ABSTRACT
This work aims at studying the effect of porosity in particulate reinforced metal-matrix composites on the statistical amplitude distribution of backscattered laser-induced ultrasonic pulses in these composites. A special laser-ultrasonic transducer used in experiments combines laser excitation and piezoelectric detection of broadband ultrasonic pulses in composite specimens with only one plane surface available for laser irradiation. We studied stir cast hypereutectic aluminium-silicon alloy A336 matrix composites reinforced with the SiC micro particles (volume fractions of 0.033-0.135) and in-situ reactive cast aluminum matrix composites reinforced with the Al3Ti intermetallic particles (volume fractions of 0.04-0.115). The amplitude distribution width of the backscattered ultrasonic pulse was determined by approximating the experimental data by the Gaussian probability distribution applicable for statistics of large number of independent random variables. The results show that the amplitude distribution width increases with the growth in the specimen porosity independent of sizes and fractions of the reinforcing particles. The empirical relationship between the local porosity and distribution width of the backscattered ultrasonic signal amplitudes was obtained for porosities up to 4.5%. This relationship can be used for nondestructive testing of the local porosity in engineering products fabricated from the studied composite materials. The proposed laser-ultrasonic technique is especially promising for structural health monitoring of particulate reinforced metal-matrix composites during their service.
ABSTRACT
In this work, we propose the laser-ultrasonic method for nondestructive evaluation of porosity in particulate reinforced metal-matrix composites fabricated by stir and in-situ reactive casting techniques. The method is based on the influence of porosity on dispersion of the phase velocity of longitudinal acoustic waves, which is measured by the broadband acoustic spectroscopy with laser excitation of ultrasound (laser-ultrasonic spectroscopy). We studied stir cast hypereutectic aluminum-silicon alloy A336 matrix composites reinforced with the SiC micro particles (3.3-13.5â¯vol%) and in-situ reactive cast Al/Al3Ti composites reinforced with the Al3Ti intermetallic particles (4-11.5â¯vol%). In the spectral range of 3-40â¯MHz, the phase-velocity dispersion in both types of composites was observed: the high-frequency velocity in the range of 20-40â¯MHz increases with the increase of the reinforcement content independent of porosity, whereas the low-frequency velocity in the range of 3-10â¯MHz decreases with the increase of porosity independent of the reinforcement content. As a result, the relative dispersion grows up with the increase in the composite porosity independent of the variation in the reinforcement content. The empirical dependence between the porosity in a scanning composite region and the relative phase-velocity dispersion in this region is approximated by the same unified function. For the first time, such unified porosity-phase velocity functional relationship is obtained for particulate reinforced metal-matrix composites completely different in fabrication techniques as well as in chemical composition and elastic properties of reinforcing particles.
