research: n. triantafyllidis

RESEARCH: N. TRIANTAFYLLIDIS

RECENT WORK ON:

• Shape Memory Alloys (at different scales)

• Coupled Physics Problems (Mechanical -Magnetic - Electrical, at different speeds)

• Composites and Microstructured Media

RESEARCH: N. TRIANTAFYLLIDIS

RESEARCH: N. TRIANTAFYLLIDIS Shape Memory Alloys (Atomic M-Lattices)

• Idea: Use atomistic models to find stablecrystal configurations at differenttemperatures and stresses

• Tools: Numerical calculations guided bysymmetry arguments and L(yapunov)-S(chmidt)-K(oiter) asymptotic techniques

• Collaborators: J. Shaw, R. Elliott

RESEARCH: N. TRIANTAFYLLIDISShape Memory Alloys (NiTi)

Satellite Frangibolts

RESEARCH: N. TRIANTAFYLLIDISShape Memory Alloys (Atomic M-Lattices)

Transformations under [110] stressStress-free Transformations

RESEARCH: N. TRIANTAFYLLIDIS Shape Memory Alloys (Atomic M-Lattices)

Stress-stretch ratio for A-Mphase transformation

Energy-stress for A-Mphase transformation

RESEARCH: N. TRIANTAFYLLIDIS Shape Memory Alloys (Honeycomb Structures)

• Idea: Use cellular architecture to increaserecoverable strain and decrease response time

• Tools: New experimental technique allowsbonding of NiTi strips. FEM calculations and useof Bloch wave theory predict interesting behavior

• Collaborators: J. Shaw, D. Grummon, P.Michailides

RESEARCH: N. TRIANTAFYLLIDISShape Memory Alloys (Honeycomb Structures)

Isothermalresponse

Shapememoryresponse


Unit cell and finite structure response


Morphing effect in isothermal response (due to stability & phase transformation)

RESEARCH: N. TRIANTAFYLLIDIS Coupled Physics (Magneto-Elasticity)

• Idea: Use iron particle-impregnated rubber tochange composite’s stiffness under externalmagnetic fields. Devices for sensors/actuators,vibration damping

• Tools: Developed theoretical framework formodeling these materials (direct and variationalapproach). Method is basis for FEM calculations.

• Collaborators: S. Kankanala


VNV A

V

NV

A

MRE torsional mounth

!

t

!

"#

!

t

Trac

tion

t (M

pa)

Stretch Ratio λ


Experimental set-up to determine constitutive properties

Displacement

MR Elastomer

Equipotential

Magnetic Plates

To Fluxmeter

(to measure B)

Gaussmeter(to measure H)

N S

Poles ofElectromagnet

Nonmagnetic

Yoke

Electromagnet

Load Cell

Displacement


• We solve the magnetoelasticbuckling of a rectangular blocksubjected to transverseuniform magnetic field.

• We use experiments to selectmaterial parameters.

• We use asymptotic analysis tocheck structural solutions ofthe corresponding beamproblem

SN

h0

Magnetoelastic beam buckling


Critical magnetic field as a function of aspect ratio (antis. & sym. modes).

RESEARCH: N. TRIANTAFYLLIDIS Coupled Physics (Electro-Magnetic Forming)

• Idea: Use consistent formulation to model EMFprocesses and avoid current staggering schemesof solving independently and sequentially amechanics and an electromagnetic problem

• Tools: Developed theoretical framework formodeling these materials (direct andvariational approach). Method is basis for FEMcalculations using variational integratortechniques

• Collaborators: J. Bradley, G. Daehn, J. Thomas


EMF increases ductility overconventional methods (i.e. higherstrains attained before necking)

EMF used in aerospace andautomotive applications (i.e. fuel cells)

RESEARCH: N. TRIANTAFYLLIDISShort Tube Simulations (free expansion)

R

Z

RESEARCH: N. TRIANTAFYLLIDISLong Tube Simulations (free expansion)

R

Z

RESEARCH: N. TRIANTAFYLLIDIS Composites and Microstructured Media

• Idea: Consistent derivation of “onset of failuresurfaces” (in stress or strain space) in solids withregular micro-structures

• Tools: Develop consistent nonlinear macroscopicmodels and check failure predictions againstmicroscopic case when failure starts as a localbuckling mode

• Collaborators: J.-C. Michel, D. Kosinsky, P.Ponte-Castaneda, O. Lopez-Pamies, W.Scherzinger


Macroscopic instability detected asloss of ellipticity in “averaged” moduli

Microscopic instability detected as firstbifurcation away from periodic solution


Macro and micro failure surfaces for finitely strained porous elastomers

RESEARCH: N. TRIANTAFYLLIDISt

l

w

Reinforced composite

Room temperature collapse

High temperature collapseOnset of instabilities at 20% strain

Compressed reinforced honeycomb

research: n. triantafyllidis

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