Aerospace materials are studied in various points of view. The materials are discussed in the points of users and suppliers, the safety concept, the strength criteria, the macroscopic and microscopic points, mechanical properties and classification of each materials.
The author has been proposing a so-called "experimental micro-mechanics of composites" to bridge the gap between material fabrication and macroscopic mechanical properties. Based on in-situ observation using optical, scanning electron, and/ or scanning acoustic microscopes with loading devices, microscopic deformation and damage has been quantified. Moreover, theoretical models have been established for damage evolution. In real applications, however, since the strains applied to the composite structures are random and uncertain, the real-time strain monitoring is necessary to predict the present damage status in composites based on the above durability evaluation method. If the damage can be detected by using sensors, more reliable estimation of the damage status or the residual life can be made. In Japan, a new university-industry collaboration project on smart material/structure system supported by MITI has started, where the structural health monitoring is one of the major themes. The author acts as a group leader in the health monitoring group, which is currently studying the following items: (1) development of high-performance sensor system technology, (2) development of self-diagnosis and damage suppression systems for structural integrity, and (3) development of implementation technology for model smart structures. Some details are also presented for this project.
Ultrasonic wave propagation characteristics can be related to elastic properties of mediums. Hence, the ultrasonic wave has been applied to nondestructive quantitative evaluation of various materials. In the present paper, some examples of the application for composite materials are shown. At first, as a measurement technique of macroscopic elastic properties, a double-through-transmission method is explained. Using this method, changes in elastic constants and viscous coefficients of moisture-induced CFRP were evaluated quantitatively. Next, the measurement of acoustic reflection coefficients with an ultrasonic micro-spectrometer is explained. This is a novel technique for quantitative evaluation of elastic properties in a local area. This technique was applied to the determination of microscopic elastic constants of CMC and a thin resin layer formed in an interlaminar-toughened CFRP composite. Through these examples, the availability of ultrasonic waves is indicated for nondestructive quantitative evaluation of composite materials, which have many unknown elastic constants.
The behaviour of the fatigue crack propagation under the repeated mode II loading was investigated using the rolled plate specimens with a fatigue precrack. Tests on the alminium alloys (7075-T 6, 2017-T 4) and the mild steel (SS 400) were conducted, and the macroscopic crack paths and the microscopic fracture surfaces were examined. In these tests under the repeated mode II loading to the precrack, two modes of the fatigue crack growth were observed ; the mode I crack growth and the mode II crack growth. In 2017-T 4 specimens, the mode II crack growth was always observed while in SS 400, only the mode I crack growth was observed. However, in 7075-T 6 specimens, the mode I growth was observed in low ΔKII and the mode II growth in high ΔKII.