Introduction to Stress-Strain Diagram Note: This is a supplementary material for Chap. 15 Fundamentals of Metal Forming. This memo is prepared especially for those who have not taken a material science course covering this technical issue.

Tensile testing to study material properties A material specimen such as the one shown in the figure below is often used for a tensile test to study some important material properties Fig. 2-4 on p.30 Textbook Two common types of tensile test specimens: (a) round and (b) flat.

Tensile force is applied and increased along a given specimen. A diagram called a “stress-strain diagram” such as the one shown below is obtained. Fig. A1: A typical stress-strain diagram

The Greek letter ‘σ’ is often used for stress. A stress is given by Commonly used physical units for stress include psi (pound forces per square inch) and Pa (Pascal: Newtons per square meter) where F is the tensile force applied to a given specimen and A is the cross section area of the same specimen.

The Greek letter ‘ε’ is often used for strain. A strain is given by Strain is a dimensionless quantity but it is also expressed using physical units such as in/in and m/m. Where ΔL is the elongation amount of a given specimen and L 0 is the original length of the specimen.

A permanent deformation of a material starts when the applied stress to the material exceeds the material’s “yield strength (σ Y )”. As the tensile force is further increased and the elongation of the specimen continues, it will pass through the peak stress point called “ultimate tensile strength (σ U ). After passing the ultimate strength, if the specimen continue to be elongated, it will eventually break into two pieces. This point is called “fracture point.”

Questions: –Are σ Y and σ U material properties? (Y/N) –Are σ Y and σ U influenced by the geometry or the shape of the specimen (Y/N) –What amount of stress should be applied to do cold working deformation processes?