In the 3-point bending mode, an offset force has to be specified in the method in addition to the maximum displacement and force amplitudes. In this article, we describe a possible procedure for determining the offset force and the other measurement parameters. We also estimate the uncertainty of the resulting 3-point bending modulus values.
Ceramics, metals and composite materials are usually measured in DMA in the 3-point bending mode (see Figure 1). In this mode, the sample is held in position in the clamping assembly by an offset force (preload force) applied to the sample. The offset force must be greater than the amplitude of the force applied to the sample otherwise the sample will lose contact with the clamping assembly. In 3-point bending measurements, the offset force is usually provided as a “constant current offset force” (details are given in the next section).
In this article, we explain how measurement parameters can be determined for 3-point bending experiments. We also estimate the uncertainty of the resulting 3-point bending modulus values.
In the Constant Current Offset control mode, the drive motor generates a constant offset force, Fcc, in addition to the dynamic force with amplitude FA. The force generated by the drive motor is applied to the sample via a membrane. The membrane and the sample (with the sample holder and the drive shaft) are connected in parallel. Part of Fcc is therefore used to deform the membrane. If the stiffness of the sample is more than twice the stiffness of the membrane, then practically the entire offset force, Fcc, is applied to the sample. The stiffness of the membrane is determined during the mechanical adjustment of the instrument and is typically about 6 N/mm for the 40-N drive motor and about 3 N/mm for the 12-N or 18-N drive motors. This means that from a sample stiffness of about 12 N/mm or more, the entire offset force is applied to the sample.
If the sample stiffness is about the same as that of the drive motor membrane, the offset force actually exerted on the sample is reduced by half. With softer samples, it is correspondingly smaller. To a certain extent, this effect can be made use of when the sample softens during a glass transition. The offset force actually applied decreases and the predeformation of the sample is reduced making it possible to measure softer samples.
The “Force Offset” measurement curve that can be displayed in the Evaluation Window is the static offset force actually exerted on the sample. The constant current offset mode is mainly used for experiments in 3-point bending. If the samples are sufficiently stiff (above 12 N/mm), the force specified in the method corresponds to the offset force acting on the sample.
The force amplitude is not influenced by the membrane because if necessary it is controlled by the force sensor.
The sample stiffness, Ss, is given by
Ss = E/G (1)
where E is the modulus of the sample and g the geometry factor. For a rectangular bar, the geometry factor in 3-point bending is given by
g= 13/4*b*h3
Here l, b and h are the dimensions of the sample (l = clamping length, b = width, h = thickness).
In the Autooffset control mode, the offset force just sufficient to hold the sample in place during the measurement is first determined (autooffset = 100%; this corresponds to the condition Fs = FA (see Figure 1)).
Tips on Method Development for DMA Measurements in 3-Point Bending | Thermal Analysis Application No. UC 283 | Application published in METTLER TOLEDO Thermal Analysis UserCom 28