Dynamic mechanical analysis, or DMA, provides a means of evaluating
mechanical properties at a wide range of frequencies. DMA instrumentation,
however, only subjects the sample to a fixed quasi-static strain. Nanomechanical
characterization demands the provision of accurate information on
mechanical properties for small volumes of material—such as fibers and wires
only microns in diameter or films that are tens of nanometers in thickness.
Until recently, accurately determining dynamic
properties for specimens in this range has been
limited to fixed strain investigations—posing
significant technical challenges for improving
understanding of material behavior at the
nanoscale. The Continuous Dynamic Analysis
technique, developed by MTS NANO INSTRUMENTS,
offers a solution to these challenges.
As an optional component on NANO UTM™
universal testing systems, the Continuous Dynamic
Analysis™ (CDA) extension provides a simple
means of determining dynamic properties such
as storage and loss modulus. In conventional
universal testing systems, dynamic properties
are ascertained through fatigue regimes, where
an oscillation is applied by the crosshead and
subsequently measured by the flexures within the
load cell. The CDA extension, however, utilizes a
technique whereby a nanomechanical actuating
transducer, or NMAT, applies an oscillatory force
that is superimposed over the nominal force.
The amplitude of the oscillation is measured by
a capacitive sensor that is an integral part of the
NMAT. As a result, the CDA technique can be
used to impose oscillatory forces at a higher frequency
than what could be accomplished by imposing
the oscillation via the crosshead. The CDA
technique also presents the advantage of measuring
high frequency oscillatory specimen response.
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The phase-lock amplifier unit, shown above the electronics chassis, powers the CDA extension.
The CDA extension offers a direct and accurate
measurement of the specimen’s stiffness
at each point in the experiment, enabling mechanical
properties to be determined continuously
as the specimen is strained. By measuring
both the amplitude and phase relationships
between the load and displacement oscillations,
the CDA extension makes it possible
to determine storage and loss modulus.
The most significant difference between DMA
as a technique and the means by which the
CDA extension performs is inherent in how the
material is strained. In DMA testing, a small oscillation
is imposed on a material that has been
subjected to a fixed quasi-static strain. The
CDA extension, however, imposes a small oscillation
on a material that is being simultaneously
subjected to a variable quasi-static strain. The
CDA extension enables the user to gain access
to dynamic properties information continuously
through the force curve, providing a wealth
of information on the material’s response.
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