May be the pipe material becomes more brittle as time increased.

Hi,

In that case, as the aging time increases, the stresses in the material are released, which reduces the amount of stress, and, on the other hand, the magnitude of strain in the material increase by increasing the aging time.

Regarding the stress and strain behavior and the following equation, it can be concluded that increasing aging time reduces Young's modulus.

E = σ/ε

Bests,

Thank you everyone for all the answers

I think you must treated with PVC materials as polymers and you can used the standards polymers in your testing .