Small strain high frequency behaviour of rubber. (DMA & other techniques)

A discussion meeting organised by the Rubber in Engineering Committee
of the Institute of Materials, Minerals & Mining

Event: Afternoon Technical Discussion Meeting
Date: Friday 5th October 2007
Start time: 2pm
Venue: IOM3, 1 Carlton House Terrace, LONDON.
Location: map and directions

Programme

Testing of low amplitude dynamic properties of damping materials in the audio frequency range for a range of temperatures.

By: N. Ahmad, D. Thompson, C. Jones (all Southampton University) and A.Muhr (TARRC)

Abstract: Dynamic testing of elastomeric materials is usually carried out at relatively low frequency and high strain. For damping applications in the audio frequency range, the relevant parameters are the stiffness and damping loss factor at small amplitudes and over a wide range of frequencies. It can also be important to be able to quantify the temperature dependence.
A simple test rig has been developed which can be used to measure the complex dynamic shear modulus. Excitation is provided by a simple coil and magnet arrangement. The input vibration is measured using an accelerometer and the transmitted force is measured by a force gauge mounted on a seismic mass. The target frequency range is 100 to 3000 Hz. The rig is quite compact and can readily be placed in a temperature cabinet to study the effects of temperature (a range of -20°C to 40°C has been used). However, initial experience showed that the measured results were of variable quality, particularly for low temperatures where the material is quite stiff but also for higher temperatures where the material is softer.
A simple analytical model has been produced to explain the dynamic effects of the rig itself. As a result of this the rig has been modified to increase the size of the seismic mass used and to isolate the seismic mass from the excitation. It has also been found that, particularly for softer materials (or higher temperatures), it is necessary to use thinner samples than initially proposed in order to avoid internal resonances in the samples.

Errors in dynamic mechanical testing with specific reference to rubbers and high frequency measurements

By: John C Duncan (Triton Technology Ltd)

Abstract: As in all analyses errors arise from incorrect procedures and inappropriate use of the instrument. Dynamic mechanical analysers are relatively complex instruments and manufacturers’ attempts to simplify their operation frequently result in many of their deeper features being hidden. For simple glass transition determinations at a single frequency of 1 Hz, this is of little consequence. However as we push the envelope and attempt to measure low stiffness samples at high frequencies, various corrections within the measurement system start to become far more significant. The following parameters and how they are treated will be covered in this presentation: Phase measurement errors; Instrument Compliance; Inertia of system; Stiffness range of DMAs and Clamping errors.
Results will be shown, where the magnitude of the above corrections will be compared with the sample data. Techniques to perform the best experiment for more demanding conditions will be discussed.

Determination of high frequency properties determination.

By: David Boast (Avon Rubber)

Abstract: The discussion will include a comparison of measurement of stiffness by three different methods: Small loops of a large sample in a tensile machine, DMTA (Polymer labs type) and a Resonance sample in shaker (1 DOF). The particular material was silicone and had a small frequency dependence. The talk will also consider general problems of testing rubber at high frequencies, such as the problems of inertial loading of samples with even small accelerometers, and the use of close microphones. Effects such as Mullins can also have a surprisingly large effect on acoustic performance of certain products and some of these issues will also be discussed.

These talks are all followed by discussion and tea will be served at 4pm.

All times are approximate. Programme is subject to change.

Attendance at the meeting is free of charge for members of the IOM3, however a small nominal charge (less than £20) may be made on the day to non members. To attend you must first reserve your place by contacting Dave Boast (see below) giving your contact details (company, telephone number, address and email).

Dave Boast
Avon Rubber

E-mail: dave.boast@avon-rubber.com