Strain Gages and Rosettes Study Notes for Mechanical Engineering

By Mallesham Devasane|Updated : August 2nd, 2017

Measurement of Strain

Strain measurement can be performed in two ways namely,

  • Direct (electrical-type gauges based on Resistive, Capacitive, Inductive or Photoelectric principles)
  • Indirect (optical methods, such as Photoelasticity, the Moiré technique, or Holographic interferometry)

The majority of strain gauges are foil types, available in a wide choice of shapes and sizes to suit a variety of applications. They consist of a pattern of resistive foil which is mounted on a backing material. They operate on the principle that as the foil is subjected to stress, the resistance of the foil changes in a defined way.


The strain gauge is connected into a Wheatstone Bridge circuit with a combination of four active gauges (full bridge),two gauges (half bridge) or less commonly, a single gauge (quarter bridge). In the half and quarter circuits, the bridge is completed with precision resistors.

The complete Wheatstone Bridge is excited with a stabilised DC supply and with additional conditioning electronics, can be zeroed at the null point of measurement. As stress is applied to the bonded strain gauge, a resistive change takes place and unbalances the Wheatstone Bridge.

This results in a signal output, related to the stress value. As the signal value is small, (typically a few millivolts) the signal conditioning electronics provides amplification to increase the signal level to 5 to 10 volts, a suitable level for application to external data collection systems such as recorders or PC Data Acquistion and Analysis Systems.


Most manufacturers of strain gauges offer extensive ranges of differing patterns to suit a wide variety of applications in research and industrial projects.

They also supply all the necessary accessories including preparation materials, bonding adhesives, connections tags, cable, etc. The bonding of strain gauges is a skill and training courses are offered by some suppliers. There are also companies which offer bonding and calibration services, either as an in-house or on-site service. Rosette Strain Gage:

  • A strain gage only measures strain in one direction, in order to get principal strains,it is necessary to use a strain rosette
  • A strain rosette is a cluster of 3 strain gages oriented at different angles

A strain gage rosette is, by definition, an arrangement of two or more closely positioned gage grids, separately oriented to measure the normal strains along different directions in the underlying surface of the test part.

  • Rosettes are designed to perform a very practical and important function in experimental stress analysis.
  • Rosettes are manufactured from different combinations of grid alloy and backing material to meet varying application requirements.
  • They are also offered in a number of gage lengths, noting that the gage length specified for a rosette refers to the active length of each individual grid within the rosette.
  • To determine the three independent components of plane strain, three linearly independent strain measures are needed, i.e., three strain gages positioned in a rosette-like layout.

For example,Consider a strain rosette attached on the surface with an angle a from the x-axis. The rosette itself contains three strain gages with the internal angles b and g, as illustrated on the right.


Suppose that the strain measured from these three strain gages are ea, eb, and ec, respectively.

The following coordinate transformation equation is used to convert the longitudinal strain from each strain gage into strain expressed in the x-y coordinates,


Applying this equation to each of the three strain gages results in the following system of equations,


These equations are then used to solve for the three unknowns, ex, ey, and exy.


  1. The above formulas use the strain measure exy as opposed to the engineering shear strain gxyimage005. To use gxy, the above equations should be adjusted accordingly
  2. The free surface on which the strain rosette is attached is actually in a state of plane stress, while the formulas used above are for plane strain. However, the normal direction of the free surface is indeed a principal axis for strain. Therefore, the strain transform in the free surface plane can be applied.

Special Cases of Strain Rosette Layouts:

Case 1: 45º strain rosette aligned with the x-y axes, i.e., α = 0º, β = γ = 45º.



Case 2: 60º strain rosette, the middle of which is aligned with the y-axis, i.e., α = 30º, β = γ = 60º.



Uses of Strain Gages:
  • Strain gages attached to Wheatstone bridges can be used for measurement of tension, bending, and torsion.
  • In biomedical applications, strain gages can be used for determining forces in bones
  • Slightly modified strain gages can be used for muscle contraction and blood pressure measurement.


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