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INfrared STRUctural monitoring of Cracks using Thermoelastic analysis In production enVironmEnts

Periodic Reporting for period 1 - INSTRUCTIVE (INfrared STRUctural monitoring of Cracks using Thermoelastic analysis In production enVironmEnts)

Reporting period: 2016-01-01 to 2017-06-30

This project is built around a well-established set of laboratory-based techniques, which use Thermoelastic Stress Analysis (TSA) for characterising strain fields associated with structural features. The project will extend their applicability to aircraft structure tests and to enhance their productivity through the use of automation for data acquisition and processing in an industrial environment. Thermoelastic stress analysis is well-established as a laboratory-based technique and its use for structural analysis, fracture mechanics and damage mechanics has been explored extensively. Most of the work so far has been performed using relatively small test coupons with simple structural features. In the current work the aim is to prove the feasibility of applying thermoelastic stress analysis in a structural test environment for detecting stress hotspots. The structural test tools developed will enable fast acquisition of data-rich stress fields in large aerospace components during structural tests at relatively low cost and the post-processing capabilities will allow meaningful comparison of experimental results with those from computational models and service life evaluations. The scientific and technical objectives are designed to lead to innovative methodologies that will enable more detailed stress information to be acquired during aircraft structure tests. These methodologies will be faster, lower cost and provide higher confidence in computational models than is possible at the moment.

Overall project objectives:

(i) to develop methods and protocols for the application of thermoelastic stress analysis (TSA) to aircraft structure fatigue and damage tolerance tests;
(ii) to demonstrate the applicability of TSA for quantitative stress analysis of complex three dimensional components;
(iii) to evaluate TSA for qualitative and quantitative assessment of hot-spots during structural testing;
(iv) to design, build and demonstrate a robotic platform for TSA data acquisition in an industrial test environment;
(v) to propose an approach for the implementation of the technology on large components and full-scale tests with the provision of quantitative results.
A robust methodology of data collection using thermoelastic stress analysis (TSA) during constant amplitude cyclic loading of fatigue coupon specimens has been developed. Post-processing of the TSA signal magnitude data is carried out using an algorithm implemented in MATLAB. This method allows the time of crack initiation to be determined, and the propagation of those cracks to be tracked and measured. Results show that crack initiation is observed when cracks are at sub-millimetre lengths, and propagating cracks can be continuously tracked up to 10 mm in length.

Thermoelastic stress analysis has also been carried out on simple specimens with a primer paint coating, complex specimens under constant amplitude cyclic loading, and on simple specimens under spectrum (flight cycle) loading. Preliminary results show that TSA can be used to observe cracks in each of these complex situations before they would be apparent using traditional methods of non-destructive inspection.

A prototype electro-mechanical positioning rig for scanning of a component for hotspots or the presence of cracks has been developed which makes use of an Arduino control interface board connected to a laptop computer via USB connection, with the Arduino permitting the use of the laptop keyboard to control the movement of the rig. Control steps are executed both automatically and with user interaction, via both stepper motor and rotary servo actuators, and example translations and rotations are executed which will be suitable for infrared camera positioning.
Progress beyond the state of the art - the current project deliverables document a methodology for successful automated real-time tracking of crack growth in planar specimens under both cyclic and flight spectrum loading, which is a significant step forward in the current state of the art for non-contact crack monitoring methods.

Expected results until the end of the project - the methodology developed in the work so far will be extended in the second period of the project, including application to complex aerospace structural components, more advanced flight spectrum loading, non-planar components and low frequency excitation during fatigue loading. The project will also consider scanning methodologies so that large areas of a component undergoing structural test may be monitored by a single thermoelastic stress analysis system.

Potential impact of the project so far - the methodologies developed in this first project period offer a complete methodology for continuous crack tip tracking data to be reported in real time during complex fatigue loading of components, removing the requirement for regular stops in fatigue test programmes for visual inspection of crack tip positions, which represents a significant, quantifiable cost saving for the end user and also an improvement in the crack length accuracy over traditional monitoring methods.