Issue 33

D. Nowell et alii, Frattura ed Integrità Strutturale, 33 (2015) 1-7; DOI: 10.3221/IGF-ESIS.33.01 1 Focussed on characterization of crack tip fields Measurement and analysis of fatigue crack deformation on the macro- and micro-scale D. Nowell, S.J. O’Connor, K.I. Dragnevski University of Oxford, UK david.nowell@eng.ox.ac.uk, samuel.oconnor@eng.ox.ac.uk, kalin.dragnevski@eng.ox.ac.uk A BSTRACT . The paper describes an experiment which performs in-situ loading of a small compact tension specimen in a scanning electron microscope. Images are collected throughout a number of successive loadincg cycles. These are then analysed using digital image correlation (DIC) in order to produce crack flank displacements as a function of load. This data is then compared with a simple elastic approach, and it is concluded that elastic-plastic analysis is required in order to accurately capture the displacements close to the crack tip. A simple approach due to Pommier and Hamam is therefore employed. This gives a better representation of the data, but predicts a variation of crack tip displacement,  , which is difficult to explain from a physical perspective. The need for a more sophisticated analysis of the data is therefore highlighted. K EYWORDS . Crack Tip Displacements; Digital Image Correlation; Elastic-Plastic Fracture Mechanics. I NTRODUCTION he understanding of fatigue crack propagation is an essential pre-requisite to safe operation of many engineering structures and systems. Most damage tolerant life prediction approaches are based on the application of experimental crack propagation data to the real system. For example, the Paris Law [1] is frequently used to apply experimental da/dN vs delta K data to service loads and to the system geometry. However, most experimental data is obtained for constant amplitude loading whereas engineering systems frequently experience non-uniform loading. The presence of history effects in fatigue crack propagation is well known and this means that life prediction under service loading conditions remains a challenging problem in many cases. A detailed understanding of the crack tip response to a range of load histories is the key to improvements in this area. Recent work at Oxford has been presented at the Forni di Sopra [2,3] and Malaga [4] IJ Fatigue/FFEMS workshops and has concentrated on the use of digital image correlation to measure and analyse the displacements fields around a crack. Our work has made use of a long-range optical microscope to examine deformations in a region within 0.5 mm of the crack tip. Analysis of these deformations has allowed stress intensity factors to be calculated and crack closure assessed. In the current paper we will seek to extend this approach by reporting measurements taken during in-situ loading of a fatigue crack in a scanning electron microscope. This permits more detailed examination of the displacement field in the neighbourhood of the crack tip. T