The physical testing of plastic materials for the purpose of defining material constitutive models in finite element analysis can be very simple or incredibly complex depending on the objective of the analysis. Linear analysis of structural parts is routinely performed using only a couple simple parameters. More complex analysis may involve elevated temperatures, severe plastic deformation and strain rate sensitivity requiring customized material model development and rigorous experimentation.
There are a few experiments that are particularly useful in characterizing plastics for analysis. They are simple tension with transverse strain measurement for elasticity and Poisson's ratio determination, simple load and unload experiment to separate the plastic and elastic strain components and thermal expansion as a function of temperature. Several of these experiments are outlined below.
Typical Plastic Experiments:
The tensile test is performed by straining a plastic specimen in one direction such that the sides of the specimen are free to contract. The region of interest is the narrowed section where the desired state of strain is achieved. Basic parameters derived from a tensile stress-strain curve are the initial material stiffness (Young's Modulus), the material yielding point and the failure stress and failure strain.
Transverse strain is sometimes measured in the modulus region in combination with axial strain such that the ratio of transverse strain to axial strain may be determined. This slope is the Poisson’s ratio and is a measure of material compressibility. Like low strain axial measurements, low strain transverse measurements are typically made with a clip-on strain gage style extensometer. Full field strains may also be measured using cameras and Digital Image Correlation methods.
Plastic deformation may appear at very small strain values. A more accurate way to determine the yield point is by unloading the specimen. By unloading the specimen from a specific total strain, we can observe plastic strain directly by removing the elastic contribution during unloading to near zero stress. We can then load to the specimen to increasingly higher total strain levels and unload at each to observe the increase in the plastic strain contribution.
As a stress is applied to plastic, the material will strain. If the stress is held constant, the plastic will continue to strain. This behavior is creep or viscous behavior. Combined with elasticity, we have viscoelastic behavior. At small resulting strains over relatively short times, the release of the stress on the material will result in the material returning to its original shape. At larger strains or longer times, release of the stress will likely reveal a permanently deformed plastic.
The shear state of strain can be an important addition to the fitting of a multi-axial material model. Shear tests for plastics include various ‘notch’ based experiments including the Arcan, Isopescu and Rail specimen style. The shear experiment can provide meaningful data across a wide range of material stiffnesses and a broad strain range. The region where the pure shear state occurs is in the center region. A 3D digital image correlation system is used to measure the strain in this region.
Testing plastic film requires low force load cells and low mass gripping systems. Since contact with the material will alter the measurement, non-contacting laser or image based strain systems are employed.
The compression test is a challenging experiment in that tall specimens can create an unstable loading condition and short specimens create a constrained loading condition. In any case, direct measurement of platen displacement is required for accurate strain measurement in a short specimen condition and surface strain measuring such as digital image correlation is required for in-plane compression.
Material testing experiments on plastic materials often require test specimens to be cut from actual parts or plaques. The shop at Axel is able to cut and machine specimens in various ISO, ASTM and custom shapes based on the application.