solutions for obtaining composite material properties ebook

8/13/2019 Solutions for Obtaining Composite Material Properties eBook

http://slidepdf.com/reader/full/solutions-for-obtaining-composite-material-properties-ebook 1/13

BETWEEN THE PLIES Solutions for Obtaining Unidirectional

Composite Material Properties for a

FEA Simulation



Solutions for Obtaining Unidirectional Material Properties for a FEA Simulation

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TABLE OF CONTENTS

Introduction .......................................................................................... 2 Properties Needed to Define a Unidirectional Composite Material ...... 3 Solutions ............................................................................................... 4

Scenario #1: Limited Lamina-Level Material Property Data ........ 5 Scenario #2: No Lamina-Level Data, Only Constituent-Level

(fiber/matrix) Data ...................................................................... 7 Summary ............................................................................................ 11 About Firehole Composites ................................................................. 11 References .......................................................................................... 12




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INTRODUCTION

A major component of conducting accurate simulations of composite materials used in an

increasing number of light-weight structures is gathering the required material properties to

fully define a composite material. Composite materials have the unique characteristic of having

different stiffness and strength values depending on the direction the composite is being

loaded. For unidirectional composites, higher stiffnesses and strengths are observed in the

direction parallel to the fibers as compared to perpendicular to the fiber alignment direction.

This document describes some methods for determining the

required material property data for a finite element analysis

simulation of unidirectional composite materials.




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PROPERTIES NEEDED TO DEFINE A

UNIDIRECTIONAL COMPOSITE MATERIAL

Unidirectional composites can be assumed to be transversely isotropic and are therefore fully

defined by 5 material constants:

• E11 – longitudinal elastic modulus (fiber direction)

• E22 – transverse elastic modulus (perpendicular to fiber direction)

o Because of transverse isotropy, E33 = E22

• ν12 – longitudinal Poisson ratio

o Because of transverse isotropy, ν13 = ν12

• G12 – longitudinal shear modulus

o Because of transverse isotropy, G13 = G12

• ν23 – interlaminar Poisson ratio

o Because of transverse isotropy, G23 =E

2(1+υ)

Additionally, unidirectional composites have 6 strength values that will fully define the material:

• S11+ – tensile longitudinal strength

• S11- – compressive longitudinal strength

• S22+ – tensile transverse strength

• S22

-

– compressive transverse strength• S12 – in-plane longitudinal shear strength

o Because of transverse isotropy, S13 = S12

• S23 – transverse shear strength

However, sometimes the data a user has access to has “holes” in it. This leaves the user unable

to fully characterize a material based upon the data they have immediate access to.

There are two common scenarios that are encountered when trying to fully

characterize a composite material:

1. Limited lamina-level material property data.

2. No lamina level data, only constituent (fiber/matrix) level data.

To assist in the process, Firehole has developed methodologies to fill in these “holes”. This

eBook will present a solution for both of these scenarios. It is noted, however, that engineering

judgment should always be used when characterizing a composite material.




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SOLUTIONS

As part of the solution, Firehole has developed tools to assist in determining material properties

for composite lamina and fiber/matrix constituents. The scenarios below include descriptions on

how to use these tools to help obtain the data needed.

Helius:CompostiePro is a desktop software package that offers an array of tools to assist in

composite material design and analysis. Included in this tool suite is an extensive material

database of pre-characterized material files and a high-fidelity micromechanics module

that calculates lamina material properties from input fiber and matrix material properties.

http://www.firehole.com/products/comppro/

Helius:MatSim is a free, online micromechanics-based composite material simulator. It

enables the user to calculated lamina properties based on supplied fiber and matrix

volume fraction and elastic constants. This tool additionally offers the following options:

• Pre-populate fiber and matrix data from a built-in material library• Calculate lamina density based upon fiber and matrix density

• Enter data as orthotropic, transversely isotropic, or isotropic

http://www.firehole.com/products/matsim

Prospector:Composites is an online, searchable database of composite material data

sheets. It provides subscribers with a single location to obtain material data as well as the

ability to quickly locate, compare, and evaluate alternative candidate composites. In

addition to manufacturer’s supplied data, the database is populated with quality data from

some of the world’s foremost testing labs and research institutions, giving users

information they can trust.

http://composites.ides.com/








http://www.firehole.com/products/matsim/








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Scenario #1: Limited Lamina-Level Material Property Data

Often times, engineers can gather lamina (ply) material property data from suppliers of the

material, but this data can be less than complete. The two most commonly missing values that

prevent an engineer from fully characterizing a unidirectional composite areν23 and S23. Steps 1and 2 below provide recommendations for obtaining these two values. In the event that even

less material data is available, Step 3 will provide sources to use to find comparable

unidirectional data to use as a starting point.

Step 1: Determine ν 23 if it is not provided.

The interlaminar Poisson ration is often not provided by a manufacturer. Through

experience, Firehole has determined that common values for this constant are 0.5 for carbon

fiber and 0.41 for glass fiber composites.

Step 2: Determine S23 if it is not provided.

The transverse shear strength is often not provided by a manufacturer. Through experience,

Firehole has determined that appropriate values for this constant are obtained from the

equation below:

23=

1

3(

22− )




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Step 3: Determine any remaining elastic constants/strengths by comparing the material

being characterized with a similar, fully-characterized material found in Prospector:

Composites.

Prospector:Composites contains numerous, analysis-ready data sheets of similar materialsthat provide multiple references (Figure 1). Users can determine the missing elastic

constants/strengths based upon the ratios evident in similar materials.

Figure 1 – Prospector:Composites (composites.ides.com) can be used to determine

missing lamina-level material properties by comparing the material being characterized

with similar materials.















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Scenario #2: No Lamina-Level Data, Only Constituent-Level

(fiber/matrix) Data

Engineers also face the challenge of not having any lamina data for a unidirectional composite

that they wish to use, or they have lamina data but spcified at a different fiber volume fraction

than the material used in their design. In this case, an analysis method commonly referred to as

“micromechanics” can be used to derive lamina-level data from material properties of the fiber

and matrix constituents composing the unidirectional material. There are 2 methods available

for using fiber and matrix material properties to calculate lamina level unidirectional properties:

1. Analytical micromechanics calculations – Use equations to calculate lamina

properties from constituent properties.

o Pros

Can be used without specialized software or a computer

o Cons

There are multiple equations for calculating lamina properties (no

consensus on which equation to use)

Not all composite lamina properties have analytical solutions

Time consuming

Error Prone

2. Simulation micromechanics calculations – Use finite element methods to

simulate a unidirectional composite microstructure.

o Pros

Fast and efficient

More accurate than analytical calculations

Easy to perform design comparison studies when multiple combinations

of fibers and matrix materials are being considered

o Cons

Requires specialized software




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Method 1: Analytical micromechanics calculations

E11 (longitudinal elastic modulus)

11 = 11 + (McCullough, 1989)

Ef11 fiber longitudinal elastic modulus

Vf fiber volume fraction

Em matrix elastic modulus

E22 (transverse elastic modulus)

22 = 1− 1−

(Chamis, 1984)

Ef22 fiber transverse elastic modulus

G12 (longitudinal shear modulus)

12 = �1+++1+ (Hashin, 1979)

Gf12 fiber longitudinal shear modulus

Gm matrix shear modulus

Vm matrix volume fraction

ν12 (longitudinal Poisson ratio)

12 = 12 + (McCullough, 1989)

νf12 fiber longitudinal Poisson ratio

νm matrix Poisson ratio

S11+ and S11

- (tensile and compressive longitudinal strength)

11+ = 11+ (Chamis, 1984)

11− =

11− (Chamis, 1984)

Sf11+ Fiber tensile strength

Sf11-

Fiber compressive strength




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S22+ and S22

- (tensile and compressive transverse strength)

22+ =

1.55 (Tsai, 2009)

22− = 0.65 (Tsai, 2009)

Stmatrix tensile strength of neat resign

Scmatrix compressive strength of the neat resign

Method 2a: Use Helius:CompositePro to determine lamina level material properties

from the fiber and matrix properties.

Helius:CompositePro uses micromechanics to calculate the properties of the new composite

material based on known properties of the fiber and matrix constituent materials.

Helius:CompositePro’s micromechanics module utilizes a detailed finite element model of a

fiber/matrix unit cell that features a uniform hexagonal distribution of parallel fibers

embedded in a matrix material. The micromechanical model utilizes the properties of fiber

and matrix materials specified by the user, in addition to the fiber volume fraction specified

by the user (Figure 2). Helius:CompositePro determines the composite material’s elastic

properties (moduli and Poisson ratios) and strengths by using the micromechanical finite

element model to simulate the various fundamental load/deformation relationships of the

composite material.

Figure 2 –The micromechanics calculator available in Helius:CompositePro

can derive lamina properties based upon constituent properties and ratios of

fiber and matrix content.








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Method 2b: Use Helius:MatSim to determine remaining lamina-level material

properties from the fiber and matrix properties.

(This is the same calculator that is available in Helius:CompositePro except that the calculator in

Helius:CompositePro derives lamina strengths from fiber and matrix properties while theHelius:MatSim calculator does not.)

Helius:MatSim (Figure 3) is a free online calculator that calculates lamina constants (moduli,

Poisson ratios, coefficients of thermal expansion, etc.) from matrix and fiber material

properties (http://www.firehole.com/products/matsim).

Figure 3 – Helius:MatSim is a free, online calculator that calculates lamina material

constants from fiber and matrix material properties

(http://www.firehole.com/products/matsim).
















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SUMMARY

Firehole Composites has assembled these tools to provide useful solutions for composite

analysis. We are happy to answer any questions regarding the above scenarios or any other

composite material characterization questions. Contact Firehole Composites at

[email protected] or call us at 307-460-4763.

ABOUT FIREHOLE COMPOSITES

Firehole Composites provides innovative software tools and engineering services designed to

significantly improve structural design and analysis with composite materials. Their mission is to

help engineers create lighter, stronger, safer and more efficient composite designs through

superior analysis capability. Firehole’s team of engineers has extensive study and experience in

analysis of composite materials and software development. For more information, visit

www.firehole.com.

http://www.firehole.com/






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REFERENCES

Chamis, C. (1984, January). Simplified Composite Micromechanics Equations for Strength,

Fracture Toughness, and Environmental Effects. NASA Report TM-83696 .

Hashin, Z. (1979). Analysis of Properties of Fiber Composites with Anisotropic Constituents.

Journal of Applied Mechanics , 546-550.

McCullough, R. (1989). Mechancial Materials Modeling, Delaware Composites Design

Encyclopedia, Vol. 2. Micro-Models for Composite Materials-Continous Fiber Composites , pp. pp.

64-68.

Tsai, S. W. (2009, March 31). Opportunities in Composites, Composite Design Tutorial 4.

Aeronautics & Astronautics .

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