Fatigue behaviour of composite tubes under multiaxial loading

M-H R Jen, Y-C Sung, etc. / Fabrication of ti/apc-2 nanocomposite laminates and their fatigue response at elevated temperature
frequency=5Hz, sinusoidal wave form under load-controlled mode at elevated temperatures, such as
25 o C(RT), 75, 100, 125, 150 o C (slightly above APC-2 Tg=143 o C). An MTS 651 environmental chamber
was also installed to keep and control the corresponding temperature of a specimen for testings. A
25mm MTS-634.11F-25 extensometer was used to monitor the strain continuously during the tests.
4. Results
The nanoscale structure of oxide layer which has been observed by SEM was shown in Fig. 3. A
columnar and porous layer grown on titanium with fluorinated chromic acid electrolyte was found
uniform. There exists a kink angle in stress-strain curves at all elevated temperatures. That results in the
initial tangent modulus and secant modulus in longitudinal direction. The early yielding of Ti sheets is
the main reason. Fig. 4 is an example of ζ-ε curve at RT. The mechanical properties, such as ultimate
tensile strength and longitudinal stiffness, of Ti/APC-2 cross-ply nanocomposite laminates at elevated
temperatures were listed in Table 1.
Table 1. The averaged mechanical properties of Ti/APC-2 cross-ply nanocomposite laminates at various temperatures.
Temperature ( o C) Ultimate Load (KN) ζult (MPa) εmax E11i (GPa) E11s (GPa)
25(RT) 46.77±0.60 719.59± 9.22 0.015±0.0005 109.79±5.19 33.84±2.39
75 44.39±1.09 682.87±16.79 0.014±0.0003 107.96±1.05 32.95±0.50
100 41.48±1.33 638.15±20.39 0.013±0.0001 106.39±6.02 33.30±1.72
125 40.26±0.56 619.38± 8.66 0.013±0.0001 104.83±8.38 33.16±0.64
150 39.21±1.33 603.28±20.50 0.013±0.0001 101.71±0.53 33.71±1.71
Kink angle is due to yielding of Ti sheets
E11i (Initial Longitudinal Stiffness) E11s (Secant Modulus)
25 o C: 0.001≤ε≤0.0022 0.0022≤ε≤0.006
75 o C: 0.001≤ε≤0.0020 0.0020≤ε≤0.006
100 o C: 0.001≤ε≤0.0019 0.0019≤ε≤0.006
125 o C: 0.001≤ε≤0.0018 0.0018≤ε≤0.006
150 o C: 0.001≤ε≤0.0017 0.0017≤ε≤0.006
Fig. 3. Scanning electron microscopy image of oxide layer on the titanium surface by chromic acid anodic method.
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