Hi,
I want to perform DMA test for injection molded dumbbell shaped CFPP specimens however, I am having some trouble to choose the right testing mode and specimen dimensions for this test.
According to the ASTM D5026 DMA test standard for Polymeric composite materials in Tension mode, the specimen size should be (76 X 13 X 01 mm, L:W:T) however, my specimen size is (76 X 10 X 3 mm). Interestingly, it is also mentioned in the ASTM standard file that "Rectangular test specimens of the other dimensions can also be used but should be clearly identified in the report section".
The trouble is that the DMA machine (TA Q800) available in our college recommends to test the polymeric sheet material having around 1mm thickness). If I follow the machine's standard, I have to rub my specimens on an emery paper machine to reduce its thickness to 1 mm, which would affect the specimen itself. I am confused what should I do? How and which specimen sized should I choose?
In addition, if I just want to find Tan delta, Storage, Loss Modulus value then on which criterion should I chose the testing mode i.e. Single-cantilever/DCB, Three-point bending/Tension etc. After all, all these tests can provide us the above mentioned values then which one should we choose?
Thank you.
Dear Ahmed Khalil
We worked on DMA of natural fibre-reinforced composites. We have used 3mm thickness sample for testing. Please refer our published paper for your kind perusal.
A study of filler content influence on dynamic mechanical and thermal
characteristics of coir and luffa cylindrica reinforced hybrid composites
Best wishes
PVR
The dimensional difference should not make huge difference if the material is isotropic, homogeneous polymer. However, your sample being a composite material, the edge effect cannot be ignored. This effect is particularly strong for anisotropic composite (your sample is relatively isotropic, though due to the flow pattern near the mold surface, you will have anisotropy, or skin effect). In a composite, it is said that the certain distance from the edge would have different stress field than in the bulk. So, you do not want to use too narrow a sample. Regarding the thickness, each rheological instruments have its characteristic rigidity. TA Instruments' machine is rather weak and if you have a very thick sample, the sample strength will influence the machine stability. This is not the case for the machine made by the predecessor of TA Instruments, Rheometrics. The machine, RMA-800 has an extremely rigid H-frame and even metallic sample can be evaluated with high reproducibility and thus thickness difference does not matter. Our sample for this RMA-800 machine is typically a few mm thick even with unidirectionally reinforced composites, much stronger than your short fiber reinforced composite.
The reason why you need to clearly indicate the dimension is that G' and G" are shape specific properties. Thus, if the dimension changes the value changes. On the other hand, tan delta is dimension independent as tan delta is calculated by G" devided by G', eliminating the dimension specific contribution from both quantities. This is the reason why tan delta spectrum exhibits better baseline and reproducibility.
The type of testing depends on what is the sample you want to use. If it is a homogeneous polymer, for example, any method is fine. Again, if it is a thin film, three point bending is not appropriate. A rigid slab can be used with any methods, whereas weak, flexible film is with tension. If you have a very strong composite, three point bending or single point cantilever may be appropriate.
Ahmed Khalil
As you said it is only "Recommended"... not the maximum permissible limit.. 1.5 mm thick samples can also be measured.. regarding mode of testing, cantilever method is best, but the choice of single or dual cantilever is dependent on the stiffness of your sample.
Use a torsion pendulum. With most other techniques the reinforcing fibers tend to mask resin and interface contributions.
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