Multiscale DIC Characterization of the Mechanics of Composite Sandwich

Structures with Fiber-reinforced Foam Cores

Prof. Hugh A. Bruck

Department of Mechanical EngineeringUniversity of Maryland, College Park, MD

Research supported by ONR, NAWCAD-Pax River, MR&D

Motivation: Aircraft Sandwich Structures

• Pin reinforced foam cores can form contoured shapes, tailor reinforcement for high stress regions, and reduce weight in low stress regions better than honeycomb

• Models are needed to understand effects of material properties, pin configurations, pin densities in core, and structural geometry on strength/stiffness for design (σ/ρ>700 kN-m/kg@0.26g/cc)

10 inch3

FEA Modeling: Homogenizing Core Properties

Core

Facesheets

Pin-Facesheet Interaction Modeling

• When computing stiffness, out-of-plane displacement is assumed to be zero.• When computing strength, pin buckling typically assumed with clamped ends. • New pin analysis controls out-of-plane displacement with spring constant. • Results show that the magnitude of the spring constant (hence out-of-plane

displacement) can have a large effect on the computed stiffness.

Kv

+

5

Pin-Facesheet Stress Interactions

Both the compressive and shear strength of X-cor are controlled by buckling of the reinforcing pins. Therefore, it is reasonable to expect an interaction between crossply and shear stresses.

Compression Shear

P/D=160 N/mmF=1063 N

V U

X

Y

Specimen length: 12 inchSupport distance: 10 inch

10 inch

6

Experimental Results using DICThree Point Bend w/ Pin Supports

Tests of 6-inch specimen using pin supports resulted in a stiffness of 160 N/mm and a failure load of 1063 N.

[Haldar, Bruck, et al, Comp Struct (2015)]

DIC data from individual pins, in global or local coordinates, show sensitivity to end constraints from specimen size and/or pin configuration that changes the effective pin stiffness.

Enhancement of Pin-Facesheet Modeling: Pin Deformation Measurements using DIC

(130 GPa){1-exp[-(nspec/4)0.2]

[ ])1]/sinh(kL)1][cosh(kL[cosh(kx)sinh(kx)kxkPVv −−+−=

Eeff =Ik

P2

.

[Haldar and Bruck, Exp Mech (2015)]

SXY

F = 189 lbs. = 840 NP/D = 164 N/mm

SY

FEA Results

Model stiffness of 164 N/mm agrees well with measured value of 160 N/mm. But based on core shear strength of 76 psi, model computes failure load of 840 N while measured failure load is 1060 N.

SXY

SY

STRESS INTERACTIONS: Model vs Data

Plotting crossply and shear stresses on failure surface shows that at 840 N (189 lb) the specimen will survive, but at 1060 N (233 lb) the specimen will fail. Therefore stress interaction criterion agrees with measured failure load and buckling response of pins.

Safe

Failed

ASTM CompressiveStrength

ASTM ShearStrength

Bioinspired Sandwich StructuresPalmetto Wood

Multifunctional properties can be potentially enhanced across multiple length scales similar to natural materials (bio-inspired)

Sandwich Structure with Bio-inspired Core

Hierarchical Structure(reinforcement of macrofiber)

Multifunctionality

Multifunctional sandwich:(Structural Battery)• Energy absorbance• Damage resistant• Power capability

Fort Sumter

Charleston, South Carolina

Structural Battery

“Nature's pool of ideas is valuable only if it can be translated into concepts that engineers can incorporate in their design” ~Ball (2001)

Historical benefit as protective structure

during Revolutionary and Civil War

Bioinspiration http://tehrkotmedia.photoshelter.com/image/I0000jYiIfDY2IKw

[Gershon, Bruck, Sutton, et al, Mat. Sci. and Eng. C (2010)]

Multi-scale DIC Characterization of Damage Mechanisms in Palmetto Wood

MicroscaleMacroscale

Debonding Plastic Pore

collapse

Shearcracking aligned in fiber direction

Video-2

Video-1

[Haldar, Bruck, Sutton, et al, Exp Mech (2011)]

Multiscale DIC Characterization of Sandwich Structures with Bioinspired Cores

Multiscale DIC strain fields under static and dynamic 3PB loading indicate:- more homogeneous behavior like Palmetto Wood at macroscale- evolution of damage mechanisms at microscale similar to Palmetto wood

Quasi-static Macroscale DIC

Quasi-staticMacroscale DIC

Quasi-static Flexure

Plastic Pore

collapse

Shearcracking aligned in fiber direction

1 mm

Modeling of Damage Evolution in Fiber-reinforced Porous Matrix Structure

( )( )[ ]pyttp

opt

a

DE

εεεε

σεε

−−−=

−+=

exp1

)1( 2

Mechanical behavior can be tuned to that of Palmetto wood by increasing the reinforcement volume fraction of pultruded carbon rod

Fiber Reinforcement Effects

Constitutive Model

Debonding and Shear Cracking:

Plasticity Effect:

[Haldar and Bruck, Mech of Materials (2013)]

Summary of Damage and Plastic Strain Evolution in Different Sandwich Structures

Using Palmetto wood as template for bioinspired cores leads to sandwich structures with evolution of plastic strain relative to damage that is a better balance of strength, stiffness, and energy absorption

[Haldar and Bruck, Comp Struct (2014)]

• Models of fiber-reinforced composite sandwich structures have been developed by modeling pins as beams and homogenizing the behavior of the pin reinforcement in foam cores

• Detailed FEA and multiscale DIC measurements of individual pins were performed to improve the model. It was found that allowing limited transverse displacement at the pin ends can have a measurable effect on the computed stiffness. Alternatively, the assumed end rotation was found to have a large effect on computed strength. Therefore, BCs in the contoured specimen model have been modified to allow for estimates of end rotation in individual pins from DIC measurements

• Natural structures like Palmetto wood also have fiber-reinforced foams that evolved to provide superior mechanical performance

• Multiscale DIC measurements of Palmetto wood have enabled mechanics of fiber-core interfacial behavior to be identified and damage mechanisms for pore collapse and interfacial debonding/shear cracking to be quantified to develop new models for developing sandwich composite structures with bioinspired cores

• Sandwich composite structures with bioinspired cores with evolution of damage mechanisms approaching those of Palmetto wood have exhibited superior mechanical performance to conventional composite structures

Conclusions

Collaborators

• Dr. Alan Gershon, UMD• Dr. Sandip Haldar, UMD• Prof. Michael A. Sutton, Univ. of South Carolina• Dr. Vikrant Tiwari, IIT-Delhi• Dr. Shaowen Xu, Georgia Southern University• Dr. Nikhil Gheewala, Rice University• Prof. Jane Grande-Allen, Rice University• Kent Buesking, MR&D• Derek Caputo, MR&D• Curtis Sharkey, NAWCAD-Pax River• Bianca Brandveen, Northrop Grumman