Issue 42

M. Peron et alii, Frattura ed Integrità Strutturale, 42 (2017) 223-230; DOI: 10.3221/IGF-ESIS.42.24 227 E XPERIMENTAL PROCEDURE mong giant magnetostrictive materials, the commercially named Terfenol-D alloy, supplied by Etrema Products, Inc. (USA) was used in all tests and analyses. The material properties are listed in Tab. 2. Test were performed with the aim to measure the fracture load, P c, of single edge precracked specimens, subjected to three point bending, in presence and in absence of the magnetic field and at various loading-rates. Specimens were 5 mm thick, 3 mm wide and 15 mm long. Before testing, all specimens were weakened on one side by a 0.5 mm deep crack, which was introduced using a tungsten cutter. Tested specimen is showed in Fig. 1. Figure 1 : Specimen’s geometry and edge micrograph of the introduced crack. The load P has been impressed at the midpoint of the specimens, which were simply supported with span of 13 mm, by means of a 250 N load cell (resolution: 0.01 N). The load was applied for different loading-rates: 0.05, 0.5 and 3.0 Ns -1 . A uniform magnetic field, with magnetic induction B 0 , has been applied in the longitudinal direction through an electromagnet. As devices in which Terfenol-D is employed commonly work in magnetic induction range which varies from 0.02 T to 0.05 T, the representative value of 0.03 T has been adopted in all tests. It is due to point out that, as alloying elements in Terfenol-D are Terbio and Disprosio, which are very expensive rare earths, the number of tested specimens was limited: from two to three at each condition. By means of experimental procedure it has also been possible to assess the second order magnetoelastic constant, m 33 . Let consider a Cartesian coordinate system, O-x y z , which origin is located at the top center of an uncracked specimen. Varying the intensity of the magnetic field applied in the z -direction (longitudinal direction), the trend of magnetostriction has been measured through a strain gauge located at x = y = z = 0 mm. By comparison between the measured strain ε zz and the numerically obtained one, it has been found that the proper value for the second order magnetoelastic constant is 4.82×10 -12 m 2 A -2 . This value has been used in the analyses to compute the SED. R ESULTS AND DISCUSSION racture load, P c , in presence and absence of the magnetic field have been experimentally measured at each loading- rate. Data, in terms of fracture load, are summarized in Tab. 3. Bold numbers represent the average value at each condition, whereas numbers in brackets represent the relative standard deviations. Average fracture loads are presented in Fig. 2. The error bars indicate the maximum and minimum values of P c The average fracture load at 0.05 Ns -1 , 0.50 Ns -1 and 3.0 Ns -1 are decreased respectively about 7%, 9% and 14% in the presence of the magnetic field. It has also been found that Terfenol-D shows a decrease in fracture load as the loading-rate decreases. Similar behavior has been observed for other materials such as TiAl alloys, by Cao et al. ([22]) and piezoelectric ceramics, by Shindo et al. ([23-24]) and Narita et al. ([25]). To take into account the effect of the loading-rate on Terfenol-D fracture load, here it is assumed that the critical radius R c , which depends on the material and on the notch opening angle, varies also with the speed at which the load is applied. By plotting the averaged SED related to the mean values of critical loads in Tab. 3, in presence and in absence of the magnetic field, as a function of control volume radius, it is possible to determine different intersections for each loading- rate. The intersections have been found at 0.05, 0.056 and 0.1 mm respectively for the loading-rates 0.05, 0.5 and 3.0 Ns -1 . A F