Chapter 3

 

Constitutive Models  Relations between Stress and Strain

 

 

 

 

3.2 Linear elastic material behavior

 

You are probably familiar with the behavior of a linear elastic material from introductory materials courses.

 

3.2.1 Isotropic, linear elastic material behavior

 

If you conduct a uniaxial tensile test on almost any material, and keep the stress levels sufficiently low, you will observe the following behavior:

 The specimen deforms reversibly:  If you remove the loads, the solid returns to its original shape.

 The strain in the specimen depends only on the stress applied to it  it doesn’t depend on the rate of loading, or the history of loading.

 For most materials, the stress is a linear function of strain, as shown in the picture above.  Because the strains are small, this is true whatever stress measure is adopted (Cauchy stress or nominal stress), and is true whatever strain measure is adopted (Lagrange strain or infinitesimal strain).

 For most, but not all, materials, the material has no characteristic orientation.  Thus, if you cut a tensile specimen out of a block of material, as shown in the figure, the the stressstrain curve will be independent of the orientation of the specimen relative to the block of material.  Such materials are said to be isotropic.

 If you heat a specimen of the material, increasing its temperature uniformly, it will generally change its shape slightly.  If the material is isotropic (no preferred material orientation) and homogeneous, then the specimen will simply increase in size, without shape change.

 

 

 

3.2.2 Stressstrain relations for isotropic, linear elastic materials. Young’s Modulus, Poissons ratio and the Thermal Expansion Coefficient.

 

Before writing down stressstrain relations, we need to decide what strain and stress measures we want to use.  Because the model only works for small shape changes

 Deformation is characterized using the infinitesimal strain tensor  defined in Section 2.1.7.  This is convenient for calculations, but has the disadvantage that linear elastic constitutive equations can only be used if the solid experiences small rotations, as well as small shape changes. 

 All stress measures are taken to be equal.  We can use the Cauchy stress  as the stress measure.

 

You probably already know the stressstrain relations for an isotropic, linear elastic solid.  They are repeated below for convenience.

 

Here, E and  are Young’s modulus and Poisson’s ratio,  is the coefficient of thermal expansion, and  is the increase in temperature of the solid.  The remaining relations can be deduced from the fact that both  and  are symmetric. 

 

The inverse relationship can be expressed as

 

 

HEALTH WARNING: Note the factor of 2 in the strain vector.  Most texts, and most FEM codes use this factor of two, but not all.  In addition, shear strains and stresses are often listed in a different order in the strain and stress vectors.  For isotropic materials this makes no difference, but you need to be careful when listing material constants for anisotropic materials (see below).

 

We can write this expression in a much more convenient form using index notation.  Verify for yourself that the matrix expression above is equivalent to

 

 

The inverse relation is

 

 

The stress-strain relations are often expressed using the elastic modulus tensor  or the elastic compliance tensor  as

 

In terms of elastic constants,  and  are

 

 

 

 

3.2.3 Reduced stress-strain equations for plane deformation of isotropic solids

 

For plane strain or plane stress deformations, some strain or stress components are always zero (by definition) so the stress-strain laws can be simplified. 

 

 

 For a plane strain deformation .  The stress strain laws are therefore

 

 

 

In index notation

 

where Greek subscripts  can have values 1 or 2.

 

 For a plane stress deformation  

 

 

 

 

 

 

 

 

3.2.4 Representative values for density, and elastic constants of isotropic solids

 

Most of the data in the table below were taken from the excellent introductory text `Engineering Materials,’ by M.F. Ashby and D.R.H. Jones, Pergamon Press.  The remainder are from random web pages…

 

Note the units  values of E are given in ; the G stands for Giga, and is short for .  The units for density are in  - that’s Mega grams.  One mega gram is 1000 kg.

 

Material

Mass density

 

Youngs Modulus

 

Poisson Ratio
 

Expansion coeft  

Tungsten Carbide

14  17

450650

0.22

 

Silicon Carbide

2.5  3.2

450

0.22

 

Tungsten

13.4

410

0.30

 

Alumina

3.9

390

0.25

 

Titanium Carbide

4.9

380

0.19

 

Silicon Nitride

3.2

320 - 270

0.22

 

Nickel

8.9

215

0.31

 

CFRP

1.5-1.6

70  200

0.20

 

Iron

7.9

196

0.30

 

Low alloy steels

7.8

200 - 210

0.30

 

Stainless steel

7.5-7.7

190 - 200

0.30

 

Mild steel

7.8

196

0.30

 

Copper

8.9

124

0.34

 

Titanium

4.5

116

0.30

 

Silicon

2.5-3.2

107

0.22

 

Silica glass

2.6

94

0.16

 

Aluminum & alloys

2.6-2.9

69-79

0.35

 

Concrete

2.4-2.5

45-50

0.3

 

GFRP

1.4-2.2

7-45

 

 

Wood, parallel grain

0.4-0.8

9-16

0.2

 

Polyimides

1.4

3-5

0.1-0.45

 

Nylon

1.1  1.2

2  4

0.25

 

PMMA

1.2

3.4

0.35-0.4

 

Polycarbonate

1.2  1.3

2.6

0.36

 

Natural Rubbers

0.83-0.91

0.01-0.1

0.49

 

PVC

1.3-1.6

0.003-0.01

0.41

 

 

 

 

3.2.5 Other Elastic Constants  bulk, shear and Lame modulus.

 

Young’s modulus and Poisson’s ratio are the most common properties used to characterize elastic solids, but other measures are also used.  For example, we define the shear modulusbulk modulus and Lame modulus of an elastic solid as follows:

 

 

A nice table relating all the possible combinations of moduli to all other possible combinations is given below.  Enjoy!

 

 

Lame

Modulus

 

Shear

Modulus

 

Young’s

Modulus

 

Poisson’s

Ratio

 

Bulk

Modulus

 

 

 

 

 

 

 

 

 

Irrational

 

Irrational

Irrational

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3.2.6 Physical Interpretation of elastic constants for isotropic solids

 

It is important to have a feel for the physical significance of the two elastic constants E and .

 

  Young’s modulus E is the slope of the stressstrain curve in uniaxial tension.  It has dimensions of stress (  ) and is usually large  for steel, . You can think of E as a measure of the stiffness of the solid. The larger the value of E, the stiffer the solid.  For a stable material, E>0.

 

 Poisson’s ratio  is the ratio of lateral to longitudinal strain in uniaxial tensile stress. It is dimensionless and typically ranges from 0.20.49, and is around 0.3 for most metals.  For a stable material, . It is a measure of the compressibility of the solid.  If , the solid is incompressible  its volume remains constant, no matter how it is deformed.  If , then stretching a specimen causes no lateral contraction.  Some bizarre materials have  --  if you stretch a round bar of such a material, the bar increases in diameter!!

 

 Thermal expansion coefficient quantifies the change in volume of a material if it is heated in the absence of stress.  It has dimensions of (degrees Kelvin)-1 and is usually very small.  For steel,  

 

 The bulk modulus quantifies the resistance of the solid to volume changes.  It has a large value (usually bigger than E).

 

 The shear modulus quantifies its resistance to volume preserving shear deformations.  Its value is usually somewhat smaller than E

 

 

3.2.7 Strain Energy Density for Isotropic Solids

 

Note the following observations

 If you deform a block of material, you do work on it (or, in some cases, it may do work on you…) 

 In an elastic material, the work done during loading is stored as recoverable strain energy in the solid.  If you unload the material, the specimen does work on you, and when it reaches its initial configuration you come out even.

 The work done to deform a specimen depends only on the state of strain at the end of the test.  It is independent of the history of loading. 

 

Based on these observations, we define the strain energy density of a solid as the work done per unit volume to deform a material from a stress free reference state to a loaded state.

 

To write down an expression for the strain energy density, it is convenient to separate the strain into two parts

 

where, for an isotropic solid,

 

represents the strain due to thermal expansion (known as thermal strain), and

 

is the strain due to mechanical loading (known as elastic strain).

 

Work is done on the specimen only during mechanical loading.  It is straightforward to show that the strain energy density is

 

You can also re-write this as

 

Observe that

 

 

 

 

 

3.2.8 Stress-strain relation for a general anisotropic linear elastic material  the elastic stiffness and compliance tensors

 

The simple isotropic model described in the preceding section is unable to describe the response of some materials accurately, even though the material may deform elastically.  This is because some materials do have a characteristic orientation.  For example, in a block of wood, the grain is oriented in a particular direction in the specimen.  The block will be stiffer if it is loaded parallel to the grain than if it is loaded perpendicular to the grain.  The same observation applies to fiber reinforced composite materials. Generally, single crystal specimens of a material will also be anisotropic  this is important when modeling stress effects in small structures such as microelectronic circuits. Even polycrystalline metals may be anisotropic, because a preferred texture may form in the specimen during manufacture.

 

A more general stressstrain relation is needed to describe anisotropic solids. 

 

The most general linear stressstrain relation has the form

 

Here,  is a fourth order tensor (horrors!), known as the elastic stiffness tensor, and  is the thermal expansion coefficient tensor. The stress strain relation is invertible:

 

where  is known as the elastic compliance tensor

 

At first sight it appears that the stiffness tensor has 81 components.  Imagine having to measure and keep track of 81 material properties!  Fortunately,  must have the following symmetries

 

This reduces the number of material constants to 21. The compliance tensor has the same symmetries as .

 

To see the origin of the symmetries of , note that

 The stress tensor is symmetric, which is only possible if  

 If a strain energy density exists for the material, the elastic stiffness tensor must satisfy  

 The previous two symmetries imply , since  and .  

 

To see that , note that by definition

 

and recall further that the stress is the derivative of the strain energy density with respect to strain

 

Combining these,

 

Now, note that

 

so that

 

These symmetries allow us to write the stress-strain relations in a more compact matrix form as

 

where , etc are the elastic stiffnesses of the material.  The inverse has the form

 

where , etc are the elastic compliances of the material.

 

To satisfy Drucker stability, the eigenvalues of the elastic stiffness and compliance matrices must all be greater than zero.

 

HEALTH WARNING: The shear strain and shear stress components are not always listed in the order given when defining the elastic and compliance matrices.  The conventions used here are common and are particularly convenient in analytical calculations involving anisotropic solids.  But many sources use other conventions.  Be careful to enter material data in the correct order when specifying properties for anisotropic solids.

 

 

 

 

3.2.9 Physical Interpretation of the Anisotropic Elastic Constants.

 

It is easiest to interpret , rather than .  Imagine applying a uniaxial stress, say , to an anisotropic specimen.  In general, this would induce both extensional and shear deformation in the solid, as shown in the figure.

 

The strain induced by  the uniaxial stress would be

 

All the constants have dimensions .  The constant  looks like a uniaxial compliance, (like  ), while the ratios  are generalized versions of Poisson’s ratio: they quantify the lateral contraction of a uniaxial tensile specimen.   The shear terms are new  in an isotropic material, no shear strain is induced by uniaxial tension.

 

 

3.2.10 Strain energy density for anisotropic, linear elastic solids

 

The strain energy density of an anisotropic material is

 

 

 

 

3.2.11 Basis change formulas for anisotropic elastic constants

 

The material constants  or  for a particular material are usually specified in a basis with coordinate axes aligned with particular symmetry planes (if any) in the material.  When solving problems involving anisotropic materials it is frequently necessary to transform these values to a coordinate system that is oriented in some convenient way relative to the boundaries of the solid.  Since  is a fourth rank tensor, the basis change formulas are highly tedious, unfortunately. 

 

Suppose that the components of the stiffness tensor are given in a basis , and we wish to determine its components in a second basis, .  We define the usual transformation tensor  with components , or in matrix form

 

This is an orthogonal matrix satisfying . In practice, the matrix can be computed in terms of the angles between the basis vectors. It is straightforward to show that stress, strain, thermal expansion and elasticity tensors transform as

 

 

The basis change formula for the elasticity tensor in matrix form can be expressed as

 

where the basis change matrix K is computed as

 

and the modulo function satisfies

 

Although these expressions look cumbersome they are quite convenient for computer implementation.

 

The basis change for the compliance tensor follows as

 

where

 

 

The proof of these expressions is merely tiresome algebra and will not be given here.  Ting’s book `Anisotropic Elasticity: Theory and Applications’ OUP (1996) has a nice clear discussion.

 

For the particular case of rotation through an angle  in a counterclockwise sense about the  axes, respectively, the rotation matrix reduces to

         

where . The inverse matrix  can be obtained simply by changing the sign of the angle  in each rotation matrix.  Clearly, applying the three rotations successively can produce an arbitrary orientation change.

 

For an isotropic material, the elastic stress-strain relations, the elasticity matrices and thermal expansion coefficient are unaffected by basis changes.

 

 

 

 

3.2.12 The effect of material symmetry on stress-strain relations for anisotropic materials

 

A general anisotropic solid has 21 independent elastic constants. Note that in general, tensile stress may induce shear strain, and shear stress may cause extension.

 

If a material has a symmetry plane, then applying stress normal or parallel to this plane induces only extension in direction normal and parallel to the plane

 

For example, suppose the material contains a single symmetry plane, and let    be normal to this plane.

 

Then the components of the elastic stiffnes matrix  (  ). (symmetrical terms also vanish, of course). This leaves 13 independent constants. 

 

Similar restrictions on the thermal expansion coefficient can be determined using symmetry conditions.  Details are left as an exercise.

 

In the following sections, we list the stress-strain relations for anisotropic materials with various numbers of symmetry planes.

 

 

 

 

3.2.13 Stress-strain relations for linear elastic orthotropic materials

 

An orthotropic material has three mutually perpendicular symmetry planes. This type of material has 9 independent material constants.  With basis vectors perpendicular to the symmetry plane, the elastic stiffness matrix has the form

 

This relationship is sometimes expressed in inverse form, in terms of generalized Young’s moduli and Poisson’s ratios (which have the same significance as Young’s modulus and Poisson’s ratio for uniaxial loading along the three basis vectors) as follows                                                                    

 

Here the generalized Poisson’s ratios are not symmetric but instead satisfy  (no sums). This ensures that the stiffness matrix is symmetric.

 

The engineering constants are related to the components of the compliance tensor by

 

or in inverse form

 

 

For an orthotropic material thermal expansion cannot induce shear (in this basis) but the expansion in the three directions need not be equal.  Consequently the thermal expansion coefficient tensor has the form

 

 

 

 

 

3.2.14 Stress-strain relations for linear elastic Transversely Isotropic Material

 

A special case of an orthotropic solid is one that contains a plane of isotropy (this implies that the solid can be rotated with respect to the loading direction about one axis without measurable effect on the solid’s response).  Choose  perpendicular to this symmetry plane.  Then, transverse isotropy requires that , , , , so that the stiffness matrix has the form                          

 

The engineering constants must satisfy

 

and the compliance matrix has the form

 

where .  As before the Poisson’s ratios are not symmetric, but satisfy  

 

The engineering constants and stiffnesses are related by

 

 

 

For this material the two thermal expansion coefficients in the symmetry plane must be equal, so the thermal expansion coefficient tensor has the form

 

 

 

 

 

3.2.15 Representative values for elastic constants of transversely isotropic hexagonal close packed crystals

 

Hexagonal close-packed crystals are an example of transversely isotropic materials.  The  axis must be taken to be perpendicular to the basal (0001) plane of the crystal, as shown in the picture.  Since the plane perpendicular to  is isotropic the orientation of  and  is arbitrary.

 

A table of values of stiffnesses (taken from Freund and Suresh, Thin Film Materials, CUP 2003) is listed below.  F&S list the original sources for their data on page 163.

 

 

 

 

 (GPa)

 (GPa)

 (GPa)

 (GPa)

 (GPa)

Be

292.3

336.4

162.5

26.7

14

C

1160

46.6

2.3

290

109

Cd

115.8

51.4

20.4

39.8

40.6

Co

307

358.1

78.3

165

103

Hf

181.1

196.9

55.7

77.2

66.1

Mg

59.7

61.7

16.4

26.2

21.7

Ti

162.4

180.7

46.7

92

69

Zn

161

61

38.3

34.2

50.1

Zr

143.4

164.8

32

72.8

65.3

ZnO

209.7

210.9

42.5

121.1

105.1

 

The engineering constants can be calculated to be

 

 

 (GPa)

 (GPa)

 

 

 

 

 

 


(GPa)

 

(GPa)

Be

289.38

335.17

0.09

0.04

0.04

162.50

132.80

C

903.69

30.21

0.04

0.08

2.25

2.30

435.00

Cd

83.02

30.21

0.09

0.26

0.72

20.40

38.00

Co

211.30

313.15

0.49

0.22

0.15

78.30

71.00

Hf

139.87

163.07

0.35

0.26

0.22

55.70

51.95

Mg

45.45

50.74

0.36

0.25

0.23

16.40

16.75

Ti

104.37

143.27

0.48

0.27

0.20

46.70

35.20

Zn

119.45

35.28

-0.06

0.26

0.87

38.30

63.40

Zr

98.79

125.35

0.40

0.30

0.24

32.00

35.30

ZnO

127.30

144.12

0.44

0.32

0.28

42.50

44.30

 

 

 

 

3.2.16 Linear elastic stress-strain relations for cubic materials

 

A huge number of materials have cubic symmetry  all the FCC and BCC metals, for example.  The constitutive law for such a material is particularly simple, and can be parameterized by only 3 material constants.  Pick basis vectors perpendicular to the symmetry planes, as shown.

 

Then

 

or in terms of engineering constants

 

This is virtually identical to the constitutive law for an isotropic solid, except that the shear modulus  is not related to the Poisson’s ratio and Young’s modulus through the usual relation given in Section 3.1.6.   In fact, the ratio

 

provides a convenient measure of anisotropy.  For  the material is isotropic. 

 

For this material the thermal expansion coefficient matrix must be isotropic.

 

The relationships between the elastic constants are

 

 

 

 

 

3.2.17 Representative values for elastic properties of cubic crystals and compounds

 

A table of elastic constants for various cubic crystals and compounds (modified from Simmons and Wang ‘Single Crystal Elastic Constants and Calculated Aggregate Properties’ MIT Press (1970)) is given below

 

Material

 

 

 (GPa)

 (GPa)

 (GPa)

 (GPa)

 

 

 (GPa)

 

 

Ag

(fcc)

124.00

46.10

93.40

43.75

0.43

46.10

3.01

Al

(fcc)

107.30

28.30

60.90

63.20

0.36

28.30

1.22

Au

(fcc)

192.90

41.50

163.80

42.46

0.46

41.50

2.85

Cu

(fcc)

168.40

75.40

121.40

66.69

0.42

75.40

3.21

Ir

(fcc)

580.00

256.00

242.00

437.51

0.29

256.00

1.51

Ni

(fcc)

246.50

127.40

147.30

136.31

0.37

127.40

2.57

Pb

(fcc)

49.50

14.90

42.30

10.52

0.46

14.90

4.14

Pd

(fcc)

227.10

71.70

176.00

73.41

0.44

71.70

2.81

Pt

(fcc)

346.70

76.50

250.70

136.29

0.42

76.50

1.59

Cr

(bcc)

339.80

99.00

58.60

322.56

0.15

99.00

0.70

Fe

(bcc)

231.40

116.40

134.70

132.28

0.37

116.40

2.41

K

(bcc)

4.14

2.63

2.21

2.60

0.35

2.63

2.73

Li

(bcc)

13.50

8.78

11.44

3.00

0.46

8.78

8.52

Mo

(bcc)

440.80

121.70

172.40

343.86

0.28

121.70

0.91

Na

(bcc)

6.15

5.92

4.96

1.72

0.45

5.92

9.95

Nb

(bcc)

240.20

28.20

125.60

153.95

0.34

28.20

0.49

Ta

(bcc)

260.20

82.60

154.50

145.08

0.37

82.60

1.56

V

(bcc)

228.00

42.60

118.70

146.72

0.34

42.60

0.78

W

(bcc)

522.40

160.80

204.40

407.43

0.28

160.80

1.01

C

(dc)

949.00

521.00

151.00

907.54

0.14

521.00

1.31

Ge

(dc)

128.40

66.70

48.20

102.09

0.27

66.70

1.66

Si

(dc)

166.20

79.80

64.40

130.23

0.28

79.80

1.57

GaAs

 

118.80

59.40

53.70

85.37

0.31

59.40

1.82

GaP

 

141.20

70.50

62.50

102.85

0.31

70.50

1.79

InP

 

102.20

46.00

57.60

60.68

0.36

46.00

2.06

KCl

 

39.50

6.30

4.90

38.42

0.11

6.30

0.36

LiF

 

114.00

63.60

47.70

85.86

0.29

63.60

1.92

MgO

 

287.60

151.40

87.40

246.86

0.23

151.40

1.51

NaCl

 

49.60

12.90

12.40

44.64

0.20

12.90

0.69

TiC

 

500.00

175.00

113.00

458.34

0.18

175.00

0.90

 


 

 

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