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Statics Activities

Statics Activities. Stress (s). Force per unit area ( s ) Typical engineering units p si (lb f /in 2 ) N/m 2 Stress = Force/Area Applied by external agents Area is X-section area Same definition as pressure. Stress (σ). applied to the material by external agents

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Statics Activities

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  1. Statics Activities

  2. Stress (s) • Force per unit area (s) • Typical engineering units • psi (lbf/in2) • N/m2 • Stress = Force/Area • Applied by external agents • Area is X-section area • Same definition as pressure

  3. Stress (σ) • applied to the material by external agents • force per unit area (same definition as pressure) • stress = force / area = F / A • unit: N/m2

  4. Types of Stress • tensile: object is pulled apart • compressive: object is pushed together • shear: equal and opposite forces applied across object’s faces

  5. Strain (ε) • material’s response to the stress • ratio of the change in length to the original length • strain = change in length / original length • strain = ΔL / Lo • units: dimensionless

  6. Stress • Example • Calculate the stress in a cylindrical rod if a 200N force is applied to one end. The radius of the cylinder is 4cm. A = p(0.04)2 = 0.0.05m2 s = 200N/0.005m2 = 40000N/m2 200 N A = pr2

  7. Shear stress • When stress develops in an area parallel to the force direction • Sometimes when a force affects surfaces, or materials with “grain” (i.e. wood, some crystals, shale rock), or specific geometries.

  8. Strain (e) • The ratio of change in length (DL) to original length (Lo) (dimensionless) • = DL/Lo • Brought about by a stress in a material • Can be temporary or permanent deformation

  9. Strain

  10. Elastic Modulus • Elastic modulus (E) (or Young’s modulus) • Indicator of relative strength of material E = s/e = (stress/strain)

  11. Modes of Failure • Materials can fail • Damaged (excessive strain- ductile failure) • Catastrophic (fracture)

  12. Ultimate Strength & Fracture • Ultimate Strength – A level of stress in a material that causes failure • Compression • Tension • Shear • Safety factors are applied to protect against reaching this level (2x,3x,4x, and higher).

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