12.12:
Plasticity
Plasticity is the property of a body by virtue of which it remains in its deformed shape and size even after the forces are removed.
The plastic deformation in a body is produced under certain load conditions and can be studied using the stress and strain curve.
The point at which the body ceases to be elastic and begins to deform plastically is known as the yield point. Permanent deformation of the body occurs once the yield point is passed.
During plastic deformation, atoms of the body are permanently displaced from the original lattice site and retain the new site even after the external forces have been removed.
The point on the stress-strain curve that represents the maximum stress that the body can endure before breaking apart is known as the ultimate stress point, and the point beyond which the body breaks or ruptures is called the fracture point.
12.12:
Plasticity
Plasticity is the property where an object loses its elasticity and undergoes irreversible deformation, even after the deformation forces are eliminated. If a material deforms irreversibly without increasing stress or load, then this is called ideal plasticity. For example, when a force is applied to an aluminum rod, it changes its shape, but it does not return to its original shape once the force is removed. Plastic deformation or ductility is thus a permanent deformation or change in the shape of a solid caused by continuous force.
Once a material goes beyond its elasticity limit and experiences plastic deformation, it remains plastically deformed until the stress reaches the fracture point (breaking point). The value of stress at the fracture point is called breaking stress (or ultimate stress). Materials with similar properties, such as two metals, can have very different breaking stresses. For example, the ultimate stress for aluminum is far lower than that of steel. Beyond the fracture point, the body is fractured into pieces.
Table 1. Approximate breaking stresses of various materials
| Materials | Breaking Stress (Pa or N/m2) |
| Aluminum | 2.2× 108 |
| Brass | 4.7 × 108 |
| Glass | 10.0 × 108 |
| Iron | 3.0 × 108 |
| Steel | 20.0 × 108 |
This text is adapted from Openstax, University Physics Volume 1, Section 12.4: Elasticity and Plasticity.
Suggested Reading
- Young, H.D and Freedman, R.A. (2012). University Physics with Modern Physics. San Francisco, CA: Pearson. pp.
- OpenStax. (2019). University Physics Vol. 1. [Web version]. Retrieved from https://openstax.org/books/university-physics-volume-1/pages/1-introduction
- University Physics with Modern Physics ( 13th) Young and Freedman. Pp. 358