11.3 Deformation of solids Flashcards
What is needed to stretch or twist an object
A pair of forces
What is the elasticity of a solid material
Its ability to regain its shape after it has been deformed or distorted and the forces that deformed it have been released
Deformation that stretches an object is…
…tensile
Deformation that compresses an object is…
…compressive
How to test materials to see how easily they stretch
Material is held at its upper end and loaded by hanging weights at its lower end
A set square or pointer attached to the bottom of the weights may be used to measure the extension of the material, as the weight of the load is increased in steps then decreased to zero. Tension of the material is equal to its weight
What is the extension of a material
Its increase of length from its unloaded length
What sort of graph does the extension of a steel spring show
A straight line in accordance with Hooke’s law
Stretching a rubber band
At first extends easily when it is stretched, however it becomes fully stretched and very difficult to stretch further when it has been lengthened considerably
Stretching a polythene strip
Gives and stretches easily after its initial stiffness is overcome.
However, after giving easily, it extends little and becomes difficult to stretch
What setup is used to measure the extension of a wire under tension
Searle’s apparatus
Micrometer attached to the control wire is adjusted so the spirit level between the control and test wire is horizontal. Test wire is loaded causing it to extend slightly meaning that the spirit level to drops on one side. Micrometer readjusted to make spirit level horizontal again.
For a wire of length L and area of cross section A under tension, the tensile strength of the wire δ
δ=T/A
Where:
T - tension
A - area of cross section under tension
What is the unit of stress
Pascal
Pa
Equal to 1Nm^-2
For a wire of length L and area of cross section A under tension, the tensile strain in the wire ε
ε = ΔL/L
Where ΔL is the extension of the wire
What is the unit of strain
No unit as it is a ratio
What to remember about measuring the extension
Always measured from the original unstratched length of the object
From 0 to the limit of proportionality P, the tensile stress is ______ to the tensile strain
Proportional
What is the Young modulus of a material
The value of stress/strain which is a constant
Young modulus E =
tensile strength/tensile strain = TL/AΔL
What happens beyond the limit of proportionality P
The line curves and continues beyond the elastic limit E to the yield point Y1 which is where the wire weakens temporarily.
What is the elastic limit
The point beyond which the wire is permanently stretched and suffers plastic deformation
What is beyond maximum tensile stress
The ultimate tensile stress
What happens at the ultimate tensile stress
The wire loses its strength, extends, and becomes narrower at its weakest point
What causes the increase of tensile stress
Due to the reduced area of cross section at this point until the wire breaks
What is the ultimate tensile stress also referred to as
The breaking stress
How can the stiffness of different materials be copared
By using the gradient of the stress-strain line which is equal to the Young modulus of the material
What is the strength of a material
Its ultimate tensile stress
Which is its maximum tensile stress
Why is steel stronger than copper
Its maximum tensile stress is greater
A brittle material ______ without any ______ ______.
Snaps
Noticeable yield
A ductile material can be ______ into a ______.
Drawn
Wire
