Embedment techniques Flashcards
What are some critical issues of smart structures?
- Invasivity on the host material (initiation of damage)
- Invasivity on the manufacturing process
- Accuracy of the sensros
- Performance of the actuators
Which smart materials need preparation for embedment?
- SMA needs training (OWSM or TWSM)
- PZT needs etching to avoid shortcuts
- FO doesn’t need preparation
What does the choice of actuators and sensors depend on?
It depends on:
- Morphology
- Compatiblity with host material
- Invasivity (passive and active)
- Performance (depends on the application)
What are the technological requirements for actuators and sensors?
1) Low Invasivity on the host structure, the sensors and actuators and the manufacturing process
2) Shape and dimensions
3) Minimizing introduction of defects inside composite laminates
4) Adequate mechanical properties
5) High load transfer capability (great adhesion at interface or coating of fibers)
6) Easy to be applied (embedded)
What is the preparation needed for SMA embedment (OWSM)?
OWSM training is a thermo-mechanical process that teaches the material a particular shape to be remembered. In OWME the shape is associated to the austenitic phase. Τhe procedure is the following:
1) To give the desired shape to the material (the material is in martensite twinned) at room temperature.
2) Apply a thermal treatment (about 450 deg C for 3 minutes) with wires constrained in the desired shape but free to lengthen and shorten. The temperature is over the recrystallization temperature. Recrystallizationn is a process by which deformed grains are replaced by a new set of defect-free grains that nucelate and grow until the original grains have been entirely consumed. Recrystalization is usually accompanied by a reduction in the strength and hardness and a simultaneous increase in the ductility. The material transforms into austenite and we continue to heat until the recrystallization of the structure, where the lattice is rearranged and the stress dissapears.
3) Quench (as cooling). The material deforms towards martensite twinned but the marcroscopic shape remains the same
What is the preparation needed for SMA embedment (TWSM)?
The TWSM training exploits very localized yielding zones that occur in specific points during the transformation. The yielding zones are the responsible for the ability of the TWSM to remember a specific configuration in the martensitic state. THe process is the following:
1) Heating (105C) over Af in order to transform the material in Austenite
2) Apply strain (max 6%) to obtain SIM (stress induced martensite). If we want to work in a lot of cycles, maximum 3%.
3) Cooling (25C) under Mf constraining wires in deformed shape
4) Heating in order to restore original undeformed shape
5) Repeat the sequence for 10 times
The yielding zones can drive the detwinning of the material when we pass from an austenitic state to a martensitic state. When we cool the material without stress applied to the material, the transformation towards martensite is driven by the presence of the yielding zones. Some orientations of the martensite state inside the material occur during the cooling without any additional stress applied. So we have the presence of martensitic detwinned state in specific points inside the material without applying mechanical load.
Transformation temperatures and mechanical characteristics can be changed after training (Differential Scanning Calorimeter Analysis and Tensile Tests).
What is the preparation needed for PZT?
The preparation of PZT is characterized by two problems.
1) PZT needs to be connected to the interrogation system
2) PZT needs to guarantee the electrical insulation between electrodes
The procedure followed is:
1) Remove the electrodes near the edges in order to ensure electrical insulation between the electrodes. We need to avoid shortcuts that can be induced by the fact we use two poles, therefore the distance between two poles is small. We have to consider also that when we cut the plate to obtain the shape needed for our application, it is difficult to obtain smooth surfaces. Therefore there is a concentration of electrical charges.
2) The second problem is the need to apply the connecting cables by using soldering. The soldering is a weak point.
It is possible to remove the electrodes by a chemical etching: Using a photosensitive film that can protect the part where we want to maintain the electrodes. This way, shortcuts are avoided.
What are the FO embedment problems and solutions?
1)
i) Problem
When the FO is embedded with an angle (misalignment) with respect to the reinforcing fibers of the composite layers, it is possible to observe different problems.
This can create problems such as continuous bending along the FO due to the presence of the reinforcing fibers, since the FO is between two layers which are pressed against each other during the curing cycle. This phenomenon is called microbending and induces localized losses of signal (where the critical angle is overpassed). In addition to that, the presence of reinforcing fibers with respect to the the FO deforms the sensor (and the shape of the spectrum reflected by the sensor) and this can compromise its functioning. Also, defects inside the laminates may be created.
ii) Solution
A possible solution is the embedment of the FO with the same orientation of the reinforcing fibers.
Another solution is to pre-embed the fiber between two thin laminates. During this pre-embedment phase we can set properly the parameters of the curing cycle (low tempearture and pressure) in order to obtain a very good component with no defects. The created laminate is called quick-pack and it is introduced into the final laminate. The material of the quick-pack has to be compatible with the one of the final laminate.
2)
i) Problem
Incompatibility of standard coating (polyacrilate coated fiber) with curing cycles.
The polyacrilate is incompatible for our applications from the thermal point of view. The polyacrilate has a glass transision temperature of less than 100 degrees (around 86). During the curing cycle we overpass this temperature, so the polyacrilate coating becomes soft and a mechanical action can deform the coating. Load transfer capability is guaranteed only in absense of any coating deformation.
ii) Solution
The first solution is using a polymid coating with a Tg of around 187 degrees. The second solution is again using a quick-pack. It can protect the FO against the pressure which can occur during the final embedment.
3)
i) Problem
The third problem is the presence of FO exiting from the laminate, since the FO needs to be connected to electronic devices. The resin after the curing can create a compressive stress state around the FO at the exiting point from the laminate.
ii) Solutions
To avoid this problem it is possible to use a PTFE tube to protect the FO. The tube is introduced inside the laminate for 5mm in order to protect the FO when it exits the laminate. The resin coming from the laminate can enter in the space between the the tube and the FO. Due to capilarity, the resin can travel inside the space. To avoid this, we apply a small quantity of glue so that it can fill the space.
Another solution is the connecters embedment. At the end of the fiber, inside the laminate, there is already a connector. The connector is protected by a special capsule to prevent any passing of the resin during the curing. At the end of the lamination process we can remove the capsule in order to discover the connector and then it is possible to apply a cable.
How can the quick-pack be made?
It is possible to create a quick-pack with thin CFRP fabrics with a high resin content and low thickness. The high content of resin permits to avoid the presence of void near the FO. The obtained laminate can be cured at very low pressure sto that the integrity of the fiber can be guaranteed. The quick-pack can be easility embedded inside the final laminate.
What are the PZT problems and solutions?
The problems are the following:
1) Needs to insulate in case of host material is made of carbon fibers
2) Presence of soldering and wires create a concentration of stress.
The high temperatures during the curing can overpass the Curie temperature, which leads to a partial depolarization of the material. There are ceramics which have a Curie temperature higher than the curing one.
The solutions are:
1) We can use again the quikc-pack with two layer of GFRP. In this way it is possible to cure at very low pressure. The soldering can be placed at the external of the PZT.
It is possible to realize the embedment in two ways:
i) Direct embedment, where we maintain the continuity of the reinforcing fibers within the component (this is the preffered solution)
ii) Embedment through cut-out, where we cut some layrs of the composite materials in order to create the space to host the PZT.
Also, monolithic PZTs are not embeddable into curved laminates. We can overcome this limit by using PZT made of fibers (Micro Fiber Composite MFC) and the same techniques developed for monolithic PZT can be adopted.
How can SMA wires be embedded?
1) They can be embeded in deformed shape. When we deform the structure, also the actuators will be deformed. The actuators are activated in order to recover their initial shape.
2) They can be embeded in undeformed shape.
What is the process for SMA embedment?
It is possible to use resins for the fabrication of the component that exhibit curing temperature lower than the activation temperature of the SMA actuators. During lamination phase, a pre-deformed shape is kept in position during low temperature curing cycle. The problem is the weakness of the interface and the poor mechanical properties. Another solution is the embedment by using special tubes made of cured rubber. The problem is the fact that the load transfer happens by external frame.
How can you evaluate the maximum load that can be transferred without itnerface degradation?
By taking a block of resin, where we embed a NiTiNOL cable or a FO and we apply a pull-out force. In this way, we can extract the wire from the resin, so the breakage of the interface will occur and we can evaluate the stress at which this happens. The interfacial shear stress is τ = Fmax/πdl.
Using a FO coating poly-imide, the failure can happen at very high stress values compared to standard coating (acrylic).
