The generation and detection of longitudinal guided waves in thin fibers using a conical transformer

A Review of NDT/Structural Health Monitoring Techniques for Hot Gas Components in Gas Turbines

The need for non-destructive testing/structural health monitoring (SHM) is becoming increasingly important for gas turbine manufacturers. Incipient cracks have to be detected before catastrophic events occur. With respect to condition-based maintenance, the complex and expensive parts should be used as long as their performance or integrity is not compromised. In this study, the main failure modes of turbines are reported. In particular, we focus on the turbine blades, turbine vanes and the transition ducts of the combustion chambers. The existing monitoring techniques for these components, with their own particular advantages and disadvantages, are summarised in this review. In addition to the vibrational approach, tip timing technology is the most used technique for blade monitoring. Several sensor types are appropriate for the extreme conditions in a gas turbine, but besides tip timing, other technologies are also very promising for future NDT/SHM applications. For static parts, like turbine vanes and the transition ducts of the combustion chambers, different monitoring possibilities are identified and discussed.

Acoustic emission technique for monitoring the pyrolysis of composites for process control

Carbonization is the first step in the heat and pressure treatment (pyrolysis) of composites in preparing carbon-carbon parts. These find many uses, including aircraft brakes, rocket nozzles and medical implants. This paper describes the acoustic emissions (AE) from various stages of the manufacturing process of carbon-carbon composites. This process involves carbonization at a high temperature and this results in both thermal expansion and volume change (due to pyrolysis in which a sacrificial polymer matrix is converted to carbon). Importantly the resultant matrix is porous and has a network of small intra-lamina cracks. The formation of these microcracks produces AE and this paper describes how this observation can be used to monitor (and eventually control) the manufacturing process. The aim is to speed up manufacture, which is currently time-consuming. The first section of the paper describes the design of unimodal waveguides to enable the AE to propagate to a cool environment where a transducer can be located. The second part of the paper describes various experimental observations of AE under a range of process conditions. In particular, this paper presents a technique based on detecting acoustic emissions and (1) uses wire waveguides to monitor parts within the autoclave to 800 degrees C, (2) monitors microcracking during pyrolysis, (3) uses a four-level threshold to distinguish between low- and high-amplitude cracking events, (4) recognizes the occurrence of harmful delaminations, and (5) guides the control of the heating rate for optimum efficiency of the pyrolysis process. In addition, supporting data are presented of in situ measurements of porosity, weight loss, cross-ply shrinkage, and mass spectroscopy of gases emitted. The process evolution is illustrated by the use of interrupted manufacturing cycle micrographs obtained by optical, scanning acoustic (SAM) and scanning electron (SEM) microscopy. The technique promotes in-process monitoring and control but also contributes to current understanding of pyrolysis.