Ultrasound Characterization of IM7/PEEK Composite Materials

IM7/PEEK composite materials may experience large mechanical and thermal stresses during service, and their failure mechanisms can be complex and have various modes. In this study, IM7/PEEK specimens for interlaminar shear strength testing were evaluated by both nondestructive ultrasound method and standard destructive test. First, IM7/PEEK specimens were fabricated under various processing parameters including roller speed, torch temperature, roller temperature, compaction level, N2 flow rate, and material tension level. Then ultrasound longitudinal wave velocity that is normal to fiber direction was evaluated and the influence of measuring locations and ultrasound frequencies from 0.5 MHz to 10.0 MHz were studied. Experimental results showed that 5.0 MHz is the most sensitive frequency to fabrication conditions and that ultrasound velocity can be related to some processing parameters such as materials tension, N2 flow rate and temperature. Finally, interlaminar shear strength experiments were carried out by standard short beam bending destructive tests. The relationship between ultrasound velocity and standard destructive tests were analyzed and the potential of ultrasound velocity for interlaminar shear strength evaluation was discussed.</jats:p

Phased Array Ultrasonic Technique Parametric Evaluation for Composite Materials

A series of ultrasonic elements arranged in a phased array transducer can provide the capability to activate each element separately but in a programmed sequence. This will help the acoustic signal to be generated at desired focusing distances and anticipated angles for specific materials and structures. In case of composite material inspection, this characteristic of the phased array method can improve the undesirable effects of the high attenuation and anisotropic structure of composite materials on response signals. In this study different phased array probes and wedges which are commercially available were evaluated for their response signals’ characteristics. First, the capability and resolution of bulk wave generation were studied for each set of probe and wedge, and the response signals were compared to that of the conventional single element ultrasonic transducers for different thicknesses composite plates. Then the resolution of the response signals and their sensitivity to defect size were evaluated and compared to the single element transducers as well. Next, each phased array probe and wedge set was used to generate plate waves in aluminum plates based on plate wave propagation theory, probe and wedge physical properties and the definition of delay law. Results show a general improvement in response signals’ strength and resolution for phased array method in comparison to the single element transducers. Also some plate wave modes could be generated with optimized signal generation parameters in phased array system.</jats:p

Interface Integrity Evaluation of Explosively Welded Metallic Structures

Explosively bonded tubes have been applied in high pressure and high temperature environments, such structures are usually consisting of dissimilar metals which are very difficult to be welded together by other conventional welding methods. In this study, various experimental techniques were used for the evaluation of interface integrity between steel major tube and tantalum donor tube. First, X-ray diffraction technique was used to profile the residual stress levels in the tantalum donor tube before bonding and steel major tube after bonding, especially the hoop stress level which is critical to quality control. Also, other experimental techniques including digital microscopy for interface geometrical features, X-ray fluorescence spectroscopy for interface compositions, and nano-hardness testing for materials strength at micro/nano scale were further applied to explain bonding mechanisms and to quantify bonding interface conditions. Then nondestructive ultrasound C-scan technique was used to create acoustic images of the bonding interfaces and the results showed that the technique has the potential to find weak interface and to further quantify good and poor bonds. Finally, destructive testing techniques were used to measure the interface shear and tensile strength and to study the interface failure mechanisms, and the results agree well with other experimental techniques.</jats:p

Effect of Alkyl Chain Length on Interfacial Performance of Alkyl Gallates in Supramolecular Oxidation of Soybean Bulk Phase Oil

From an interfacial phenomena standpoint, the effect of length of alkyl chain was evaluated on activities of gallic acid and its alkyl ester derivatives in supramolecular oxidation of stripped soybean oil. The gallic acid molecules (log P = -0.47) with an electron-donating carboxylate anion had a higher scavenging activity against DPPH• than alkyl gallates. Interfacial activities of gallates in emulsion and bulk oil improved as the lengths of the alkyl chain were reached until a critical chain length; after that, further chain length extension causes a decrease in potency. In β-carotene bleaching and Rancimat assays higher inhibitory effect was observed for dodecyl and propyl gallate, respectively. Evaluation of interfacial tension, water content, and droplet size of hydroperoxides reverse micelles in parallel with lipid peroxidation indicated that the inhibitory effect of the alkyl gallates, especially methyl gallate, was significantly better than the gallic acid in the bulk phase oil.</jats:p