SPIE Press Book

Digital Shearography: Theory and Application of Digital Speckle Pattern Shearing Interferometry

The ever-increasing requirements of product quality and reliability demand more efficient measuring and testing methods. Developed in the past decade, shearography--also called speckle pattern shearing interferometry--is a coherent-optical measuring and testing method, similar to holographic interferometry. This book describes the development of second-generation digital shearography, which is especially effective in nondestructive testing, strain measurement, and vibration analysis due to its relative insensitivity to environmental disturbances.

1. Introduction and approach to problem: 1.1 Introduction; 1.2 Speckle patterns: a historical retrospect; 1.3 Laser speckle phenomenon; 1.4 Speckle statistics; 1.5 Reduction and exploitation of speckle noise; 1.6 State of the shearographic art
2. Shearographic metrology: 2.1 Fundamentals of the optical setup for shearography; 2.2 Refraction and interference of light waves passing a shearing element; 2.3 Relationship between the mechanics of components and the fringe; interpretation of the shearogram; 2.4 Generation of a shearogram; 2.5 Method of conventional shearography; 2.6 Fringe readout by Fourier filtering; References
3. Simple digital shearography: 3.1 Real-time observation of a shearogram; 3.2 Fringe interpretation of a shearogram; 3.3 Shearograms of elementary components; 3.3.1 Bending strain of a cantilever; 3.3.2 Tensile strength; 3.3.3 Steadily varying cross section of a tensile bar; 3.3.4 Torsion of a tube; 3.3.5 Evolution of the shearograms; References; 3.4 Rigid-body motion of the object; References
4. Phase-shifting shearography: 4.1 Fundamentals of the phase-shifting technique; 4.2 Arrangement of phase-shifting shearography; 4.3 Calculation of the relative phase difference ( in a shearogram; 4.4 The cranked tensile bar h/6 as a calibration object; References
5. Evaluation of the interferogram: 5.1 Filtering of the phase map; 5.2 Demodulation; 5.3 Determination of the absolute magnitude of the fringe order independent of boundary conditions; 5.3.1 General method for determining the absolute magnitude of the fringe order independent of the boundary conditions created by static loading; References
6. Nondestructive testing of materials: 6.1 Principle of shearographic testing of materials; 6.2 Phase-shifting shearography for nondestructive testing; 6.3 The experimental setup for NDT; 6.3.1 Introduction; 6.3.2 Laser diodes for interferometric measurements; 6.3.3 Experimental results by laser diode measurement; 6.3.4 General process of interferometric analysis; 6.3.5 Conclusions; References; 6.4 Applications of interferometric NDT; 6.4.1 Investigations of GFRP and CFRP components; 6.4.2 Types of loading for detecting emerging defects; 6.4.3 Investigation of compound materials and sintered metals; 6.5 Conclusions; References
7. Strain measurement: 7.1 Introduction; 7.2 State of the art with respect to strain measurement; 7.3 State of shearographic strain measurement; 7.3.1 Geometrical relations; 7.3.2 Optical relations; 7.4 Determination of the pure in-plane strains and the pure out-of-plane component using two illuminating beams; References; 7.5 Complete determination of the 2D deformation tensor; 7.5.1 Principle and setup for the direct measurement of (u/(x, (u/(y,(v/(x, (v/(y, and (w/(x, (w/(y for a single external loading; 7.5.2 Feasibility of the arrangement; 7.5.3 Operation of the Shearwin program; 7.6 Application of shearographic strain measurement; 7.6.1 Full-field and direct strain measurement; 7.6.2 Shearographic strain measurement using a tensile testing machine; 7.6.3 Shearographic investigation of strain concentration at a crack tip; 7.6.4 Shearographic investigation of local strain on the surface of concrete; References; 7.7 Shape identification by the shearographic technique; References; 7.8 Complete determination of flexural strains in thin plates under a single loading; References
8. Vibration analysis using continuous wave lasers: 8.1 Introduction; 8.2 Real-time observation of vibration by continuous wave illumination; 8.2.1 Real-time subtraction with a fixed reference frame; 8.2.2 Real-time subtraction with a continuously refreshed reference frame; 8.2.3 Quality assurance applications of real-time observation using time-averaged shearography; 8.2.4 General method for determining the absolute fringe order independent of the boundary conditions created by dynamic loading; 8.2.5 Industrial applications investigated by time-averaged digital speckle pattern shearing interferometry; 8.2.6 Accuracy of time-averaged interferometry; 8.2.7 Single- and multi-(composite) mode vibration; References; 8.3 Quantitative evaluation of a digital shearogram of a vibrating object; 8.3.1 Stroboscopic illumination used in conjunction with the phase-shifting technique; 8.3.2 Implementation of the arrangement for measuring vibrations by stroboscopic illumination; 8.3.3 Determination of the vibration mode and the dynamic bending and shear strains of the shearogram; References; 8.4 Numerical evaluation of stroboscopically illuminated shearograms; 8.4.1 Phase maps for different trigger positions; 8.4.2 Determination of the displacement field from the deformation gradients; 8.4.3 Theory and development of an advanced analysis program; 8.4.4 Conclusions; References
9. Digital shearography for quantifying heat flow rate: 9.1 Measurement tool; 9.2 Measurement principle; 9.3 Experimental investigation; 9.4 Quantitative evaluation; 9.5 Optimization of a heat exchanger; 9.6 Conclusions and outlook; References
10. Analysis of deviations: 10.1 Comparison of results obtained by different experimental methods; 10.2 Shearing amount; 10.3 Sensitivity vector; 10.3.1 Out-of-plane measurement; 10.3.2 In-plane measurement; 10.4 Phase shifting; 10.5 Demodulation; 10.6 Rigid-body motion; 10.7 Speckle size and numerical aperture; 10.8 Measurement range of shearography; References
11. Assessment of digital speckle pattern shearing interferometry: 11.1 State of the art; 11.1.1 Sources of measurement error using strain gauges; 11.2 Applications of digital shearography

Preface

The aim of this book is to present the shearographic method of strain analysis. The availability of instruments such as the laser and laser diodes and the personal computer itself in conjunction with the development of new methods and more sophisticated interferometric and evaluating techniques have greatly increased the range of problems for which digital shearography is the most viable means for investigating prototypes.

The author has endeavored to present the subject in such a manner that the book will meet the requirements of the beginner as well as being of assistance to the experienced research worker.

The author takes this opportunity to express his appreciation for the laboratory assistance of Dr.-Ing. M. Schuth, Dr.-Ing. L.X. Yang, Labor-Ing. Ing. grad. G. Kupfer, Dipl.-Ing. F. Vossing and Dipl.-Ing. P. Mackel, who wrote Sections 9.1 - 9.4. The author also wishes to express his gratitude to the mechanical and electronic workshops of University Gh Kassel for manufacturing the precision devices.

The author sincerely appreciates the help of Mrs. W. Barte in the careful preparation of the manuscript and its continuous revision and completion, as well as the design office of the University Gh Kassel for the figures and graphics.

Wolfgang Steinchen