Download Wear modeling and material testing of elastomers

Wear modeling and
material testing of
elastomers
G
lobal demand for material efficiency increases the need for
practical understanding of material wear behavior. Wear control and
process optimization are important steps
to sustainable development. Wear commonly limits the lifetime of components
and can create component failures. Consequently, the attempts to improve and
model the processes having wear challenges cause beneficial effects on resource efficiency. Improving component lifetime three times longer will decrease the
demand of raw material to one third
within the same period of time. The importance of rubber materials, for example the natural rubber, currently produced
mainly from hevea brasiliensis in South
East Asia is high. Elastomer material
products, such as material handling hoses, mill liners, screen panels and pump
liners are key components in increasing
material efficiency.
Wear modeling of elastomers is not
well known, because of the complicity on
the behavior of elastomers, e.g. rubbers
and modeling enables more thorough and
deeper understanding of wear phenomena. However, improved material solutions
and speeding up the product design cycle
can be guaranteed only by comprehending the underlying mechanisms. This
also decreases the experimental laboratory testing time and improves the accuracy of exploiting the results. Novel modeling methods can gradually be applied in
a modeling assisted material design process to couple material nano-microstructure and composition to performance of
a component. Moreover, multiscale material modelling is the only scientifically
sound way to predict the lifetime in materials that meet high wear, which for a
wear problem can contain finite and discrete element means enabling computational analysis of material behavior in
process environments. Nevertheless, modeling of elastomer behavior at that accuracy is a very demanding task. Modeling relies on experimental testing of
actual rubber material mechanical properties and failure and wear models on
material behaviour under impact type
surface attacks and erosive environments.
VTT has developed and offers solid experience and advanced computational
tools, VTT ProperTune™, to tackle even
Modeled single impact with several rubber
thickness on the substrate material.
the most complex material design challenges.
The constitutive model of the elastomer
material developed comprises of hyperelastic and viscoelastic contributions. The
material model is normally calibrated by
fitting using a single material point model
to experimental results. The experimental
results for hyperelasticity comprise of tensile, compression and shear tests at different strain rates. Viscoelasticity is determined from creep tests by utilizing the
determined creep curves and dynamic
mechanical testing. The tensile, compression and shear tests are conducted with
servohydraulic material testing machine.
Dynamic mechanical analysis can be carried on with the dynamical mechanical
analyser (DMA) most reliable in shear
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Centrifugal accelerator produces
dry erosion tests simultaneously
for 15 samples.
mode in order to obtain frequency dependent viscoelastic material parameters, such
as storage and loss modulus values, also in
a function of temperature.
The material can be subjected to several different kinds of conditions during its
service. Thus, more than one type of testing with various conditions and parameters is needed to assess the wear properties
of the materials. The model verification is
reasonable to conduct by several performance testing devices in laboratory conditions such as scratching and sliding
conditions, impacting and erosive condition as well as in severe abrasive conditions. Tampere Wear Center (TWC) at
the Department of Materials Science,
Tampere University of Technology has
rapidly developed into an internationally
recognized concentration of advanced
wear expertise and test facilities. Several
devices have been designed to simulate
actual wear conditions especially in the
field of heavy abrasive and impact wear.
Many of the methods are suitable also for
wear testing of rubber materials.
Experimental rubber surface single
scratch testing is used as model verification. Penetration depth dependence on
the applied force in scratch testing for
rubber materials simulates the hard par-
ticle influence in the industrial slurries
and effect on the rubber liner surfaces.
Scratch tester (CSM Micro Combi Tester) in VTT Technical Research Centre
Espoo is in constant environment in tribology laboratory (22 ± 1 °C, RH 50 ±
5 %). The parameters for scratch testing
can be for instance following: scratch
length 5 mm and linearly increasing load
from 100 mN to 1000 mN with loading
rate of 1800 mN/min during the experiments. The penetration depth is measured
during the scratch test and post scan can
be carried out for measuring the residual
depth.
High velocity particle impactor
(HVPI) at Tampere Wear Center conduct
High speed video frames of the HVPI tests for rubber.
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impact tests where the experimental parameters such as the shape and size of the
impacting particle and its velocity and
impact angle are strictly controlled. The
HVPI data is used to verify and calibrate
the numerical simulations based on the
high-speed video recording. The device is
capable of shooting 9 mm projectiles of
different shape and composition to a fixed
target at velocities ranging from 30 to
200 m/s.
Erosion testing at Tampere Wear Center using centrifugal accelerator produces
impacts with lower loads but higher volume. The amount of abrasive is up to
10 kg. However, the particle size of the
abrasive is smaller than 1 mm. The samp-
High-speed slurry-pot
type erosion tester is
suitable for testing of
metals, coatings,
polymers, and
elastomers.
le angles vary from 15° to 90° and all angles can be tested simultaneously.
High-speed slurry-pot type erosion
tester was developed at TWC for slurry
or dry erosion testing with high speeds
and various sizes of particles. In this pin
mill type tester, several samples are attached horizontally to a rotating central
shaft on various levels. The strong and
durable structure enables testing of 8 materials simultaneously also with large size
abrasives using rotating speeds up to 2000
rpm.
The crushing pin-on-disc test method
at TWC, which is based on the common
pin-on-disc principle, is used to determine the abrasion wear resistance with a
loose abrasive. The abrasive is placed between the rotating disc and the pin, which
is cyclically pressed against the abrasive.
The pin and the disc are not in direct contact at any point of the test. Thus, the abrasive is rolling and sliding, producing
demanding abrasion wear conditions.
Friction and Wear of Polymers project
(2009–2014) is the TEKES funded DEMAPP program of FIMECC Ltd that
concentrates on wear modeling and material testing of elastomers. The material
data for modeling was collected using
DMA, and tensile, compression and shear
tests at different strain rates. Scratch testing and high velocity particle impactor
tests were utilized for verification of the
models. Moreover, various wear tests, such
as dry erosion and slurry erosion tests,
increased understanding of the material
behavior and wear mechanisms of various
rubber materials under erosive environments. The general target of the project
was to develop a suitable and reliable modeling method for elastomer life time
estimation and extension.
Päivi Kivikytö-Reponen and
Anssi Laukkanen, VTT
Kati Valtonen and Marian Apostol,
Tampere Wear Center,
Department of Materials Science,
Tampere University of Technology
Heavy abrasion
wear tests are
produced by
crushing
pin-on-disc.
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