Stress Measurement Using Strain Rosette

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BACKGROUND

There are some drawbacks in stress and strain analysis using a strain gauge as it is only possible to measure the strain in a direction in which the gauge is oriented. There is no any direct method the principle strain as its directions are not usually known. A strain rosette is a combination of three independent strain gauges which are connected in perpendicular axes direction. These gauges are attaches in any three directions to measure the shear strain.1

It is well known from the rotated normal strain theory that strain is a function of the co-ordinate axes Ex, Ey, and Sxy. Thus, if three strain gauges in a rosette-like structure is rotated, it will give three equations with all unknown values, which is given by:2

  1. Ea = (Ex + Ey)/2 + {(Ex – Ey)/2} Cos 2a + {Sxy/2} Sin 2a
  2. Eb = (Ex + Ey)/2 + {(Ex – Ey)/2} Cos 2b + {Sxy/2} Sin 2b
  3. Ec = (Ex + Ey)/2 + {(Ex – Ey)/2} Cos 2c + {Sxy/2} Sin 2c

The resistance on the strain gauges are proportional to its displacement. Thus, by measuring the resistance through a point, we can easily calculate the value of strain at that point.

The main objective of this experiment is to measure the acting strain by using a strain rosette.

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REQUIREMENTS

Materials:   A Strain Rosette

Cantilever Beam Weights

Ammeter Voltmeter

Multimeter

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PROCEDURE

First of all, apply a single load on beam and note down the strain at different points to compare it with experimental data. Then, arrange the cantilever beam, ammeter, and voltmeter in a proper circuit. Take the three values by using each rosette to determine the shear strain. Determine the state of strain by using the equation given in the background section. The theoretical value of strains can be compared with experimental value by using a Mohr’s Circle.

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CONCLUSION

We have studied the methods of strain measurements by using a strain rosette. The theoretical stress and strain value can be used to show the stress distribution from finite element analysis for the beam under a unit load.3

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REFERENCES

  1. C. D. F. M. F. A. F. D.B.Burr, “In vivo measurement of human tibial strains during vigorous activity,” Department of Anatomical Sciences, University of Queensland, Brisbane, Queensland, Australia, vol. 18, no. 5, pp. 405-410, 1995.
  2. P. H. P. J. E. F. D. A. Gorham, “An improved method for compressive stress-strain measurements at very high strain rates,” 18 February 1992.
  3. D. R. Carter, “Anisotropic analysis of strain rosette information from cortical bone,” Journal of Biomechanics, vol. 11, no. 4, pp. 199-202, 1978.