Measurement of Small Cracks by Photomicroscopy: Experiments and Analysis

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Abstract
A procedure for using photomicroscopy to record the growth of small fatigue cracks is presented. Using a specially designed fatigue specimen, the method is applicable to both naturally initiated cracks and cracks initiated from a small electro-discharge machined notch. Components of the experimental apparatus, which are low cost and readily available, include a standard metallurgical microscope, a 35-mm camera with bulk film capability, an electronic flash, and a microcomputer to control the fatigue machine and record test data. The photographic record provides a direct measurement of surface crack length and documents crack interactions with microstructural features; measurement precision less than 1 μm is possible. Following a test, the photographs of small cracks are projected on a computer digitizing tablet for convenient measurement of crack length. The crack length data are then combined with fatigue cycle-count data and reduced to the form of da/dN versus ΔK (or some other appropriate crack driving force). The capabilities of the photomicroscopic method are illustrated using typical data from specimens of the alloy Ti-6Al-2Sn-4Zr-6Mo, and an assessment is made of the practical advantages and limitations of the technique. Finally, some commonly unrecognized pitfalls that routinely arise in the analysis of small-crack data are discussed, and an alternative procedure for the analysis of such data is presented.

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