Best Practices for Testing Thin Sheets on the DMA 850

The DMA 850 tension film clamp (two-screw), shown in Figure 1B, is designed for thin films up to 2 mm thick, allowing for the determination of elastic modulus (E’), also referred to as Young’s Modulus or the storage modulus. E’ quantifies a material’s resistance to elastic (reversible) deformation under load. It is a measure of the stiffness of a material in tension [6, 7]. The sample is held in tension, with a small preload to prevent wrinkles or folding, between a fixed upper clamp and a movable lower clamp.

The differences between cantilever and tension clamps in measuring modulus primarily revolve around the type of deformation they apply and the materials they are best suited for. Cantilever clamps measure modulus by applying a bending force to the sample. This setup is ideal for materials that can maintain their shape under bending stress, providing insights into their flexural properties. In contrast, tension clamps measure modulus by applying a tensile force, stretching the sample along its length. This method is particularly suitable for thin films, fibers, and other elongated materials where tensile properties are critical. Tension clamps ensure uniform clamping pressure, reducing the risk of sample slippage and providing accurate tensile property measurements.

The dual surface film tension clamp (single screw), shown in Figure 1C, features a second clamp face with two thin, ridged surfaces at the top and bottom. This ridged clamp face is specifically designed for situations where the thin film under-study slips out of the clamp during testing.

It is crucial to note that the choice of clamps significantly impacts measurement accuracy. Clamps must be properly calibrated before each experiment, with specific calibrations required for different clamps, including mass and position. Ensuring successful calibration prior to the experimental procedure is essential for the reliability of the data. Additionally, the dimensions of the sample must be accurately measured before the experiment and clamped with appropriate tightening [1, 8].

Strain amplitude sweep tests were conducted using a two screw tension film clamp on samples from the electrode sheet to determine E’.

Figure 3 demonstrates that the E’ of the cathode sheet measured ranges between 2−3×10⁹ Pa. This discrepancy may stem from the combined influence of the coating of the sheet or the natural variability of the DMA measurements.

A comparative test was conducted to evaluate E’ of uncoated versus coated aluminum foil. Due to the small thickness of 0.02 mm and the smooth surface of the aluminum foil, a dual-face single screw tension clamp was used to prevent slippage during testing (see Figure 1C). It is important to be aware of both types of tension clamps TA Instruments offers and choose the proper one for the process under-study. Any form of slippage during the test will significantly affect the data.

The results in Figure 4 show that the elastic modulus of the coated aluminum foil was lower than that of the uncoated aluminum foil. This reduction is attributed to the coating, which increases the film’s thickness fourfold with a material possessing lower stiffness than aluminum. Consequently, the overall stiffness or elastic modulus of the coated sheet is diminished. These findings underscore the necessity of considering both dimensional and material properties when assessing the mechanical performance of composite-like structures.

Best practices for testing in bending mode

For bending mode, a 3-point bending clamp is unsuitable for thin sheets. Due to the low stiffness of the sheet, a 10 mm single cantilever was selected to enhance stiffness and achieve more reliable results. Available sizes for single cantilever clamps are 4 mm, 10 mm, and 17.5 mm.

For DMA measurements, the machine stiffness range is fixed at 10² – 10⁷ N/m. If the sample’s stiffness is insufficient, the sample length can be reduced or the thickness increased. The film thickness should not exceed 5 mm, and the width of the rectangular film should be between 5 and 15 mm.

When using the single cantilever clamp, the sample length is defined by the distance between the fixed and movable clamps. Due to the flexibility of the drive shaft, the actual sample length may slightly differ from the default values after tightening. It is crucial to ensure the sample is clamped in an almost perfectly horizontal position. To achieve this, first mount the sample on the movable clamp, then stretch it carefully. Once it is perfectly horizontal, lock the shaft and mount the sample on the fixed clamp.

To achieve the best results with either single or dual cantilever clamps, TA Instruments recommends that the length-to-thickness ratio of the sample be greater than or equal to ten.

Pre-load is not required for the cantilever clamp, and maintaining a constant frequency throughout the study remains important.

In bending, the storage modulus is referred to as the flexural modulus and measures a material’s resistance to bending. It represents the ratio of stress to strain during bending. Considering that tensile force results in uniform stress distribution, while bending force creates a gradient of stress across the sample’s cross-section, flexural modulus can differ from elastic modulus, particularly in composite-like materials [7].The flexural modulus is often higher than the tensile modulus for the same material because the bending test involves both tensile and compressive stresses, making the material appear stiffer [7, 9].

Figure 5 shows that the measured flexural modulus is an order of magnitude higher than the obtained elastic modulus. Additionally, the results indicate slight discrepancy, as previously observed through measurements using a two-screw film tension clamp [4, 10, 3].

Figure 6 illustrates that clamping the sheets in different directions yielded different results. When the sheets were clamped with the coating side up, higher flexural modulus values were obtained compared to clamping with the aluminum side up in the cantilever clamp. This difference stems from the anisotropy of these samples. When testing materials with a composite-like structure in bending, the influence of surface layers—specifically, which layer is outer or inner—is significant [2, 10, 9], thus its influence on measured modulus values should be noted.