13344-020-0067-8
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Abstract: With the rapid development of ocean technology, the deep-sea manned submersible is regarded as a high-tech equipment for the exploration and exploitation of ocean resources. The safety of manned cabin has a decisive effect on the whole system. Ti-6Al-4V with the superior strength-to-weight ratio and corrosion resistance has been used for the manned cabin. The manned cabin experiences loading spectrum with different maximum stresses and different dwell time during their service life. The load sequence effects on dwell fatigue crack growth behavior of Ti-6Al-4V under different dwell time are investigated experimentally in this paper. The experimental results show that the crack tip plastic zone is enlarged by the dwell time and the overload retardation zone increases with dwell time under the same overload rate. A dwell fatigue crack growth model is proposed by modifying the crack tip plastic zone under the loading history with combinations of the single overload and dwell time factors are included in the modified model. Based on the experimental data, the overload retardation zone and the crack growth rates of Ti-6Al-4V are predicted by the modified model. A reasonable model for the load sequence effect on the dwell fatigue crack growth rates of Ti-6Al-4V is verified.
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Key words:
- manned cabin /
- dwell fatigue crack growth rate /
- Ti-6Al-4V /
- load sequence effect
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Figure 9. Comparison between the prediction and experiments under constant amplitude loading.
Table 1. Chemical composition of Ti-6Al-4V (wt%)
Ti Al V Fe C N H O Base 6.2 4.1 0.12 0.01 0.01 0.002 0.11 
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Table 2. Tensile properties of Ti-6Al-4V
No. d (mm) Fm (kN) Yield strength RP0.2 (MPa) Ultimate strength Rm (MPa) Elastic modulus E (MPa) Elongation A(%) #1 10 84.83 1032.86 1080.12 131838.18 15.44 #2 9.98 82.79 1025.45 1058.3 131628.25 7.14 #3 9.98 83.67 1023.75 1069.57 131404.9 13.46 Average 9.99 83.76 1027.35 1069.33 131623.78 12.01
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Table 3. Fracture toughness test data of titanium alloy Ti-6Al-4V
No. #1 #2 #3 Average KIC( ${\rm{MPa}}\sqrt {\rm{m}} $) 61.278 62.371 63.277 62.278
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Table 4. Experimental results of transient zone size (mm)
Fatigue type Overload ratio OLR=1.2 OLR=1.5 Without dwell time 0.325 0.795 Dwell time=10 s 0.617 0.945 Dwell time=60 s 0.752 1.238
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Table 5. Model parameters
Parameter Value Parameter Value Parameter Value ${A_1}$ $\left({{\rm{MPa}}{^{ - {m_1}}}{{\rm{m}}^{1 - {m_1}/2}}} \right)$ 5×10−9 ${{\textit{σ}} _{\rm{u}}}$(MPa) 1069 k (m−1) 20874 ${A_2}$ $\left({{\rm{MPa}}{^{ - {m_2}}}{{\rm{m}}^{1 - {m_2}/2}}{{\rm{s}}^{ - 1}}} \right)$ 2.2×10−12 ${n_1}$ 6 $n$ 14.96 $\Delta {K_{{\rm{thR}}}}$ $\left({{\rm{MPa}}\sqrt {\rm{m}} } \right)$ 5.6 ${n_2}$ 9 $B$ 4.79×10−53 $\Delta {K_{{\rm{th - s}}}}$ $\left({{\rm{MPa}}\sqrt {\rm{m}} } \right)$ 1.1 ${m_1}$ 3.62 $E$(MPa) 131624 ${K_{\rm{C}}}$ $\left({{\rm{MPa}}\sqrt {\rm{m}} } \right)$ 62.278 ${m_2}$ 2 $In$ 3.7925 ${{\textit{σ}} _{\rm{y}}}$(MPa) 1027 R 0.03 ${F_{{\rm{cr}}}}_{\left({\textit{θ}} \right)}$ 0.3985
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Table 6. Experimental results and predicted results of transient zone size
Fatigue type Overload ratio Experiment (mm) Prediction (mm) Error (%) Without dwell time OLR=1.2 0.325 0.608 46.54 OLR=1.5 0.795 0.863 7.87 Dwell time =10 s OLR=1.2 0.617 0.633 2.52 OLR=1.5 0.945 1.091 13.38 Dwell time=60 s OLR=1.2 0.752 0.794 5.29 OLR=1.5 1.238 1.043 18.69
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