Strain analysis of a disk subjected to diametral compression by means of holographic interferometry

Detection of surface strain by three-dimensional digital holography

Three-dimensional digital holography with three object-illuminating beams has been successfully used for the detection of surface strain in metallic objects. The optical setup that uses illuminating beams to irradiate the object from three directions means that all three object surface displacement components, x, y, and z, can be independently calculated and used to find the strain gradients on the surface. The results show the conversion of the complete surface displacement field into a surface strain field. The method is capable of measuring microstrains for out-of-plane surface displacements of less than 10 microm.

Statistical analysis of errors in holographic interferometry

A statistical analysis of the effect of measurement errors on the determination of small vector displacements of diffusely reflecting objects by holographic interferometry is reported. There are two sources of error: inaccurate measurement of fringe-order number and inaccurate measurement of illumination and observation directions. Standard deviations of errors in measured displacements due to these two sources are presented in graphical form for a variety of holographic system geometries. Absolute fringe orders should be measured whenever possible, because fringe readout by scanning leads to very large errors.

Implementation of hologram interferometry with a continuously scanning reconstruction beam

A system for fringe evaluation in hologram interferometry is studied in which a conjugate reconstruction beam scans the hologram, and the resulting fringe motion is observed in the real image of the object. The principle is suitable for semiautomatic evaluation. An earlier article presented a detailed theoretical analysis of the measuring principle, and the present paper deals with the practical aspects. It is shown that the system is capable of interpolating fringes to 1/100 of a fringe, which is experimentally verified by the determination of 3-D displacement components with an accuracy of about 40 nm and of strain with an accuracy of about 10 microstrain over an observation length of 1 cm. An analysis of the different error sources shows that the mechanical measures of the recording and evaluation setup have to be known to a small fraction of a millimeter.

Techniques of holographic displacement measurement: an experimental comparison

Several of the existing methods of holographic displacement measurement have been discussed, procedures for their use outlined, and the techniques themselves compared experimentally for a variety of test situations. The results show that each technique has a unique application depending on the degree of accuracy and precision required for a particular measurement.

Homogeneous deformations: determination by fringe vectors in hologram interferometry

When an object undergoes a combination of minute rigid-body motion and homogeneous deformation, its vectorial displacements can be described by a general, 3 x 3 matrix transformation. This same matrix can be used to transform the sensitivity vector of hologram interferometry into a fringe vector that defines the fringes as laminae that are intersected by the surface of the object. When fringes are observable on more than one surface of a three-dimensional object whose shape is known, it is possible to determine the fringe vector from the shape and spacing of the fringes on the object. If three holographic views are available, from which three fringe vectors can be determined for three known sensitivity vectors, it is possible to determine the transformation matrix, and this, in turn, can be decomposed into deformation and rotation matrices. More than three views allow the use of least-square-error theory to minimize errors in data taking.

Measurement of three-dimensional displacements by scanning a double-exposure hologram

Measurement of displacements and strains is performed by extracting with a new scanning method the information from a double-exposure hologram. A real image is formed by a conjugate reference beam that illuminates a small part of the plate defining an observation direction for each point of the object under test. Sweeping the beam across the plate, which corresponds to a variation of the viewing direction, produces a movement of the fringes on the surface of the observed object. The fringes are counted during each scanning by a detector placed at the observed point. A minimum of three scannings provides all the information required to determine the three-dimensional displacement of the point itself.