THESIS
2018
xv, 83 pages : illustrations (some color) ; 30 cm
Abstract
The cyclic compressive responses of polycrystalline pseudoelastic NiTi shape memory alloy
(SMA) are investigated under frequency ? ranging from 0.0007?? to 50?? with a controlled
maximum strain of ?
??? = 4.2%. The cyclic compressive fatigue behaviors of this material are
investigated under stress levels of ?
???= 800??? , 950??? and 1200??? with and
without fabricated holes of different sizes. For the cyclic compressive responses, we have found
a critical frequency ?
?????
, below which the temperature and stress oscillations are dominated by
the ? dependent coupling between phase transition (PT) and heat transfer and above which the
macroscopic plastic strain ?
? of austenite phase gets involved due to heat accumulation in the
transient stage and interacts with PT and heat transf...[
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The cyclic compressive responses of polycrystalline pseudoelastic NiTi shape memory alloy
(SMA) are investigated under frequency ? ranging from 0.0007?? to 50?? with a controlled
maximum strain of ?
??? = 4.2%. The cyclic compressive fatigue behaviors of this material are
investigated under stress levels of ?
???= 800??? , 950??? and 1200??? with and
without fabricated holes of different sizes. For the cyclic compressive responses, we have found
a critical frequency ?
?????
, below which the temperature and stress oscillations are dominated by
the ? dependent coupling between phase transition (PT) and heat transfer and above which the
macroscopic plastic strain ?
? of austenite phase gets involved due to heat accumulation in the
transient stage and interacts with PT and heat transfer. The accumulation of ?
? causes the
reduction of PT volume fraction, and thus the latent heat and hysteresis heat in the subsequent
cycles. Eventually the thermomechanical responses are brought to a steady-state stage with the
saturation of ?
?. Theoretical modelling is performed to quantify those steady-state responses
resulting from the interactions among PT, heat transfer and ?
?. For the compressive fatigue
behaviors, with stress intensity factors ?
1 in the order of 10
0~10
-3???√?, the life span ?
?
of all the samples with no pre-existing hole exceed 1 million cycles, among which a ?
? of 20
million cycles under ?
??? = 800??? is recorded with stable thermomechanical responses and
an average coefficient of performance (COP) of 17.6. ?
? is significantly reduced with the
increasing hole size due to the great increase in ?
1. Quasi-brittle splitting and chipping caused
by compression-parallel cracks are the main failure modes of the samples under cyclic
compression. The studies in this thesis establish a broader scenario of frequency dependent
interactions among PT, heat transfer and plasticity in SMA and provide a solid mechanical base
for the application of NiTi in solid-state cooling technology.
Keywords: NiTi shape memory alloy (SMA); Phase transition (PT); Cyclic compression;
Frequency effect; Thermomechanical coupling; Plasticity; Fatigue crack; Fracture; Size effect
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