Issue 30

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 30 (2014) 369-374; DOI: 10.3221/IGF-ESIS.30.44 371 Resonance machines Resonance testing machines provide a simple means for tests involving a large number of fields. These machines can operate at much higher frequencies than servohydraulic machines and have other salient advantages including reduced size, servicing requirements and energy consumption (1/100)[4]. However, resonance machines pose some problems in implementing DIC on fatigue loaded specimens. Thus, the resonance frequency cannot be accurately controlled by the user as it depends on the rigidity of the target specimen and changes lengthwise and widthwise in it. Also, the specimen cross-section decreases as cracks grow, which alters its rigidity and hence its resonance frequency. The inability to accurately set and maintain the resonance frequency throughout a test hinders the use of stroboscopic cameras, which are frequently used on servohydraulic machines. Stroboscopic cameras can take photographs at user- defined frequencies. In fixed-frequency tests, this allows the camera to be set at the test frequency and a static video filmed for subsequent DIC processing with a view to calculating deformations due to crack formation and growth [3]. One other technique used with servohydraulic machines involves periodic halting of the test to photograph the specimen. This technique is difficult to implement with resonance machines because they are mounted on springs and hence in continuous motion. The oscillations are insubstantial to the naked eye but can have a considerable impact on DIC results. As stated above, in this work we developed a simple solution to the above-described problems. N EW ACQUISITION SYSTEM he difficulties encountered in applying DIC to a resonance testing machine were overcome by developing a new method to acquire images involving the use of very small Digimicro microscopic cameras with magnifications of 10–100× and 40–200× (see Fig. 4). Figure 4 : Microscope with a USB interface used for in situ measurements of crack growth. This type of camera had previously been successfully used to monitor crack growth uninterruptedly in planar specimens [6–8]. With DIC, however, even a very small displacement can lead to error in calculating deformations. This required altering the cameras for sticking to the specimen in roughly the same manner as a contact extensometer in order to prevent relative motion between the specimen and acquisition system. Fig. 5 depicts the camera assembly used, which included two cameras in order to allow both sides of the specimen to be photographed at once. The cameras were mounted by means of elastic bands and fixed by means of two small metal pins along their equator. Figure 5 : Microscopes attached to the specimen surface. T