Issues with SLS Model for Rubber and Foam Compression Tests

[joshwarner]

TL;DR Summary: I'm comparing the viscoelastic properties of rubber and foam using compression tests. The SLS model predicts exponentially decaying stress for linear displacement, but my results show exponentially increasing stress, especially for rubber. How can I reconcile this?

Hello everyone,

I'm currently working on comparing the viscoelastic properties of different materials, specifically rubber and foam, using results from compression tests performed under displacement control. My initial aim was to compare the energy dissipation characteristics of these materials using hysteresis cycles. However, I’ve encountered a significant discrepancy between my experimental results and the theoretical predictions provided by standard linear solid (SLS) models.

According to the SLS model, the stress response to a linearly increasing displacement should decay exponentially. However, in my experiments, particularly with rubber, I observe that the stress increases approximately exponentially with time under a linear displacement. This behavior is contrary to the SLS model's predictions and poses a challenge for my comparative analysis.

Here are my specific questions:

1. How can I reconcile the observed stress response with existing viscoelastic models, given that the SLS model does not seem to fit my experimental data?
2. Are there alternative viscoelastic models or modifications to the SLS model that could better represent the exponential increase in stress observed in materials like rubber under linear displacement?
3. What methods or approaches can I use to accurately compare the energy dissipation (e.g., from hysteresis cycles) between rubber and foam, considering these discrepancies in model predictions?

Any insights or suggestions on how to proceed with my comparative analysis would be greatly appreciated. Thank you!

 

[berkeman]

Hi Josh,

Is this set of questions for your schoolwork? Last year you were finishing up high school, are you in undergrad now?

 

[joshwarner]

Hi Josh,

Is this set of questions for your schoolwork? Last year you were finishing up high school, are you in undergrad now?

Yes this is for a physics project.

 

[berkeman]

Okay, I'll move it to the schoolwork forums then. We ask that all schoolwork-related threads go in the schoolwork forums. Thanks.

 

[berkeman]

According to the SLS model, the stress response to a linearly increasing displacement should decay exponentially.

I'm probably not understanding the situation, but I would expect stress and strain to be linearly related up until yield (or some plastic limit for rubber). But the model says some part of the relationship should be exponential? Can you link to where it says this?

However, in my experiments, particularly with rubber, I observe that the stress increases approximately exponentially with time under a linear displacement.

Can you show some of your data and describe the actual setup? Thanks.

 

[joshwarner]

This is a compression test that applies a linear displacement to the material (in this case, rubber) over time and records the resulting stress. In my understanding, the exponentially decreasing stress evolution is predicted by the first-order differential equation with respect to stress when the displacement is fixed linearly. However, my experimental results show an exponentially increasing stress for rubber. Below are the results for reference.