Practical Considerations In Determining Material Properties

Susan I. HillStructures and Materials Evaluation Laboratory

University of Dayton Research Institute(937) 229-4704 hillsi@udri.udayton.edu

www.udri.udayton.edu

Future of Modeling in Composites Molding Processes Workshop

June 9-10, 2004

Defining Needs• Depends on application and model

– Impact can be compressive event but material failure is tensile event

– Localized delaminations, cracking, interfacial bonding in composites

• Necessary test data are defined by selected model– Tensile, compression, and shear data– Energy absorption– Temperature effect– Strain rate effect– Failure

Types of tests required

• Uniaxial compression• Confined compression (bulk modulus)• Cyclic tension• Stress relaxation• Resonant Beam• ?

May need to go beyond the typical tensile strength, modulus, failure strength data, e.g.

Impact-related models

• Material models exist for structural polymers• Lacking for composites• FE codes may not incorporate correct

material models– Current models have poor handling of viscoelastic

effects, plastic flow, strain rate effects, and fracture

– Use of quasi-static data will underestimate material response at higher impact rates

Polyolefin

0 50 100 150 200 250 300 350

0.5/s5/s50/s250/s450/s

Engi

neer

ing

stre

ss [M

Pa]

Engineering strain [%]

Polyolefin

Polyolefin relationship with strain rate

0.1 1 10 100 1000

PolyolefinPeak StressBreaking Stress

Engi

neer

ing

stre

ss [M

Pa]

Engineering post-yield strain rate [1/s]

Polycarbonate

0 50 100 150 200

0.4/s40/s300/s

TE

NSI

LE

EN

GIN

EE

RIN

G S

TR

ESS

ENGINEERING STRAIN [%]

Relationship with Strain Rate

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

0.01 0.1 1 10 100 1000NO

RM

AL

IZE

D Y

IEL

D S

TR

EN

GT

H

STRAIN RATE [SEC -1]

Highly glass-filled polymer

0.001 0.01 0.1 1 10 100 1000

Highly glass-filled polymerFailure stress

Engi

neer

ing

stre

ss [M

Pa]

Engineering post-yield strain rate [1/s]

Failure stress relativelyconstant across strain rates

Background information

Defining Needs

• Strain rate defines test method– Quasi-static -- Screw-type test machines

• 0.0001 to 0.1/s– Intermediate (“High”) -- Servo-hydraulic test

machines• 0.1 to 200-700/s

– Bar Impact -- Split Hopkinson Bar• 200 to 10,000/s

Types of high rate problems

Structural Dynamics Wave PropagationProblem Areas Vibration Impact, shockTime Duration Milliseconds — seconds Nano — millisecondsStrain Rates < 10 1 — 106

Frequency Content Low — moderate Moderate — highSolution Method Implicit ExplicitFEA Codes ABAQUS, ANSYS,

NASTRANAutoDyn, DYNA,

PAMCrash, RADIOSS

Comparison of tensile specimens used for quasi-static and dynamic tests

ASTM D638 Type V

ASTM D638 Type I

Dimensions in mm.