The linear slope of the stress-strain diagram describes the Young’s Modulus of a material.
This reflect the proportional relationship between stress and strain that can be seen by the deformation of a solid body with linear-elastic behavior.
The Young’s Modulus is dependent of the direction and varies therefore especially if anisotropic behavior in a material can be found.
This is also true for cristalline materials.
The higher the Young’s Modulus the more the material resiststhe elastic deformation.
Plastics have a rather small Young’s Modulus compared to steel. In addition to that the modulus depends on temperature as well.
Exmaples at 20 °C:
ABS: 1,9 – 2,6 GPa
PA6: 3,0 – 3,5 GPa
PC: 2,2 – 2,4 GPa
PP: 1,3 – 1,8 GPa
HDPE: 0,5 – 1,2 GPa