In light microscopy, the spatial transverse resolution is a function of the wavelength and numerical aperture. The depth resolution is another function of these parameters. The factors that enable the detection of fine detail, make the sharp focusing of more than a thin slice of the depth in an object impossible. When the examination of fracture surfaces is attempted using light reflection microscopy, the roughness will often restrict the in-focus parts of an image to a small portion of the field of view. Several authors have presented methods that enable a set of digitised images to be processed into a single composite image which contains the in-focus parts from each image. These methods are effective, unfortunately the noise present in each digital image is accumulated, resulting in increasingly noisy composite images as the number of images in a set is increased. During processing, a separate image depicting the heights in the surface, i.e. a contour map, may be produced. This image is the key that enables the production of an in focus composite image which does not accumulate noise. Image analysis under computer control will frequently require the use of automatic focusing. Several authors have published criteria which may be used to determine the state of focus of an image. Such criteria have a clear application to the above process. This paper presents an evaluation of some methods used for the processing of such images, and also some procedures used for the determination of sharpness of focus and demonstrates a sensitive method for the evaluation of such procedures. Finally, an implementation of a method which uses the one of the simplest focus criteria is presented, and a procedure for the production of deep focus images which are free from the accumulation of noise.
Quantitative fractography techniques have been in use at the Aeronautical Research Laboratory (ARL), Melbourne, Australia, for more than 15 years, and they have been developed into a specialized facility for deriving crack growth histories from fracture surfaces. This capability has played a fundamental role in a number of pioneering life extension programs which have permitted the continued operation of several aircraft types in current service with the Royal Australian Air Force (RAAF). The increasing RAAF demand for advanced fractography has required the development over a number of years of computer-based semiautomated data acquisition and data processing facilities for fractographic analysis; these developments have permitted a significant improvement in the speed with which fracture surfaces may be analyzed and are now used routinely in ARL's quantitative fractography.
This paper describes current ARL activities in the use and further development of quantitative fractographic techniques and the specialized facilities employed to derive and process data from fracture surfaces observed by both scanning electron microscopy and optical microscopy. The results able to be obtained are illustrated by reference to case histories of defects in components from service aircraft, from full-scale component tests, and from laboratory test samples. Procedures for minimizing or overcoming some of the difficulties associated with interpretation of fracture surface markings are discussed. The paper also describes the way in which fracture mechanics concepts need to be used in conjunction with fracture surface analysis to achieve a correct understanding of the crack growth history. In particular, some empirically based approaches developed for interpolation and extrapolation of incomplete crack growth histories may now be understood by reference to fracture mechanics concepts. Finally, current and future developments in fracture surface analysis are discussed, with particular reference to the benefits to be gained by the introduction of modern image processing and image enhancement techniques.
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