Microwave, millimeter wave and terahertz (MMT) techniques for materials characterization

Electromagnetic waves (EMW) in the frequency range of microwaves, in the millimeter range and in the terahertz range (MMT) can be used to nondestructively characterize many materials. There is no unique definition of the term microwaves. It can encompass decimeter and centimeter waves corresponding to a wavelength from 1 m to 10 cm to 1 cm, respectively. The millimeter waves extend from 1 cm to 1 mm (30-300 GHz), while the terahertz waves (THz) have wavelengths less than 1 mm (300 GHz). The upper limit of the range of THz waves - also termed far infrared - is not clearly defined. In contrast to conventional ultrasound testing, no coupling agent is needed to couple the EMW into the test object to be investigated. Principally the distance between the test object and test device is not limited. The interior of metallic materials cannot be tested by MMT waves since the high electrical conductivity generates a total reflection of the incident waves thus reducting the penetration depth to a few µm or less

The Acoustic emission analysis of the crack processes in alumina

The crack processes in single-phase alumina specimens have been investigated by acoustic emission (AE) analysis with regard to the increase of the crack resistance. During loading of a notched specimen, individual AE signals are observed at first, which are probably due to the generation of microcracks in a process zone around the notch. At higher loads signal clusters are found, which should be due to the coalescence of microcracks. By these coalescence events the main crack is formed. At microscopic crack propagation most AE events are located within the crack tip zone. However, up to about 20% of all events are located within the crack flank zone behind the crack tip. Thus, it can be concluded that there is an energy dissipation at the crack tip at beginning of the loading, which determines the starting value of the crack resistance, too. However, it cannot be decided from AE if the contribution of the process zone at the crack tip or of the crack flanks in the wake of the crack tip lay the major role in increasing the crack resistance