18-02-2013, 09:55 AM
Universal testing machine for mechanical properties of thin materials
Universal testing.pdf (Size: 2.69 MB / Downloads: 50)
ABSTRACT
In this work, the design, construction, calibration and compliance measurement of a universal testing machine for tension tests of materials in
film geometry are presented. A commercial load cell of 220 N and sensitivity of 1.2345 mV/V is used to measure the applied load. Material
strain is measured by movement of the crosshead displacement of the machine with a digital indicator with 0.001 mm resolution and 25 mm
maximum displacement, connected to a PC through an interface. Mechanical strain is achieved by an electric high precision stepper motor
capable to obtain displacement velocities as low as 0.001 mm/s. The stress-strain data acquired with a GPIB interface are saved as a file
with a home-made program developed in LabView 7.0. Measurements of the elastic modulus and yield point of a commercial polymer film
(500 HN Kapton) were used to validate the performance of the testing machine. The obtained mechanical properties are in good agreement
with the mean values reported by the supplier and with the values obtained from a commercial machine, taking into account the limitations
of thin film testing and experimental conditions.
Introduction
New methodologies to measure the physical properties of
thin films are currently required. Particularly, reported mechanical
properties of materials at these dimensions are currently
controversial in the scientific literature. Thus, it is
necessary to propose techniques for determining mechanical
properties of thin films, such as elastic modulus, Poison’s ratio
and strength. Properties of materials at micro and nanoscale
are of considerable interest because of the unique properties
associated with small volumes. These unique properties
are increasing the importance of thin films and nanostructured
materials used in several technological applications.
Design and construction
The testing machine was designed to determine the stressstrain
curves of thin materials such as polymers and particularly
metallic films deposited onto polymeric substrates.
Figure 1 shows a 3D illustration of the designed device
with 15 cm wide, 55 cm length and 45 cm high as total dimensions.
The equipment is capable of analyzing samples up
to 10 cm of length. The mechanical design minimizes effects
of load introduction in the main frame, drive screws, and the
relative movement between the movable crosshead and the
drive screws.
Calibration and data acquisition
The calibration of the load cell was conducted by collecting
data of different known applied loads (weights) and measuring
its corresponding output voltage.
Calibration measurements were conducted by steps over
a range of 0 to 12 N with an elapsed-time of 1 min between
each calibrated load in order to avoid hysteretic effects; i.e,
the voltage returns to cero value after removing the load. A
series of calibrated loads were applied in increasing order.
The output voltage of the load cell was captured through
a high-resolution programmable voltmeter HP 3458A. Figure
3 shows the obtained linear behavior between the applied
load (P) and the output voltage (V) as obtained from
the load cell.
Machine compliance determination
In mechanical testing of materials, when a strain gage or an
in-situ element cannot be used to measure the real material
strain, it is customary to use the machine crosshead displacement
to measure the applied strain. Measurements conducted
by crosshead displacement need to be calibrated by taking
into account the machine compliance Cm. In order to calibrate
the machine compliance (Cm=1/km = ±/P, where km
is the stiffness constant, ± the crosshead displacement, and
P the applied load)
Results and validation
A commercial polymer, 500 HN Kapton, was initially used
as a benchmarking specimen by its well-known properties
reported by the DuPont Co. supplier [23]. Tensile tests
were conducted using rectangular geometries of Kapton films
of 40 mm length, 5 mm wide and 0.125 mm thick. The gage
length of the samples was always 20 mm. The displacement
velocity of the movable crosshead during tensile experiments
was maintained at 0.01 mm/s in all cases.
Figure 6 shows a plot of the displacement of the movable
crosshead vs. time, where high stability can be observed
when it moves along the drive screw with the sample gripped.
From Fig. 6, a constant behavior of the velocity and very low
mechanical noise during the crosshead displacement is evident.
Therefore, the movement of the movable crosshead
does not have additional effects, such as vibrations or speed
changes that could affect the tensile tests.
Figure 7 shows a group of six superimposed stress-strain
curves as obtained from different samples of the Kapton foil.
The six curves are difficult to visualize given their high reproducibility.
From this figure, it can be observed that Kapton
has a linear elastic behavior below 1.8% strain. The total
strain applied to the samples was always 24%, which is far
from the ultimate strain reported by the supplier (72%). This
high strain can not be achieved by our machine giving the
mechanical limits of the device.
Conclusions
The design, construction, calibration and compliance measurement
of a universal testing machine for tensile tests on
thin and soft materials were discussed. The design has the
capability to obtain displacement as small as 0.001 mm and
maximum loads of 220 N. The estimated compliance machine
value was 0.16 ¹m/N as measured with a stiff material.
The mechanical properties of a 500HN Kapton polymer
film were measured and compared with the mean values reported
by the supplier as well as independent testing using a
commercial testing machine. The corrected average elastic
modulus and the yield point of Kapton film determined with
our home-made testing machine were 2.7 GPa, and 61.0 MPa.
This elastic modulus value was corrected in about 1.1% for
the case of Kapton, and represents a slight difference as compared
with the mean values reported by the supplier; and is
also in reasonable agreement with the values obtained with
the Shimadzu machine.