=Paper= {{Paper |id=Vol-1853/p02 |storemode=property |title=Analysis of the transversely isotropy, due to the production process, of carbon black filled SBR |pdfUrl=https://ceur-ws.org/Vol-1853/p02.pdf |volume=Vol-1853 |authors=Gianfilippo Dottore |dblpUrl=https://dblp.org/rec/conf/system/Dottore17 }} ==Analysis of the transversely isotropy, due to the production process, of carbon black filled SBR== https://ceur-ws.org/Vol-1853/p02.pdf
     Analysis of the transversely isotropy, due to the
     production process, of carbon black filled SBR
                                                        Gianfilippo Dottore
                                          Department of Civil Engineering and Architecture
                                                    University of Catania (Italy)
                                                email: gianfilippo.dottore@gmail.com


   Abstract—Through tensile tests of uniaxial and planar type,              Transversely isotropic behavior, caused by the rubber cal-
was proceeded to the mechanical characterization of SBR rubber           endering process [20], has made more difficult the analysis.
sheets filled with particles of carbon black. The analysis has           The phenomenon is well known in the literature, so that were
allowed defining the elastic features, the storage modulus, the
transversal contraction coefficient (both obtained from uniaxial         already made attempts to characterize elastomers which have
testing) and the shear modulus (from the planar/pure shear               transverse isotropy [21], [22], [23].
tests). Due to the calendering process by which was obtained, the           Therefore, tests were performed at different loading di-
elastomer sheet it’s shown transversely isotropic, with different        rections, in order to assess the behavioral differences of the
mechanical behavior in the direction of processing with respect to       material between the direction of calendering and the direction
the transverse direction. Therefore, it was essential to evaluate
two different values for each storage modulus, depending that            orthogonal to it.
the sample was loaded in the direction of calendering or in that
orthogonal to it.                                                                       II. C ONSTITUTIVE EQUATIONS
   Index Terms—Transversal Isotropy, Hyperelasticity, SBR, Car-            Most of engineering materials belongs to the category of
bon Black, Calendering.                                                  simple materials, for which the stress tensor T is a function
                                                                         of the history of the position gradient F = ∇ x [2]:
                        I. I NTRODUCTION
   The Styrene Butadiene Rubber (SBR) arises from copoly-                            T (x, t) = T (F (x, τ ), x0 , t) τ ∈ [0, t]      (1)
merization of Butadiene and Styrene. The latter, present to the
extent of 20-25% of the copolymer, increases both strength and              Among the simple materials, those elastic exhibit a stress
stiffness from SBR with respect to the butadiene rubber (BR)             state as a function of the single value of the deformation
[1], [2]. Often, it is filled with carbon black, carbon charges          gradient:
of nanometric particles obtained from the combustion of
hydrocarbons, the presence of which improves the mechanical                                T (x, t) = T (F (x, τ ), x0 , t)           (2)
properties and slows the aging. The SBR is the most usage                   According to this constitutive relationship, stress does not
rubber [3], [4], [5], [6], [7], [8], [9], being able to provide          depend on the deformation path (Cauchy elasticity), but it
mechanical properties similar to those of natural rubber at a            is not the same for the deformation work that it performs.
lower cost. The elastomer is used mainly where there is a need           The hyperelastic materials are, instead, conservative elastic
for high-friction surfaces, so that the 70% of the production            materials (Green elasticity), i.e. for which, there being an
is for tires tread [10], [11], [12], [13].                               elastic potential function ϕ = ϕ(F), which can be derived
   The mechanical characterization of a material consists in             from stress, the differential form of the deformation work is
determining the current constitutive law between stress and              exact
strain [3], [14], [15], [16].
                                                                                                           t
   The characterization process is particularly critical for elas-
                                                                                                  
                                                                                                      ∂ϕ
tomers, due the large deformations that, for the same elastomer                                                 = TI                  (3)
                                                                                                      ∂F
definition [17], [18], can tolerate without reaching the failure.
   The aim of this experimental analysis is reached the                     where TI := JTF−t is the first tensor of Piola-Kirchhoff,
mechanical characterization of styrene-butadiene rubber, im-             with J = det F.
proved in its quality by the introduction of fillers of carbon              The simple assumption of linear elastic material is correct if
black (25%) in the blend [4], [19].                                      you can overlook the time-dependent effects (such as sliding
   The analysis developed is focused on tracking of math-                and relaxation) [2], [24], [25] and for small deformations.
ematical functions, which correlated stress and strain, and              Circumstance, the latter, in which it is allowed the stress-
on evaluation, starting with these functions, of the storage             strain relationship linearization. The linear stress-strain law
modules assumed at the beginning of the rubber test.                     is represented in classic form, tensorial, from equations Tij =
                                                                         P  3                                     4
                                                                            h,k=1 Cijhk Ehk , which contain the 3 = 81 constants Cijhk
  Copyright c 2017 held by the authors.                                  of the elastic tensor di of order 4. Since the stress and strain

                                                                     7
symmetry of the tensors requires six independent components                     and
of the stress tensor as a function of the six independent                                    1             v         v
                                                                                                      − Exyy       − Exyy        0           0            0
                                                                                                                                                                  
components of the strain tensor, only 36 independent constants                              Ex
                                                                                             v                       v
                                                                                          − yx         1
are needed to determine the linear elastic relationship.                                   Ex         Ey          − Eyzy        0           0            0        
                                                                                                                                                                   
                                                                                          − vyx        v
                                                                                                      − Eyzy         1
                                                                                                                                 0           0            0        
                                                                                C−1 = 
                                                                                           Ex                      Ey                                             
                                                                                       0                  0         0         1/Gxy         0            0
                                                                                                                                                                   
                                                                                                                                                           
          σx      σ(1)                             ε̇x     ε̇(1)                       0                  0         0           0         1/Gxy          0
                                                                                                                                                                  
                                                                                                                                                                   
         σy   σ(2)                           ε̇y   ε̇(2)                                                                                       2(1+vyz )
                                                                                0                  0         0           0           0            Ey
       σz   σ(3)                         ε̇z   ε̇(3) 
   σ :=   = 
                    ;                 ε̇ :=   = 
                                                              
        τxy   σ(4)                          γ̇xy   ε̇(4)                where the 6 modules present are defined as follows
                                                                
        τxz   σ(5)                          γ̇xz   ε̇(5) 
          τyz    σ(6)                             γ̇yz    ε̇(6)                                       σi
                                                                                              Ei =             σ(i) 6= 0,      σ(k) = 0 ∀k 6= i
                                                                                                      εi
  We can express the linear relationship in a matrix simplified
using independent components vectors                                                                  εj
                                                                                              vji =            σ(i) 6= 0,      σ(k) = 0 ∀k 6= i
                                                                                                      εi
       
         σx
             
               c11       c12     c13     c14     c15     c16
                                                              
                                                                 εx                                                           τij
                                                                                                                   Gij =
        σy  c21       c22     c23     c24     c25     c26   εy                                                          γij
                                                           
        σz  c31       c32     c33     c34     c35     c36 
                                                               εz 
                                                                
        =                                                                       The elements on the main diagonal are the modules of
       τxy  c41       c42     c43     c44     c45     c46 
                                                              γxy 
                                                                
         
       τxz  c51       c52     c53     c54     c55     c56  γxz            normal deformability 1/Ei and tangential 1/Gij ; those that
        τyz    c61       c62     c63     c64     c65     c66    γyz             appear outside of the same diagonal such as the modules
                                                                                of transverse deformability −vji /Ei . The symmetry of the
                                                                                previous matrix adds the condition vyx /Ex = vxy /Ey , which
   The linear elastic relationship is defined by 6 × 6 = 36                     reduces to 5 the independent components. In the case of
components of the C tensor of order 2 of elastic constants.                     transversely isotropic material plates, which present the x or
Assuming the occurrence of a potential elastic (hyperelastic                    y axis of transverse symmetry, assuming the status of plane
material), according to the Schwartz theorem, the elasticity                    stress σz = τxz = τyz = 0 and writing separately the
tensors are symmetrical and the constants are reduced to 21                     equations:
[3].
   They are called transversely isotropic those materials, which                                               vyx      1
have an isotropic response in the plane orthogonal to an axis,                                    εz = −           σx +    (σz − vyz σy )
                                                                                                               Ex       Ey
said axis of transverse symmetry. By choosing a base having
the x-axis parallel to symmetry transverse
                                                                                                                    γxz = 0

   
     σx
         
           c11       c12       c13      0       0        0
                                                               
                                                                    εx
                                                                                                                   γyz = 0
    σy  c12       c22       c23      0       0        0        εy 
                                                                                   the constitutive relation can be narrowed to only three
                                                              
    σz  c13       c23       c33      0       0        0        εz 
    =
                                                                                stress and three strain components in the x and y directions,
                                                                 
   τxy   0         0         0      c44      0        0       γxy 
                                                                                becoming [27], [3]:
                                                              
   τxz   0         0         0       0      c44       0       γxz 
                                                       c22 −c23
    τyz     0         0         0       0       0          2       γyz
                                                                                             Ex                         vxy Ex
                                                                                                                                               
                                                                                                                                           0
                                                                                                                                                    
                                                                                        σx       1−vxy vyx               1−vxy vyx                 εx
  In this case, the elastic response is described by 5 indepen-                        σy  =   vyx Ey                    Ey
                                                                                                                                           0  =  εy 
                                                                                                                                              
dent parameters [26].
                                                                                                1−v xy vyx              1−vxy vyx
                                                                                       τxy           0                        0           Gxy     γxy
  The constitutive equation can be formulated in terms of
engineering constants [3], inverting the stress-strain relation
                                                                                                     1                  v              
                                                                                                                      − Exyy         0
                                                                                              
                                                                                                εx       Ex                                  σx
                                                                                                         vyx              1
                     
                       εx
                               
                                 σx
                                                                                               εy  = 
                                                                                                        Ex              Ey          0 
                                                                                                                                        =  σy 
                                                                                               γxy                                   1      τxy
                      εy      σy                                                                      0               0         Gxy
                              
                      εz 
                       = C−1  σz 
                                
                     γxz 
                      
                               τxy 
                                                                                 where there is a symmetry condition vyx /Ex = vxy /Ey ,
                     γxz     τxz                                            whose effect is that only 4 of the 5 elastic constants appearing
                      γyz       τyz                                             in the above matrix are independent.

                                                                            8
              III. P URE SHEAR IN PLANAR TEST
   A valid constitutive model must properly describe the
material behavior for each stress state. A single test as a
simple uniaxial tensile test does not allow the construction
of a reliable model in every situation. It then requires the
programming of tests set that provide different stress mode.
The uniaxial tensile test is then flanked by planar tensile and
equibiaxial tests [4], [19].
   The planar tensile test replaces the pure shear test (torsion of
cylindrical sample), not applicable on samples cut from rubber
sheets. The central portion of the specimen is subject to a pure
shear strain and stress [4], [24]. To prove this assumption, we
consider an orthogonal axes system, formed by the loading                                  Fig. 1.    Reference system, x-y plane
direction, from the direction perpendicular to this plane in the
specimen and the direction normal to the sample itself. Noted
these directions, respectively, with the letters x, y, z (Figg. 1,
2):
   The test consists to creating a stretching in the x-direction.
For implementation of the test (high ratio width/length of the
sample), it is considered negligible the strain along the y-
direction

                       λy = 1 ⇒ ε̇y = 0

  Consequently, the incompressibility condition J = det F =
1 ⇒ ε̇V = ε̇x + ε̇y + ε̇z = 0 for planar test is written [2]:

                                                 1
                 J = λx λz ⇔= 1 ⇒ λz =
                                                λx

                   ε̇V = ε̇x + ε̇z ⇔ ε̇z = ε̇x

   Therefore, assuming the incompressibility of the mate-
rial, the coordinates of tensors F and V=1/2(grad v +
grad vt )(strain rate) respect to the system of axes x, y, z
are.,                                                                                      Fig. 2.    Reference system, x-z plane
                                                      
                            λx          0        0
              F xyz = FD xyz =  0          1        0                                                                    
                                                  1                                                      σ˙x          0    0
                                 0          0    λx
                                                                                              dT                   σ˙x
                                                                                               dt xyz
                                                                                                      = 0
                                                                                                                      2    0
                                                                                                        0           0    0
                           ε˙x     0  0
                    V xyz =  0      0  0                                   Unlike from strain, a condition of pure shear not realized
                              0      0 −ε˙x                               for stress. However, the deviator stress tensor T0 still takes a
                                                                          pure shear connotation (see Fig. 4):
   being FD the right tensor of the deformation gradient. As                                           σ˙x            
can be seen from the following image (Fig. 3), relative to the                            h 0i           2    0     0
circumferences of Mohr for the strain rate V, at each instant                              dT       =0 0           0 
                                                                                            dt xyz
of the test, the strain occurs in pure shear mode.                                                       0 0 − σ2˙x
   The deformation in the z-direction is free, therefore in the
                                                                          n and t are the orthogonal directions to the y-axis and
absence of external loads in this direction is not generated
                                                                          forming angles of 45◦ with the x- and z-axis (Fig. 2). Since
stress (σz = 0). Moreover, from the generalized Hooke’s law
                                                                          τ̇nt = σ̇x /2, and being γ̇nt = 2ε̇x , the calculation of the shear
ε̇y = σ̇y − ν(σ̇x + σ̇z ), being for incompressible materials
                                                                          modulus G is carried out using the formula
ν = 1/2 [3], for the y-direction stress is σ̇y = ν σ̇x = σ̇x /2. In
conclusion, during the planar test, the stress tensor T follows                                          dτnt   1 dσx
the variation equation                                                                           G=           =
                                                                                                         dγnt   4 dεx

                                                                      9
       Fig. 3.   Circumferences of Mohr of the strain rate tensor.



                                                                                           Fig. 5.   Grid for the optical relief of the strain.




                                                                               20 mm wide and 180 mm long, with a gauge length of 120
                                                                               mm.
                                                                                In the planar tests, was adopted a rectangular specimen, 150
                                                                               mm wide and 70 mm long, having a gauge length of 50 mm.
                                                                                  For the measurement of deformations of the specimen was
                                                                               adopted an optical method. In effect, the large deformations
                                                                               do not allow easy application of the classic method of the
                                                                               strain gages. The used solution is based on tracing a grid on
                                                                               the specimen surface, which allows to highlight the location
                                                                               of certain specific points. By known distances between various
                                                                               points of the grid in the absence of load, just evaluate their
                                                                               new position for load applied to achieve the deformation.
                                                                                  For image acquisition, carried out at a constant rate, it used
                                                                               a digital camera Basler acA1300-30gm. It is equipped with a
                                                                               Sony CCD sensor ICX445 that provides 8-bit greyscale images
   Fig. 4.   Circumferences of Mohr of the stress rate deviator tensor.        at a maximum frame rate of 30 per second, with a resolution
                                                                               of 1280x960 px.
                                                                                  To provide an accurate strain measurement, the Vision
                    IV. E XPERIMENTAL TESTS
                                                                               Builder AI software of National Instruments acquires and
  The tested material is SBR filled with 25% of carbon black.                  processes images through the Vision Assistant command. At
The tests were performed on specimens cut from a sheet                         first a convolution filter is applied to emphasize the grid.
having a 3 mm thickness. These were carried out with a test                    In fact, the alteration of the grid, caused by the specimen
machine Zwick/Roel, model Z100, equipped with 1 kN load                        stretching, reduces the brightness and thus the contrast with the
cell, to imposed deformation, while maintaining constant the                   background, making difficult the identification of the points by
speed of the movable crossbar.                                                 the software [33]. Finally, a median filter removes the image
  The uniaxial tests were performed on rectangular specimens,                  residual noise.

                                                                          10
                                                                                Fig. 8.   Localization of points on the processed image.



                   Fig. 6.    Uniaxial Test Setup.




                                                                                Fig. 9.   Uniaxial Test: Normal Stress vs Normal Strain.


                   Fig. 7.    Not processed image.
                                                                        to the growing of the test speed. A decrease of the failure
                                                                        strain is associated. It also occurs a different location of the
                             V. R ESULTS                                sample crack, depending on the execution test rate. At the
   By the stress and strain values, the relationships of vari-          ends, near the gripping point, for tests with crossbar that moves
ous functional dependencies between the physical quantities             5 mm/min; at the center of the specimen for the tests with
involved is obtained. The storage modules were determined               speed of 60 mm/min.
from the mathematical derivation of these functions.                       In all performed tests, the value of Young’s modulus E at the
   The uniaxial testing allows to express both the normal stress        O(0, 0) ranged between 6.45 and 7.70 MPa, comparable with
as a function of the normal strain in the vertical direction            values reported in the literature for PDMS filled by carbon
(loading direction), and the normal strain in the horizontal            black (25%) [34]. The stretching at failure is much lower than
direction (orthogonal to the loading direction) as a function           the average obtained in the styrene-butadiene rubber [34].
of vertical strain. From the first of these applications the               In addition, at the O(0, 0), the Poisson’s ratio ν is greater
normal storage modulus E = dσ/dεν [24] is obtained, from                for the high-speed tests (0.460 and 0.490) compared to those
the second application the transverse contraction coefficient           at low speed (0.360 and 0.387). For the same rate of the
ν = −dεo /dεν .                                                         crossbar, instead, the tested elastomer shows a greater Young’s
   As already verified during a previous study on a silicone,           modulus and Poisson’s ratio in the transverse test, but a lesser
developed at the University of Catania [25], a small strain             failure strain. It is interesting to observe that for faster tests
rate, lower than ASTM standards, could rise viscoelastic                the coefficient ν, evaluated by means of real strains, assumes
relaxation phenomena in elastomers (decreasing of stress level          constant value and close to the theoretical limit value of 0.5,
compared to the strain achieved). Increasing the execution              maximum value for isotropic materials [26], which character-
test rate, this aspect can be kept under control, drastically           izes the incompressibility, during the entire test. This statement
mitigating the effects. As a consequence of this, the stress-           is true only for the Hencky’s tensor. Adopting instead other
strain curve is higher for the higher test rate, then the rubber        tensors of deformation, it decreases during the experimental
manifests an increase of stiffness and mechanical resistance            test, being equal to about 0.5 only at the beginning of the test

                                                                   11
                                                                             account the highly hyperelastic material characteristics. Then,
                                                                             they applied optical methods for overcoming this limit to
                                                                             characterize the elastomer.
                                                                                The collected data were analyzed to determine the main
                                                                             elastomer storage modules. Different responses in relation
                                                                             to the different direction of load application are obtained,
                                                                             studying anisotropy expressed by the material due to the
                                                                             production process of calendering elastomeric sheet. Given
                                                                             the high variability of the materials properties, the results
                                                                             were compared with those indicated in the literature for the
                                                                             styrene-butadiene rubber filled with carbon black. The normal
                                                                             storage modulus (stiffness) and the transverse contraction
                                                                             coefficient (Poisson’s ratio) of the SBR as a function of
                                                                             the filler percentage have appeared, in accordance with the
      Fig. 10.   Uniaxial Test: Horizontal Strain vs Vertical Strain.        data available, lower for the tests with load applied in the
                                                                             calendaring direction.

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                                                                        12
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