Hello everyone,

I am currently performing some experiments on thin (0.5 - 1mm thickness) elastomer layers reinforced with aramid fabric to identify its mechanical properties under equi-biaxial loading.

For this, I am subjecting a clamped specimen to pressure on one side and measuring the 3D deformation using a Digital Image Correlation device as shown below.

My goal is to fit the biaxial stress-strain behaviour using constitutive hyperlastic material. I am measuring the stretch ratio of the specimen and calculating the stress at the pole as follows :

where

p: internal pressure,

R : Curvature radius during the expansion,

λ : Stretch Ratio (measured by laying a line on the specimen and measuring the stretch at each pressure step),

t = thickness

My questions :

Am I calculating the strain correctly in order to fit my hyperelastic model?

My interpretation of R in the stress formula above is the Radius of the sphere that basically forms at each pressure step as shown in the image below. This radius essentially decreases as the pressure increases. So the stress is actually reducing in my experiment since R decreases with increase in pressure. Is my understanding correct?

Looking forward to an interesting discussion on this topic. Thank you all.