1. Muhammad Istiaque Haider and Nathan Salowitz
Residual Stresses Produced by Nickel Titanium after Undergoing Constrained Recovery
Muhammad Istiaque Haider and Nathan Salowitz
Department of Mechanical Engineering, University of Wisconsin - Milwaukee

2. Truck Steering Brackets
Motivation
Vehicles face damage from fatigue, impacts, and overload events
Truck steering knuckle stress concentrations
Strength and fatigue issues prevent use of lighter weight materials
Helicopter components face high cycle fatigue
Armor faces damage from high velocity, projectile impacts
Truck Steering Brackets
Helicopters

3. Solution: Self-Healing Material
Micro-cracks
Atomic diffusion
Encapsulated adhesive
Fracture
SMA fiber composites
Paradigm shift in design for realiaility

4. Limitations SMA cast in parent geometry
Intentionally (thermally cycling wire)
Through manufacturing (casting in metal or exothermic epoxy reaction)
At best returning fracture faces to adjacent positions
Assuming no unrecovered strain
No resulting load carrying capability

5. Approach: Residual Loads
Create residual loads in low temperature Martensitic state (after actuation, in un-actuated state)
© Lagoudas

6. Approach
Constrained recovery with return to low temperature state (martensite) results in residual loads- Post Constrained Recovery Residual Stress (PCRRS).
Explore properties past constrained recovery
Application of small strain
Re-application of full (pre-working) strain
Validation of the result for multiple formulations
Microstructural analysis (ongoing)
Publications in 2017 SMASIS and JIMSS

7. Methods Dynalloy Flexinol 90 NiTi
Ms = 75˚C
Mf = 65 ˚C
As = 90˚C
Af = 110˚C
TA Instruments Q800 Dynamic Mechanical Analyzer
Integrated heating and cooling capabilities and
Thermal extremes of 20 C and 115 C
Maximum strains of 3.4, 3.9, 4.4 & 5.0%
Note: temps up to 150 explored but found not to make a difference

8. Post Constrained Recovery Residual Stress (PCRRS) Generation
0) Thermally cycle free material
Strain NiTi
3.4% strain
Return to 0 stress state
0 stress, recoverable strain
Hold strain & heat to 115 C
“Blocking stress”
Hold strain and cool to 23 C
Residual Stress

9. Interest in Martensite
Approximate phase diagram
Superimposed experimental data 1 3 6 7 5 ♦ 4 2 8 Ms As Af Mf
Manufacturer published transition temperatures
Slope of transition lines approximated
Point: data of interest thoroughly in martensite region
Max temp of 115C used, tests were performed up to 150 with the same results

10. Continue Testing- 0.5% Small Strain Application
Cyclic small strains 0.5%
Linear elastic
Modulus between loading (early 1) & relaxation (3)
Variation over 4 cycles < 20 MPa
Decreasing trend (settling) 1 3 6 7 5 ♦ 4 2 8 10
Consistent & stabilizing values suggest the small strain cycling is not inducing detwinning

11. Further Investigation with Exposure to Repeated 0
Further Investigation with Exposure to Repeated 0.5% Strain Application
To clarify the cause and nature of the reduction of the PCRRS two more experiments was done-
Subjected to a repeated thermomechanical cycle to train them and reduce potential variation that could be caused by grain boundary stabilization from cycle to cycle.
Investigated the material response when subjected to a series of sets of 0.5% strain cycles followed by another thermal cycle and then more 0.5% strain cycles

12. Effects on Training
Repeated cycling, or training, of SMAs can reduce variation in the stress- strain relation between thermal cycles.
Samples were trained by exposing to thermal cycling (heating and cooling) in a fixed stress.
To stabilize the hysteric response, this cycles were repeated for 20 cycles.
Samples were later subjected to the same o.5% small strain application.

13. Effects on Training
Comparison between the trained and untrained samples’ behavior beyond the PCRRS point is analyzed.
Trained specimens produced slightly smaller initial PCRRS.
After 0.5% strain application, the residual stress still decreased after each cycle.
The trend even showed the same slope, or reduction in stress from cycle to cycle in the trained and untrained samples.

14. Regeneration of PCRRS
Initial PCRRS could be recovered by subsequently exposing the sample to thermal actuation and constrained recovery.
The thermal cycle through the reverse and forward transformations restored the residual stress with small, 10% variation.
Repeating the full sequence of loads stabilized the material response by the 3rd full thermomechanical sequence.

15. Future Work Testing on other formulations Microstructural analysis
Theoretical development
Extrapolation to other applications

16. Vision
Generate residual compression in materials to inhibit fatigue damage
Generate crack closing loads:
Inhibit crack growth
Generate residual strength in cracked structures

17. Integrate Technologies to Create Closed Loop System
Vision: SHM2
Integrate Technologies to Create Closed Loop System
Structural health monitoring to initiate and evaluate self-healing materials
Improved self-healing materials designed to support structural health monitoring

18. Acknowledgement Funding from the University of Wisconsin – Milwaukee
Advanced Structures Laboratory seed funding, contact Nathan Salowitz
UWF SURF program
Materials provided by Dynalloy
SMArt Program
Materials provided by SAES Getters S.p.A.
SAES® Smart Materials (SSM)

19. Thanks!