Statics Activities
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
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
Types of Stress
• tensile: object is pulled apart• compressive: object is pushed together• shear: equal and opposite forces applied across
object’s faces
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
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
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.
Strain (e)
• The ratio of change in length (DL) to original length (Lo) (dimensionless)e = DL/Lo
• Brought about by a stress in a material
• Can be temporary or permanent deformation
Strain
Elastic Modulus
• Elastic modulus (E) (or Young’s modulus)• Indicator of relative strength of material
E = s/e = (stress/strain)
Modes of Failure
• Materials can fail– Damaged (excessive
strain- ductile failure)– Catastrophic (fracture)
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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