PSI - Issue 24

Riccardo Panciroli et al. / Procedia Structural Integrity 24 (2019) 593–600

599

R. Panciroli and F. Nerilli / Structural Integrity Procedia 00 (2019) 000–000

7

500

V

400

300

200

II

I

Stress [MPa]

100

IV ↓

III

0

0

1

2

3

4

5

6

Strain [%]

Fig. 6. Stress strain curve on an representative element of the wire during the whole simulation. The prestrain is applied and the wire yields pseudo-plastically in (I); as the maximum prestrain is reached (II) the load is removed and the wire shortens due to the spring-back (III). It follows a minor stretch due to the increased curvature of the panel upon cooling (IV). Finally, the wire shortens and the stress increases (V) due to the thermal actuation of the wire.

(III), to later minimally elongate as the plate cools down to assume the post-cured shape (IV). The final back-facing ramp is due to the activation of the wire, which would shorten back to a zero strain if it was free, but the panel opposes the contraction and a large stress is attained. Such stress generated in the wire is the one responsible for the snap-back, as it is seen as a distributed bending moment by the plate, having the wire an o ff set with respect to the neutral plane. The results show that the stress in the wire might reach rather high values, and a careful analysis to the mechanical and functional fatigue resistance of the wires should be conducted (Maletta et al., 2014; Scire` Mammano and Dragoni, 2014; Kang and Song, 2015) .

5. Conclusion

Primary objective of this work was to propose a numerical scheme capable to predict the bistable behavior of composite laminates introduced by thermal elongation mismatches between the layers after the curing cycle. The model can be utilized during the design phase to optimize panel dimensions, stacking sequences, and curing cycles as a function of the desired bistable shapes. The numerical model has been further expanded to account for SMA wires embedded in the composite structures for actuation purposes. The wires material model has been validated against experimental results and has been utilized to develop a predictive tool capable to simulate the panel’s actuation. Results reveal that the SMA wires are indeed capable to actuate the composite bistable structure, but the minimum numbers of wires needed for the actuation might be very large. In this vision, the proposed numerical scheme can be utilized to optimize number and location of the wires. In fact, the switching between the two stable configurations, which correspond to the minimum energy content, passes through a series of unstable configurations with higher internal energy. An optimum location of the wires would allow to move along the path with lower energy, hence allowing to identify the lowest amount of wires to perform the actuation.

Acknowledgements

This work was supported by the Italian ministry for higher education through the grant PRIN2015 n.2015RT8Y45. Views expressed herein are those of the authors and not of the funding agency.