Most rocks will exhibit permanent (plastic) deformation when loaded at large strains . Plastic deformation includes plastic compression strains and plastic shear strains. The theory of elasto-plasticity is covered in the Advanced Geomechanics course https://www.youtube.com/playlist?list=PLv0npDbE5HXssC2CwCAssJs0fTkKquQFj. Figure 3.23 shows an example of permanent deformation during a typical deviatoric loading test to measure Young's modulus. First-time loading usually involves plastic deformation and creep. Therefore the loading Young's modulus results smaller than the unloading modulus . While calculation lumps elastic, plastic, and creep strains, involves mostly elastic strains. Notice the the re-loading modulus is similar to the unloading modulus because a re-loading path is not a first-time loading.