numero25

D. Nowell et alii, Frattura ed Integrità Strutturale, 25 (2013) 1-6 ; DOI: 10.3221/IGF-ESIS.25.01 2 studies of deformations in the crack tip region. The latter work has led to a suggested life prediction approach which obviates the need to characterize the number of cycles experienced by the crack [9]. Despite the existence of rainflow and similar approaches, cycle counting can be a difficulty in non-uniform loading situations. The experimental work reported in the current paper springs from an investigation of crack closure originally carried out by de Matos [10]. However, the data was later re-processed to compare with a two-parameter crack tip field model proposed by Pommier and co-workers [11]. This work was reported at the Forni di Sopra workshop [12] and in the subsequent special issue [13]. However, the majority of the work reported was concerned with constant amplitude loading. A few experiments on simple overload cycles were carried out and the results of one of these experiments were presented in [13]. The displacement fields were converted into the two parameter description ( K,  ) and provided an interesting contrast to the results obtained from constant amplitude loading. The current paper therefore sets out to explore these results in more detail and to analyse a wider range of variable amplitude experiments. E XPERIMENTAL WORK he fatigue experiments carried out by de Matos have been reported elsewhere, but a brief summary is given here for clarity. Fig. 1 shows the main experimental set-up, which employs a CT specimen manufactured from 6082 T6 aluminium alloy. The specimen is loaded in fatigue and the area along the crack flanks is monitored using a Questar long range microscope and a digital camera (a low-cost USB webcam). A second camera is used to measure the crack length on the opposite side of the specimen, but this does not provide sufficient resolution for displacement measurement. The use of the microscope means that only a small area (approx. 600 x 400  m) close to the crack tip is imaged, but this permits high resolution and high accuracy evaluation of displacements. It should be noted that, because the primary focus of the investigation was crack closure, most of the image is along the crack wake, and there is very little information collected from the region ahead of the crack tip. Figure 1 : Experimental configuration: side and top views. Images were collected during fatigue loading at 30fps, whilst the specimen was loaded in a servo-hydraulic test machine at a frequency of 0.25 Hz. A set of 360 images was collected over four loading cycles at a number of discrete points during crack propagation. The loading frequency was increased between these measurement steps in order to propagate the crack in a reasonable time interval. Each set of images was analysed using a public domain Matlab script [14]. Rather than analyse a full displacement field, the relative displacement of five pairs of points along the crack flank was determined (Fig. 2). This information proved useful in investigating crack closure directly, but may also be employed to determine crack tip loading history. The elastic model for a sharp crack predicts that the displacement along the crack faces is given by: T