Stress and Strain

- Internal force: The resisting force that, together with deformation, develops when an external force is applied to a body and balances that external force.

- Stress: The magnitude of internal force per unit area.

1. Normal stress (= axial stress): Stress acting perpendicular to a material surface.

   A. Formula:Normal stress = load, internal force, or external force / cross-sectional area = P/A [Pa(N/m^2), MPa(N/mm^2)]

     [1000000Pa = 1000KPa =1MPa]

     B. When tensile stress makes a specimen elongate: Generally marked +.

     C. When compressive stress makes a specimen shorten: Generally marked -.

2. Shear stress: Stress acting tangentially along a cross section.

A. Formula:Tau = τ = P/A[Pa(N/m^2), MPa(N/mm^2)]

- Strain: The relative deformation of a material caused by a load.

1. Longitudinal strain: Strain occurring in the longitudinal direction under a longitudinal load.

      A. Formula:ε = (l'-l)/l  = λ/l

2. Transverse strain (compressive strain): Strain occurring in the transverse direction under a longitudinal load.

      A. Formula: ε' = (d'-d)/d = δ/l (where d' is the contracted length and d is the transverse length)

3. Shear strain: The strain of a body when it deforms under the influence of shear stress.