Module 3 Stress, Strain, Young Modulus Flashcards
Define (tensile) stress?
(tensile) force per unit cross sectional area
Unit for tensile stress?
pascals Pa
symbol for tensile stress?
σ - sigma
What is tensile strain?
the extension per unit original length
Unit for tensile strain?
no unit
Symbol for tensile strain?
ε - epsilon
Definition of Young Modulus
Ratio of stress to strain of an object measured in pascals
Equation for Young Modulus?
E=σ/ε
What is an alternative equation for Young Modulus
E=FL/Ax
What does the gradient of a stress strain graph represent?
Young Modulus
Describe the apparatus used to find the Young Modulus of a wire
Control wire kept taught by a fixed weight with a reference scale fixed to it
Wire being tested is attached to the same support with a vernier scale opposite reference scale to measure extension
Mass hanger to vary weights
Give a suitable value for the length of a wire when finding the Young Modulus?
2 meters or over
What is deformation?
the change change in size or shape of an object when put under stress
What is elastic behaviour?
when an object returns to its original shape when unloaded
What is plastic behaviour?
when an objects shape is permanently deformed once unloaded from a stress
What kind of forces can deform an object?
compressive and tensile forces
What is another name for plastic behaviour?
inelastic
Describe the characteristics of a brittle material
Behaves elastically until it reaches its breaking point
Obeys Hooke’s law
Describe the characteristics of a ductile material
Has a small elastic region before behaving plastically (deforming) and ultimately breaking
Below the elastic limit it obeys Hooke’s law
Describe the characteristics of a polymeric material?
does not obey Hooke’s law - deforms when stress is applied
How can stiffness be compared?
comparing gradients of stress strain graphs - higher gradient the higher the stiffness (can be compared by young modulus as well)
What is the strength of a material defined by?
it’s ultimate tensile strength (UTS) - it’s maximum stress it can withstand before breaking
What is breaking stress?
the stress at the moment of fracture/breaking
can be different from ultimate tensile strength
How do you find the toughness from a stress strain graph?
area = energy stored
indicates toughness
When plotting an unloading line how should it be done?
straight dashed line parallel to loading curve
How can work done to deform a material be found from a force extension graph?
difference between area under loading curve and area under unloading curve
Explain why despite the fact rubber does not obey Hooke’s law, it is still considered elastic?
extension is zero when load is removed/returns to original length when load is removed
Explain the difference in area between the loading and unloading curves?
Difference in area = difference in energy
When stretched, some of the energy in rubber is stored and some internal energy is retained
In the form of heat (molecules oscillate with greater amplitude)
