Thermomechanical behaviour of shape memory alloys (SMAs)

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Experimental investigation

Micromechanical model

Phenomenological model

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Experiments on NiTi shape memory alloy (Nitinol)

A series of experiments were carried out on NiTi shape memory alloys.

These experiments include:

DSC tests;

tension tests at different constant temperatures;

tension tests with different strain rates;

tension tests under different instant loads;

thermal cycling tests with fixed length;

and thermal cycling tests under different constant forces, etc.

Transformation front movement in tension test at different strain rate was observed.

Transformation front movement

strain rates	1.30e-4/s	2.60e-4/s	1.30e-3/s

Micromechanical model

Behaviour of shape memory alloy was investigated based on micromechanic and phase transformation theory. Besides ordinary behavior,

Yielding surfaces of different shape memory alloys were studied.

Yielding surfaces of SMAs

SMAs	NiTi	NiAl	CuAlNi	CuZnGa
yielding surfaces

By using this Yielding surface theory,

The fundamental reason for non-symmetry of NiTi shape memory alloy in tension and compression was explained by means of maximum transformation strain in each crystal.

Maximum transformation strain in Nitinol crystals

tension	compression

And the behavior of SMA in compression and torsion was predicated based on tension result.

From tension to compression and torsion

tension to compression	tension to torsion

Further work in this field will lead to a real multi-dimentional model for real shape memory alloys.

V-shape Yielding stress at different temperatures was expalined. According to the following figure, there can be two V-shapes for cooling process and heating process, respectively.

Yielding stress vs temperature

in cooling process _{(a-o-b-h-f-g)}	in heating process _{(g-f-h-i-j-b-a)}

Phenomenological constitutive model

A phenomenological constitutive model based on two kinds of tests (the standard tension test, carried out at different constant temperature, and the thermal cycling test under different constant loads) were suggested for engineering applications (details of this model refer Technical Report).

This phenomenological constitutive model has been used to simulate:

strain rate effetcs	transformation front movement		thermal cycling (fixed length)⁺
	sample (1)	sample (2)

(+ Temperature is not uniform.)

and in shape memory alloy based Actuator designs (refer Publications).

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Last updated on the 27th of March, 1998

W Huang - mwmhuang@ntu.edu.sg