Strain gauge installation on a test prototype

  • Date:
  • Category: Aerospace

We have recently worked with an Aerospace manufacturer on a specialist project; a strain gauge installation for structural monitoring and integrity testing of a prototype aerofoil component, an outboard flap.

The manufacturer had a proposed design for a new upgraded flap, but before it could get anywhere near being placed in production for use on new passenger aircraft the structure needed to be thoroughly tested, improved, and finally validated.

For this testing process prototype structures had to be built for each stage: static load testing, fatigue life testing and flight testing. The purpose of the whole test series was to hone the design model, to predict the component life span in operating cycles and for final safety validation.

We provided the strain gauge installation team, who worked closely with the manufacturer’s build staff, as new or improved prototypes were constructed for each successive test stage. Our staff instrumented each of the prototype structures comprehensively as they were built, so that every possible stress, strain and load could be measured.

The first prototype was subjected to an iterative process of ultimate static strength tests. Each test provided a set of numbers for the actual structure strength of the flap component. These numbers were fed back into the theoretical design model, refining it as understanding of static strengths of the component increased. Each test highlighted different structural issues so strain gauge positioning was adjusted to analyse these issues as testing progressed.

Once the strength tests were completed, using the now updated and refined theoretical model, a second prototype was built, again with our team instrumenting the structure during the build.

This second prototype was then used in fatigue tests, as a fatigue failure is different from an ultimate strength failure. The fatigue tests try to measure the flap performance when subjected to the repeated stress loadings and unloadings experienced during the length of an aircraft’s service. The component was put in a test rig and basically wobbled in a defined programme of cycles, based on the theoretical what-we-think-the-aeroplane-will-see-in-reality-of-its-service. This enabled data to be gathered from the strain gauges as potential issues appeared over the period of the test, seeing what the stresses were, seeing when the stresses started to rise as cracks propagated, as components within the prototype flap structure failed, enabling a much more realistic estimate to be made about the service life of the new flap design.

This fatigue data was then also fed back into the theoretical model. The understanding of how the structure is likely to behave and its and strengths and weaknesses was growing. The theoretical model was also getting closer to how the real structure will behave throughout the flight envelope, and what it would withstand.

A final prototype was constructed, thoroughly strain gauged, which will actually fly. The structure was fitted to a test aircraft and flown right through the expected flight envelope, with the strain gauge installation recording all the stresses that the prototype experienced. These final data were now fed back into the model, and this enabled an extremely accurate service life prediction and optimum flap structure to be determined for production.