Heriot-Watt Institute of Photonics use Deben 2kN tensile stage to characterise ceramics and engineering plastics

Dr Peter Schemmel is a postdoctoral researcher in Applied Photonics at Heriot-Watt University. He is part  of  the  team  at  the  Institute  of  Photonics  and  Quantum  Sciences  in  a  group  which  focuses  on stress  analysis  in  ceramics  and  engineering  plastics.  They  are  particularly  interested  in  ceramic thermal  barrier  coatings  used  on  aeroplane  engine  turbine  blades  or  power  generating  turbines. Typically,  the  air  around  these  blades  is  hotter  than  the  melting  point  of  the  blades  themselves. Therefore, they must be coated with a ceramic thermal barrier coating. These coatings degrade over time  and  must  be  replaced  after  a  certain  number  of  hours determined  by  the  manufacturer  and application. The  group  is  studying  ways  to  increase  the  useable  lifetime  of  these  coatings  by understanding the stresses built up in the coatings during manufacture and over time in service. A quantity known as the “stress optic coefficient,” is used to convert measured sample birefringence into stress distributions.  Each  material  has  a  different  coefficient,  which  can  depend  on  several factors. Some things that can alter the stress optic coefficient of a material include the frequency of the radiation used in the test, the manufacturing methods, material density and atomic structure. This list is not exhaustive.  Part  of  the  research  tries  to  understand  how  the  stress  optic  coefficient  of  a material can change.

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Dr Schemmel describes how he applies the Deben 2kN tensile stage: “In order to find the stress optic coefficient  of  a material,  we  must measure  the  birefringence  whilst  a  known  stress  is  applied  to  the sample. Deben’s tensile stage is indispensable to this operation. Constant loads can be applied to the sample, over the course of each birefringence measurement.  Such measurements take around ten minutes. If the stress on the sample changes during this time, the measurement is invalid. Without the Deben tensile stage, we would not be able to accurately calibrate our materials.” The benefits of the stage are fourfold.  Dr Schemmel continues: “Firstly, it is small.  We  required  a small  stage  to  sit  on  our  optical  bench,  inside  an  anechoic  chamber.  Secondly, Deben was able to machine a specifically sized window in the back of our tensile stage so that we could pass our GHz – THz beam through the sample under test. Thirdly, and most importantly, the Deben system allows us to control the applied load with great finesse. We are able to maintain a constant load on each sample during the birefringence measurement period, which is critical to achieve accurate results. Finally, the system is extremely easy to operate. It is really a “plug-and-play-system,” which allows us to be more productive.” Research in the group applying the stage is moving rapidly with the first publication due shortly. This will show interesting results on the stress optic coefficient of YTZP ceramic and PTFE plastic. It was found that the stress optic coefficient of some materials can depend on sample preparation variables, e.g. cutting specimens parallel or perpendicular to the extrusion direction.

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