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bridge null errors,
apparent strain error during the test, and
variations in gauge factors and gauge heating.
The test data do not include uncertainties resulting from temporal variation of the measurands
or procedural variations under loading, instrument calibration errors, temperature variation effects,
and electrical noise induced by operation of the loading test of the specimen, dynamic effects on the
gauges, including differences in creep and fatigue, or reduction curve fit errors.
11.7 OPTICAL STRAIN MEASURING TECHNIQUES
Optical methods for experimental stress analysis can provide fundamental information concerning directions and magnitudes of the stresses in parts under design loading conditions. Optical
techniques have been developed for the measurement of stress and strain fields, either in models
made of materials having appropriate optical properties, or through coating techniques for
existing specimens. Photoelasticity takes advantage of the changes in optical properties of certain
materials that occur when these materials are strained. For example, some plastics display a
change in optical properties when strained that causes an incident beam of polarized light to be
split into two polarized beams that travel with different speeds and that vibrate along the principal
axes of stress. Since the two light beams are out of phase, they can be made to interfere;
measuring the resulting light intensity yields information concerning applied stress. To implement this method, a model is constructed of an appropriate material, or a coating is applied to an
existing part.
A second optical method of stress analysis is based on the development of a moir e pattern,
which is an optical effect resulting from the transmission or reflection of light from two overlaid grid
patterns. The fringes that result from relative displacement of the two grid patterns can be used to
measure strain; each fringe corresponds to the locus of points of equal displacement.
Recent developments in strain measurement include the use of lasers and holography to very
accurately determine whole field displacements for complex geometries.
Basic Characteristics of Light
To utilize optical strain measurement techniques, we must first examine some basic characteristics
of light. Electromagnetic radiation, such as light, may be thought of as a transverse wave with
sinusoidally oscillating electric and magnetic field vectors that are at right angles to the direction of
propagation. In general, a light source emits a series of waves containing vibrations in all
perpendicular planes, as illustrated in Figure 11.19. A light wave is said to be plane-polarized
if the transverse oscillations of the electric field are parallel to each other at all points along the
direction of propagation of the wave.
Figure 11.19 illustrates the effect of a polarizing filter on an incident light wave;
the transmitted light is plane polarized, with a known direction of polarization. Complete
extinction of the light beam could be achieved by introduction of a second polarizing filter,
with the axis of polarization at 90 degrees to the first filter (labeled an Analyzer in Fig. 11.19).
These behaviors of light are employed to measure direction and magnitude of strain in
photoelastic materials.
492 Chapter 11 Strain Measurement
13:14:4 Page 492
bridge null errors,
apparent strain error during the test, and
variations in gauge factors and gauge heating.
The test data do not include uncertainties resulting from temporal variation of the measurands
or procedural variations under loading, instrument calibration errors, temperature variation effects,
and electrical noise induced by operation of the loading test of the specimen, dynamic effects on the
gauges, including differences in creep and fatigue, or reduction curve fit errors.
11.7 OPTICAL STRAIN MEASURING TECHNIQUES
Optical methods for experimental stress analysis can provide fundamental information concerning directions and magnitudes of the stresses in parts under design loading conditions. Optical
techniques have been developed for the measurement of stress and strain fields, either in models
made of materials having appropriate optical properties, or through coating techniques for
existing specimens. Photoelasticity takes advantage of the changes in optical properties of certain
materials that occur when these materials are strained. For example, some plastics display a
change in optical properties when strained that causes an incident beam of polarized light to be
split into two polarized beams that travel with different speeds and that vibrate along the principal
axes of stress. Since the two light beams are out of phase, they can be made to interfere;
measuring the resulting light intensity yields information concerning applied stress. To implement this method, a model is constructed of an appropriate material, or a coating is applied to an
existing part.
A second optical method of stress analysis is based on the development of a moir e pattern,
which is an optical effect resulting from the transmission or reflection of light from two overlaid grid
patterns. The fringes that result from relative displacement of the two grid patterns can be used to
measure strain; each fringe corresponds to the locus of points of equal displacement.
Recent developments in strain measurement include the use of lasers and holography to very
accurately determine whole field displacements for complex geometries.
Basic Characteristics of Light
To utilize optical strain measurement techniques, we must first examine some basic characteristics
of light. Electromagnetic radiation, such as light, may be thought of as a transverse wave with
sinusoidally oscillating electric and magnetic field vectors that are at right angles to the direction of
propagation. In general, a light source emits a series of waves containing vibrations in all
perpendicular planes, as illustrated in Figure 11.19. A light wave is said to be plane-polarized
if the transverse oscillations of the electric field are parallel to each other at all points along the
direction of propagation of the wave.
Figure 11.19 illustrates the effect of a polarizing filter on an incident light wave;
the transmitted light is plane polarized, with a known direction of polarization. Complete
extinction of the light beam could be achieved by introduction of a second polarizing filter,
with the axis of polarization at 90 degrees to the first filter (labeled an Analyzer in Fig. 11.19).
These behaviors of light are employed to measure direction and magnitude of strain in
photoelastic materials.
492 Chapter 11 Strain Measurement