ASTM D638, Type 5 specimen cut from a polyamide sheettortuetec GmbH

Transient Water Content Simulation in Polyamide (PA)

The strength and toughness of components made of polyamide is humidity dependent. Additionally, humidity changes cause dimensional changes and warping. So, a good understanding of a component’s water content is sometimes necessary.

In our laboratory we condition and test polyamide specimens. Accordingly, we created a FE model, which takes thickness, temperature and transient water content into account. By solving the diffusion equation, the transient (time dependent) water content of each point through the thickness was computed. Then the volume-weighted mean water content was derived. This value was directly compared to measured changes of specimen weight.

Test and FE model together serve some purposes:

  • material model parameter calibration (swelling)
  • model validation
  • for developing and checking conditioning guidelines
Graph, showing time (hours) on the x-axis and water content (weight %) on the y-axis. The green curve shows the increase of water content in the specimen center. The Blue curve shows the water content at the specimen surface, which is assumed to be in equilibrium to the surrounding medium. The Red curve shows the weight averaged water content of the specimen as a whole.
Result of a transient water content simulation inside a polyamide test specimen. It is assumed, that the specimen is initially dry. The surface is immediately in equilibrium with the surrounding medium (blue curve). Then the specimen core takes up humidity as given by the green curve. The volume averaged curve (red) can be used to compare the simulation with specimen weight measurements.
Yellowish bend specimen, made from polyamide after conditioning. It lays on a green sheet with a grid indicating 10 mm distances. The points indicate 2 mm distances.
Bend Specimen made of polyamide after conditioning. The specimen was conditioned to standard conditions a saline solution and a heating cabinet. The yellowish color is caused by the salts.