Lecture 12 part 2 - Strain Gauge Background

Question 1

Background of Strain gauges

Most widely used of strain/stress analysis methods

Basic principle discovered by Lord Kelvin (1856)

He discovered change in ……….. of iron and copper wires when loaded

Also that

• resistance change was function of ……
• different materials have different …………..
• use of Wheatstone ………

Answer 1

resistance

strain

sensitivities

bridge

Question 2

Strain Gauge Alloys

Advance or Constantan (45% .. , 55% .. )

Features

1. S_A is ……… over large range of strain (2.1)
2. S_A does not change significantly as material goes ……….
3. high ……….. resistance (small gauge size possible)
4. good thermal …………

Answer 2

Ni, Cu

linear

plastic

specific

stability

Question 3

Strain Gauge Alloys - 2

Karma (74%.., 20%.., 3%.., 3%..)

Features

1. Similar to Advance (S_A=2.0)
2. Higher ……. limit than Advance
3. Excellent temporal ………
4. can be used at ……. …………. than Advance (260deg C vs 204deg C)

Answer 3

Ni, Cr, Al, Fe

fatigue limit

stability

higher temperatures

Question 4

Strain Gauge Alloys - 3

Isoelastic (36%.., 8%.., 0.5%.., 55.5%..)

Features

1. higher S_A than Advance (3.6)
2. high ……. strength

Limitation

1. extremely sensitive to …………. ……… (can only be used in ……….. stable environments)

Answer 4

Ni ,Cr, Mo, Fe

fatigue

temperature changes

temperature

Question 5

Strain Guage Designs

sketch a 3 different strain guage designs

Answer 5

Question 6

_Strain Gauge Adhesives _

Name 2 types of adhesives used for strain gauges and their associated features

Answer 6

Epoxy Resin

kit form
high bond strength
30 mins drying time

Polyester Adhesive

low curing temperature

Question 7

Strain Gauge Cements

Name 2 types of strain gauge cements and their associated features

Answer 7

Cyanoacrylate Cement

• 10 mins drying time
• not long term
• waterproofing required

Ceramic Cements

• high temperature use / radiation environs

Question 8

Bridge Circuit 1

Strain gauge at ..

Used for ……. and ………. measurements when ………… ………… is not important

R2, R3 andR4 adjusted so bridge is initially ………. and ……. is maximised

Most …….. bridge operating case

Answer 8

R1

static , dynamic

temperature compensation

balanced

sensitivity

nonlinear

Question 9

Bridge Circuit - 2

Strain gauge at ..

Dummy gauge at ..

Temperature compensation provided by ……. ……

Can be used at top and bottom of ….. in ………

ΔR1/R1 = -ΔR2/R2

Major disadvantage, r=1 so circuit ……….. is only 50%

Answer 9

R1

R2

dummy gauge

beam , bending

efficiency

Question 10

Bridge circuit - 3

Strain gauge at ..

Dummy gauge at ..

Again, temperature compensation provided by …… …….

Circuit efficiency can be ……….. from ..% to near .. or ..% for r = 4 to 9

Answer 10

R1

R4

dummy gauge

50, 80, 90

Question 11

Bridge circuit - 4

Four active ……. ……… used

r must be ……

Temperature ……………

Linear behaviour if ΔR’s are of the …… magnitude

Answer 11

strain guages

unity

compensation

same