Unit 1 - Mechanical properties of materials & structures Flashcards
What is the difference between a material & a structure?
Structure - arrangement of 1/more materials in a way designed to sustain loads
Material - things used to construct a structure
How to calculate stress?
σ = F/A
Unit = Pa (N/m^2)
Equation for strain?
ε = Δl/l
No units because it’s a ratio
What is the proportional limit on a stress/strain curve?
Between origin & proportional limit (P) the strain-strain curve is a straight line
What is the elastic limit on a stress-strain curve?
(E) is the greatest stress that may be applied to the material without causing permanent deformation
What is elastic behaviour?
When a material deforms instantaneously on loading & returns immediately to original shape & size when load removed
What is plastic behaviour?
Material deforms instantaneously under applied load however retains a new size & shape when load is removed
What is the yield point on a stress-stain curve?
Once (Y) passed the material will undergo considerable elongation without a corresponding increase in stress
Stress at yield point = yield strenght
What is the strain hardening region of a stress-strain curve?
After material undergoes large strain that occurs during yielding the material begins to strain/work harden - undergoes changes in atomic & crystalline structure resulting in increased resistance to further deformation
What is the ultimate strength point on a stress-strain curve?
(U) occurs at highest point of stress-strain curve
In region after ultimate strength point stretching occurs with reduction in stress due to necking of the material whereby CSA decreases
What is the rupture point on a stress strain curve?
(R) is the point at which the material breaks (rupture strength)
What is Hooke’s Law?
Up to a certain level of stress, the strain produced is proportional to the applied stress
What is the equation for Young’s modulus & what is it a measure of?
E = σ/ε (only up to proportional limit)
Is a measure of how stiff a material is:
- small value for E means material is flexible
- large value for E mean material is stiff
How do you calculate rigidity?
Rigidity = E/A
This is an indication of a materials ability to resist axial deformation
Units = N
How do you calculate stiffness (k)?
k = F/Δl = EA/l (from equation for E)
The force required to produce a unit of deflection (elongate/shorten)
What is the equation for flexibility?
f = Δl/F = l/EA = 1/k
Increase in length of a bar will reduce stiffness but increase flexibility
Describe viscous behaviour
Does not deform instantaneously when load applied, strain is prolonged
Once load removed, material will not return to original shape & size
Means no energy is stored in material and all energy required to deform it is dissipated as heat
What is the equation for the coefficient of viscosity?
η = stress/strain rate
Units = N/m^2 .s or Pa .s
What is viscoelastic behaviour?
Exhibits both:
- Viscous: responds to rate of loading
- Elastic: returns to original size & shape after loading
What is the difference between creep & stress relaxation?
Creep - increase in strain in a material which is constantly loaded over time
Stress relaxation - reduction is stress in a material which is constantly deformed over time
