Skip to main content
main-content

Über dieses Buch

Rapid strides have been made in the use of digital image processing tech­ niques for data acquisition in photoelasticity in the last two decades. Techniques such as fringe thinning, fringe clustering, fringe tracing, phase shifting, polarization stepping and Fourier transform methods have significantly contributed to the automation of data acquisition. The recent developments in colour image processing and development of tricolour light source have added a new dimension. The use of time delay and integration (TDI) camera techniques has extended digital photoelasticity for dynamic analysis. Now the field of Digital Photoelasticity has matured to a level where it could be used to solve problems in industries. Apart from developments in data acquisition techniques, several methods have also come into existence for efficient processing of experimental data. Extensive use of computer graphics has found a unique place in presenting the experimental results in a meaningful way. Though there has been significant developments in data processing and data acquisition in the last two decades, there is no book available yet to present these developments in a comprehensive way. The motivation for this book is based on the experience of teaching the course on Experimental Methods in Stress Analysis at lIT Kanpur for the last 10 years. I have always felt a need for introducing Digital Image Processing in an appropriate way, which will be useful for an experimentalist.

Inhaltsverzeichnis

Frontmatter

1. Transmission Photoelasticity

Abstract
Photoelasticity is an experimental method for analysing stress or strain fields in mechanics. The technique of photoelasticity is very well developed and many standard textbooks [1–7] have been written. An early description of the method of photoelasticity was provided by Coker and Filon [1] in 1931. Then, in 1937, Oppel [8] introduced the concept of frozen stress photoelasticity, which has facilitated the analysis of three-dimensional problems with the use of two-dimensional concepts. This is achieved by initially stress freezing the model and then mechanically slicing it. The mechanical slicing was replaced by optical slicing with the use of scattered light by Weller [9] in 1939. A new branch of photoelasticity viz., the scattered light photoelasticity was then developed [10]. Analysis of a three-dimensional model as a whole was proposed by Aben [11] and the technique is known as integrated photoelasticity.
K. Ramesh

2. Reflection Photoelasticity

Abstract
Reflection photoelasticity is an extension of transmission photoelastic analysis for the analysis of opaque prototypes. A thin temporarily birefringent coating is pasted on the prototype with a reflective backing at the interface. The prototype is loaded appropriately and a reflection polariscope is used for collecting the optical information.
K. Ramesh

3. Digital Image Processing

Abstract
The term digital image processing (DIP) generally refers to the processing of a two-dimensional picture by a digital computer. A digital image is an array of real numbers represented by a finite number of bits. An image given in the form of a photograph or a slide is first digitised and stored as a matrix of binary digits in computer memory. This digitised image can then be processed and/or displayed on a high-resolution monitor. Early systems of image processing were configured around big computers such as a PDP 11 system. Recent advances in computer technology have brought in the development of plug-in cards, which can make a conventional PC into an image processing station. These cards are known as frame grabbers. Monochrome and colour frame grabbers are available in the market.
K. Ramesh

4. Fringe Multiplication, Fringe Thinning and Fringe Clustering

Abstract
Isochromatics and isoclinics are the two fringe contours obtained in a photoelastic experiment. Isochromatics are contours of constant principal stress difference and isoclinics are the loci of points along which the principal stress orientation (with respect to a reference axis) is a constant. For stress analysis at any point, the photoelastic data required are,
1.
The isochromatic fringe order and
 
2.
The isoclinic parameter at that point.
 
K. Ramesh

5. Phase Shifting, Polarization Stepping and Fourier Transform Methods

Abstract
In the previous chapter, data acquisition by fringe skeleton identification was discussed. The skeleton identification became much simpler and effective if intensity variations over the fringe field were also taken into account. In view of skeleton identification, the data being collected is restricted to these zones. In the early stages of automatic acquisition of photoelastic data, several point-by-point methods were proposed which also utilised intensity information for automation. In these techniques, either the analyzer/polarizer or the compensator is rotated continuously to produce a modulated intensity signal at the point of interest. Data is recorded based on either the intensity signal is monitored for its minimum value or the phase of the modulated signal is compared with that of a reference signal [1-11]. Thus, the use of intensity information in some form has always attracted researchers to improve the methodology of data acquisition in photoelasticity.
K. Ramesh

6. Phase Unwrapping and Optically Enhanced Tiling in Digital Photoelasticity

Abstract
In the previous chapter, various techniques to obtain fractional retardation over the complete model domain have been presented. In these techniques, the fractional retardation is represented as a phase map. In the phase map, the fractional fringe orders in the range of 0-1 are represented as an intensity map of 0-255. For practical utilisation of the data, one has to find the total fringe order over the domain. This is achieved by a process called phase unwrapping. For phase unwrapping to be effective, the phase map should be free of noise and discontinuities.
K. Ramesh

7. Colour Image Processing Techniques

Abstract
With rapid advancements in computer technology, colour image processing systems are now available at affordable prices. A proper understanding of the colour image processing techniques requires an understanding of the various models for colour representation. Apart from this, one needs to know the spectral response of the colour camera and the light source. The type of colour image processing hardware used for transmission or reflection photoelastic analysis has an influence on the quality of data acquisition. Data interpretation and analysis requires the understanding of the intensity of light transmitted, for various polariscope arrangements, in white light
K. Ramesh

8. Evaluation of Contact Stress and Fracture Parameters

Abstract
In many mechanical devices, there exist contact between two or more parts such as in gears, rolling element bearings, locomotive wheels and rails etc. The stresses caused by the pressure distribution between the bodies in contact are of importance in the design of these parts. Hertz [1] pointed out that in the absence of friction, the maximum shear stress occurs beneath the surface of contacting bodies. This leads to pitting of the contacting surfaces. The material lost from the surface due to pitting may get trapped into the contacting surfaces, causing abrasive wear. Smith and Liu [2] studied the effect of friction between the contacting surfaces. They reported that under certain conditions, the point of maximum shear stress could also occur at the surface of contacting bodies. The knowledge of contact zone and coefficient of friction between the contacting bodies is essential for evaluating the design of such contacting elements.
K. Ramesh

9. Stress Separation Techniques

Abstract
Photoelasticity directly provides the information of principal stress difference and the orientation of the principal stress direction at the point of interest. Using these, one can find the normal stress difference and in-plane shear stress by invoking equations in mechanics of solids or Mohr’s circle.
K. Ramesh

10. Fusion of Digital Photoelasticity, Rapid Prototyping and Rapid Tooling Technologies

Abstract
The recent advances in rapid prototyping (RP) [1–5] have made it possible to produce prototypes of very complicated parts directly from three-dimensional computer aided design (CAD) models without using part specific tooling. Application of these techniques for producing prototypes leads to considerable reduction in the total cycle time. There are a number of RP techniques developed so far, such as stereolithography (STL), fused deposition modelling (FDM), solid ground curing (SGC), selective laser sintering (SLS) etc. This technology has emerged in response to the need for reducing the lead-time to produce physical prototypes. These models could be directly used for checking form, fit and function, getting management approval for detailed design and development, conducting market research, submitting for price negotiations and as models for wind tunnel testing.
K. Ramesh

11. Recent Developments and Future Trends

Abstract
The application of digital techniques for data acquisition in 2-D transmission photoelasticity and reflection photoelasticity is well developed and these were discussed in the earlier chapters. The extension of digital techniques for data acquisition to integrated photoelasticity and scattered light photoelasticity have just made a beginning. In integrated photoelasticity, one has to determine three parameters experimentally and these are known as characteristic parameters. In scattered light photoelasticity, the illumination levels are quite low due to light scattering. This necessitates the use of high-resolution CCD cameras. The ultimate aim of integrated photoelasticity or scattered light photoelasticity is to evaluate the stress field interior to the model. The current developments on tensorial tomography are discussed in this chapter.
K. Ramesh

Backmatter

Weitere Informationen