The monitoring of energy flow plays a critical part in stress analysis in a range of different markets and processes. By measuring often tiny temperature changes in a material under strain, it becomes possible to gauge the possibility of cracks or weaknesses in a material or component.
This is the specialist area of Chesterfield-based Strain Solutions, which has been providing stress analysis consultancy services, education and training to industry and academia in the United Kingdom, continental Europe and North America since 2004. With a range of experimental mechanics and non-contacting infrared measurement tools at our disposal, it offers an unusually focused service for the design, commission and optimisation of stress analysis hardware and test methodologies, and associated mechanical test rigs.
Dividing its workload roughly equally between applications involving aviation, bio-mechanical medical implants (metallic and polymeric ) and chemical pressure vessels, Strain Solutions undertakes photoelastic stress consultancy, thermoelastic stress analysis consultancy, as well as thermal NDE (Non-Destructive Evaluation) using state of the art micro-bolometer array infrared cameras.
Dr Richard John Greene, Strain Solutions' managing director, explains in detail how the process works, saying: "If, for example, you are running a fatigue test for a piece of aeroplane or a gas turbine, as you fatigue the structure, you are putting energy into the system – vibrating it or cycling it in a test frame or something – that energy has got to go somewhere. Some of it goes into the elastic straining of the material, but inevitably, some of it goes into irreversible damage of the specimen.
"The classic thing is, if you have a crack in there, as you load and unload tensile compressive stresses on your bit of wingspar, at the crack tip, you get an energy loss from the system, which makes the crack grow. That energy loss produces an increase in surface temperature that you can see. So one of the things we do is, during cyclic test work, we monitor the surface temperature of aerospace and biomechanical structures. By looking for hot spots – very small changes in surface temperature, we can locate fatigue crack initiation and growth."
Another function undertaken by Strain Solutions is stress monitoring within the plastic forming of materials. This is performed in applications where there is a component that needs to be plastically deformed as part of a fabrication. If, for instance, there is a plate and the application requires it to be made it into a bracket by bending it through a right angle, the energy being put into the system as the bend is created causes millions of dislocations through the structure. That energy then produces a temperature rise and, as the material is formed, the quality of the microstructure after forming is strongly rate-dependent. In other words, Thus, if this process is performed quickly, a very large temperature rise is created, leading to a very particular, very dense microstructural change in the component.
Says Greene: "If you do it more slowly, the microstructural change is more gentle, the temperature rise is much lower and you basically get less work hardening. So, by monitoring the surface temperature during the forming process you can control the resulting work hardening and therefore the microstructural condition and therefore the hardness of the material and the fatigue properties."
Clearly, these processes call for some fairly specialised equipment, not the least of which are the infrared cameras used to measure the changes in temperature. Dr Greene's previous experience of these processes involved working with nitrogen-cooled thermal imaging cameras for the military. He says: "Because the military want to do high-speed differential imaging, they're very, very sensitive, have a very, very fast response time and very programmable. They're great things, but they're way out of the price range of a little UK start-up like us. Thus, when I started the company in 1994, I was looking around for a piece of kit that could do 90-95% of what I needed them to do, but at a tenth of the price."
The equipment that was considered to best fit the bill was Micro-Epsilon's thermoIMAGER TIM160 inline radiometric thermal imaging camera. Powered and operated via a USB 2.0 interface, the thermoIMAGER TIM160 is a low cost inline radiometric thermal imaging camera that provides temperature images and profiles of a target area. This plug-and-play unit is supplied with a full software package, TIM Connect, that enables the user to configure all device parameters, as well as enabling the real time capture (at 120Hz full frame rate) and storage of images or video of an event for slow motion play back or snapshots at a later date – an important feature in R&D and failure diagnostics work.
The cameras are sufficiently sensitive to be able to pick up changes of a tenth of a degree. The ability to monitor a steady state temperature rise of a few tenths of a degree, allows the user to track the growth of a fatigue crack.
Greene is unequivocal that what marks these cameras out is their capability relative to their cost (it costs less than £3,000 plus VAT). He says: "If I could afford it, I'd love to spend £100,000 on a military spec camera. This is not the best thing out there – that's not the point. The point is that this thing is amazing for the price."
He continues: "The TIM Connect software provided with the camera is easy to use and surprisingly fully featured for such a reasonably priced camera. We particularly like the software's ability to stream every frame to memory, which allows the full, uncompressed, raw data to be captured for post-processing at a later stage. The Software Developer's Kit [SDK] also provided with the camera has enabled us to make a rapid start in creating our own software applications to capture and process the raw data stream from the camera. The sample C++ code is well laid out and comprehensively annotated and demonstrates most of the useful camera functions."