Basic Mechanical Engineering

1. STRESS
AND
STRAIN
MEASUREMENTS

2. Stress measurement:
Stress = Load / unit area
= P/A
Strain e = Change in length / Original length
=
Young's modulus E =
e

3. Strain gauges:
 A metallic conductor is stretched or compressed, its resistance
changes due to the change in length and diameter of the conductor.
 Strain gauge is a measurement transducer used to measure strain
( change in dimensions).
 Converts information about relative change in dimension to a
change resistance.

4. Positive strain:
 Strain gauge(Metallic conductor) is subjected to tension, it is said to be
Positively strained.
 Length increases and its area of the cross-section decreases.
 Resistance of the conductor is proportional to its length and inversely
proportional to its area of cross-section the resistance of the gauge increases
with positive strain.
Negative strain:
 Strain gauge(Metallic conductor) is subjected to compression it is said to
be Negative strain.
 Length decreases and its area of the cross-section increases.
 Resistance of the conductor is proportional to its length and inversely
proportional to its area of cross-section the resistance of the gauge decreases
with negative strain.

5. Piezo resistivity:
Change in resistivity of a conductor when it is strained and this property is
called Piezo resistivity.
Poisson's ratio:
Poisson's ratio = Lateral strain / Longitudinal strain
= dD/D
dL/L
Gauge factor (strain sensitivity factor):
Gauge factor F= dR/R
dL/L

6. Types of strain gauges:
The common types of strain gauges are
1. Unbonded strain gauges
2. Bonded strain gauges
a) Fine wire strain gauge
b) Metal foil strain gauge
c) Semi-conductor or Piezo resistive strain gauge

7. 1. Unbonded strain gauges

8. 2. Bonded strain gauges a) Fine wire strain gauge

9. b) Metal foil strain gauge

10. c) Semi-conductor or Piezo resistive strain gauge

11. Surface preparation and Bonding techniques
Surface preparation and bonding techniques are mainly
three types
1. Backing, Base or Carrier material
2. Bonding material or Cement
3. Surface preparation and mounting of strain gauges

12. 1. Backing, Base or Carrier material:
The purpose providing in this materials are
 Support to the resistance wire (Grid).
 Protection to the sensing resistance wire.
 Stability for the resistance wire.
Characteristics for baking material:
 The backing material should be an insulator of electricity.
 Should not absorb humidity.
 Very thin material.
 The adhesive material is used to fix(Bond).
 Effected by temperature changes.
 Transmit the force from the structure under study to the sensing resistance wire

13. 2. Bonding materials or cement (Adhesive):
 The strain gauge has to be fixed (Bonded) structure under study
using an adhesive or paste.
 These adhesives are called Bonding materials or cements.

14. Characteristics:
 The bonding material should be an insulator of electricity.
 Should not absorb humidity.
 The backing material is fixed (Bonded) rigidly on the structure under study.
 Should not effected by temperature changes.
 Should have good shear strength to transmit the force.
 Should be easy to apply, spread easily.
 The bonding material should have high creep resistance.

15. 3. Surface Preparation and Mounting of Strain Gauges:
 The structure under study is made Even and Free from Dust and dirt by
rubbing with an emery sheet or by sand blasting.
 The even surface is then cleaned by Volatile solution (Acetone) using a
cloth to remove oil/grease.
 The Bottom side of the backing is also cleaned by a solvent using a cloth.

16. WHEATSTONE BRIDGE CIRCUIT

17. TEMPERATURE COMPENSSTION
1. Temperature compensation using an adjacent arm compensating gauge
a) Method-I --- Using Dummy Gauge
b) Method-II --- Poisson’s method
c) Method-III --- Use of Two Active gauges on Cantilever
2. Self Temperature Compensation
a) Selected melt gauge
b) Dual element gauge

18. 1. Temperature compensation using an adjacent arm compensating gauge
a)Temperature compensation using Dummy Gauge

19. a)Temperature compensation using Poisson’s method

20. c)Temperature compensation using Two Active gauges on Cantilever

21. Gauge Rosettes (Multiple Grid)
Two Element Rosettes

22. Rectangular Rosettes

23. Delta Rosettes