# Biophysics Q: Impact of stiffness (Young's modulus) on stress failure

## Summary:

I'm trying to determine the impact of stiffness (Young's modulus) on the tendency of 'stress failure' with a given strain.
Hi. I'm a physician trying to understand the micromechanics of lung injury due to overdistension. The basic idea is that overstretching of the plasma membrane of the lung epithelial cell causes "stress failure" --> i.e. plasma membrane rupture --> cell death. The concepts of stress, strain, and Young's modulus (E) are utilized to study this: Stress = E x Strain, or Strain = Stress/E

Say object 1 has a stress = x, strain = y

Say object 2 has Young's modulus twice that of object 1, and the same stress (x) is applied
hence for stress = x, strain = y/2

So object 2 sees same stress (x) but half the strain (y/2) compared to object 1. What can be said about its chances/tendency for 'stress failure' compared to object 1: same or half?!

Would appreciate any insight.

Best.

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jrmichler
Mentor
One way to look at it:
Stress breaks things.
Strain is how far it stretches before it breaks.
That would imply that stress is the sole determinant of cell death.

But you have to be careful. Do the less flexible cells (higher Young's modulus) die at the same stress as the more flexible cells?

Side question: Is this related to the operation of ventilators?

DNA105, Tom.G and berkeman
Thank you for the response. So the stress at which failure occurs may vary if alteration/remodeling occurs in an object. Makes sense!

And yes, this is in relation to ventilation induced lung injury (specifically overdistension injury due to large tidal volumes)

jrmichler
Chestermiller
Mentor
The failure criterion of an object (or membrane) is somewhat separate from the stiffness behavior of the material. The failure is usually characterized by a critical stress level (expressed 3 dimensionally in terms of the principal stresses of the material). Google failure criteria.

DNA105
Tom.G