1. Integrating Self-Healing Materials and Structural Health Monitoring
Ameralys Correa, Professor Nathan Salowitz
University of Wisconsin-Milwaukee Advanced Structures Lab
Motivation
Force Shoulders
Sample
Catastrophic failures can be prevented by integrating self healing materials and structural health monitoring. This poster illustrates the recent progress of creating closing loads on non self actuating self healing material. The primary goal of this research is to contribute to a closed loop system where
A structure or system is in service,
Incident or fatigue damage occurs,
Structural health monitoring detects damage,
Self healing material fixes damage,
Sensor in a structural health monitoring set up evaluates structure’s healing, and
Process repeats without having to take the structure or system out of service. structure out of service
By plotting extension (mm) vs force (N), the first shoulder that appears on the plot is the amount of force needed to create a gap in the healed sample. We performed several tensile tests to study how the force shoulder would change through several cracking and healing cycles.
Figure 4. An Untested Translucent Sample From Earlier in Sample Development
Method
Challenges Solved
The self healing materials used in this research are composed of a thermoset epoxy matrix reinforced with shape memory alloy Nickel Titanium wires in a single plane. The NiTi wires are prestrained and casted at a length longer than their trained martensitic length. Next the sample matrix is pulled apart so that the wires transverse the crack. Then the sample is heated to the wire’s activation temperature that transforms the NiTi wires into their austenitic phase. In the wire austenitic phase, the wires contract to try to return to their trained martensitic length before casting. However, the epoxy matrix halves press together at the crack’s seam while the wire contracts in the higher temperature phase. This creates a compressive forces on the crack’s seam that keeps the wires from reaching their trained length and keeps the crack closed. Once the sample cools to room temperature, the sample returns to its martensitic phase that is remains at until heated again.
Mold Design that:
Did not adhere to the cured sample
Prevented leaks while curing epoxy
Did not react with the curing epoxy
Frame Design that:
Decreased scrapped epoxy
Increased the reusability of parts of frame
Kept the inner NiTi wires in tension within the epoxy sample
Curing Process that:
Kept the exothermic reaction from prematurely activating the NiTi wires
Fully cured the samples
Conclusions
NiTi Epoxy Composites:
an heal repeatedly and retain a force shoulder within 10% of first cycle’s force shoulder for several cycles.
are strongest when wires are kept in a linear form and loaded in a solely tensile direction.
can heal in the heated state even when overly plastically deformed, but will release hold on seam when cooled to room temperature.
can heal even when cracked in more than simple straight line.
Future work is to place sensors on the samples to evaluate if a crack is open or closed. Also, trials to increase the forces on the healed seam by varying the pre-strain in the wire and increased the number of wires in the samples will be done.
Figure 1: Tensile Tester Cracks Sample and Wires Transverse The Crack
Acknowledgments
We would like to thank Dynalloy for supplying their NiTi Flexinol Wires for our self healing material composites. We would also like to thank the UWM Tensile Lab, UWM Foundry, and UWM Machine Shop for their cooperation in making and testing of our samples.
Figure 2: Sample After Tensile Test
Figure 3: Sample After Healing