If you are looking for free foundational material on classical plasticity theory, consider these open-access academic sources:

: Provides a limited preview and summary of the text's contents, covering yield conditions, flow rules, and hardening rules. Open & Academic Alternatives

While a direct, legally "free" full download of the recent 2022 textbook Elements of Classical Plasticity Theory by Andreas Öchsner is generally restricted by copyright, you can access several legitimate alternatives and previews. Official & Legal Resources

: Hosts older but foundational texts for free borrowing, such as Alexander Mendelson's Plasticity: Theory and Application and L.M. Kachanov's Foundations of the Theory of Plasticity.

: A comprehensive Introduction to Plasticity PDF that covers standard tests, assumptions, and basic models.

: Authors often upload similar content, such as A Short Introduction to the Theory of Plasticity or Classical Elasticity and Plasticity.

: The official publisher offers the eBook for purchase . You may have free access if you are affiliated with a university or library that has a SpringerLink subscription.

Elements of Classical Plasticity Theory | Springer Nature Link