PSI - Issue 3

3

G.M. Domínguez Almaraz et al. / Procedia Structural Integrity 3 (2017) 562–570 Author name / Structural Integrity Procedia 00 (2017) 000–000

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Fig. 1. (a) Mold used for the fabrication of PMMA specimens, and (b) PMMA specimens after the heat curing polymerization process.

The cylindrical bars of PMMA shown in Figure 1b, have been machined to obtain the hourglass shape ultrasonic testing specimens: the specimen profile was determined by modal numerical simulation using some of the physical and mechanical properties of testing specimen (density, elastic modulus and Poisson coefficient), and its dimensions. Under resonance, which is the condition to carry out ultrasonic fatigue tests, the natural frequency of vibration of testing specimen must be close to the frequency of the excitation source: 20 KHz. This was achieved by modal numerical simulation and varying the PMMA specimen dimensions to obtain the longitudinal frequency of vibration close to 20 KHz. The Figure 2a shows the modal simulation result with the obtained frequency of vibration for the PMMA ultrasonic fatigue testing specimen, and Figure 2b presents its resonance dimensions at 20 KHz.

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Fig. 2. (a) Modal analysis to obtain the natural frequency of vibration of testing specimen, (b) Dimensions (mm) of testing specimen, and c) specimen and attachment to ultrasonic fatigue machine. The dimensions of testing specimen presented on Figure 2b correspond to the lowest length of specimen (28 mm); in this study was investigated by modal analysis the lowest dimensions for the testing specimen in order to reduce the temperature in this poor thermal conducting material. Three precautions were undertaken to carry out ultrasonic fatigue tests on this polymer: 1) the dimensions of the hourglass shape specimen were as small as possible, fitting the resonance condition, to limit the temperature gradient at neck section, 2) The applied load was low: about 9 to ~14% of the ultimate tensile stress, and 3) The testing specimens were immersed in a liquid simulating the human saliva (pH = 7), to increase the heat dispassion (Dominguez et al, 2015). The frequency vibration of 20457 Hz was adequate, since the specimen was attached to the ultrasonic fatigue machine by a steel screw bolt (5 mm of diameter and 9 mm length), Figure 2c; this implies that PMMA material is