characterization of viscoelastic properties of a material
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APPROVED: Jaehyung Ju, Major Professor Xun Yu, Committee Member Sheldon Shi, Committee Member Yong Tao, Chair of the Department of
Mechanical and Energy Engineering Costas Tsatsoulis, Dean of the College of
Engineering Mark Wardell, Dean of the Toulouse Graduate
School
CHARACTERIZATION OF VISCOELASTIC PROPERTIES OF A MATERIAL
USED FOR AN ADDITIVE MANUFACTURING METHOD
Shaheer Iqbal
Thesis Prepared for the Degree of
MASTER OF SCIENCE
UNIVERSITY OF NORTH TEXAS
December 2013
Iqbal, Shaheer. Characterization of Viscoelastic Properties of a Material Used for an
Additive Manufacturing Method. Master of Science (Mechanical & Energy Engineering),
December 2013, 58 pp., 8 tables, 34 figures, references, 25 titles.
Recent development of additive manufacturing technologies has led to lack of
information on the base materials being used. A need arises to know the mechanical behaviors
of these base materials so that it can be linked with macroscopic mechanical behaviors of 3D
network structures manufactured from the 3D printer. The main objectives of my research are
to characterize properties of a material for an additive manufacturing method (commonly
referred to as 3D printing). Also, to model viscoelastic properties of Procast material that is
obtained from 3D printer. For this purpose, a 3D CAD model is made using ProE and 3D printed
using Projet HD3500. Series of uniaxial tensile tests, creep tests, and dynamic mechanical
analysis are carried out to obtained viscoelastic behavior of Procast. Test data is fitted using
various linear and nonlinear viscoelastic models. Validation of model is also carried out using
tensile test data and frequency sweep data. Various other mechanical characterization have
also been carried out in order to find density, melting temperature, glass transition
temperature, and strain rate dependent elastic modulus of Procast material. It can be
concluded that melting temperature of Procast material is around 337ยฐC, the elastic modulus is
around 0.7-0.8 GPa, and yield stress is around 16-19 MPa.
ACKNOWLEDGEMENTS
I would like to express the deepest appreciation to my advisor, Dr. Jaehyung Ju, for his
constructive guidance, understanding and support. I feel really obliged to have worked with
him on his research. Without his guidance and persistent help this thesis would not have been
possible.
I would also like to thank my committee member Dr. Sheldon Shi and Dr. Xun Yu, for
granting me permission to use their equipments. Also, I would like to thanks Dr. Nandika Anne
Dโsouza for allowing me to use her lab and equipments.
I want to express my thanks to all other faculty members and my friends in the
Mechanical and Energy Engineering Department for their assistance and excellent help which
have helped me on my thesis.
iii
TABLE OF CONTENTS Page
ACKNOWLEDGEMENTS ................................................................................................................... iii
LIST OF TABLES ................................................................................................................................ vi
LIST OF FIGURES ............................................................................................................................. vii
CHAPTER 1 INTRODUCTION ........................................................................................................... 1
1.1 Motivation ........................................................................................................................ 2
1.2 Objectives ......................................................................................................................... 2
CHAPTER 2 LITERATURE REVIEW .................................................................................................... 3
2.1 Additive Manufacturing ........................................................................................................ 3
2.2 Mechanical Characterization ........................................................................................... 3
2.2.1 Tensile Test ............................................................................................................... 3
2.2.2 Creep Test ................................................................................................................. 4
2.2.3 Dynamic Mechanical Analysis (DMA) ....................................................................... 4
2.3 Viscoelastic Models .......................................................................................................... 5
2.3.1 Maxwell Model ......................................................................................................... 5
2.3.2 Generalized Maxwell Model ..................................................................................... 6
2.3.3 Voigt Model ............................................................................................................... 7
2.3.4 Voigt-Kelvin Model.................................................................................................... 8
2.3.5 Prony Series .............................................................................................................. 9
2.3.6 Schapery Model ...................................................................................................... 14
CHAPTER 3 MECHANICAL TESTING ............................................................................................... 16
3.1 Tensile Testing ................................................................................................................ 16
3.2 Creep Test ...................................................................................................................... 18
3.3 Dynamic Mechanical Analysis Test ................................................................................ 19
3.4 Differential Scanning Calorimetry (DSC) ........................................................................ 23
3.5 Density ............................................................................................................................ 24
CHAPTER 4 NONLINEAR REGRESSION .......................................................................................... 26
iv
CHAPTER 5 LINEAR VISCOELASTIC MODELS ................................................................................. 28
5.1 Maxwell Model ................................................................................................................... 28
5.2 Voigt-Kelvin Model ......................................................................................................... 30
5.3 Prony Series .................................................................................................................... 34
CHAPTER 6 NON-LINEAR VISCOELASTIC MODEL .......................................................................... 39
CHAPTER 7 VALIDATION OF MODEL ............................................................................................. 46
CHAPTER 8 CONCLUSIONS AND FUTURE WORK .......................................................................... 55
REFERENCES .................................................................................................................................. 56
v
LIST OF TABLES
Table 1 : Different strain rates and speed of crosshead ............................................................... 17
Table 2 : Young's Modulus and Yield Strength as a function of strain rate .................................. 18
Table 3 : Density of Procast material ............................................................................................ 25
Table 4 : Material Parameters for Maxwell Model ....................................................................... 28
Table 5: Material Parameters for Voigt-Kelvin Model .................................................................. 31
Table 6 : Creep Material Parameters for Prony Series ................................................................ 36
Table 7 : Material Parameters for Schapery Model ..................................................................... 42
Table 8 : Polynomial Constants for Nonlinear Material Paramters ............................................. 44
vi
LIST OF FIGURES
Fig. 1 : Projet HD 3500 3D Printer ................................................................................................... 1
Fig. 2 : Some 3D Printed Samples .................................................................................................. 2
Fig. 3 : Maxwell Model .................................................................................................................... 6
Fig. 4 : Generalized Maxwell Model ............................................................................................... 7
Fig. 5 : Voigt Model ......................................................................................................................... 7
Fig. 6 : Voigt-Kelvin Model ............................................................................................................. 8
Fig. 7 : Shimadzu AGS-X Series Universal Testing Machine ......................................................... 16
Fig. 8 : Stress-Strain Behavior of Procast at different strain rates................................................ 17
Fig. 9 : Creep Test Results from 1-10 MPa .................................................................................... 19
Fig. 10 : Storage Modulus and Loss Modulus plot over temperature of Procast at 1 Hz ............ 21
Fig. 11 Storage Modulus and Tan Delta plot over temperature of Procast at 1 Hz .................... 22
Fig. 12 : Frequency Sweep Response of Loss Modulus at Room Temperature ............................ 22
Fig. 13 : Frequency Sweep Response of Storage Modulus at Room Temperature ...................... 23
Fig. 14 : DSC Graph for Procast ..................................................................................................... 24
Fig. 15 : Creep Strain for Maxwell model for 1 -3 MPa stress ...................................................... 29
Fig. 16 : Creep Strain for Maxwell model for 1-8 MPa stress ....................................................... 29
Fig. 17 : Creep Strain for Voigt-Klevin Model for 1 -3 MPa stress ................................................ 32
Fig. 18 : Creep Strain for Voigt-Kelvin Model for 1-8 MPa stress ................................................. 33
Fig. 19: Creep Strain prediction for Prony Series .......................................................................... 36
Fig. 20 : Schapery Model Prediction of Creep Test ...................................................................... 43
Fig. 21 : Nonlinear Parameters for Schapery Model at Various Stress Levels .............................. 43
vii
Fig. 22 : Strain-Rate Dependent Tensile Test Data Including Yielding Region .............................. 46
Fig 23 : Validation for Maxwell Model at Various Strain Rates .................................................... 47
Fig. 24 : Validation for Voigt-Kelvin Model at Various Strain Rates ............................................. 47
Fig. 25 : Validation for Prony Series at Various Strain Rates ........................................................ 48
Fig. 26 : Validation for Schapery Model at Various Strain Rates .................................................. 49
Fig. 27 : Loss Modulus response for Maxwell model ................................................................... 50
Fig. 28 : Loss Modulus response for Voigt-Kelvin model ............................................................. 50
Fig. 29 : Loss Modulus response for Prony model ....................................................................... 51
Fig. 30 : Loss Modulus response for Schapery Model .................................................................. 51
Fig. 31 : Storage Modulus response for Maxwell model .............................................................. 52
Fig. 32 : Storage Modulus response for Voigt-Kelvin model ....................................................... 53
Fig. 33 : Storage Modulus response for Prony Series ................................................................... 53
Fig. 34 : Storage Modulus response for Schapery model ............................................................. 54
viii
CHAPTER 1
INTRODUCTION
Recent development of additive manufacturing technologies has led to a lack of
information on the base materials being used. A need arises to know the mechanical behaviors
of these base materials so that it can be linked with macroscopic mechanical behaviors of 3D
network structures manufactured from the 3D printer. The base material used by our 3D
printer is Procast. The objective of my research is to investigate non-linear stress-strain
behaviors of the Procast obtained from the 3D printer.
Fig. 1 : Projet HD 3500 3D Printer
For this purpose, different mechanical tests are conducted on Procast such as tensile
test, and creep test. Some dynamic mechanical analysis is also being carried out. For this
purpose, a 3D CAD model is made using ProE and 3D printed using Projet HD3500 plus Fig. 1.
For post-processing, a Projet finisher oven is used to melt the support material, and digital
ultrasonic cleaner is used for cleaning the remaining support material. Tensile and creep tests
1
are performed using Shimadzu AGS-X series Universal Testing Machine following ASTM
standards [1]-[5][8][9]. Rheometric Scientific equipment is used to carry out dynamic
mechanical analysis. Experimental data is fitted using different viscoelastic models both in time
domain and frequency domain. Validation of the data is carried out using experimental test
data and viscoelastic model data.
Fig. 2 : Some 3D Printed Samples
1.1 Motivation
Additive manufacturing (3D printing) is a relatively new technique and the material used
in this method is unknown. 3D printer can make complex cellular structures which were not
possible the conventional manufacturing technique.
When considering a cellular structure, its geometry and material gives the material
behavior. In my research I am working on the material aspect and will be providing research
community the information regarding the base material (which in our case is called Procast)
and its properties.
1.2 Objectives
The main objectives of my research are to characterize properties of a material for an
additive manufacturing method (commonly referred to as 3D printing). Also, to model
viscoelastic properties of Procast material that is obtained from 3D printer.
2
CHAPTER 2
LITERATURE REVIEW
2.1 Additive Manufacturing
Additive manufacturing is a method in which 3D solid objects are printed using a 3D
printer. In common terminology, additive manufacturing is also termed as 3D printing. The
general principle of additive manufacturing is layer by layer deposition of base material to make
3D parts. Additive manufacturing uses a 3D CAD model as a soft copy for 3D part. The
corresponding CAD file is converted into STL format which is the required format for the 3D
printer [1]. Upon receiving the STL file, the 3D printer starts printing the 3D part layer by layer.
There are differing types of printing process available [2]; the following two processes
are used by our Projet HD 3500 plus, 3D Systems printer.
a) Fused deposition modeling (FDM): In this method, layer by layer deposition of base
material is made by the 3D printer.
b) Stereolithography (SLA): This process uses a laser to solidify the base material as it is
being printed by FDM method.
2.2 Mechanical Characterization
Since 3D printing is a new technique, and base materials being used for it are relatively
new as well, a need arises to conduct different mechanical tests in order to find their
mechanical properties.
2.2.1 Tensile Test
Tensile tests were carried out using ASTM Standards D412 [1] and D638 [5]. For our
Procast material, dumbbell shaped specimen Fig. 2 was printed using Projet HD3500 Fig. 1. The
3
same dimensions were considered for Procast as Die C in ASTM standard D412 [1]. Once the
sample is printed, it is placed in the Projet finisher oven to melt the support material; in our
case it is wax. The Procast sample is further cleaned using Digital Ultrasonic Cleaner, and the
material obtained is dried by placing it in a desiccator.
For the tensile test, Shimadzu AGS-X series Universal Testing Machine Fig. 7 is used with
a load cell of 5kN. The test is conducted at room temperature 23 ยฑ 2ยฐC as per ASTM standards.
For different strain rates, the cross-head speed of the machine is adjusted to obtain the desired
strain rate.
2.2.2 Creep Test
Creep tests were carried out using Shimadzu AGS-X series Universal Testing Machine. In
a creep test, stress is held constant for some time duration at a specified temperature. We
conducted the test using different stress levels at room temperature 23 ยฑ 2ยฐC for 1800 seconds.
ASTM Standard D2990 [3] was followed in preparing the sample. The sample dimension was in
standing with Type 1 in ASTM Standard D638 [5].
2.2.3 Dynamic Mechanical Analysis (DMA)
Dynamic mechanical properties refer to the response of the material when it is
subjected to dynamic loading. These properties may be expressed in terms of a storage
modulus, loss modulus and a damping term.
Temperature sweep test is conducted where we study the response of the material at
fixed frequency over a wide range of temperature. This temperature sweep allows in studying
the transitions in the material including the glass transition temperature (๐๐๐๐).
4
A Frequency sweep test is conducted where we study the response of the material at
fixed temperature but over a wide range of frequencies, we use this data for time temperature
superposition. We find the material properties by the following equations
๐๐ก๐๐๐๐๐ ๐๐๐๐ข๐๐ข๐ (Eโ) = ๐๐ะ๐๐๐๐ ๐ฟ (2. 1)
๐ฟ๐๐ ๐ ๐๐๐๐ข๐๐ข๐ (Eโโ) = ๐๐ะ๐๐ ๐๐๐ฟ (2. 2)
๐โ๐๐ ๐ ๐๐๐๐๐ โถ ๐๐๐๐(๐๐๐๐ก๐) = EโโEโ
(2. 3)
2.3 Viscoelastic Models
Viscoelasticity is made up of two words: viscous and elastic. If a material exhibits both
elastic and viscous nature, then it is termed as viscoelasticity, and such material exhibits a time-
dependent strain. A viscoelastic material has an elastic component and a viscous component.
This viscous component gives a time-dependent strain. The elastic component is modeled as
spring (๐ = ๐ธ๐), while the viscous component is modeled as dashpot (๐ = ๐ ๐๐๐๐ก
)
There are various viscoelastic material models that give a good time dependent
response of the material. Few viscoelastic material models are studied here.
2.3.1 Maxwell Model
Maxwell model is the simplest model that gives linear viscoelastic material response
[11] [13]. In this model, spring is connected in a series with a dashpot. Here, strain in both the
elements is different while stress remains same ๐๐๐๐๐ด๐ฟ = ๐๐ท = ๐๐
Total strain is the sum of strains in spring and dashpot ๐๐๐๐๐ด๐ฟ = ๐๐ท + ๐๐
5
Fig. 3 : Maxwell Model
To obtain strain rate,
๐๐๐๐๐๐ด๐ฟ๐๐ก
=๐๐๐ท๐๐ก
+๐๐๐๐๐ก
=๐๐
+1๐ธ๐๐๐๐ก
(2. 4)
๐ฬ =๐๐
+๏ฟฝฬ๏ฟฝ๐ธ
(2. 5)
For Stress Relaxation test, the above equations becomes
0 =๐๐
+๏ฟฝฬ๏ฟฝ๐ธ
(2. 6)
๏ฟฝฬ๏ฟฝ๐ธ
= โ๐๐
(2. 7)
๐๐๐
= โ๐ธ๐๐๐ก (2. 8)
๐ = ๐๐๐โ๐ก ๐๏ฟฝ (2. 9)
Here, ฯ is the relaxation time given by ๐ = ๐๐ธ๏ฟฝ
And Relaxation Modulus ๐ธ(๐ก) is given by
๐ธ(๐ก) = ๐ธ๐โ๐ก ๐๏ฟฝ (2. 10)
And for Creep Compliance,
๐ท(๐ก) = ๐ท(1 + ๐ก ๐๏ฟฝ ) (2. 11)
2.3.2 Generalized Maxwell Model
Generalized Maxwell model is obtained by adding parallel combinations of n Maxwell
units. This model is used to model relaxation behavior [12]-[13].
๐ธ
๐
6
Fig. 4 : Generalized Maxwell Model
The relation for stress relaxation for the generalized Maxwell model is given by
๐ธ(๐ก) = ๏ฟฝ๐ธ๐๐โ๐ก ๐๐๏ฟฝ
๐
๐=1
(2. 12)
The equation for Generalized Maxwell model in frequency domain is given by
(๐๐ก๐๐๐๐๐ ๐๐๐๐ข๐๐ข๐ )๐ธโฒ(๐) = ๏ฟฝ๏ฟฝ๐ธ๐๐๐2๐2
1 + ๐๐2๐2๏ฟฝ๐
๐=1
(2. 13)
(๐ฟ๐๐ ๐ ๐๐๐๐ข๐๐ข๐ )๐ธโฒโฒ(๐) = ๏ฟฝ๏ฟฝ๐ธ๐๐๐๐
1 + ๐๐2๐2๏ฟฝ๐
๐=1
(2. 14)
2.3.3 Voigt Model
Voigt model is another type of linear viscoelastic material model in which spring and
dashpot are connected in parallel.
Fig. 5 : Voigt Model
Here, stress is different in both the elements while strain remains same: ๐๐๐๐๐ด๐ฟ = ๐๐ท = ๐๐
๐ธ ๐
7
Total stress is sum of stresses in spring and dashpot: ๐๐๐๐๐ด๐ฟ = ๐๐ท + ๐๐
๐(๐ก) = ๐ธ ๐(๐ก) + ๐๐๐(๐ก)๐๐ก
(2. 15)
For creep test,
๐๐ = ๐ธ ๐(๐ก) + ๐๐๐(๐ก)๐๐ก
(2. 16)
๐(๐ก) =๐๐๐ธ๏ฟฝ1 โ ๐โ๐ก ๐๏ฟฝ ๏ฟฝ (2. 17)
Here, ฯ is the relaxation time given by ๐ = ๐๐ธ๏ฟฝ
Thus, creep compliance is given by
๐ท(๐ก) = ๐ท ๏ฟฝ1 โ ๐โ๐ก ๐๏ฟฝ ๏ฟฝ (2. 18)
2.3.4 Voigt-Kelvin Model
The Voigt-Kelvin model is a generalization of the Voigt model, in which Voigt elements
are added in series [11] [12]. This model gives good response for creep behavior, but it is
relatively poor for relaxation behavior:
Fig. 6 : Voigt-Kelvin Model
8
So, for n number of Voigt elements, the creep compliance is given by
๐ท(๐ก) = ๏ฟฝ๐ท๐ ๏ฟฝ1 โ ๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ
๐
๐=1
(2. 19)
Applying Fourier Transformation,
(๐๐ก๐๐๐๐๐ ๐๐๐๐ข๐๐ข๐ ) ๐ทโฒ(๐) = ๏ฟฝ๏ฟฝ๐ท๐
1 + ๐๐2๐2๏ฟฝ๐
๐=1
(2. 20)
(๐ฟ๐๐ ๐ ๐๐๐๐ข๐๐ข๐ ) ๐ทโฒโฒ(๐) = ๏ฟฝ๐ท๐ ๏ฟฝ๐๐๐
1 + ๐๐2๐2๏ฟฝ๐
๐=1
(2. 21)
2.3.5 Prony Series
A common form for the linear viscoelastic response is given by Prony Series by the
following equation [10]:
๏ฟฝ๐ผ๐๐โ๐ก ๐๐๏ฟฝ
๐
๐=1
(2. 22)
Here, ๐๐ are the time constants and ๐ผ๐ are the exponential coefficients.
In order to determine material parameters, creep and relaxation tests are used mostly.
There are various approaches to determine Prony coefficients; we have used a nonlinear
regression approach to determine the coefficients.
We know that for a relaxation test, its constitutive equation is given by the following equation:
๐(๐ก) = ๐(๐ก)ะ0 (2. 23)
Here, ๐(๐ก) is the relaxation function, and its response under Prony Series is given by
๐(๐ก) = ๐ธ0.๏ฟฝ1 โ๏ฟฝ๐๐(1 โ ๐โ๐ก ๐๐๏ฟฝ
๐
๐=1
๏ฟฝ (2. 24)
Here,
9
๐๐ is the ith Prony constant ๐ = (1,2,3, โฆ )
๐๐ is the ith Prony Retardation time constant ๐ = (1,2,3, โฆ )
๐ธ0 is the instantaneous modulus
When time ๐ก = 0, ๐(0) = ๐ธ0
And when time ๐ก = โ, ๐(โ) = ๐ธโ(1 โ โ๐๐)
To determine the stress state at a particular time, the deformation history must be
taken into account. For linear viscoelastic materials, a superposition of hereditary integrals
gives a time dependent response. In the case for stress relaxation, the specimen is under no
strain level prior to time ๐ก = 0, at which a strain is applied and its corresponding stress
response for time ๐ก > 0 is given by
๐(๐ก) = ะ0๐(๐ก) + ๏ฟฝ ๐(๐ก โ ๐)๐ะ(๐)๐๐
๐๐๐ก
0 (2. 25)
Here, ๐(๐ก) is the relaxation function and ๐ะ(๐)๐๐
is the strain rate.
The process in general relaxation test is divided into 2 segments [10]: loading response
(increasing strain rate) and constant strain response (zero strain rate). Their functions are given
below:
ะ(๐ก) = ๏ฟฝะ1๐ก
(๐ก1 โ ๐ก0)๏ฟฝ ; ๐ก0 < ๐ก < ๐ก1
ะ1 ; ๐ก1 < ๐ก < ๐ก2๏ฟฝ
๐ะ๐๐ก
= ๏ฟฝะ1
(๐ก1 โ ๐ก0)๏ฟฝ ; ๐ก0 < ๐ก < ๐ก1
0 ; ๐ก1 < ๐ก < ๐ก2๏ฟฝ
Here, ะ0 = 0, ะ1 is the strain level at which the strain is kept constant, and ๐ก0 = 0
10
Using the above strain response, the stress function for the loading response can be given as
[10]
Step 1(๐๐ < ๐ก โค ๐๐)
๐1(๐ก) = ะ0๐(๐ก) + ๏ฟฝ ๐(๐ก โ ๐)๐ะ(๐)๐๐
๐๐๐ก
0 (2. 26)
๐1(๐ก) = 0 + ๏ฟฝ ๐ธ0. ๏ฟฝ1 โ๏ฟฝ๐๐(1 โ ๐โ(๐กโ๐)
๐๐๏ฟฝ๐
๐=1
๏ฟฝะ1๐ก1
๐๐๐ก
0 (2. 27)
๐1(๐ก) = ๐ธ0ะ1๐ก1
๏ฟฝ ๏ฟฝ1 โ๏ฟฝ๐๐(1 โ ๐โ(๐กโ๐)
๐๐๏ฟฝ๐
๐=1
๏ฟฝ ๐๐๐ก
0 (2. 28)
๐1(๐ก) = ๐ธ0ะ1๐ก1
๏ฟฝ ๏ฟฝ1 โ๏ฟฝ๐๐
๐
๐=1
+ ๏ฟฝ๐๐๐โ(๐กโ๐)
๐๐๏ฟฝ๐
๐=1
๏ฟฝ ๐๐๐ก
0 (2. 29)
๐1(๐ก) = ๐ธ0ะ1๐ก1
๏ฟฝ๐ โ๏ฟฝ๐๐๐๐
๐=1
+ ๏ฟฝ๐๐๐๐๐โ(๐กโ๐)
๐๐๏ฟฝ๐
๐=1
๏ฟฝ0
๐ก
(2. 30)
๐1(๐ก) = ๐ธ0ะ1๐ก1
๏ฟฝ๐ก โ๏ฟฝ๐๐๐ก +๏ฟฝ๐๐๐๐ โ๏ฟฝ๐๐๐๐๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ (2. 31)
Here, n is the number of terms in the Prony Series.
Step 2(๐๐ < ๐ก โค ๐๐)
In this step, the strain is kept constant.
๐2(๐ก) = ะ0๐(๐ก) + ๏ฟฝ ๐(๐ก โ ๐)๐ะ(๐)๐๐
๐๐๐ก1โ
0+ ๏ฟฝ ๐(๐ก โ ๐)
๐ะ(๐)๐๐
๐๐๐ก
๐ก1+ (2. 32)
๐2(๐ก) = 0 +๐ธ0ะ1๐ก1
๏ฟฝ๐ โ๏ฟฝ๐๐๐๐
๐=1
+ ๏ฟฝ๐๐๐๐๐โ(๐กโ๐)
๐๐๏ฟฝ๐
๐=1
๏ฟฝ0
๐ก1
+ 0 (2. 33)
๐2(๐ก) = ๐ธ0ะ1๐ก1
๏ฟฝ๐ก1 โ๏ฟฝ๐๐๐ก1 +๏ฟฝ๐๐๐๐๐โ(๐กโ๐ก1)
๐๐๏ฟฝ โ๏ฟฝ๐๐๐๐๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ (2. 34)
11
Using a non-linear regression technique [10], the above stress function can be determined by a
stress relaxation test.
To get material response in frequency domain for Prony Series, we apply Fourier
transformation. Prony Series is represented in terms of shear relaxation modulus by the
following expression [20]:
๐๐ (๐ก) = 1 โ๏ฟฝ๐๐ ๏ฟฝ1 โ ๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ
๐
๐=1
(2. 35)
Here, ๐๐ and ๐๐ are material parameters and ๐๐ (๐ก) is the dimensionless relaxation modulus
given by
๐๐ (๐ก) =๐บ๐ (๐ก)๐บ0
(2. 36)
Apply Fourier Transformation:
(๐๐ก๐๐๐๐๐ ๐๐๐๐ข๐๐ข๐ ) ๐บโฒ(๐) = ๐บ0 ๏ฟฝ1 โ๏ฟฝ๐๐
๐
๐=1
๏ฟฝ + ๐บ0๏ฟฝ๐๐
๐
๐=1
๏ฟฝ๐๐2๐2
1 + ๐๐2๐2๏ฟฝ (2. 37)
(๐ฟ๐๐ ๐ ๐๐๐๐ข๐๐ข๐ ) ๐บโฒโฒ(๐) = ๐บ0๏ฟฝ๐๐
๐
๐=1
๏ฟฝ๐๐๐
1 + ๐๐2๐2๏ฟฝ (2. 38)
We know that for creep compliance [10],
ะ(๐ก) = ๐ฝ(๐ก)๐0 (2. 39)
Here, ๐ฝ(๐ก) is the creep compliance function and its response under Prony Series is given by
๐ฝ(๐ก) = ๐ฝ0.๏ฟฝ1 โ๏ฟฝ๐๐๐โ๐ก ๐๐๏ฟฝ
๐
๐=1
๏ฟฝ (2. 40)
ะ(๐ก) = ๐0๐ฝ(๐ก) + ๏ฟฝ ๐ฝ(๐ก โ ๐)๐๐(๐)๐๐
๐๐๐ก
0 (2. 41)
12
๐(๐ก) = ๏ฟฝ๐1๐ก
(๐ก1 โ ๐ก0)๏ฟฝ ; ๐ก0 < ๐ก < ๐ก1
๐1 ; ๐ก1 < ๐ก < ๐ก2๏ฟฝ
๐๐๐๐ก
= ๏ฟฝ๐1
(๐ก1 โ ๐ก0)๏ฟฝ ; ๐ก0 < ๐ก < ๐ก1 0 ; ๐ก1 < ๐ก < ๐ก2
๏ฟฝ
Here, ๐0 = 0, ๐1 is the stress level at which the stress is kept constant, and ๐ก0 = 0
Step 1(๐๐ < ๐ โค ๐๐)
ะ1(๐ก) = ๐0๐ฝ(๐ก) + ๏ฟฝ ๐ฝ(๐ก โ ๐)๐๐(๐)๐๐
๐๐๐ก
0 (2. 42)
ะ1(๐ก) = 0 + ๏ฟฝ ๐ฝ0.๏ฟฝ1 โ๏ฟฝ๐๐๐โ(๐กโ๐)
๐๐๏ฟฝ๐
๐=1
๏ฟฝ๐1๐ก1
๐๐๐ก
0 (2. 43)
ะ1(๐ก) = 0 + ๏ฟฝ ๐ฝ0.๏ฟฝ1 โ๏ฟฝ๐๐๐โ(๐กโ๐)
๐๐๏ฟฝ๐
๐=1
๏ฟฝ๐1๐ก1
๐๐๐ก
0 (2. 44)
ะ1(๐ก) = ๐ฝ0๐1๐ก1
๏ฟฝ๐ โ๏ฟฝ๐๐๐๐๐โ(๐กโ๐)
๐๐๏ฟฝ๐
๐=1
๏ฟฝ0
๐ก
(2. 45)
ะ1(๐ก) = ๐ฝ0๐1๐ก1
๏ฟฝ๐ก โ๏ฟฝ๐๐๐๐ +๏ฟฝ๐๐๐๐๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ (2. 46)
Step 2(๐๐ < ๐ โค ๐๐)
In this step, the stress is kept constant.
ะ2(๐ก) = ๐0๐ฝ(๐ก) + ๏ฟฝ ๐ฝ(๐ก โ ๐)๐๐(๐)๐๐
๐๐๐ก1โ
0+ ๏ฟฝ ๐ฝ(๐ก โ ๐)
๐๐(๐)๐๐
๐๐๐ก
๐ก1+ (2. 47)
ะ2(๐ก) = 0 +๐ฝ0๐1๐ก1
๏ฟฝ๐ โ๏ฟฝ๐๐๐๐๐โ(๐กโ๐)
๐๐๏ฟฝ๐
๐=1
๏ฟฝ0
๐ก1
+ 0 (2. 48)
ะ2(๐ก) = ๐ฝ0๐1๐ก1
๏ฟฝ๐ก1 โ๏ฟฝ๐๐๐๐๐โ(๐กโ๐ก1)
๐๐๏ฟฝ + ๏ฟฝ๐๐๐๐๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ (2. 49)
13
2.3.6 Schapery Model
Schapery model is used to model the behavior of non-linear viscoelastic materials. We
know that for creep test,
ะ๐
= ๐ท(๐ก) (2. 50)
Here, ๐ท(๐ก) is the creep compliance. For this creep test, the stress-strain equation is given by
[18]- [19]:
ะ = ๐ท0๐ + ๐ฅ๐ท(๐ก)๐ (2. 51)
Here, ๐ท0 is the initial value of compliance and
๐ฅ๐ท(๐ก) = ๐ท(๐ก) โ ๐ท0 (2. 52)
Similar response can be obtained for stress relaxation test.
When we have creep test data, i.e. ๐ท(๐ก) is known, we can calculate strain by using Boltzmann
superposition principal which is given by,
ะ = ๐ท0๐ + ๏ฟฝ ๐ฅ๐ท(๐ก โ ๐)๐ก
0
๐๐๐๐
๐๐ (1) (2. 53)
This was the linear response of the material [14]-[19], slight changes are to made in the above
equation to obtain nonlinear constitutive equation, which is given below
ะ(๐ก) = ๐0๐ท0๐ + ๐1 ๏ฟฝ ๐ฅ๐ท[๐(๐ก) โ ๐โฒ(๐)]๐ก
0
๐๐2๐๐๐
๐๐ (2) (2. 54)
The above equation gives 1-D representation for Schapery model. Here, ๐ท0 and ๐ฅ๐ท(๐) are the
instantaneous and transient linear viscoelastic creep compliance components which have been
defined previously, and ๐ is reduced-time given by
14
๐ = ๏ฟฝ๐๐กโฒ
๐๐[๐(๐กโฒ)]
๐ก
0
(2. 55)
And
๐โฒ = ๐(๐) = ๏ฟฝ๐๐กโฒ
๐๐[๐(๐กโฒ)]
๐
0
(2. 56)
By comparing equations (2. 53 and (2. 54 and 1 we see that ๐0,๐1,๐2,๐๐ = 1 when the stress is
sufficiently small. For case of creep test, constant stress ๐ is used and the strain rate ๐๐๐2๐๐๐
๐๐
goes to zero. So the equation modifies to [14]-[19]
๐ท๐ =ะ๐
= ๐0๐ท0 + ๐1๐2๐ฅ๐ท ๏ฟฝ๐ก๐๐๏ฟฝ (2. 57)
If we have ๐0,๐1,๐2,๐๐ = 1 then above equation becomes
๐ท๐ =ะ๐
= ๐ท0 + ๐ฅ๐ท(๐ก) (2. 58)
This is the response for linear Schapery model. The transient tensile component ๐ฅ๐ท(๐) is
expressed in terms of Prony series [17]
๐ฅ๐ท(๐) = ๏ฟฝ๐ท๐[1 โ exp (โ๐๐๐)]๐
1
(2. 59)
Here, ๐ท๐ and ๐๐ are Prony constants which can be determined by tensile creep compliance
data and non-linear material parameter ๐0,๐1,๐2,๐๐ can be determined by creep-recovery
tests.
15
CHAPTER 3
MECHANICAL TESTING
In this chapter we found mechanical properties of 3D printed material Procast obtained
from a 3D printer. Procast is a new 3D material, and its properties are unknown. Tensile tests
are conducted at 4 different strain rates to study the effect of elastic modulus. Stress relaxation
and creep test are also conducted to study the viscoelastic response of the material.
3.1 Tensile Testing
For Uniaxial Testing Shimadzu AGS-X Series universal Testing Machine Fig. 7 is used with
a load cell of 5kN. The maximum speed to elongate with the current universal testing machine
was 1000 mm/sec.
Fig. 7 : Shimadzu AGS-X Series Universal Testing Machine
A uniaxial tension test was performed with sample dimensions as per ASTM D412
standards [1]. Testing speed was taken from 1.5 mm/sec to 75 mm/sec, which corresponds to a
strain rate of 0.001 secโ1 to 0.05 secโ1.
16
Tests were performed at 4 different strain rates.
Table 1 : Different strain rates and speed of crosshead
Strain Rate ะฬ Velocity of crosshead
0.001 secโ1 1.5 mm/min
0.005 secโ1 7.5 mm/min
0.01 secโ1 15 mm/min
0.05 secโ1 75 mm/min
The quasi-static stress-strain behavior of the Procast material is shown in Fig. 8. The material
was tested at four different strain rates, and as the graph indicates, the material becomes
stiffer as we increase the strain rate.
Fig. 8 : Stress-Strain Behavior of Procast at different strain rates
0
10
20
30
40
50
60
0.00% 5.00% 10.00% 15.00% 20.00%
Stre
ss (M
Pa)
Strain (%)
Stress-Strain
Strain Rate = 0.001
Strain Rate = 0.005
Strain Rate = 0.01
Strain Rate = 0.05
17
The material response can also be studied from Table 2, where Youngโs modulus and yield
strength are listed at various strain rates.
Table 2 : Young's Modulus and Yield Strength as a function of strain rate
Strain Rate
(๐๐๐โ๐)
Youngโs Modulus
(๐ฎ๐๐)
Yield Strength
(๐๐๐)
Yield Force
(๐)
0.001 0.727ยฑ0.051 16.33ยฑ3 184.66ยฑ34.7
0.005 0.77ยฑ0.05 16.66ยฑ2.62 188.66ยฑ29.17
0.01 0.788ยฑ0.056 18ยฑ2.9 203.33ยฑ33.29
0.05 0.831ยฑ0.059 19ยฑ1.63 222ยฑ17.57
From Fig. 8, we can see that initially the material experiences a linear response. After some
time, it undergoes non-linear deformation and fails around 10-12% strain. As we increase the
strain rate the material becomes more stiff and its Youngโs modulus increases which is given in
Table 2.
3.2 Creep Test
Creep tests were carried out Shimadzu AGS-X Series Universal Testing Machine Fig. 7.
Procast sample was hold at different stress levels (1-10 MPa) for 1800 seconds.
Their responses are given below:
18
Fig. 9 : Creep Test Results from 1-10 MPa
Fig. 9 shows strain versus time response for creep test. It can be seen from these figures that as
the stress values are held constant, strain increases exponentially for that period.
3.3 Dynamic Mechanical Analysis Test
The DMA test was conducted using a Rheometric Scientific Equipment in bending mode.
The sample was placed on a clamping fixture and the strain amplitude was applied with by a
movable clamp at the center of the sample. Distance between clamps was 25 mm. The
response from the sample was measured as stress. The dynamic modulus (Eโ), dynamic loss
modulus (Eโ) was determined by phase angle, the strain applied and the measured stress and
their formula is given below [7]-[8].
0.00%
0.50%
1.00%
1.50%
2.00%
2.50%
0 500 1000 1500 2000
Stra
in
Time (sec)
Strain at 1-10 MPa
1 MPa2 MPa3 MPa4 MPa5 MPa6 MPa7 MPa8 MPa9 MPa10 MPa
19
๐๐ก๐๐๐๐๐ ๐๐๐๐ข๐๐ข๐ (Eโ) = ๐๐ะ๐๐๐๐ ๐ฟ (3. 1)
๐ฟ๐๐ ๐ ๐๐๐๐ข๐๐ข๐ (Eโโ) = ๐๐ะ๐๐ ๐๐๐ฟ (3. 2)
๐โ๐๐ ๐ ๐๐๐๐๐ โถ ๐๐๐๐(๐๐๐๐ก๐) = EโโEโ
(3. 3)
Initially a strain sweep test was conducted in order to find the strain amplitude that can
be used for the material to follow Hookโs Law. Strain amplitude used for this polyurethane was
0.005%. Once strain amplitude was found a dynamic temperature sweep test was conducted at
a frequency of 1 Hz over a temperature range of -100 ยฐC to 150 ยฐC with a scanning rate of
3ยฐC ๐๐๐โ1. A sinusoidal strain was applied and material response was measured as stress. By
approximating the applied sinusoidal strain wave with a triangular strain wave, the average
strain was calculated as โฬ=2 โ ๐.
Fig. 10 gives temperature sweep response for Procast at 1 Hz. Maximum value of Eโ is
observed in the region of -65ยฐC till -35ยฐC which is in the order of 13 GPa. The values of Eโ is 9.4
GPa at around 25ยฐC. It can be clearly seen that Eโ decreases around 25ยฐC and this steep
decrease is observed till 100ยฐC. From 100ยฐC till 150ยฐC material maintains almost a constant
value Eโ of 0.1-0.2 GPa so this can be taken In account where material application comes in.
20
Fig. 10 : Storage Modulus and Loss Modulus plot over temperature of Procast at 1 Hz
Fig. 11 gives response for Tan Delta curve response for Procast at 1 Hz. Since, there is an
inverse relation between Storage Modulus (Eโ) and Tan ฮด given by Tan ฮด = Eสน๐ธสนสน
, so Tan ฮด peak is
observed corresponding to decrease in Eโ in similar temperature range Fig. 11. So, peak in Tan
delta region gives material glass transition ๐๐๐๐ value, which in for Procast is around 81ยฐC.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.1
1
10
100
-150 -100 -50 0 50 100 150 200
E" (G
Pa)
E' (G
Pa)
Temp (C)
Temperature Sweep @ 1 Hz
E'(Storage Modulus)
E''(Loss Modulus)
21
Fig. 11 Storage Modulus and Tan Delta plot over temperature of Procast at 1 Hz
A frequency sweep test 0.01 Hz to 80 Hz on Procast material was also conducted at
room temperature (25ยฐC) using same material dimension as stated in ASTM Standard.
Fig. 12 : Frequency Sweep Response of Loss Modulus at Room Temperature
0.01
0.1
1
0.1
1
10
100
-150 -100 -50 0 50 100 150 200
Tan_
Delta
E' (G
Pa)
Temp (C)
Temperature Sweep @ 1 Hz E'(Storage Modulus)Tan_Delta
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.00E+00
1.00E+08
2.00E+08
3.00E+08
4.00E+08
5.00E+08
6.00E+08
7.00E+08
8.00E+08
0 20 40 60 80 100
Tan_
Delta
E'' (
Pa)
Freq (Hz)
Loss Modulus
Tan_Delta
22
Fig. 12 gives frequency sweep response for Loss Modulus of Procast material at room
temperature. In this figure, we can see that Loss modulus values slightly increase from 0.2 GPa
at around 0.01 Hz to 0.7GPa at around 80 Hz.
Fig. 13 : Frequency Sweep Response of Storage Modulus at Room Temperature
Fig. 13 gives frequency sweep response for Storage Modulus of Procast material at room
temperature. In this figure, we can see that Storage modulus values slightly increase from 3.6
GPa at around 0.01 Hz to 5.1 GPa at around 80 Hz.
3.4 Differential Scanning Calorimetry (DSC)
To determine the melting temperature point of Procast material, a differential scanning
calorimetry (DSC) is used. In a typical DSC experiment, the difference of power required to heat
a reference pan and a sample pan is measured over wide temperature range. Typically DSC
pans are made up of Aluminium, while in certain specific experiments pans of gold, platinum
and stainless steel are also used.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.00E+00
1.00E+09
2.00E+09
3.00E+09
4.00E+09
5.00E+09
6.00E+09
0 20 40 60 80 100
Tan_
Delta
E' (P
a)
Freq (Hz)
Storage Modulus
Tan_Delta
23
In our experiment we have used a DSC 6 Perkin Elmer machine. Procast sample was
prepared in Aluminium pans (30ยตL), weighed and crimped. For reference a black Aluminium
pan was used. Peak observed in the following graph indicated Procast melting temperature
which is 337.5 ยฐC.
Fig. 14 : DSC Graph for Procast
3.5 Density
We also measured density of Procast sample. For this purpose we printed out 5 by 5 by 5
rectangular samples from 3D printer at high definition, ultra high definition and extreme
ultra high definition. Three samples for each resolution level were measured to obtain the
density whose result is given in Table 3. The densities of the three samples are
1125.797ยฑ1.356 kg/m3, 1155.503ยฑ0.984 kg/m3, and 1162.283ยฑ0.835 kg/m3 for High
Definition, Ultra High-Definition, and Extreme Ultra High-Definition resolution respectively.
0
5
10
15
20
25
30
0 50 100 150 200 250 300 350 400 450
Heat
Flo
w (m
W)
Temperature (oC)
DSC Graph for Procast
๐๐๐๐ = 97.591 oC
๐๐๐๐ = 337.105 oC
24
Table 3 : Density of Procast material
High Definition Ultra High-Definition Extreme Ulta High-Definition
Sample
1
Sample
2
Sample
3
Sample
1
Sample
2
Sample
3
Sample
1
Sample
2
Sample
3
Volume
mm3 124.99 124.248 125.482 124.744 124.744 124.744 124.998 124.747 124.998
Mass
mg 140.67 139.71 141.5 144.15 143.99 144.29 145.17 145.13 145.26
Density
kg/m3 1125.3 1124.44 1127.65 1155.56 1154.27 1156.68 1161.37 1163.39 1162.09
25
CHAPTER 4
NONLINEAR REGRESSION
Nonlinear regression analysis is a technique in which experimental data is modeled by
function which is a combination of material/model parameters. The experimental data is fitted
by using an algorithm or a fitting approach to best predict the behavior. Curve fitting technique
is used to describe experimental data with mathematical equations.
Suppose there is a function y=f(x). Where x is the independent variable and y is dependent
variable, which is measured; and f is the function which uses one or more model parameters to
describe y. Better prediction of these material parameters by any algorithm, the more accurate
the function describes the data. There are many approaches and software available to find out
the best model parameters that gives a good fit. I have used Excel, to obtain my material
parameters. Excel contains the SOLVER function, which comes with every MS office package. It
uses an iterative approach to fit the data with non-linear functions [22][25].
The method used for this approach is called iterative non-linear least square fitting. In a
linear regression (least square approach), we try to minimize the value of squared sum of the
difference between experimental value and predicted/fitted value.
๐๐ = ๏ฟฝ๏ฟฝ๐ฆ โ ๐ฆ๐๐๐ก๏ฟฝ2
๐
๐=1
(4. 1)
Here, y is the experimental value, ๐ฆ๐๐๐ก is the predicted/fitted value, and SS is the sum of the
squares. The difference between linear regression and non-linear regression is that, in the later
we use iterations to get this SS value to minimum.
26
Starting point for this method is to assume or predict good initial parameters for the
function. This good starting value provides less iteration to compute the function and obtain
best result.
In the first iteration, once we have given some initial starting value, the algorithm runs
the function and obtains some SS value. In second iteration, SOLVER makes small changes in the
initial parameters values and recalculates value of SS.
This method is repeated many times to ensure we have the smallest possible value of
SS. Several different algorithms can be used for non-linear regression, such as the Guass-
Newton, the Marquardt-Levenberg, and the Nelder-Mead. However, Excel SOLVER uses
another iterative approach called GRG (generalized reduced gradient) method. A detailed
description about this code can be found elsewhere [23]-[25].
All of these algorithms have similar properties. Each of them requires the user to input
initial parameters, and based on that it predicts or gives the best fit that function.
27
CHAPTER 5
LINEAR VISCOELASTIC MODELS
In this chapter we will study linear viscoelastic models and try to determine which
model is better for our 3D printed material Procast. We have used 3 linear viscoelastic models:
Maxwell Model, Generalized Voigt-Kelvin Model, and Prony Series. Material parameters are
determined using creep test previous chapter.
5.1 Maxwell Model
The following equation gives the response for creep strain in Maxwell model
๐(๐ก) = ๐๐๏ฟฝ๐ท๏ฟฝ1 + ๐ก ๐๏ฟฝ ๏ฟฝ๏ฟฝ (5. 1)
To find material parameters ๐ท and ๐ we use Creep Test data and optimizing technique to fit our
model with the experimental data. After doing non-linear regression, we got the following
material parameters that give a good response for creep behavior for low stress levels.
Table 4 : Material Parameters for Maxwell Model
๐ท 1.5 ร 10โ9
๐ 2 ร 104
Using the above equation we obtain the creep strain response for Procast at different creep
stress levels which is given by the following figures
28
Fig. 15 : Creep Strain for Maxwell model for 1 -3 MPa stress
Fig. 15 shows creep strain prediction for Maxwell model at low stress levels. It can be seen from the
figure that at low stress levels the response is a good fit. For
Fig. 16 we can see that Maxwell model fails to capture the response for higher stress values.
Fig. 16 : Creep Strain for Maxwell model for 1-8 MPa stress
0.00%
0.50%
1.00%
1.50%
2.00%
0 500 1000 1500 2000
Stra
in
Time (sec)
Maxwell Model
3 MPa experiment2 MPa experiment1 MPa experiment3 MPa prediction2 MPa prediction1 MPa prediction
0.00%
0.50%
1.00%
1.50%
2.00%
0 500 1000 1500 2000
Stra
in
Time (sec)
Maxwell Model 8 MPa experiment7 MPa experiment6 MPa experiment5 MPa experiment4 MPa experiment3 MPa experiment2 MPa experiment1 MPa experiment8 MPa prediction7 MPa prediction6 MPa prediction5 MPa prediction4 MPa prediction3 MPa prediction2 MPa prediction
29
To obtain material response from time domain to frequency domain, Fourier transformation is
used on the relaxation modulus equation for Maxwell model.
๐ธ(๐ก) = ๐ธ๐โ๐ก ๐๏ฟฝ (5. 2)
Apply Fourier Transformation,
๐ธ(๐) = ๐ธ ๏ฟฝ1
1 + ๐๐๐๏ฟฝ (5. 3)
Here, j is the imaginary number with a value of โโ1
๐ธ(๐) = ๐ธ ๏ฟฝ๐๐
๐๐ + ๐๏ฟฝ (5. 4)
Multiply and divide by conjugate:
๐ธ(๐) = ๐ธ ๏ฟฝ๐๐
๐๐ + ๐๏ฟฝ ๏ฟฝ๐๐ โ ๐๐๐ โ ๐
๏ฟฝ (5. 5)
๐ธ(๐) = ๐ธ ๏ฟฝ๐2๐2 โ ๐๐๐
1 + ๐2๐2 ๏ฟฝ (5. 6)
Separating real and imaginary parts, we get
๐๐ก๐๐๐๐๐ ๐๐๐๐ข๐๐ข๐ ๐ธโฒ(๐) = ๏ฟฝ๐ธ๐2๐2
1 + ๐2๐2๏ฟฝ (5. 7)
๐ฟ๐๐ ๐ ๐๐๐๐ข๐๐ข๐ ๐ธโฒโฒ(๐) = ๏ฟฝ๐ธ๐๐
1 + ๐2๐2๏ฟฝ (5. 8)
5.2 Voigt-Kelvin Model
The Voigt-Kelvin model is a generalization of Voigt models in which Voigt elements are
connected in series. The Voigt-Kelvin model gives a better viscoelastic response than the simple
Voigt model.
30
The following equation gives the relation for creep compliance for this Voigt-Kelvin model
[11]-[13].
๐ท(๐ก) = ๏ฟฝ๐ท๐ ๏ฟฝ1 โ ๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ
๐ง
๐=1
(5. 9)
For our model, we considered two Voigt elements connected in series and so the above
equation becomes
๐ท(๐ก) = ๐ท1 ๏ฟฝ1 โ ๐โ๐ก ๐1๏ฟฝ ๏ฟฝ+ ๐ท2 ๏ฟฝ1 โ ๐
โ๐ก ๐2๏ฟฝ ๏ฟฝ (5. 10)
To find material parameters ๐ท1,๐ท2, ๐1, and ๐2 we use Creep Test data and optimizing technique
to fit our model with the experimental data.
After doing non-linear regression, we got the following material parameters that give a good
response for creep behavior
Table 5: Material Parameters for Voigt-Kelvin Model
๐ท1 1.12 ร 10โ10
๐ท2 1.5 ร 10โ9
๐1 399
๐2 3.406
Using the above equation we obtain the creep strain response for Procast at different creep
stress levels which is given by the following figures
31
Fig. 17 : Creep Strain for Voigt-Klevin Model for 1 -3 MPa stress
Fig. 17 shows creep strain prediction for Voigt-Kelvin model at low stress levels. It can be seen
from the figure that at low stress levels the response is a good fit. For Fig. 18 we can see that
Voigt-Kelvin model fails to capture the response for higher stress values.
0.00%
0.50%
1.00%
1.50%
2.00%
0 500 1000 1500 2000
Stra
in
Time (sec)
Voight-Kelvin Model
3 MPa experiment
2 MPa experiment
1 MPa experiment
3 MPa prediction
2 MPa prediction
1 MPa prediction
32
Fig. 18 : Creep Strain for Voigt-Kelvin Model for 1-8 MPa stress
Above figures give material response in time domain; to see material response in frequency
domain, we apply Fourier transform following equation.
๐ท(๐ก) = ๏ฟฝ๐ท๐ ๏ฟฝ1 โ ๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ
๐
๐=1
(5. 11)
Applying Fourier Transformation,
๐ท(๐) = ๏ฟฝ๐ท๐
๐
๐=1
๏ฟฝ1 โ ๏ฟฝ1
1 + ๐๐๐๐
๏ฟฝ๏ฟฝ (5. 12)
Here, j is the imaginary number with a value of โโ1
๐ท(๐) = ๏ฟฝ๐ท๐
๐
๐=1
๏ฟฝ1 โ ๏ฟฝ๐๐๐
๐๐๐ + ๐๏ฟฝ๏ฟฝ (5. 13)
0.00%
0.50%
1.00%
1.50%
2.00%
0 500 1000 1500 2000
Stra
in
Time (sec)
Voight-Kelvin Model 8 MPa experiment7 MPa experiment6 MPa experiment5 MPa experiment4 MPa experiment3 MPa experiment2 MPa experiment1 MPa experiment8 MPa prediction7 MPa prediction6 MPa prediction5 MPa prediction4 MPa prediction3 MPa prediction2 MPa prediction1 MPa prediction
33
Multiply and divide by conjugate
๐ท(๐) = ๏ฟฝ๐ท๐
๐
๐=1
๏ฟฝ1 โ ๏ฟฝ๐๐๐
๐๐๐ + ๐๏ฟฝ ๏ฟฝ๐๐๐ โ ๐๐๐๐ โ ๐
๏ฟฝ๏ฟฝ (5. 14)
๐ท(๐) = ๏ฟฝ๐ท๐ ๏ฟฝ1 โ ๏ฟฝ๐๐2๐2 โ ๐๐๐๐๐๐2๐2 โ ๐2
๏ฟฝ๏ฟฝ๐
๐=1
(5. 15)
๐ท(๐) = ๏ฟฝ๐ท๐ ๏ฟฝ1 โ ๏ฟฝ๐๐2๐2 โ ๐๐๐๐๐๐2๐2 + 1
๏ฟฝ๏ฟฝ๐
๐=1
(5. 16)
๐ท(๐) = ๏ฟฝ๐ท๐ ๏ฟฝ1 + ๐๐2๐2 โ ๐๐2๐2 + ๐๐๐๐
๐๐2๐2 + 1๏ฟฝ
๐
๐=1
(5. 17)
๐ท(๐) = ๏ฟฝ๐ท๐ ๏ฟฝ1 + ๐๐๐๐
1 + ๐๐2๐2๏ฟฝ๐
๐=1
(5. 18)
Separating real and imaginary parts, we get
(๐๐ก๐๐๐๐๐ ๐๐๐๐ข๐๐ข๐ ) ๐ทโฒ(๐) = ๏ฟฝ๏ฟฝ๐ท๐
1 + ๐๐2๐2๏ฟฝ๐
๐=1
(5. 19)
(๐ฟ๐๐ ๐ ๐๐๐๐ข๐๐ข๐ ) ๐ทโฒโฒ(๐) = ๏ฟฝ๐ท๐ ๏ฟฝ๐๐๐
1 + ๐๐2๐2๏ฟฝ๐
๐=1
(5. 20)
5.3 Prony Series
A common form for the linear viscoelastic response is given by Prony Series by the
following equation [10]-[20]:
๏ฟฝ๐ผ๐๐โ๐ก ๐๐๏ฟฝ
๐
๐=1
(5. 21)
Here, ๐๐ are the time constants and ๐ผ๐ are the exponential coefficients.
For creep test we use the creep compliance equation [10]-[20]
34
ะ(๐ก) = ๐ฝ(๐ก)๐0 (5. 22)
Here, ๐ฝ(๐ก) is the creep compliance function and its response under Prony Series is given by
๐ฝ(๐ก) = ๐ฝ0.๏ฟฝ1 โ๏ฟฝ๐๐๐โ๐ก ๐๐๏ฟฝ
๐
๐=1
๏ฟฝ (5. 23)
ะ(๐ก) = ๐0๐ฝ(๐ก) + ๏ฟฝ ๐ฝ(๐ก โ ๐)๐๐(๐)๐๐
๐๐๐ก
0 (5. 24)
๐(๐ก) = ๏ฟฝ๐1๐ก
(๐ก1 โ ๐ก0)๏ฟฝ ; ๐ก0 < ๐ก < ๐ก1
๐1 ; ๐ก1 < ๐ก < ๐ก2๏ฟฝ
๐๐๐๐ก
= ๏ฟฝ๐1
(๐ก1 โ ๐ก0)๏ฟฝ ; ๐ก0 < ๐ก < ๐ก1 0 ; ๐ก1 < ๐ก < ๐ก2
๏ฟฝ
Here, ๐0 = 0, ๐1 is the stress level at which the stress is kept constant, and ๐ก0 = 0
Step 1(๐๐ < ๐ โค ๐๐)
ะ1(๐ก) = ๐0๐ฝ(๐ก) + ๏ฟฝ ๐ฝ(๐ก โ ๐)๐๐(๐)๐๐
๐๐๐ก
0 (5. 25)
ะ1(๐ก) = ๐ฝ0๐1๐ก1
๏ฟฝ๐ก โ๏ฟฝ๐๐๐๐ +๏ฟฝ๐๐๐๐๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ (5. 26)
ะ1(๐ก) = ๐ฝ0๐1๐ก1
๏ฟฝ๐ก โ ๐1๐1 โ ๐2๐2 + ๐1๐1๐โ๐ก ๐1๏ฟฝ + ๐2๐2๐
โ๐ก ๐2๏ฟฝ ๏ฟฝ (5. 27)
We obtained a two-term Prony Series
Step 2(๐๐ < ๐ โค ๐๐)
In this step, the stress is kept constant.
ะ2(๐ก) = ๐ฝ0๐1๐ก1
๏ฟฝ๐ก1 โ๏ฟฝ๐๐๐๐๐โ(๐กโ๐ก1)
๐๐๏ฟฝ + ๏ฟฝ๐๐๐๐๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ (5. 28)
ะ2(๐ก) = ๐ฝ0๐1๐ก1
๏ฟฝ๐ก1 โ ๐1๐1๐โ(๐กโ๐ก1)
๐1๏ฟฝ โ ๐2๐2๐โ(๐กโ๐ก1)
๐2๏ฟฝ + ๐1๐1๐โ๐ก ๐1๏ฟฝ + ๐2๐2๐
โ๐ก ๐2๏ฟฝ ๏ฟฝ (5. 29)
35
We used the following material parameters to predict our creep response
Table 6 : Creep Material Parameters for Prony Series
๐ฝ 1.75 ร 10โ9
๐1 0.12
๐2 0.2
๐1 25
๐2 1000
Fig. 19 gives creep strain prediction using Prony Series. We can see that it gives a perfect
response till 5 MPa stress, and at higher stress levels it does not show a good response. It gives
better response than previous two viscoelastic models as it catches the trend of the creep
strain and shows a better fit.
Fig. 19: Creep Strain prediction for Prony Series
0.00%
0.50%
1.00%
1.50%
2.00%
2.50%
0 500 1000 1500 2000
Cree
p St
rain
Time (sec)
Prony Series 10 MPa9 MPa8 MPa7 MPa6 MPa5 MPa4 MPa3 MPa2 MPa1 MPa10 MPa prediction9 MPa prediction8 MPa prediction7 MPa prediction6 MPa prediction5 MPa prediction4 MPa prediction3 MPa prediction2 MPa prediction1 MPa prediction
36
To get material response in frequency domain for Prony Series, we apply Fourier
transformation. Prony Series is represented in terms of shear relaxation modulus by the
following expression [20]:
๐๐ (๐ก) = 1 โ๏ฟฝ๐๐ ๏ฟฝ1 โ ๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ
๐
๐=1
(5. 30)
Here, ๐๐ and ๐๐ are material parameters and ๐๐ (๐ก) is the dimensionless relaxation modulus
given by
๐๐ (๐ก) =๐บ๐ (๐ก)๐บ0
(5. 31)
๐บ๐ (๐ก)๐บ0
= 1 โ๏ฟฝ๐๐ ๏ฟฝ1 โ ๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ
๐
๐=1
(5. 32)
๐บ๐ (๐ก)๐บ0
= 1 โ๏ฟฝ๐๐
๐
๐=1
+ ๏ฟฝ๐๐
๐
๐=1
๐โ๐ก ๐๐๏ฟฝ (5. 33)
๐บ๐ (๐ก) = ๐บ0 ๏ฟฝ1 โ๏ฟฝ๐๐
๐
๐=1
+ ๏ฟฝ๐๐
๐
๐=1
๐โ๐ก ๐๐๏ฟฝ ๏ฟฝ (5. 34)
๐บ๐ (๐ก) = ๐บ0 ๏ฟฝ1 โ๏ฟฝ๐๐
๐
๐=1
๏ฟฝ + ๐บ0๏ฟฝ๐๐
๐
๐=1
๐โ๐ก ๐๐๏ฟฝ (5. 35)
Apply Fourier Transformation:
๐บ(๐) = ๐บ0 ๏ฟฝ1 โ๏ฟฝ๐๐
๐
๐=1
๏ฟฝ + ๐บ0๏ฟฝ๐๐
๐
๐=1
1
1 + ๐๐๐๐๏ฟฝ
(5. 36)
Here, j is the imaginary number with a value of โโ1
37
๐บ(๐) = ๐บ0 ๏ฟฝ1 โ๏ฟฝ๐๐
๐
๐=1
๏ฟฝ + ๐บ0๏ฟฝ๐๐
๐
๐=1
1
1 + ๐๐๐๐๏ฟฝ
(5. 37)
๐บ(๐) = ๐บ0 ๏ฟฝ1 โ๏ฟฝ๐๐
๐
๐=1
๏ฟฝ + ๐บ0๏ฟฝ๐๐
๐
๐=1
๐๐๐๐๐๐ + ๐
(5. 38)
Multiply and divide by conjugate:
๐บ(๐) = ๐บ0 ๏ฟฝ1 โ๏ฟฝ๐๐
๐
๐=1
๏ฟฝ + ๐บ0๏ฟฝ๐๐
๐
๐=1
๏ฟฝ๐๐๐
๐๐๐ + ๐๏ฟฝ ๏ฟฝ๐๐๐ โ ๐๐๐๐ โ ๐
๏ฟฝ (5. 39)
๐บ(๐) = ๐บ0 ๏ฟฝ1 โ๏ฟฝ๐๐
๐
๐=1
๏ฟฝ + ๐บ0๏ฟฝ๐๐
๐
๐=1
๏ฟฝ๐๐2๐2 โ ๐๐๐๐๐๐2๐2 โ ๐2
๏ฟฝ (5. 40)
๐บ(๐) = ๐บ0 ๏ฟฝ1 โ๏ฟฝ๐๐
๐
๐=1
๏ฟฝ + ๐บ0๏ฟฝ๐๐
๐
๐=1
๏ฟฝ๐๐2๐2
๐๐2๐2 + 1๏ฟฝ โ ๐๐บ0๏ฟฝ๐๐
๐
๐=1
๏ฟฝ๐๐๐
๐๐2๐2 + 1๏ฟฝ (5. 41)
We know that
๐บ(๐) = ๐บโฒ(๐) + ๐๐บโฒโฒ(๐) (5. 42)
Separating real and imaginary parts, we get
(๐๐ก๐๐๐๐๐ ๐๐๐๐ข๐๐ข๐ ) ๐บโฒ(๐) = ๐บ0 ๏ฟฝ1 โ๏ฟฝ๐๐
๐
๐=1
๏ฟฝ + ๐บ0๏ฟฝ๐๐
๐
๐=1
๏ฟฝ๐๐2๐2
1 + ๐๐2๐2๏ฟฝ (5. 43)
(๐ฟ๐๐ ๐ ๐๐๐๐ข๐๐ข๐ ) ๐บโฒโฒ(๐) = ๐บ0๏ฟฝ๐๐
๐
๐=1
๏ฟฝ๐๐๐
1 + ๐๐2๐2๏ฟฝ (5. 44)
38
CHAPTER 6
NON-LINEAR VISCOELASTIC MODEL
In this chapter we will discuss methods of characterization non-linear viscoelastic
materials. We will be using constitutive equations from Chapter 1 and using experimental data
(creep test data) to obtain material parameters. In small deformation materials, the total strain
is the sum of viscoelastic strain and viscoplastic strain [14]-[19].
ะ(๐ก) = ะ๐ฃ๐(๐ก) + ะ๐ฃ๐(๐ก) (6. 1)
While, the incremental strain is given by
๐ฅะ(๐ก) = ๐ฅะ๐ฃ๐(๐ก) + ๐ฅะ๐ฃ๐(๐ก) At time t>0 (6. 2)
Here, ะ๐ฃ๐refers to viscoelastic strain and ะ๐ฃ๐ refers to viscoplastic strain and ๐ฅะ๐ฃ๐ gives the
incremental viscoelastic strain while ๐ฅะ๐ฃ๐ gives the incremental viscoplastic strain. In our
model we are only concerned with elastic deformation and have omitted the viscoplastic part.
Schapery equation in 1-D form is given by the following equation [14]-[19],
ะ(๐ก) = ๐0๐ท0๐ + ๐1 ๏ฟฝ ๐ฅ๐ท[๐(๐ก) โ ๐โฒ(๐)]๐ก
0
๐๐2๐๐๐
๐๐ (1) (6. 3)
Here, ๐ท0 and ๐ฅ๐ท(๐) are the instantaneous and transient linear viscoelastic creep compliance
components which have been defined previously, and ๐ is reduced-time given by
๐ = ๏ฟฝ๐๐กโฒ
๐๐[๐(๐กโฒ)]
๐ก
0
(6. 4)
And
๐โฒ = ๐(๐) = ๏ฟฝ๐๐กโฒ
๐๐[๐(๐กโฒ)]
๐
0
(6. 5)
39
In a 3-D representation of Schapery model, stress and strain are given by its deviatoric and
hydrostatic components.
๐๐๐ = ๐๐๐ โ13๐ฟ๐๐๐๐๐ (6. 6)
๐๐๐ = ะ๐๐ โ13๐ฟ๐๐ะ๐๐ (6. 7)
Here, ๐ฟ๐๐ is Kronecker Delta and 13๐๐๐ and 1
3ะ๐๐ are the hydrostatic stress and strain
respectively.
For an isotropic linear elastic material, the relation between stress and strain can be expressed
as [14]-[19]
ะ๐๐ =12๐ฝ๐๐๐ +
19๐ต๐ฟ๐๐๐๐๐ (6. 8)
Here, ๐ฝ is the shear compliance and ๐ต is the bulk compliance.
So, using our 1-D Schapery equation, we can get 3-D non-linear viscoelastic model as
ะ๐๐(๐ก) =12๐0๐ฝ0๐๐๐(๐ก) +
12๐1 ๏ฟฝ ๐ฅ๐ฝ[๐(๐ก) โ ๐โฒ(๐)]
๐ก
0
๐๐2๐๐๐๐๐
๐๐
+19๐0๐ต0๐ฟ๐๐๐๐๐(๐ก) +
19๐1๐ฟ๐๐ ๏ฟฝ ๐ฅ๐ต[๐(๐ก) โ ๐โฒ(๐)]
๐ก
0
๐๐2๐๐๐๐๐
๐๐
(6. 9)
The instantaneous shear compliance ๐ฝ0 and instantaneous bulk compliance ๐ต0 can be given by
the following equations, where ฮฝ is the Poisonโs ratio [14]-[19].
๐ฝ0 = 2(1 + ๐)๐ฅ๐ท(๐) (6. 10)
๐ต0 = 3(1 โ 2๐)๐ฅ๐ท(๐) (6. 11)
The transient tensile component ๐ฅ๐ท(๐) is expressed in terms of Prony series [17]
40
๐ฅ๐ท(๐) = ๏ฟฝ๐ท๐[1 โ exp (โ๐๐๐)]๐
1
(6. 12)
Here, ๐ท๐ and ๐๐ are Prony constants which can be determined by tensile creep compliance
data.
Using the relation between shear compliance and tensile compliance we can write the following
equation as;
๐ฅ๐ฝ(๐) = ๏ฟฝ๐ฝ๐[1 โ exp(โ๐๐๐)]๐
1
(6. 13)
Similarly,
๐ฅ๐ต(๐) = ๏ฟฝ๐ต๐[1 โ exp(โ๐๐๐)]๐
1
(6. 14)
Here,
๐ฝ๐ = 2(1 + ๐)๐ท๐ (6. 15)
๐ต = 3(1 โ 2๐)๐ท๐ (6. 16)
The Prony constants ๐ท๐ and ๐๐ can be obtained creep test data by using curve fitting
approach, and by using the relation between shear compliance and tensile compliance we can
obtain the remaining terms. At first linear response of the material is considered (at low value
of stress) and non-linear material parameters ๐0,๐1,๐2, and ๐๐ is considered as 1.
Transient creep compliance can be given by the following by using simple power law [17]-[19]
๐ฅ๐ท(๐) = ๐ถ๐๐๐ (6. 17)
Here, ๐ถ and ๐ are material constants. In the case of creep test where ฯ is held constant for
time> 0, equation (6. 3 becomes
41
ะ๐(๐ก) = ๐0๐ท0๐ +๐1๐2๐ถ๐๐๐๐
๐ก๐๐ (6. 18)
Taking low stress region into account, in our case creep test at 1 MPa, where the response of
the material is almost linear and non-linear material parameters ๐0,๐1,๐2, and ๐๐ is
considered as 1, above equation deduced into [17][19]
ะ๐(๐ก) = ๐ท0๐ + ๐ถ๐ก๐๐ (6. 19)
Using curve fitting technique to find out the constants ๐ท0, ๐ถ and ๐. Using the above
equations, creep strain response is predicted by using the following equation.
ะ๐(๐ก) = ๏ฟฝ๐0๐ท0 + ๐1๐2๐ฅ๐ท๏ฟฝ๐ก ๐๐๏ฟฝ ๏ฟฝ๏ฟฝ ๐ (6. 20)
This is the required equation we will be using in order to predict our response. We use the
following material parameters to get the response, while ๐0,๐1,๐2,๐๐ = 1 is used for linear
visco-elastic response. Here ๐ท0 = 1.43 ร 10โ3 MPaโ1.
Table 7 : Material Parameters for Schapery Model
๐ต ๐ซ๐(๐ด๐ท๐โ๐) ๐๐(๐โ๐)
1 1.11 ร 10โ5 1
2 1 ร 10โ5 0.1
3 1 ร 10โ5 0.01
4 4.6 ร 10โ5 0.001
5 7.5 ร 10โ4 0.0001
6 9 ร 10โ4 0.00001
42
Fig. 20 : Schapery Model Prediction of Creep Test
Fig. 20 gives creep strain prediction of Procast using Schapery model. We can see that it gives a
better material response than Prony series, even at higher stress values.
Fig. 21 : Nonlinear Parameters for Schapery Model at Various Stress Levels
0.00%
0.50%
1.00%
1.50%
2.00%
2.50%
0 500 1000 1500 2000
Cree
p St
rain
Time (sec)
Schapery Model 10 MPa9 MPa8 MPa7 MPa6 MPa5 MPa4 MPa3 MPa2 MPa1 MPa10 MPa prediction9 MPa prediction8 MPa prediction7 MPa Prediction6 MPa Prediction5 MPa Prediction4 MPa prediction3 MPa Prediction2MPa Prediction1MPa Prediction
0.5
1.5
0 1 2 3 4 5 6 7 8 9 10 11 12
Non
linea
r vis
coel
astic
par
amet
ers
Stress (MPa)
g0
g1
g2
a
43
Nonlinear viscoelastic parameters can be obtained for other stress levels by using the constants
listed in Table 8 for polynomial equations, where subscript of ฮฑ denotes exponent of stress for
example;
๐(๐0,๐1,๐2,๐) = ๐ผ๐๐๐ + ๐ผ๐โ1๐๐โ1 + โฏ+ ๐ผ0๐0
Table 8 : Polynomial Constants for Nonlinear Material Paramters
๐ถ๐ ๐ถ๐ ๐ถ๐ ๐ถ๐ ๐ถ๐
๐0 - - 0.0032 -0.0111 1.0037
๐1 0.0005 -0.0107 0.0719 -0.1091 1.0119
๐2 0.0004 -0.0085 0.057 -0.0866 1.0095
๐ 0.0001 -0.0026 0.0171 -0.0254 1.0026
Equation (6. 20 gives material response in time domain, to obtain material response in
frequency domain we apply Fourier transformation.
ะ๐(๐ก)๐
= ๏ฟฝ๐0๐ท0 + ๐1๐2๐ฅ๐ท๏ฟฝ๐ก ๐๐๏ฟฝ ๏ฟฝ๏ฟฝ (6. 21)
Using equation (6. 12, we get
๐ท(๐ก) = ๐0๐ท0 + ๐1๐2๏ฟฝ๐ท๐๏ฟฝ1 โ exp ๏ฟฝโ๐๐ ๐ก ๐๐๏ฟฝ ๏ฟฝ๏ฟฝ๐
1
(6. 22)
Applying Fourier Transformation,
๐ท(๐) = ๐0๐ท0 + ๐1๐2๐ท๐ ๏ฟฝ1 โ ๏ฟฝ1
1 + ๐๐ ๏ฟฝ
๐๐๐ ๏ฟฝ
๏ฟฝ๏ฟฝ (6. 23)
44
๐ท(๐) = ๐0๐ท0 + ๐1๐2๐ท๐ ๏ฟฝ1 โ ๏ฟฝ๐
๐ + ๐ ๏ฟฝ๐๐๐ ๏ฟฝ๏ฟฝ๏ฟฝ (6. 24)
Multiply and divide by conjugate
๐ท(๐) = ๐0๐ท0 + ๐1๐2๐ท๐ ๏ฟฝ1 โ ๏ฟฝ๐
๐ + ๐ ๏ฟฝ๐๐๐ ๏ฟฝ๏ฟฝ๏ฟฝ
๐ โ ๐ ๏ฟฝ๐๐๐ ๏ฟฝ
๐ โ ๐ ๏ฟฝ๐๐๐ ๏ฟฝ๏ฟฝ๏ฟฝ (6. 25)
๐ท(๐) = ๐0๐ท0 + ๐1๐2๐ท๐
โฃโขโขโก1 โ
โ
โ๐2 โ ๐๐ ๏ฟฝ๐๐๐ ๏ฟฝ
๐2 + ๏ฟฝ๐๐๐ ๏ฟฝ2
โ
โ
โฆโฅโฅโค (6. 26)
๐ท(๐) = ๐0๐ท0 + ๐1๐2๐ท๐
โ
โ๐2 + ๏ฟฝ๐๐๐ ๏ฟฝ
2โ ๐2 + ๐๐ ๏ฟฝ๐๐๐ ๏ฟฝ
๐2 + ๏ฟฝ๐๐๐ ๏ฟฝ2
โ
โ (6. 27)
๐ท(๐) = ๐0๐ท0 + ๐1๐2๐ท๐
โ
โ๏ฟฝ๐๐๐ ๏ฟฝ
2+ ๐๐ ๏ฟฝ๐๐๐ ๏ฟฝ
๐2 + ๏ฟฝ๐๐๐ ๏ฟฝ2
โ
โ (6. 28)
Separating real and imaginary parts, we get
(๐๐ก๐๐๐๐๐ ๐๐๐๐ข๐๐ข๐ ) ๐ทโฒ(๐) = ๐0๐ท0 + ๐1๐2๐ท๐
โ
โ๏ฟฝ๐๐๐ ๏ฟฝ
2
๐2 + ๏ฟฝ๐๐๐ ๏ฟฝ2
โ
โ (6. 29)
(๐ฟ๐๐ ๐ ๐๐๐๐ข๐๐ข๐ ) ๐ทโฒ(๐) = ๐1๐2๐ท๐
โ
โ๐ ๏ฟฝ๐๐๐ ๏ฟฝ
๐2 + ๏ฟฝ๐๐๐ ๏ฟฝ2
โ
โ (6. 30)
45
CHAPTER 7
VALIDATION OF MODEL
In this chapter we will try to validate our models with the uniaxial tensile tests we
performed. For this purpose we have taken tensile test data in Fig. 8 and plotted its response
for various models. We have considered only the elastic region which in our case is under 2.5%
strain, which can be seen by following figure.
Fig. 22 : Strain-Rate Dependent Tensile Test Data Including Yielding Region
The yielding region is considered to be under 2.5 % strain. This is due to the fact that we are
considering the elastic response, rather than the plastic deformation. We will try to fit the
validation of different material model under 2.5 % strain.
0
10
20
30
40
50
60
0.00% 5.00% 10.00% 15.00% 20.00%
Stre
ss (M
Pa)
Strain (%)
Stress-Strain
Strain Rate = 0.001
Strain Rate = 0.005
Strain Rate = 0.01
Strain Rate = 0.05
Yield Point
46
Fig 23 : Validation for Maxwell Model at Various Strain Rates
Fig 23 shows response of Maxwell model at different strain rates. It can be seen that current
Maxwell model is unable to capture complete deformation at various strain rates. This is due to
the fact of very high time constant.
Fig. 24 : Validation for Voigt-Kelvin Model at Various Strain Rates
0
5
10
15
20
25
0.00% 0.50% 1.00% 1.50% 2.00% 2.50%
Stre
ss (M
Pa)
Strain
Maxwell Model
Model 0.05
Model 0.01
Model 0.005
Model 0.001
Experiment 0.05
Experiment 0.01
Experiment 0.005
Experiment 0.001
0
10
20
30
40
50
60
70
0.00% 0.50% 1.00% 1.50% 2.00% 2.50%
Stre
ss (M
Pa)
Strain
Voigt-Kelvin Model
Model 0.05
Model 0.01
Model 0.005
Model 0.001
Experiment 0.05
Experiment 0.01
Experiment 0.005
Experiment 0.001
47
Fig. 24 shows response of Voigt-Kelvin model at different strain rates. It can be seen that at low
strain rates the response matches but at higher strain rates it starts deviating. The main reason
for this is due to high values of time constants.
Fig. 25 : Validation for Prony Series at Various Strain Rates
Fig. 25 shows response of Prony series at different strain rates. It can be seen that Prony series
captures very well the deformation at various strain rates.
0
5
10
15
20
25
0.00% 0.50% 1.00% 1.50% 2.00% 2.50%
Stre
ss (P
a)
Strain
Prony Series
Model 0.05
Model 0.01
Model 0.005
Model 0.001
Experiment 0.05
Experiment 0.01
Experiment 0.005
Experiment 0.001
48
Fig. 26 : Validation for Schapery Model at Various Strain Rates
Fig. 26 shows response of Schapery model at different strain rates. It can be seen that Schapery
model also captures the deformation very well at various strain rates.
Using the equations for dynamic response, we try to predict the loss modulus and
storage modulus values of frequency sweep of Procast data. Fig. 27Fig. 30 gives loss modulus
response for all viscoelastic models. We can see that all models do not capture a complete
response for loss modulus behavior.
0
5
10
15
20
25
0.00% 0.50% 1.00% 1.50% 2.00% 2.50%
Stre
ss (M
Pa)
Strain
Schapery Model
Model 0.05
Model 0.01
Model 0.005
Model 0.001
Experiment 0.05
Experiment 0.01
Experiment 0.005
Experiment 0.001
49
Fig. 27 : Loss Modulus response for Maxwell model
Fig. 28 : Loss Modulus response for Voigt-Kelvin model
1.00E+00
1.00E+01
1.00E+02
1.00E+03
1.00E+04
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E+09
0 20 40 60 80 100
Loss
Mod
ulus
(Pa)
Frequency (Hz)
Maxwell Model
Experiment
Maxwell Model
1.00E+001.00E+011.00E+021.00E+031.00E+041.00E+051.00E+061.00E+071.00E+081.00E+091.00E+101.00E+111.00E+12
0 20 40 60 80 100
Loss
Mod
ulus
(Pa)
Frequency (Hz)
Voigt-Kelvin Model
Experiment
Voigt-Kelvin Model
50
Fig. 29 : Loss Modulus response for Prony model
Fig. 30 : Loss Modulus response for Schapery Model
1.00E+00
1.00E+01
1.00E+02
1.00E+03
1.00E+04
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E+09
0 20 40 60 80 100
Loss
Mod
ulus
(Pa)
Frequency (Hz)
Prony Series
Experiment
Prony Series
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1.00E+10
1.00E+11
1.00E+12
1.00E+13
0 20 40 60 80 100
Loss
Mod
ulus
(Pa)
Frequency (Hz)
Schapery Model
Experiment
Schapery
51
Using the equations for dynamic response (storage modulus), we try to predict the
storage modulus values of frequency sweep of Procast data. Fig. 30 Fig. 31Fig. 34 gives storage
modulus response for all viscoelastic models. We can see that Maxwell and Voigt-Kelvin models
do not capture the complete dynamic response while Prony series and Schapery model gives a
better prediction in terms of storage modulus.
Fig. 31 : Storage Modulus response for Maxwell model
1.00E+00
1.00E+01
1.00E+02
1.00E+03
1.00E+04
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1.00E+10
1.00E+11
1.00E+12
0 20 40 60 80 100
Stor
age
Mod
ulus
(Pa)
Frequency (Hz)
Maxwell Model
Experiment
Maxwell Model
52
Fig. 32 : Storage Modulus response for Voigt-Kelvin model
Fig. 33 : Storage Modulus response for Prony Series
1.00E+00
1.00E+02
1.00E+04
1.00E+06
1.00E+08
1.00E+10
1.00E+12
1.00E+14
0 20 40 60 80 100
Stor
age
Mod
ulus
(Pa)
Frequency (Hz)
Voigt-Kelvin Model
Experiment
Voigt-Kelvin
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1.00E+10
0 20 40 60 80 100
Stor
age
Mod
ulus
(Pa)
Frequency (Hz)
Prony Series
Experiment
Prony Series
53
Fig. 34 : Storage Modulus response for Schapery model
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1.00E+10
0 20 40 60 80 100
Stor
age
Mod
ulus
(Pa)
Frequency (Hz)
Schapery Model
Experiment Schapery
54
CHAPTER 8
CONCLUSIONS AND FUTURE WORK
Additive manufacturing is a relatively new technique and mechanical response of the base
material used for printing is also unknown. In my thesis I have conducted various tests to
determine the base properties for Procast material. My major findings include
(i) Glass transition temperature of Procast, which is in the range of 81ยฐC
(ii) Melting temperature of Procast which is around 337.1ยฐC
(iii) Density of Procast material can be seen from Table 3 which is around 1125-1162
kg/m3
(iv) Strain rate dependent elastic modulus which can be seen from Table 2 which is
around 0.7-0.8 GPa
(v) Procast yield stress which is around 16-19 MPa
(vi) Linear viscoelastic stress level for Procast which is around 3 MPa
(vii) Prony Series and Schapery Model show good validation response at various strain
rates.
These finding from my thesis can benefit the research community. It can also be seen from my
viscoelastic models that Prony Series show better fit for linear viscoelastic response and
Schapery model shows good fit for all stress levels for non-linear viscoelastic region.
So far, I have studied the material response for Procast material, in the future we can also look
at complex cellular geometry behavior of Procast material.
55
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