Engineering Failure Analysis 18 (2011) 1645–1651
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Engineering Failure Analysis
journal homepage: www.elsevier.com/locate/engfailanal
Useful lifetime prediction of rubber component
Chang Su Woo ⇑, Hyun Sung Park
Korea Institute Machinery & Materials, 171, Jang-Dong, Yuseong-Gu, Daejeon 305-343, Republic of Korea
a r t i c l e
i n f o
Article history:
Available online 25 January 2011
Keywords:
Rubber mount
Failure mechanism
Heat-aging
Compression set
Arrhenius plot
a b s t r a c t
Rubber material properties and lifetime evaluation are very important in design procedure
to assure the safety and reliability of the rubber components. This paper discusses the failure mechanism and material tests were carried out to predict the useful lifetime of NBR
and EPDM for compression motor, which is used in refrigerator component. The heat-aging
process leads not only to mechanical properties change but also to chemical structure
change so called degradation. In order to investigate the heat-aging effects on the material
properties, the accelerated test were carried out. The stress–strain curves were plotted
from the results of the tensile test for virgin and heat-aged rubber specimens. The rubber
specimens were heat-aged in an oven at the temperature ranging from 70 °C to 100 °C for a
period from 1 to 180 days. Compression set results changes as the threshold are used for
assessment of the useful life and time to threshold value were plotted against reciprocal
of absolute temperature to give the Arrhenius plot. By using the compression set test, several useful life prediction equations for rubber material were proposed.
Significance: Useful lifetime estimation procedure employed in this study could be used
approximately for the design of the rubber components at the early design stage.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Rubber materials are widely used in many applications such as vibration isolators, damping and energy absorption due to
their compressibility, elongation, hyper-elastic, recoverability from deformation and other characteristics [1]. The interest of
the fatigue life for rubber components was increasing according to the extension of warranty period. A design of rubber components against fatigue failure is one of the critical issues to prevent the failures during the operation. Therefore, useful life
prediction and evaluation are the key technologies to assure the safety and reliability of mechanical rubber components [2].
When rubber is used for a long period of time, rubber becomes aged it usually becomes hardened and loses its damping
capability. This aging process results mainly from heat due to hysteric loss, and is affects not only the material property
but also the useful lifetime of rubber [3,4].
In this paper, the heat-aging effects on the material properties and useful lifetime prediction of rubber materials for compression motor, which is used in refrigerator components as shown Fig. 1 was experimentally investigated. In order to investigate the heat-aging effects on the material properties, the stress–strain curves were obtained from the results of tensile
test. The rubber specimens were heat-aged in an oven at the temperature ranging from 70 °C to 100 °C for a period ranging
from 1 day to 180 days. Compression set results changes as the threshold are used for assessment of the useful life and time
Abbreviations: EPDM, Ethylene–propylene diene M-class rubber; NBR, Acrylonitrile–butadiene rubber; UTM, Universal Testing Machine.
⇑ Corresponding author. Tel.: +82 1047267065; fax: +82 428687884.
E-mail address: cswoo@kimm.re.kr (C.S. Woo).
1350-6307/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.engfailanal.2011.01.003
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C.S. Woo, H.S. Park / Engineering Failure Analysis 18 (2011) 1645–1651
Notation
CS
d0
d1
d2
K
K(T)
A, B
E
R
T
compression set
thickness of the specimen
thickness in the compressed state
thickness after removal of the load
crosslink density change
reaction rate
constants
activation energy
Boltzmann constant
absolute temperature
to threshold value were plotted against reciprocal of absolute temperature to give the Arrhenius plot. By using the compression set test, several useful lifetime prediction equations for rubber material were proposed.
2. Failure mechanism of rubber mount
During its lifetime a rubber can be submitted to different types of degradation from different exposure conditions. The
failure mechanism which produce degradation depend not only on the degradation agents present but also on the polymer
and additives. It must be noted that aging in the broad sense is often the consequence of several factors in combination. The
contribution of each factor is often complicated to evaluate but their classification is simply stated as shown Table 1.
One of the important characteristics of rubber mount for compression motor, which is used in refrigerator component is
their lifetime, they should be maintained enough mechanical and chemical properties for actual service conditions until their
lifetime. However, it is difficult to estimate the lifetime, since their properties are sensitive to the manufacturing parameters
and are also affected by typical use conditions including temperature, moisture, UV light, ozone, chemical attack and applied
load type and so on [5]. It is necessary to get specimens having uniform properties and to find real service environments to
predict the lifetime of rubber mount. Significant improvements in the reliability of electronics components can be achieved
by retrieving fielded samples and examining the extent of degradation which have been used for several years to determine
the dominant failure sites, modes, and mechanisms. Root cause failure analysis provided clues as to the elements of the design that need to modify for improved quality and long-term reliability [6].
3. Mechanical test for rubber materials
3.1. Specimen and test conditions
The materials of rubber mount for refrigerator compression motor were a NBR and EPDM. Rubber can be considered as a
hyper-elastic material showing highly elastic isotropic behavior with incompressibility. Hyper-elastic properties of rubber
are characterized by the strain energy functions, which can be determined by the experimental stress–strain relationship
[7]. The basic tests for strain states, namely, hardness and simple tension and equi-biaxial tension test were performed to
get the constitutive relation of the NBR and EPDM. The hardness of the specimen was measured using the International Rubber Hardness Degree(IRHD). Simple tension and equi-biaxial tension test was loaded by a UTM at a speed of 100 mm/min,
and the deflection was measured using a laser extensometer in Fig. 2. In the case of thermal aging, the only way in which
Fig. 1. Rubber mount for refrigerator component.
C.S. Woo, H.S. Park / Engineering Failure Analysis 18 (2011) 1645–1651
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Table 1
Aging influence factors in rubber mount.
Factors
Type of aging
Temperature
UV light
Ionising radiation
Humidity
Fluids (gas, organic, vapours)
Mechanical loading (stress,
pressure)
Thermo-oxidation, additive migration
Photo-oxidation
Radio-oxidation, cross-linking
Hydrolysis
Chemical degradation, swelling
Fatigue, creep, stress relaxation, compression
set
Fig. 2. Mechanical properties test of rubber materials.
acceleration can sensibly be applied is by raising the temperature. Rubber specimens were heat-aged in an oven at the temperature ranging from 70 °C to 100 °C for a period from 1 day to 180 days.
3.2. Result and discussion
Modification and reformation of rubber network structures in the initial loading stages can show a lower stiffness and
changes in damping characteristics. The loading curve and unloading curve are not coincident. Strain induced stress softening in rubber material called the Mullin’s effect. Fig. 3 are shown the results of the tensile strength and Mullin’s effect for NBR
and EPDM. When the stress–strain curve no longer changes significantly, the material may be considered to be stable for
strain levels. The stress–strain curve stabilized after between 3 and 5 repetitions.
After heat aging, physical and chemical properties of rubber are likely to have changed. The monitoring of those changes
requires the choice of suitable mechanical test such as tensile stress and strain measurement. Figs. 4 and 5 are shown stress–
Fig. 3. Mechanical test results and Mullin’s effect of NBR and EPDM.
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C.S. Woo, H.S. Park / Engineering Failure Analysis 18 (2011) 1645–1651
Fig. 4. Stress–strain curve for NBR.
Fig. 5. Stress–strain curve for EPDM.
strain curve of NBR and EPDM for heat-aging conditions. We know that the stiffness increases with increasing aging temperature and days. Also, the properties change for NBR was higher more than EPDM. Indicate in Figs. 4 and 5 that the indexes
mean the number of exposition days.
4. Lifetime prediction of rubber mount
4.1. Arrhenius model
The reaction rate of a chemical reaction normally increases with increasing temperature. By exposing test pieces to a series of elevated temperatures and measuring property change, the relation between the reaction rate of degradation mechanisms and temperature can, in principle, be deduced. Estimates can then be made by extrapolation of the degree of
degradation after a given time at a given temperature [8,9].
The Arrhenius relation is the best known and most widely used of the two applications to the permanent effects of temperature. The relation rate and temperature relationship can often be represented by the Arrhenius equation:
E
KðTÞ ¼ A e RT
LogðKðTÞÞ ¼ B E
RT
ð1Þ
where K(T) is the reaction rate, A and B are constants, E is the activation energy, R is the Boltzmann constant and T is the
absolute temperature. The reaction rate at any temperature is obtained from the change in the selected property with exposure time at that temperature. Although the Arrhenius relation is generally the first choice to apply to the effect of temperature, no general rule can be given for the measure of the reaction rate to be used with it. In the example shown in Fig. 6(a)
the property parameter has been plotted against time at four temperatures, and the reaction rate taken as the time for the
property to reach a given threshold value or end of life criterion. The log of the chosen measure of reaction rate is plotted
against the reciprocal of absolute temperature, which should result in straight lines as illustrated in the Arrhenius plot in
Fig. 6(b).
C.S. Woo, H.S. Park / Engineering Failure Analysis 18 (2011) 1645–1651
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Fig. 6. Arrhenius model for lifetime prediction.
4.2. Lifetime prediction
In order to lifetime prediction, we carried out the compression set with heat-aged in an oven at the temperature ranging
from 70 °C to 100 °C for a period ranging from 1 day to 180 days. The compression set was determined according to ISO 815.
To carry out this test a simple compressive force is applied to rubber mount, usually to a fixed degree of strain. Compression
set is calculated from:
Compression setð%Þ ¼
d0 d2
100
d0 d1
ð2Þ
Fig. 7. Change of properties for heat-aging test.
Table 2
Arrhenius equations for rubber mount.
Material
Degradation
Arrhenius equation
NBR
10%
15%
20%
10%
15%
20%
ln (t) = 11.21 + 5351/(273 + T)
ln (t) = 10.98 + 5910/(273 + T)
ln (t) = 9.17 + 6213/(273 + T)
ln (t) = 18.73 + 8221/(273 + T)
ln (t) = 18.62 + 8921/(273 + T)
ln (t) = 14.35 + 8474/(273 + T)
EPDM
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C.S. Woo, H.S. Park / Engineering Failure Analysis 18 (2011) 1645–1651
Fig. 8. Arrhenius plot of NBR.
Fig. 9. Arrhenius plot of EPDM.
Table 3
Lifetime prediction on each temperature.
Temperature (°C)
15
20
25
30
NBR, lifetime (h)
EPDM, lifetime (h)
10%
15%
20%
10%
15%
20%
1587
1156
850
632
13,916
9805
6989
5039
243,510
168,514
118,065
83,696
18,306
11,248
7023
4447
232,117
136,899
82,125
50,111
3518,730
2129,723
1310,925
819,948
where d0 is the original thickness of the specimen, d1 is the thickness in the compressed state and d2 is the thickness after
removal of the load. Not surprisingly, in all cases compression set increased with time of exposure and with increasing temperature. Compression set results presented graphically in Fig. 7. Figures illustrate how the rate of change with time will vary
for different materials and for different temperatures. Compression set results changes as the threshold are used for assessment of the useful life and time to threshold value (10%, 15%, 20%) were plotted against reciprocal of absolute temperature to
give the Arrhenius plot. By using the compression set test, several useful life prediction equations for rubber material were
proposed as shown Table 2 and Figs. 8 and 9. Table 3 are shown on the useful lifetime for NBR and EPDM. According to Arrhenius equation, useful lifetime of EPDM was longer more than NBR.
5. Conclusion
The material properties and fatigue life evaluation of rubber materials are very important in design procedure to assure
the safety and reliability of the rubber components. In order to investigate the heat-aging effects on the material properties,
the stress–strain curves were obtained from the results of tensile test. The aged specimens were heat-aged in an oven set at a
temperature ranging from 70 °C to 100 °C for a period ranging from 1 to 180 days.
We know that the material properties were a function of heat-aged period as well as temperatures. The stiffness increases
with increasing aging temperature and days. Also, properties change for NBR was higher more than EPDM. By using the compression set test, several useful life prediction equations for rubber material were proposed. Useful life estimation procedure
employed in this study could be used approximately for the design of the rubber components at the early design stage.
C.S. Woo, H.S. Park / Engineering Failure Analysis 18 (2011) 1645–1651
Acknowledgement
This research has been supported by Ministry of Knowledge Economy, Korea.
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