Lectures 1-3 Flashcards
What is a stress-strain diagram
-A diagram used to show tensile strength, youngs modulus(m), yield strength(y-max), stiffness and toughness
Engineering stress N^2/m, σ
-force/original cross-sectional area
Engineering strain, ε, (no units)
the change in length/original
Tensile Strength
-The amount of force required to cause a rope wire or beam to the point where it breaks.
Ultimate tensile strength- is the maximum stress on a stress-strain graph
Elastic deformation
When a material changes shape when undergoing stress but will return to its original shape once the stress is removed.
Plastic deformation
When a material changes shape when it undergoes stress but do not return to its original shape
Youngs Modulus, E
A measure of elasticity of an object, equal to the stress acting on an object compared to the strain.
Yield strength
- The point at which the object under stress starts to deform plastically, usually 0.2% of the unstressed length
- E=σ/ε
What is Hooke’s law
- Stress is proportional to the strain on an object, up to the elastic limit.
- σ=Eε
Fracture stress
The point at which a specimen fails via fracture
Brittle
When an object is subjected to stress it breaks with little elastic deformation and without significant plastic deformation.
Ductility
Has the ability to withstand tensile strength and can be stretched without becoming weak/brittle.
True Stress
True stress is the stress but the cross-sectional area will change with the elastic and plastic deformation of the object
Poisson’s ratio
-The ratio of the transverse contraction strain to the longitudinal
v=-εx/εz
=-ε(lateral)/ε(longitude)
Shear stress
Stress in the direction of the cross-sectional area
How is ductility measured?
percentage or elongation
percentage of reduction in cross-sectional area
Toughness
The amount of energy an object can absorb per unit volume before fracturing.
The area under a stress-strain diagram
Name two tests for toughness
- Charpy test
- Izod test
What is a Charpy test?
Indent in the object before the impact, to focus on an imperfection
Bar placed behind the imperfection
What is an Izod test?
-Fixes into a pocket and the bar swings and takes the object off if KE>toughness
How does temperature affect the properties of an object?
-Objects become more brittle with lower temperatures
