1700

Stress

Load/over cross-sectional area

Units is N/m² —> 1 pascal (Pa)

Strain

Stress tells us how hard (how much force) the atoms at any point in a solid are being pulled apart, strain tells us how far they are being pulled apart.

Young’s Modulus and the relation with stiffness/graph

When Young’s Modulus is higher, the gradient is steeper and the material is stiffer

When Young’s Modulus is lower, the gradient is lower/ less steeper and the material is less stiffer (more compliant)

Units is same as stress N/m²

Yield Strength

sigma y = the stress at which plastic deformation begins

Difference between Elastic deformation and Plastic deformation

Elastic deformation is when the material will recover by itself

Plastic deformation is when the material won’t recover by itself (bent spoon)

Young’s Modulus only applies to elastic deformation (when stress is proportionate to strain, which is elastic deformation range and below sigma y)

Can only use Hooke’s Law when theres no plastic deformation in the material (when the stress level is lower than the yield strength)

What are the support reactions at a fixed support

Horizontal reaction, a vertical reaction and a moment

A truss is statically indeterminate if

The number of equations to solve is less than the number of unknowns (unknown reactions + unknown member forces )

A truss is statically determinate iif

the number of equations to solve is equal to the number of unknowns (r + m)

If the number of equations to solve is less the number of unknowns then it is unstable

When should you use method of joints or method of sections?

Method of Joints can be quick for simple structures (triangles) and if information is available at a node

Methods of Sections is quicker if interested in particular member forces in large or complex structures

Tue 31 March 1700 lecture goes through a past exam question paper on trusses