1. Recap- Optical extensometer

2. OPTICAL STRAIN GAUGE Marten’s mirror extensometer
Tuckerman optical strain gauge

3. Marten’s mirror extensometer

4. Working Principle To measure Elongation
Developed by Peterson and Wahl in 1936
In this extensometer fixed scale is observed though telescope by means of tilting the mirror M, when rotating fixed knife edge B
As the specimen rotates the measuring knife edge will rotate about its pivot point and tilts mirror

5. -Cont
As the mirror tilts different portions of scale seen giving a reading S which is a measure of angle of tilt.
The Magnification factor is M= 2A/B
The distance of B is 5mm
The distance A is 250 times B
Hence obtained Magnification is 500

6. Function of Magnification factor
Magnification factor is used to determine how large an object is in real life compared to the image of the object that was captured by a microscope, camera or other imaging device.

7. For example,
To determine the size of an object in a photograph when the distance to the object and the focal length (lens strength) are known, the object distance is multiplied by the vertical (or horizontal) image size and this number is then divided by focal length to determine the magnification factor

8. Tuckerman optical strain gauge

9. Auto Collimator

10. Working Principle
To remove error due to linear displacement an improvement over two mirror system has been made by Tuckerman.
It is a combination of mechanical and optical levers used to amplify the relative distance between knife edge.
The relative motion between the fixed mirror, M1 and the movable mirror M2 is measured in collimater

11. Lozenge functions as movable knife edge.
In this system relative motion between the component and auto collimator will not affect the system.
Sensitivity is 2 Micro strain
It can measure static and dynamic strains.

12. Advantage Reduced Wiring as compare to Electrical Resistance
Light weight when compared to others

13. ACOUSTICAL STRAIN GAUGE

14. Working Principle
Frequency of vibration in test gauge is measured by comparing it with natural frequency of the wire.
Vibrating steel wire tensioned between two supports at a predetermined distance.
Variation of distance alters the natural frequency of wire.

15. An electromagnet adjacent to the wire is placed to set wire in vibration.
This wire movement will then generate a oscillating electric signal.
The signal may be compared with the pitch of an adjustable standard wire
The visual display produced or CRO renders the wire adjustment easier

16. Tuning is now more usually accomplished by feeding the two signals in an oscillograph and making use of Lissajous figure formation to balance the frequencies.
Matching of both the frequencies can be made easily using earphones.