chapter 6 - materials

Question 1

tensile force

Answer 1

produces an extension

Question 2

compressive force

Answer 2

reduces the length of an object

Question 3

hookes law

Answer 3

force is proportional to extension so long as elastic limit isn’t exceeded
F = kx
- its device specific - changes for every spring

Question 4

spring constant k

Answer 4

k = F/x
gradient of an F x graph
stiffer spring = greater k

Question 5

elastic deformation

Answer 5

before elastic limit - object will return to original shape when load is removed

Question 6

plastic deformation

Answer 6

after elastic limit
permenant change in shape that remains when load is removed

Question 7

area under force extension graph

Answer 7

work done in stretching the object

Question 8

elastic material

Answer 8

will return to origional shape

Question 9

tensile stress

Answer 9

force applied per unit cross sectional area
F/A (Pa)

Question 10

tensile strain

Answer 10

fractional change in length of object
x/L

Question 11

Youngs modulus

Answer 11

stress/ strain = FL/Ax
(Pa)
stiffness - higher YM means it is more stiff
gradient of stress strain graph

Question 12

ductile material

Answer 12

can be drawn into wire or hammered into thin sheets
- behave elastically until elastic limit after it stretches more and has plastic behaviour
- a straight graph that then curves

Question 13

brittle material

Answer 13

snaps easily
- stretches slightly then fractures
- elastic behaviour up until point of fracture
- behaves same both ways (unloading and loading)
- straight graph

Question 14

limit of proportionality of a material

Answer 14

point at which a material stops obeying hooks law

Question 15

ultimate tensile strength of materials

Answer 15

maximum stress a material withstands before breaking
- end of a stress strain graphs

Question 16

measure to what dp

Answer 16

the dp you can measure to
- has to be the same for even your average
- can’t have a value more accurate than your measuring devices

Question 17

breaking strength of a material

Answer 17

stress at the point of fracture

Question 18

what is a strong material

Answer 18

one with a high ultimate tensile strength

Question 19

when does Young modulus apply to a material

Answer 19

when stress is proportional to strain OR when its obeying hooks law

Question 20

how do you find young modulus from a stress-strain graph

Answer 20

gradient of the initial straight line region

Question 21

polymeric materiaal

Answer 21

material made from long chain molecules
behaviour depends on temp as well as molecular structure

Question 22

elastic limit

Answer 22

beyond which the material becomes permanently deformed
- won’t return to normal when load removed

Question 23

young modulus experiment

Answer 23

• measure diameter
• calculate CSA
• find original length
• increase load
• record mass, and find extension
• plot graph (stress/strain)
• calculate gradient = YM

Question 24

elastic behaviour

Answer 24

returns to original length when load is removed
- all materials show it till the elastic limit

Question 25

plastic behaviour

Answer 25

shown when materials are loaded above the beyond the elastic limit - permanently deformed

Question 26

elastic potential energy

Answer 26

- energy stored when a solid is stretched/compressed - if strained elastically energy can be recovered - if plastically some work done has gone into moving atoms so is lost - area under F x graph before elastic limit - E = 1/2Fx = 1/2kx^2