Development of nondestructive non-contact acousto-thermal evaluation technique for damage detection in materials.
This paper presents the development of a new non-contact acousto-thermal signature (NCATS) nondestructive evaluation technique. The physical basis of the method is the measurement of the efficiency of the material to convert acoustic energy into heat, and a theoretical model has been used to evaluate this. The increase in temperature due to conversion of acoustic energy injected into the material without direct contact was found to depend on the thermal and elastic properties of the material. In addition, it depends on the experimental parameters of the acoustic source power, the distance between sample and acoustic source, and the period of acoustic excitation. Systematic experimental approaches to optimize each of the experimental variables to maximize the observed temperature changes are described. The potential of the NCATS technique to detect microstructural-level changes in materials is demonstrated by evaluating accumulated damage due to plasticity in Ti-6Al-4V and low level thermal damage in polymer matrix composites. The ability of the technique for macroscopic applications in nondestructive evaluation is demonstrated by imaging a crack in an aluminum test sample.
Internal friction and ultrasonic yield stress of the alloy 90 Ti 6 Al 4 V
Abstract The alloy 90 Ti 6 Al 4 V is used as a mechanical transformer to produce high strains in metal samples. The internal friction, modulus defect and limiting stable strain amplitudes have been studied over wide frequency and temperature ranges. The internal friction Q−1 of annealed samples has the low value of 5 × 10−5 which is independent of the frequency and decreases only slightly at high temperature. The result is consistent with a model in which the energy loss is produced by a non-linear motion across the Peierls kink barrier. The value found for the ratio of the internal friction to the modulus change is 0.03 which is intermediate between two theoretical calculations. This material becomes unstable at strains from 3 to 5 × 10−3. Evidence is presented which shows that this is a material with a Cottrell type pinning with pinning points on the average about 3 Burgers distances apart. Assuming that on the average one kink occurs between each pinning point, the modulus defect is consistent with about 1010 dislocations per cm2. The frequency and temperature variations are consistent with this model.
A Critical Assessment on Functional Attributes and Degradation Mechanism of Membrane Electrode Assembly Components in Direct Methanol Fuel Cells
Direct methanol fuel cells (DMFC) are typically a subset of polymer electrolyte membrane fuel cells (PEMFC) that possess benefits such as fuel flexibility, reduction in plant balance, and benign operation. Due to their benefits, DMFCs could play a substantial role in the future, specifically in replacing Li-ion batteries for portable and military applications. However, the critical concern with DMFCs is the degradation and inadequate reliability that affect the overall value chain and can potentially impede the commercialization of DMFCs. As a consequence, a reliability assessment can provide more insight into a DMFC component’s attributes. The membrane electrode assembly (MEA) is the integral component of the DMFC stack. A comprehensive understanding of its functional attributes and degradation mechanism plays a significant role in its commercialization. The methanol crossover through the membrane, carbon monoxide poisoning, high anode polarization by methanol oxidation, and operating parameters such as temperature, humidity, and others are significant contributions to MEA degradation. In addition, inadequate reliability of the MEA impacts the failure mechanism of DMFC, resulting in poor efficiency. Consequently, this paper provides a comprehensive assessment of several factors leading to the MEA degradation mechanism in order to develop a holistic understanding.