Residual Strain Measurements in Plastics ASTM D4093
Some transparent and translucent plastics display birefringence under stress. This means they can split an incident, polarized beam of light into two phase-differentiated polarized rays. The optical path difference between the two emerging rays is termed retardation. The retardation values and stress and strain distributions can be determined by the ASTM 4093 test.
Residual strains in some transparent and translucent plastics can be analysed using photoelastic measurement of birefringence as specified in standard method ASTM D4093. This optical method of analysis of stresses and strains is useful for three-dimensional plastic components manufactured through moulding, casting, extrusion, and welding. Some common causes of harmful residual stress are differential cooling rates, non-uniform temperature, material contaminants, fabrication stresses, mechanical and thermal cycling, improper annealing and sharp edges.
A polarized, incident ray of light passing through a stressed region of the transparent plastic is refracted into two polarized rays, travelling at different velocities along different paths, influenced by stress values. The optical path difference between the two refracted rays is termed retardation. Birefringence is calculated by dividing the retardation value by the thickness of the polymer sample. Birefringence is influenced by relative alignments of the polymer macromolecules (caused by stress) and anisotropy in the material. The interference patterns of the two phase differentiated waves are studied in a photoelastic polariscope.
The ASTM D4093 test yields valuable insights that can be used to improve the manufacturing process and product design.
The test apparatus includes various components like light source, polariser, a loading frame to hold and apply stress on the component and the photoelastic polariscope instrument with compensator. The polarized incident light from the source traverses the transparent specimen. Due to the stress applied by the loading frame, the incident ray will immediately split into two components in the directions of principal stresses moving with different velocities. This will create a phase difference between the emerging rays. Generally, the emerging light will be elliptically polarised. Interference between the two rays provides the stress patterns which are then analysed by numerical techniques. The measured value of retardation can then be used as a gauge of the stress value or can be converted into units of birefringence or stress (MPa or psi) by means of a simple equation.
The specimen size depends on the field of view of the polariscope.
The retardation is reported in nanometres (nm) and the stress values in Megapascals (MPa). Visual patterns of stress and strain distributions can be obtained.