Smart Structures: Types, Technology and Applications Flashcards
(9 cards)
What are the critical issues with smart structures?
- Invasivity on host material
- Accuracy of sensors
- Performance of actuators
- Invasivity on manufacturing process
What are the technological requirements of smart structures?
The material must have low invasivity on the host structure (therefore not sacrificing mechanical properties or fatigue behaviour), on sensors and actuators (working parameters), and the manufacturing process (not impose limits on temperatures, pressures, mould shapes, etc). In other words, it must be of correct shape and dimensions, minimize introduction of defects, have adequate mechanical properties and load transfer capabilities, and be easy to embed. The technological aspects to be considered are: sensors/Actuators preparation (for embedment), development of embedment techniques, and load transfer capability.
How is NiTiNOL prepared? OWSM and TWSM (one and two way shape memory effects)
For OWSM, the desired shape must be given to the material. Then, a thermal treatment (about 450C for 3 minutes) is performed whilst the wires are constrained in the desired shape (they are only allowed to lengthen and shorten but not bend/twist/etc. The material is then quenched to generate a fast cooling (guaranteeing the desired phase changes). The initial shape is given in a microscopic martensite detwinned state, the heating makes it go to austenite associated with the given macroscopic shape, and the cooling once again changes us to martensite (twinned this time) with the desired macroscopic shape. Different temperatures on the heating and quenching give different mechanical behaviors and characteristic temperatures.
For TWSM, the heating goes over austenite finish (105C) to fully transform to austenite. Strain is then added (max 6%) in order to obtain some strain-induced martensite. The material is then cooled (25C) under martensite finish temperature, fully transforming it to martensite whilst keeping the geometry constrained in the deformed shape. The material is finally heated to restore the original undeformed shape. This sequence is repeated 10x.
Transformation temperatures and mechanical characteristics may be changed after training: a tensile static test, differential scanning calorimeter analysis, constant stress temperature variation and other characterization tests can be performed to this end.
How is a PZT prepared?
PZT needs to first be connected to the interrogation system, whilst ensuring electrical insulation between the electrodes. We remove the electrodes near the edges and chemically etch the piezoelectric to this end.
Why do we develop specific embedment techniques?
Minimize the invasivity on the host structure, on sensor and actuator work parameters, and on manufacturing processes, whilst maximizing the performance (ensuring an optimal load transfer capability).
What are the problems and solutions in PZT embedment?
Incompatibility with carbon fiber due to conductivity necessitates insulation of host materials. Solders and wires also must be insulated for the same reason. High local temperatures and pressures can also lead to partial depolarization or ceramic breakage, respectively. A simple solution is to perform the embedment and curing with a quick-pack, which will guarantee insulation, guarantee uniform pressure (if the quick-pack is elastomeric) and allow for the solders to be placed externally to the PZT itself.
Another limitation/problem is that monolithic PZTs are naturally not embeddable into curved laminates. PZTs made of fibres, however, allow this functionality. The same techniques described above for other problems can then be used on the micro fiber PZT composite.
What are the problems and solutions in SMA embedment?
We encounter problems in both the lamination and post-curing phases. Mainly, the problem is that the pre-deformed shape is required to keep its position during a low temperature curing cycle (that is, during a ΔT which would ordinarily actuate the material). This causes weakness in the interface and poor mechanical properties. This could partially be handled by embedding the SMA within special tubes made of cured rubber, but the load transfer will then not be as effective as it will have to go through the rubber. The main solution, then, is to introduce a special fixture to keep the wires tense and ‘rigid’ during curing.
How can we model the interface degradation, what are parameters to keep in mind?
τ_ISS=F_max/πdl is used to estimate the interfacial shear stress (ISS) in the context of FO embedment or fiber matrix interfaces in composite material embedment. τ_ISS is the average shear stress acting along the interface between the embedded fiber and host matrix, F_max is the maximum force transferred, d is the diameter of the embedded fiber or sensor, and l the length of the fiber.
When load is applied to the host structure, some of that load is transferred to the fiber (e.g., an FBG sensor) via shear at the interface. If shear stress is too high, the bond between the fiber and matrix may fail (debonding), and the sensor may not effectively track strain. One wants the interface to be strong enough to transfer load without damage, but not so stiff that it causes stress concentrations or damages the fiber during curing or loading. This expression allows one to calculate the maximum allowable load or required embedment length to avoid degradation.
