PPubMed4 Jun 2019Cited 13×

In Situ Observation on Rate-Dependent Strain Localization of Thermo-Induced Shape Memory Polyurethane

Jian Li, Qianhua Kan, Kaijuan Chen, Zhihong Liang, Guozheng Kang

Abstract

In situ monotonic tensile experiments of thermo-induced shape memory polyurethane (SMPU) at different loading rates were carried out by the digital image correlation (DIC) method and infrared camera FLIR®-A655sc in natural convection (NC) and forced convection (FC) conditions, respectively. The multiform strain localization of SMPU was observed by the DIC method, and the influence of thermo-mechanical coupling on the strain localization was analyzed by using the FLIR to measure the temperature field caused by the internal heat generation. The experimental results show that the strain localization mode strongly depends on the strain rate and convection condition, and the strain localization mode can be transformed by changing the convection condition from NC to FC. The competition mechanism between the strain hardening induced by the increasing loading rate and strain softening induced by the internal heat generation is indicated, the transition modes of strain localization are clarified, and the influences of thermo-mechanical coupling on shape memory effect are discussed.

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In situ tensile tests reveal that loading rate and convection condition dictate strain localization modes in thermo‑induced shape memory polyurethane via competing strain hardening and internal heating.

What’s new

i.Performed in situ monotonic tensile experiments on SMPU using digital image correlation and infrared thermography.
ii.Demonstrated that strain localization mode depends on loading rate and convection condition (natural vs. forced).
iii.Identified a competition between strain hardening (rate effect) and strain softening (internal heat generation) governing transition modes.
iv.Quantified maximum/minimum strains and temperature rises across four loading rates in both convection conditions.
v.Discussed how thermo‑mechanical coupling influences the shape memory effect of SMPU.

How they did it

Monotonic tensile tests were carried out on dumbbell‑shaped SMPU specimens at loading rates of 0.1, 1, 5, and 10 mm/s under natural and forced convection. Strain fields were captured with digital image correlation, while surface temperature fields were recorded using a FLIR‑A655sc infrared camera.

What they found

Maximum strain in NC: 110.0% (0.1 mm/s), 167.2% (1 mm/s), 315.3% (5 mm/s), 343.4% (10 mm/s); in FC: 91.2%, 145.2%, 191.9%, 241.2% respectively.
Minimum strain decreased with rate, reaching as low as 1.8% in FC at 10 mm/s.
Maximum temperature rise ranged from ~1 K at 0.1 mm/s to 33 K at 10 mm/s in NC condition.
Force change during plastic flow decreased with increasing loading rate in both NC and FC, but showed strain hardening in FC and strain softening in NC.

Caveats

✕Only one SMPU material (MM4520) was tested, limiting generality.
✕Experiments were limited to monotonic tensile loading; cyclic or multiaxial loading was not explored.
✕Measurements were surface‑based (DIC, infrared); internal fields were not captured.
✕Only force‑displacement curves are provided, preventing extraction of intrinsic stress‑strain response.

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