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Spie Press Book

Robust Speckle Metrology Techniques for Stress Analysis and NDT
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Book Description

Optical techniques are usually applied inside laboratories equipped with temperature, humidity and vibration control. These techniques are very suitable for fast measurements due to their noncontact nature and their capability to measure on surfaces without special, time-consuming preparation. Among them, optical methods based on the speckle phenomenon have developed substantially over the last two decades due to the development of digital image processing, digital cameras, computers, lasers, and optical components. However, applying speckle methods outside of the laboratory becomes a challenging task. This book presents techniques and tools that will enable the development of robust measurement instruments to be used outside the laboratory for nondestructive structural-integrity-evaluation devices. Additionally, several technical solutions that combine mechanical systems to solve industrial measurement demands are described.

Book Details

Date Published: 7 October 2014
Pages: 200
ISBN: 9781628413182
Volume: PM251

Table of Contents
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Table of Contents

List of Symbols and Notations

1 NDT Applications in Engineering
1.1 Mechanical Design of Structures and Mechanical Parts
     1.1.1 Mechanical-design procedure
     1.1.2 The influence of working conditions on the mechanical performance of the structure
1.2 The Importance of Inspection: Avoiding Accidents
1.3 Application of Nondestructive Techniques
1.4 The Importance of Optical Techniques as Nondestructive Evaluation Tools
1.5 Requirements for Field Applications of Optical Techniques

2 Principles of Digital Speckle Pattern Interferometry
2.1 Introduction
2.2 Speckle Principle
2.3 Objective and Subjective Speckle
     2.3.1 Objective speckle
     2.3.2 Subjective speckle
2.4 Speckle Interferometry
2.5 Phase Shifting as a Quantitative Tool
2.6 Phase-Unwrapping Processing

3 Optical Configurations for Measurements Using DSPI
3.1 Displacement Measurements
     3.1.1 Out-of-plane sensitivity
     3.1.2 In-plane sensitivity
 Measuring with only one sensitivity direction
 Measuring along two sensitivity directions
 Measuring along radial sensitivity directions
3.2 Derivative Measurements
3.3 Concluding Remarks

4 Robust Optical Systems
4.1 Introduction
4.2 Negative Environmental Agents for Optical Methods
     4.2.1 Temperature
     4.2.2 Humidity
     4.2.3 Atmospheric conditions
     4.2.4 Shock and vibration
     4.2.5 Radiation and background illumination
4.3 Requirements for Robust Measurements
     4.3.1 Robust
     4.3.2 Flexible
     4.3.3 Compact
     4.3.4 Stable
     4.3.5 User-friendly
     4.3.6 Organized
4.4 Possible Solutions to Improve Robustness
     4.4.1 Isolation
 Environmental isolation
 Temperature isolation
 Radiation isolation
 Vibration isolation
     4.4.2 Robustness

5 Quantitative Evaluation of Stresses and Strains
5.1 Mechanical Stress and Strain Fields
5.2 Experimental Measurement of Stress and Strain Fields
5.3 Interferometric Solutions to Measure Mechanical Stress and Strains
5.4 In-Field Applications
5.5 Final Remarks

6 Quantitative Evaluation of Residual Stresses
6.1 Residual Stress Fields
6.2 Experimental Measurement of Residual Stresses
6.3 Interferometric Solutions to Measure Residual Stresses
6.4 Qualitative Evaluation of Residual Stresses by Indentation
6.5 In-Field Applications
     6.5.1 Determining the external loading of a pipeline
     6.5.2 Measurement of combined stresses in a gas pipeline in service
6.6 Final Remarks

7 Qualitative Fault Detection and Evaluation
7.1 Traditional Nondestructive Methods to Detect Defects
     7.1.1 Visual inspection
     7.1.2 Radiographic techniques
     7.1.3 Magnetic techniques
     7.1.4 Ultrasonic techniques
     7.1.5 Liquid-penetrant inspection
     7.1.6 Eddy-current methods
     7.1.7 Thermography
7.2 Shearography as a NDT Inspection Tool
     7.2.1 Optical configurations suitable for field applications
     7.2.2 The importance of shearography as a NDT inspection tool
7.3 Excitation Methods Used for Flaw Detection
7.4 Requirements for in situ Applications
     7.4.1 Uncooperative surfaces
     7.4.2 Large inspection areas
     7.4.3 Quality control for in-line production
     7.4.4 Loading adjustment
7.5 Optical Setups for Inspection
     7.5.1 Pipeline inspection
     7.5.2 Tank inspection
     7.5.3 Aeronautical inspection
7.6 Image Processing Tools for Fast Defect Identification
7.7 Commercial Systems

8 Digital Image Correlation for Structural Monitoring
8.1 Noninterferometric Methods for Monitoring
8.2 Fundamentals of Image-Matching Methods
8.3 Subset Shape Functions
8.4 Optimization Criteria for Pattern Merging
8.5 Optical Configurations Used in DIC
     8.5.1 In-plane measurements using 2D DIC
     8.5.2 Three-dimensional measurements using DIC
8.6 Application Examples

9 Closing Remarks


The invention of the laser in the early 1960s allowed for light sources with a high coherence degree, which generated many novel research lines in order to make use of them. People working with these light sources noticed that a high-contrast and fine-scale granular pattern was produced when a rough surface was illuminated with laser light. This effect was called a "speckle effect," characterized by a random distribution of scattered light. After recognizing that each speckle has a definite phase, several techniques were developed to measure deformations, displacements, stresses, vibrations, and inner defects.

Several multiauthor books have been published beyond the first one published in 1978 (Speckle Metrology, edited by R. K. Erf) - including Digital Speckle Pattern Interferometry and Related Techniques, edited by P.K. Rastogi, and Advances in Speckle Metrology and Related Techniques, edited by G. H. Kaufmann - show new branches in speckle metrology, new proposed schemes and improvements in processing techniques, and optical approaches that have occurred over the last 20 years.

The main goal of nondestructive testing (NDT) is to detect and characterize anomalies that can adversely affect the performance of the component under test without impairing its intended service.

Optical techniques can be considered as alternative approaches to traditional NDT methods. They are very attractive for NDT due to their noncontacting nature and their high relative speed of inspection. The application of digital techniques allows for automatic processing. Consequently, a fast inspection procedure enables the evaluation of large areas (e.g., aircraft wings and ship structures) or a large number of parts (e.g., automotive components). Speckle techniques have the advantages cited for optical methods. Additionally, they are adequate for the evaluation of real components without further preparation of the surface or time-intensive analysis.

This book provides tips, ideas, and examples for the successful application of optical techniques (more specifically based on the speckle phenomenon) outside the laboratory room. Readers can see that the topics presented in the following nine chapters have been selected to benefit graduate students, engineers, and scientists who are interested in the in-field application of speckle techniques to solve specific problems related to optical metrology, experimental mechanics, and NDT.

Chapter 1 discusses aspects to consider when designing mechanical parts and structures for safe and reliable products because several applications are usually related with human life and ecology. This chapter also shows the working conditions influence the performance and mechanical integrity of the part. This influence can sometimes cause an accident due to a lack of corrective actions. For this reason, the chapter highlights the use of NDT to foresee possible accidents and focuses on optical techniques, especially speckle methods.

Chapter 2 addresses the theoretical aspects of the origin and formation of the speckle phenomenon. The most important principles for speckle interferometry are then developed, showing how the phase of the speckle distribution carries essential information for measuring displacements fields, object shapes, etc. For this reason, several tools to quantify the phase of the speckle distribution are presented, as well as the phase-unwrapping principles that are used to deal with 2π jumps obtained after the use of phase-shifting techniques.

Chapter 3 presents traditional digital-speckle-pattern-interferometry (DSPI) optical configurations used to measure displacement fields and their derivatives. Measurements are divided into (a) out-of-plane and (b) in-plane displacements. For the former, the working principle is presented, as well as a possible laboratory optical setup. For the latter, traditional interferometers with in-plane sensitivity are presented; radial, in-plane interferometer setups capable of measuring polar coordinates are also presented. Finally, principles for shearography are shown.

Chapter 4 gives a more-detailed description of the requirements for robust optical setups. The chapter offers tools, tips, and reference parameters to guide the development and design of interferometers based on the speckle phenomenon for use outside of the laboratory. Additionally, various environment agents are described, showing the effect that they have on the measuring performance of the optical system.

Chapter 5 discusses the application of DSPI to measure mechanical stresses as an auxiliary tool for structural integrity assessment. After a short introduction, the principles for traditional strain-gage sensors are presented. Some interferometric solutions are shown in order to measure 3D displacements (along three sensitivity directions) and displacements in polar coordinates. For the latter, several tips are listed for the measurement of large strain fields without loss of correlation. Finally, an application example shows the effectiveness of the proposed solution.

Many service failures of structural or mechanical components are caused by a combination of residual stress fields in the material and mechanical stresses produced by applied loads. For this reason, Chapter 6 provides experimental solutions to compute residual stresses. The traditional method combines strain gages with the hole-drilling technique. In this case, a small hole is introduced into the material, allowing for local stress relief that enables stress measurements. The chapter also explores a combination of the holedrilling technique and DSPI. A practical application outside the laboratory is described, showing the high potential of the technique as an integrity-evaluation tool.

Chapter 7 begins with a list of the traditional nondestructive techniques used in defect detection. The chapter highlights shearography as a NDT tool with important applications in the automotive, aeronautical, and petroleum and gas industries. Several optical configurations suitable for in-field applications are presented. One of the most important components in a shearographic device is the loading/excitation setup. For this reason, several possible methods are described. Finally, applications in some industries, mechanical parts, and structures are shown. Available commercial systems highlight the fast growth of shearography as a NDT technique. Some significant commercial devices are illustrated in this chapter.

Previous chapters address principles, optical setups, and application examples for interferometric techniques based on the speckle phenomenon. Another optical speckle technique that has grown quickly over the last two decades is digital image correlation (DIC), which is considered a noninterferometric technique. A short review of the available literature about this technique is presented in Chapter 8, which is oriented to NDT applications.

Finally, Chapter 9 briefly discusses all of the presented techniques to help readers select the best optical setup for their needs, or, beyond that, develop new solutions (for those cases where there are none) to measure a specific measurand.

We would like to thank the following people: Prof. Guillermo Kaufmann and SPIE Press Manager Tim Lamkins for their encouragement before writing this book; Prof. Gary Schajer for his kind help and valuable collaboration with some figures obtained by residual stress measurements with the hole-drilling techniques; Prof. Gustavo Galizzi for his help during the elaboration of some simulated figures used in the phase-unwrapping section; Dr. Gordon Craggs for several fruitful discussions about Chapters 2 and 3 and for his help with some phrasing; the peer reviewers for their important comments and corrections; and Scott McNeill and the SPIE editorial department for their help and support.

Last, but not least, we are grateful to our families for their support and patience during our time "inside the book." In particular, we would like to give thanks to God for the opportunity to write this book.

Matias R. Viotti
Armando Albertazzi, Jr.

Florianopolis, Brazil
August 2014

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