of University Virginia Department of Physics
Physics 606: How Things Work II
Lecture #34 Slides: Knives and Steel
Question: Knives and Steel
If you take a steel paper clip and bend it repeatedly, will it become stiffer or less stiff with each new bend (at least initially)?
Observations About Knives
Some knives keep a better edge than others Some knives chip while others bend Some knives rust while others don't Some rust-resistant knives don't keep an edge Some rust-resistant knives aren't magnetic
Stress and Strain
When you squeeze a material, its thickness decreases
Stress: the force per unit of top surface area (pressure) Strain: the fractional change in thickness
For small stresses, strain is determined by the compressibility of the atoms themselves Since most steels contain similar atoms, most have the same relationship between stress and strain
Shear Stress and Shear Strain
When you bend a material, it flexes
Shear stress: the force per unit of cross sectional area Shear strain: the angle of bend in the material
Steel Crystals
Steel is a crystalline material Crystals have large regions of orderly atoms Surfaces of atoms can "slip" across one another
For small shear stresses, shear strain is determined by atomic properties Since most steels contain similar atoms, most have same relationship between shear stress and shear strain
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Elastic and Plastic Deformation
For small shear stresses, steel deforms elastically
Deformation no involves overall shifting of atoms Deformation vanishes when shear stress is removed Forces involved are proportional to distortion
Controlling Slip
Dislocation defects assist slip
Extra row of atoms allows a sheet of atom to slip gradually
Thermal energy assists slip
It helps dislocations move Dislocations help slip
For large shear stresses, steel deforms plastically
Deformation involves shifting atoms (slip) Deformation is permanent Forces involved are less than proportional to distortion
To stop slip
Eliminate dislocations (difficult) Lower temperature (awkward) Spoil crystal structure (easy) Hardened steel is harder to bend
Spoiling Crystal Structure
Work hardening
Working breaks up crystals into tiny bits that won't slip
Question:
If you take a steel paper clip and bend it repeatedly, will it become stiffer or less stiff with each new bend (at least i...