Compression and Hysteresis Curves of Nonlinear Polyurethane Foams Under Different Densities, Strain Rates and Different Environmental Conditions

Abstract

Polyurethane (PU) closed cell foam samples with different densities were tested under loading and unloading compression tests at different temperatures and strain rates. Quasi-static compression tests were performed using the Lloyd LR5K Plus instrument at strain rates ranges from 0.033–0.267 s−1 . Tests were conducted in a précised enclosure to control the dependency of PU foam cells on temperature and humidity. In order to have an accurate comparison in compression and hysteresis curves for all tests; all PU foam samples were selected intentionally from the same foam block but with different location densities. Furthermore, all foam samples were tested in the direction of foam rise (thickness). First, PU foam samples were compressed with a circular platen up to 70% strain at different strain rates and different temperatures. Then, the platen was raised completely from the foam samples. During the experiment; stress-strain responses were measured and plotted for loading and unloading curves to determine stored energy, dissipation energy and peak stresses were calculated at 70% strain. Results have shown that PU foam sample responses under compression testing gets softer at higher temperatures when conducted at a constant strain rates. At constant temperatures, PU foam samples get harder at higher slip rates. Finally, both stored and dissipation energies were found to be dependent heavily on foam density, ambient temperature and strain rate.

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1 Copyright © 2011 by ASME
Proceedings of IMECE11
2011 ASME International Mechanical Engineering Congress & Exposition
November 11-17, 2011, Denver, Colorado, USA
IMECE2011-6229
Compression and hysteresis curves of nonlinear polyurethane foams under
different densities, strain rates and different environmental conditions
M. F. Alzoubia, E. Y. Tanbourb, R. Al-Wakedb
aVisiting Scientist, All Cell Technologies LLC, Chicago, IL USA
bCollege of Engineering, Prince Mohammad Bin Fahd University (PMU),
Al-Khobar, Eastern Province, KSA
ABSTRACT
Polyurethane (PU) closed cell foam samples with different
densities were tested under loading and unloading compression
tests at different temperatures and strain rates. Quasi-static
compression tests were performed using the Lloyd LR5K Plus
instrument at strain rates ranges from 0.033-0.267 s-1. Tests
were conducted in a précised enclosure to control the
dependency of PU foam cells on temperature and humidity. In
order to have an accurate comparison in compression and
hysteresis curves for all tests; all PU foam samples were
selected intentionally from the same foam block but with
different location densities. Furthermore, all foam samples were
tested in the direction of foam rise (thickness). First, PU foam
samples were compressed with a circular platen up to 70%
strain at different strain rates and different temperatures. Then,
the platen was raised completely from the foam samples.
During the experiment; stress-strain responses were measured
and plotted for loading and unloading curves to determine
stored energy, dissipation energy and peak stresses were
calculated at 70% strain. Results have shown that PU foam
sample responses under compression testing gets softer at
higher temperatures when conducted at a constant strain rates.
At constant temperatures, PU foam samples get harder at higher
slip rates. Finally, both stored and dissipation energies were
found to be dependent heavily on foam density, ambient
temperature and strain rate.
Keywords: Polyurethane Foam; Compression; Peak Pressure,
Elastic Energy, Hysteresis Loss, Dissipation Energy.
1. INTRODUCTION
Viscoelastic materials such as PU foam materials were first
developed in Germany in 1937. Production of Polyurethane
fibers has increased in the last fifty years; primarily due to the
development of fibers that can move physically with the human
body [1]. Moreover, Polymeric elastomers consist chemically
of long, randomly coiled aliphatic polyethers or polyesters,
joined by stiff regions of urethane linkages. Generally; PU
foam are one of the most versatile materials in use today. Their
uses range from flexible foam in upholstered furniture and
energy absorbers to rigid foam as insulation in walls and roofs
to thermoplastic polyurethane used in medical devices and
footwear to coatings, adhesives, sealants and elastomers used
on floors and automotive interiors and multiple space and
acoustics applications. The ability of dissipating or absorbing
energy and relieving pressure are among the main
functionalities of PU foam. For their multi use; there are several
mechanic loading applications for which PU foams can be
subjected to such as; compression, tension, shear, creep and
relaxation. The focus of the current research was on
compression and energy dissipation applications. Foam usually
can be compressed up to 95% and it can be elongated up to
400% of its original dimensions. In addition, cellular foam
possesses unique elastic and viscous behavior and therefore, PU
foam can be characterized mechanically similar to viscoelastic
materials. These characteristics set the viscoelastic models
distinctly apart from elastic models.
Elastic materials in compression and tension can store
usually up to 100% of energy due to deformation. Whereas
viscoelastic materials don’t store 100% of energy under
deformation; but actually lose or dissipate some of this energy
as heat. This dissipation energy is also known as hysteresis loss.
The importance of the hysteresis measurement is that it gives a
strong indicator about the material capacity to absorb energy
and/or relief pressure. The area under the loading curve (i.e. the
blue line in Fig. 1) can be called as the total mechanical energy
input. The area under the return curve (i.e. the red line) can be
considered as the return of stored energy and the area between
the two curves is the energy which cannot be returned but
rather is dissipated and converted to heat as shown in Fig. 1.
This dissipated heat loss can be called also hysteresis loss. A
Proceedings of the ASME 2011 International Mechanical Engineering Congress & Exposition
IMECE2011
November 11-17, 2011, Denver, Colorado, USA
IMECE2011-62290

recent work PU hysteresis was conducted by Buckley, C.P., et
al [2].
Figure1.
Hysteresis loop for a typical PU foam sample. The
area between loading and unloading curves is
known to be the
hysteresis loop loss.
It is well understood that the deformation of open
foams show three main regions
of A, B and C
Fig. 2. These regions can be divided into an
initial linear elastic
region where strain energy is stored in the reversible bending of
the struts; a plateau region where struts begin to impinge upon
each other and finally the third region which is the densification
area. During final stage; the foam
essentially becomes a solid
composed of the solid material from which it is made of. This
region is called densification, this is occurs when foam cells
crush each other causing the internal compression strength to
increase rapidly as in [3, 4]. Each of th
e above mentioned
regions have their own unique advantages
and applications
Figure 2. Stress-
strain curve for PU foam sample under
compression testing. A: linear elastic region, B: plateau region
& C: densification region.
Historically; it has been known
as in [3,4]
mechanical properties of the foam are directly proportional to
the foam’s cell structures. Previously; t
he general method of
modeling the foam
cells as a cubic array of members of length l
and square cross-section of side t as shown
in Fig. 3. From this
model, the defining characteristics relative to cell structures can
be found. The principle characteristics describing foams is the
relative density (ρ*/ρs) which is the ratio between the foam’s
2
recent work PU hysteresis was conducted by Buckley, C.P., et
Hysteresis loop for a typical PU foam sample. The
known to be the
It is well understood that the deformation of open
-cell
of A, B and C
as indicated in
initial linear elastic
region where strain energy is stored in the reversible bending of
the struts; a plateau region where struts begin to impinge upon
each other and finally the third region which is the densification
essentially becomes a solid
composed of the solid material from which it is made of. This
region is called densification, this is occurs when foam cells
crush each other causing the internal compression strength to
e above mentioned
and applications
.
strain curve for PU foam sample under
compression testing. A: linear elastic region, B: plateau region
as in [3,4]
that the
mechanical properties of the foam are directly proportional to
he general method of
cells as a cubic array of members of length l
in Fig. 3. From this
model, the defining characteristics relative to cell structures can
be found. The principle characteristics describing foams is the
s) which is the ratio between the foam’s
density (i.e. ρ*) to
that of the solid m
made of (i.e. ρs).
Figure 3.
Theoretical model for a
compression loading.
The relationship between the geometry and the physical
parameters describing the cell and the relative density is given
as:
ρ*/ρ
s
α ( t/ l)
2
From this relationship, the porosity (
found:
Ф = 1 - ρ*/ρ
s
Another important property
mechanical properties of materials is the elastic modulus.
Although foams are inherently anisotropic,
relationship using the previous analysis and some empirical
results is given by:
E = E
s
(1- Ф )
2
where Es is the elasti
c modulus of the solid material, and E is
the elastic modulus of the foam.
A recent unique work in
modeling structures in
was conducted by
Li, Gao et al [
micromechanics model for three dimensional open
using a tetrakaidecahedral unit cell model an
second theorem.
Each tetrakaidecahedral unit cell
Fig. 4a, had
36 struts and was treated as uniform slender beams
undergoing linearly elastic deformations.
incorporated the struts with different cross sectional shapes
such as circular, square, equilateral triangle and Plateau border.
Out of these findings,
two closed
determining the effective Young’s modulus and Poisson’s ratio
of open-
cell foams were provided. The new formulas explicitly
show that the foam elastic properties depend on the relative
foam density, the shape and size of the st
Young modulus and the Poisson’s ratio of the strut material.
In another work,
Li, Gao et al [6
micromechanical modeling
by using
cell foam along with the matrix method
i
nstead of the Castigliano’s second theorem.
formulas for determining the effective Young’s moduli,
Copyright © 2011 by ASME
that of the solid m
aterial that the foam is
Theoretical model for a
cubic foam cell under a
compression loading.
The relationship between the geometry and the physical
parameters describing the cell and the relative density is given
(1)
From this relationship, the porosity (Ф) of the foam can be
(2)
Another important property
used in determining the
mechanical properties of materials is the elastic modulus.
Although foams are inherently anisotropic,
a general
relationship using the previous analysis and some empirical
(3)
c modulus of the solid material, and E is
modeling structures in
foam cells
Li, Gao et al [
5]. They developed a
micromechanics model for three dimensional open
-cell foams
using a tetrakaidecahedral unit cell model an
d Castigliano’s
Each tetrakaidecahedral unit cell
, shown in
36 struts and was treated as uniform slender beams
undergoing linearly elastic deformations.
They also
incorporated the struts with different cross sectional shapes
such as circular, square, equilateral triangle and Plateau border.
two closed
-form formulas for
determining the effective Young’s modulus and Poisson’s ratio
cell foams were provided. The new formulas explicitly
show that the foam elastic properties depend on the relative
foam density, the shape and size of the st
rut cross section, the
Young modulus and the Poisson’s ratio of the strut material.
Li, Gao et al [6
] developed their
by using
the tetrakaidecahedral unit
cell foam along with the matrix method
for spatial frames
nstead of the Castigliano’s second theorem.
In this model, the
formulas for determining the effective Young’s moduli,

Poisson’s ratios and shear moduli of open cell foams are
derived using the composite homogenization theory.
theory confirms that foam
elastic properties depend on the
relative foam density, the shape and size of the strut cross
section, the Young’s modulus and the Poisson’s ratio of the strut
material.
Figure 4a. A tetrakaidecahedral unit model
of an open foam
cell.
In biomechanics’ applications,
Zhang et al. [
the strain rate dependency on Finite Element (FE) model of the
Rodent Traumatic Brain Injury. Also,
Saha et al. [8
measured peak stress within the elastic range and the energy
absorption up to 15% strai
n and they concluded that these
measurements are highly depending on foam density.
furthermore
, Saha et al. investigated peak stresses and energy
absorption for open foam cells at 15% strains.
The current research
adds to the work of Saha et al [8] in
covering
closed foam cells and extends the compression
deflection up to 70% strain where the peak stress is measured in
the densification region.
Calculating peak stress at higher
strains is essential demand for
many industrial applications
such as bedding and
cushion applications especially for the top
layers that the beds are made of. Also, the hysteresis loss
measurement is an essential mechanical measurement that can
be correlated to the amount of pressure relieving
viscoelastic beds. In turn, the pr
essure relieving is an important
quantifying parameter that affects the comfort level especially
for bedding and seats’ industry. A quasi-
static loading and
unloading compression tests were performed for three PU foam
samples with densities of 78, 98 and 1
31 kg/m3. A varying
strain rate of 0.033-0.267 s-1
were used during
compression process
but was kept constant at 0.017
the unloading processes
for all tests. The low speed of 0.017
of the platen was chosen for the unloading
proce
that the recovery of the foam rate is just faster enough than
unloading speed of the platen
and also to prevent the
detachment of the platen from the foam samples
Adding to the work of Zhang et al. [7]
, PU samples were
tested under different environmental temperatures rages from
25°C-
55°C. Also, peak stresses at 70% strain were measured
and energy dissipation and energy absorption were calculated
3
Poisson’s ratios and shear moduli of open cell foams are
derived using the composite homogenization theory.
This
elastic properties depend on the
relative foam density, the shape and size of the strut cross
-
section, the Young’s modulus and the Poisson’s ratio of the strut
of an open foam
Zhang et al. [
7] validated
the strain rate dependency on Finite Element (FE) model of the
Saha et al. [8
] studied and
measured peak stress within the elastic range and the energy
n and they concluded that these
measurements are highly depending on foam density.
, Saha et al. investigated peak stresses and energy
adds to the work of Saha et al [8] in
closed foam cells and extends the compression
deflection up to 70% strain where the peak stress is measured in
Calculating peak stress at higher
many industrial applications
cushion applications especially for the top
layers that the beds are made of. Also, the hysteresis loss
measurement is an essential mechanical measurement that can
be correlated to the amount of pressure relieving
of the
essure relieving is an important
quantifying parameter that affects the comfort level especially
static loading and
unloading compression tests were performed for three PU foam
31 kg/m3. A varying
were used during
the loading
but was kept constant at 0.017
s-1 during
for all tests. The low speed of 0.017
s-1
proce
ss in such way
that the recovery of the foam rate is just faster enough than
and also to prevent the
detachment of the platen from the foam samples
.
, PU samples were
tested under different environmental temperatures rages from
55°C. Also, peak stresses at 70% strain were measured
and energy dissipation and energy absorption were calculated
from the stress-
strain curves using the Simpson’s rule
integration method.
2. TESTS MATERIALS
Among the reasons that make the mechanical
characterization of PU foam cells especially for closed cells
complex are their chemical formulation sensitivity and their gas
that get trapped within the cell walls during the foam rising
process.
Therefore; to investigate the effect of density, strain
rates and temperature on compression and hysteresis curves for
the different PU foam closed cell samples and to have a
legitimate comparison; same chemical structure and
formulation among all foam sa
mples have been assumed by
selecting foam samples from different locations within the same
foam block as shown in Fig. 4b. Samples’ density values were
78, 98 and 131kg/m3
and for testing proposes they were labeled
as P1-PU78, P2-PU98 and P3-
PU131.
Figure 4b.
A block of foam with different
locations.
Table 1 lists density of the
collected. Due to symmetry of the foam density
samples that were selected from the same foam block, only
three foam samples of P1-
PU78, P
tested and they were labeled in the next graph results as PU78,
PU98 and PU131.
3. TEST METHOD
Foam samples labeled with PU78, PU98 and PU131 were
cut with dimensions of 100 mm x 100 mm x 50 mm for which
all samples were tested
in the 50 mm (rise) direction. Quasi
static compression tests were performed using the Lloyd LR5K
Plus instrument at strain rates ranges from 0.017 s
and samples were subjected to different environmental
temperatures ranges from 25°C to 55
kept constant during each test. Each foam sample was
compressed up to 70% using a circular platen with 20 cm
diameter. An assumption was made that during the loading and
unloading process of the tests; there was no separation betwe
the platen and the foam samples.
Copyright © 2011 by ASME
strain curves using the Simpson’s rule
Among the reasons that make the mechanical
characterization of PU foam cells especially for closed cells
complex are their chemical formulation sensitivity and their gas
that get trapped within the cell walls during the foam rising
Therefore; to investigate the effect of density, strain
rates and temperature on compression and hysteresis curves for
the different PU foam closed cell samples and to have a
legitimate comparison; same chemical structure and
mples have been assumed by
selecting foam samples from different locations within the same
foam block as shown in Fig. 4b. Samples’ density values were
and for testing proposes they were labeled
PU131.
A block of foam with different
foam sample
locations.
Table 1 lists density of the
six foam samples that were
collected. Due to symmetry of the foam density
of the PU
samples that were selected from the same foam block, only
PU78, P
2-PU98 and P3-PU131 were
tested and they were labeled in the next graph results as PU78,
Foam samples labeled with PU78, PU98 and PU131 were
cut with dimensions of 100 mm x 100 mm x 50 mm for which
in the 50 mm (rise) direction. Quasi
-
static compression tests were performed using the Lloyd LR5K
Plus instrument at strain rates ranges from 0.017 s
-1 - 0.267 s-1 ,
and samples were subjected to different environmental
temperatures ranges from 25°C to 55
°C. However; temperature
kept constant during each test. Each foam sample was
compressed up to 70% using a circular platen with 20 cm
diameter. An assumption was made that during the loading and
unloading process of the tests; there was no separation betwe
en

4 Copyright © 2011 by ASME
Table 1. Foam samples densities
Property P1-PU78 P2-PU98 P3-PU131 P4-PU131 P5-PU98 P6-PU78
Density, ρ (kg/m
3
)
Density of solid foam, ρ
s
(kg/m
3
)
Relative Density ρ
s
/ ρ
78
1200
0.065
98
1200
0.082
131
1200
0.11
131
1200
0.065
98
1200
0.082
78
1200
0.11
Compression and hysteresis’ stress-strain data for loading and
unloading curves were recorded as a function of time using a
data acquisition system. Also, densification Peak Stresses
(PmaxD) at 70% strain were recorded for comparisons. The
compression Lloyd instrument tester which is positioned inside
an environmental enclosure for précising controlling of
temperature and humidity is shown in Fig. 5a and 5b.
Figure 5a. A compression test with a foam sample and a
circular platen.
Figure 5b. Lloyd LR5K Plus instrument with an enclosure
enviroment
4. RESULTS AND DISCUSSION
4.1. EFFECT OF FOAM STRAIN RATES
Typical stress-strain responses of PU78, PU98 and PU131
foams at different strain rates of 0.033 s-1, 0.133 s-1 and 0.267 s-
1 are shown in Fig. 6a. All curves show typically three stages of
deformation, an elastic behavior up to almost 5% strain which
is the elastic peak stresses within the elastic range, then
increases plateau and finally densification region. For these
foam samples there were no plastic bending post-peak
softening region noticeable in comparison with Saha et al. [6]
work.
For cellular materials, the elastic peak stresses under
compression is controlled by the elastic bending of the cell
walls. If cell walls bend plastically; collapsing of the cell occurs
and this phenomenon is called plateau. After plateau; cells start
crushing each other causing a rapid increase in compression
stresses.
Fig. 6a-6c show that compressing the foam samples at
higher strain rates cause hysteresis loss area to increase and the
densification peak stress (PmaxD) values to increase as well.
Table 2 shows summary results of the hysteresis loss energy,
elastic energy and the maximum densification stresses at 70%
strain. It should be noticed here that all energy calculations
were actually energy per unit volume i.e. “energy density”, and
all energy calculations were computed using the integration
method of Simpson’s rule.
Variations of PmaxD and energy dissipation as a function
of strain rate are shown in Fig. 7a and Fig. 7b for PU78, PU98
and PU131 foams.
Fig. 7a shows that peak stresses at densification region
increases with strain rates for all foam samples. Also, it is
shown from Fig. 7b that hysteresis loss increases with increases
strain rates for all foam density samples. However; the
increasing rates of the peak stresses and the hysteresis loss for
P98 and P131 samples are higher than that for P78 sample.
Figure 6a. Stress–strain response of PU78 at
different strain rates.
0
2
4
6
8
10
12
0 0.2 0.4 0.6 0.8
Compression Stress (Kpa)
Strain
P1
-
PU78
-
0.033 /sec
P1
-
PU78
-
0.133 /sec
P1
-
PU78
-
0.267 /sec

5 Copyright © 2011 by ASME
Figure 6b. Stress–strain response of PU98 at different strain
rates.
Figure 6c. Stress–strain response of PU131 at different strain
rates.
Figure 7a. Peak stress (PmaxD) at densification as a function of
strain rate for different density foams.
Figure 7b. Hysteresis loss or energy dissipation as a function of
strain rate for different density foams.
Table 2. Summary of total energy, hysteresis loss energy, elastic energy and the maximum densification stresses.
Strain Rate 0.033 s
-1
0.033 s
-1
0.033 s
-1
0.133 s
-1
0.133 s
-1
0.133 s
-1
0.267 s
-1
0.267 s
-1
0.267 s
-1
Measured
Result P1-PU78 P2-PU98 P3-PU131
P1-PU78 P2-PU98 P3-PU131
P1-PU78 P2-PU98 P3-PU131
Density 78 98 131 78 98 131 78 98 131
Densification
Peak Stress
(P
maxD
) (Kpa)
6.49 12.04 19.32 9.53 17.22 28.18 12.39 36.12 53.92
Total Energy
(kJ/m
3
) 0.710 1.056 1.548 1.033 1.442 2.148 1.229 2.917 4.181
Releasing
Energy (kJ/m
3
)
0.393 0.572 0.817 0.418 0.549 0.813 0.362 0.454 0.823
Dissipation
Energy (kJ/m
3
)
(Hysteresis
Loss)
0.318 0.484 0.730 0.615 0.892 1.335 0.867 1.813 1.920

6 Copyright © 2011 by ASME
4.2. EFFECT OF FOAM DENSITY
Stress-strain responses of PU78, PU98 and PU131 foams
at 25° C and loading strain rate of 0.033 s-1, 0.133 s-1 and 0.267
s-1 are shown in Fig. 8a, 8b and 8c respectively.
Figure 8a. Stress–strain responses for different PU foam
densities at strain rate loading of 0.033 s-1.
Figure 8b. Stress–strain responses for different PU foam
densities at strain rate loading of 0.133 s-1.
Figure 8c. Stress–strain responses for different PU foam
densities at strain rate loading of 0.267 s-1.
From Fig 8a, b and c; peak stresses were extracted and
hysteresis loss were calculated and plotted as a function of
foam density for the different strain rates in Fig 9a and b.
Figure 9a. PmaxD versus different foam density responses at
different strain rates.
Figure 9b. Hysteresis loss versus different foam density
responses at different strain rates.
It is observed from Fig.9a and Fig.9b that the peak stresses
and hysteresis significantly depends on foam density and this is
with agreement with 15% compression strain for Saha et al [6].
Also, this finding is with agreement of the work of Li, Gao et al
as in indicated in [4] and [5]. However; the work of Li, Gao, et
al. was performed for open foam cells; whereas this
experimental work was conducted for closed foam cells.
Therefore, a quantitative comparison can’t be conducted but it
generally shows that the foam material stiffness increases with
the foam density ratio of the foam and this is can be valid for
open and closed foam cells. In order for a meaningful
quantitative comparison between this experimental work and
the work of Li, Gao et al; the micromechanical modeling of the
tetrakaidecahedral unit foam cell should be incorporated with a
gas that is trapped within the closed cell walls. It is obvious that
at the peak stresses and hysteresis significantly increase at
higher rates for strain rate of 0.267 s-1 than those for 0.033 s-1
and 0.133 s-1. However; the increase of the rates of the peak

7 Copyright © 2011 by ASME
stresses and hysteresis for PU131 foam sample is higher than
that for PU78 and PU98 foam samples. Therefore; the intension
of increases foam density of foam should be incorporated with
the application of the foam that will be used for. In another
way; increases the foam density beyond a certain limit causes
the internal stiffness of the cell walls to increase rapidly which
may jeopardizes the mechanical and thermal properties of the
foam that was intended for certain applications.
4.3. EFFECT OF FOAM TEMPERATURE
Stress-strain responses at strain rate of 0.033 s-1of PU78,
PU98 and PU131 foams at 25° C, 40° C and 55° C are shown
respectively in Fig. 10a, 10b and 10c.
From Fig 10a, b and c; peak stresses were extracted and
hysteresis loss were calculated and plotted as a function of
foam density for the different temperatures on Fig 11a and 11b.
It is obvious from Fig. 10a, b and c that the hysteresis loss is
higher at lower temperatures than that for higher temperatures.
Figure 10a. Stress–strain responses for different PU foam
densities at 25° C temperature.
Figure 10b. Stress–strain responses for different PU foam
densities at 40°C temperature.
Figure 10c. Stress–strain responses for different PU foam
densities at 55° C temperature.
Figure 11a. Hysteresis loss versus different foam density
responses at different temperatures.
Figure 11b. PmaxD versus different foam density responses at
different temperatures.
It is observed from Fig.11a and 11b that peak stresses and
hysteresis significantly depends on foam’s temperature. Peak
stresses and hysteresis significantly increase at lower
temperatures. This is due to decreasing in the stiffness of the
foam at lower temperature. Increasing foam temperature of the
foam cells cause cells’ struts and walls’ stiffness to reduce
which causes the hysteresis loss and peak stresses to reduce as

8 Copyright © 2011 by ASME
well. Thus, capacity of cellular materials to absorb and/or to
relief pressure is higher at 25°C than 40°C and 55°C. This
conclusion is valid only for these temperature ranges. However
further investigations and testing need to be conducted for
temperatures below the 25°C.
From application point view and especially for energy
absorbers’ applications, it is very essential to watch for the
environmental conditions of the foam. Changing the
environmental conditions is indeed play an important role for
jeopardizing the functionality of the PU foams. It is interesting
also to see from Fig. 10a, 10b and 10c that all stress-strain
curves at all temperatures remain almost same for strain ranges
between 10-20%; after which the stress-strain deviates at higher
strains. From practical point view; this shows also that when
foams subjected to higher temperatures such as 40°C or 55°C,
the capacity for the foam to absorb energy gets lowered. This is
could be explained in such way that the viscous characteristics
of the foam losses its capacity to absorb more energy due to the
reduction of it is viscosity.
To investigate effect of the temperature on hysteresis and
peak stresses for each foam samples; stress-strain responses for
each foam density were plotted for temperatures of 25°C, 40°C
and 55°C in Fig 12a, 12b and 12c respectively.
Figure 12a. Stress–strain responses for P1-PU78 foam at
different temperatures.
Figure 12b. Stress–strain responses for P2-PU98 foam at
different temperatures.
Figure 12c. Stress–strain responses for P3-PU131 foam at
different temperatures.
4. SUMMARY
The closed cell PU foam samples with different density
were tested in quasi-static compression tests using the Lloyd
LR5K Plus instrument at different strain rates and different
temperature environments. Peak stress and energy absorption
were found to be significantly depended on strain rates, density
and temperature environments and this with a good agreement
with Saha et al. and Li, Gao et al. Also, strain rate dependent
behavior was found to be more pronounced at higher density
foams. Finally, peak stress and energy absorption were found to
be significantly depended on foam and environment
temperatures. Increasing foam temperature shows a reduction
in foam stiffness. Reduction of foam stiffness causes also a
reduction of the foam ability to absorb energy or relief pressure.
ACKNOWLEDGMENT
The authors would like to thank All Cell Technologies LLC
for their contribution to make this work a success.
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