Infrared thermography for test machine control and material characterisation

Abstract

The aim of the PhD is to develop Infrared thermography (IRT) so that it can be used to detect and assess damage growth in metallic and composite materials. A key component is to demonstrate that cheaper and more robust microbolometers could be used to replace the expensive photon detectors that are used currently in most laboratory settings, particularly for thermoelastic stress analysis (TSA). The possibility of using IRT, not just for monitoring and assessment of material behaviour, but additionally, for test machine control is investigated. The building blocks for a new approach to test machine control known as ‘damage evolution control’ are defined, using IRT based on the application of a microbolometer, presented as an alternative to the standard load, strain or displacement control methods incorporated in to modern test machine control systems.

The thesis contains a detailed review of the literature associated with the application of IRT in material assessments. As TSA and IRT are well established techniques only an overview of the underlying physics is provided. A detailed appraisal of the two infrared detector types (microbolometers and photon detectors) is provided alongside an overview of their architecture, advantages and limitations. The performance of each detector type is evaluated using a temperature controlled blackbody to evaluate the noise levels and sensitivity of the bolometer system in comparison with that of the photon detector and incorporated into a camera model framework. The model is then used to demonstrate the
limitations of using a bolometer based system for TSA using a metallic notched specimen.

It is demonstrated that IRT can be used to monitor and evaluate a growing crack in the metallic notched specimen and in particular that the bolometer temperature data is sufficiently accurate to determine the crack length. It is also demonstrated that IRT could be used to control a fatigue test. The IRT results from the photon detector were post processed for thermoelastic stress analysis (TSA), the output of which provide two other means to determine the crack length and confirmed the validity of the bolometer data. The TSA data was further post processed to calculate the stress intensity factor (SIF) to provide a further verification of the findings.

The design and implementation of a fatigue test on composite open hole tension (OHT) and un-notched specimens is described which characterises the temperature change generated in damaged composites and how the damage and the temperature evolves. The difference between the photon detector and microbolometer are reviewed and discussed, concluding that IRT may be used to monitor and assess damage evolution.

Integration of the IRT monitoring into the Instron test machine control system is described with an overview of the test machine control system and a variety of implementation strategies. A feasibility study of one of the integration techniques is presented, that demonstrates the microbolometers temperature output could be
connected to Instron’s WaveMatrix software via an analogue input.

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