1.5 solids under stress Flashcards
hookes law
extention is directly proportional to fore given conditions are kept constant
F = k L
k = stiffness constant
tensile stress
force applied per unit cross sectional area
tensile strain
a ratio of how the material stretches
diff between elastic and plastic deformation
elastic - when force is removed object returns to og shape
plastic - load is removed and objects doesnt return to og shape
breaking stress
minimum stress needed to break a material
a brittle material
doesn’t plastically deform but breaks when the stress reaches a certain value
elastic limit
force above which the material will b plastically deformed
area under force - extention graph represent
work done to deform material
equation to calculate elastic strain from the spring constant and extension
E = 1/2 kL^2
young modulus
describes stiffness of a material
E = stress / strain
how to find young m from a stress-strain graph
gradient
force - extension graph when hookes law is being obeyed
straight line through origin
force and extension - directly proportional
limit of proportionality
after hookes law is no longer obeyed, it is shown by the line beginning to curve on a fx graph
how is work done to stretch or compress a material stored
elastic strain energy
why loading & unloading lines are parallel on a fx graph for a plastically deformed material
stiffness constant hasnt changed, forces between the atoms are the same when loading and unloading
why all work done isnt stored as elastic strain energy when a stretch is plastic
work is done to move atoms apart, so energy is not stored as elastic stain energy but is dissipated as heat
how is dissipation of energy in plastic deformation used to design after vehicles
- crumple zones deform plastically in a crash using the cars KE energy so less is transferred to the passengers
- seat belts stretch to convert the passengers KE into elastic strain energy
energy changes when a spring is pulled and released
work done in stretching it is stored as elastic strain energy.
when the spring is released this is converted to KE whic is converted to GPE
do stress strain graphs show behaviour of a material or a specific object
material
stress - strain for a ductile material
can undergo large amount of plastic deformation pre fracturing
spring constant
force per unit extension required to stretch a spring
what is a ductile material
a material that can be drawn out into a wire - plastic deformation occurs when there is enough stress
ductile fracture (necking)
where there s local thinning, which increases the stress before material breaks
