Mechanical Properties

Question 1

define normal and shear stress

Answer 1

force acts on a surface - resolve into components. F/A
normal = tensile, σ
shear = parallel to surface, 𝝉

Question 2

define strain

Answer 2

𝜀, a ratio of dimensions:
∆ length / initial length

for shear strain, horizontal shift / initial length = tan ø

dimensionless

Question 3

define nominal stress / strain

Answer 3

stress / strain quoted with respect to original area / length
denoted by subscript n

Question 4

eqn nominal strain

Answer 4

𝜀(n) = (l - l(0)) / l(0) = l/l(0) -1

Question 5

true strain eqn

Answer 5

𝜀(t) = ln (l/l(0)) = ln(𝜀(n)+1))

Question 6

describe a stress-strain curve for a typical metal

Answer 6

linear elasticity initial region (𝜀(n) 0.001ish)
yield point –> plastic deformation region, slight curve down
necking = maximum
fracture

Question 7

gradient of the linear elastic region is

Answer 7

Young’s modulus, E
hence at low strains,
σ(n) = E 𝜀(n)

Question 8

define proof stress

Answer 8

σ(0.2%) = F / A(0) that gives permanent strain of 0,2%
easier to measure

Question 9

define tensile strength

Answer 9

σ(TS) at the onset of necking

Question 10

define tensile ductility

Answer 10

also known as plastic strain after fracture
calculated change in length after putting the broken pieces back together

Question 11

ceramics / glasses stress strain curve

Answer 11

steeply linear, immediate fracture (brittle)

Question 12

stress strain curves of polymers

Answer 12

can undergo relatively large strains
much flatter long curve