Depth Maintenance

21st February 2018: Depth Maintenance (NDT Task)

This activity was carried out under the guise of continuation of the fault diagnosis process from the first lab activity conducted on the 20th of February. We had made it this far under the premise that our engine blades had to been taken out for further inspection and testing, after extended visual inspection did not reveal anything we were looking for.

This involved a new exposure to some NDT processes and some commonly used metrology equipement and their applications.

NDT

Non-Destructive Testing (NDT), as the name suggest is a way of testing a component, part, material or a system without causing irreparable damage to the component being tested or compromising the serviceability of that component. Simply put, after this test is done, this unit should be just as useful and efficient as it was prior to the test. In the previous activity report, I made reference to using visual tools  to explore systems to further our knowledge of its inner workings, while minimising the amount of time spent doing so.

The importance of NDT cannot be overstated in the engineering industry. NDT is used in manufacturing, fabrication and in-service inspections to ensure product integrity and reliability, to control manufacturing processes, lower production costs and to maintain uniform quality level.

Metrology

Simply put, metrology is the study of measurement, both theoretical and experimental. The human eye, along with the other sensory organs, is quite good in detecting, sensing and measuring things; but it’s not that good (Yoshizawa, T, 2008) . This is where specially designed tools machines become an advantage.

Safety

Similar to the fault diagnosis activity, safety was also very crucial.

  • Face Masks: this was to keep out toxic fumes from dye penetrant used in the activity
  • Disposable Gloves: a contamination barrier against corrosive fluids getting to the hands and skin
  • Coveralls: a staple of PPE equipment, this was clothing fit for purpose of the task, preventing any further damage to clothes or skin underneath
  • Boots: Steel toe capped boots were worn to also optimise safety in the working environment, safeguarding against falling items.

Another part of the activity that was worthy of mentioning was the work surfaces, which were made of either granite or steel. Using granite surfaces in a metrology lab is advantageous for chemical resistance, impact resistance, heat resistance, nature of application, durability, aesthetics and cost effectiveness (A. Sohoni, 2011). Steel also has advantageous properties of durability, flammability, fit for purpose, cleanliness and customisation (A. Cappello, 2017).

Risk Assessment
Compliant with HSE2014 (see also), we were made aware of the risks associated with working with the tools, equipments, aerosols, and other potentially harmful substances in the environment, and throughly informed on how to mitigate these risks.
For example, with regards to safety, using gloves when dealing with potentially corrosive dye, not looking directly into lenses with bright lights, not leaving open ended drinks lying around, or even to watch where hands are going when dealing with precision instruments.
We were shown how to perform dye-penetrant testing and how to avoid harm to ourselves. We could see labels clearly at all times with caution & warning signs attached to all the equipment, as well as around the lab walls.

 

Learning Objectives
Using previously gained knowledge of NDT testing and what it entails, to carry out experimental testing on an aircraft component, and analyse the findings to determine its serviceability. Health and safety in the working environment to be of utmost importance at all times in the lab.
The compressor blades to be investigated

 

Some of the equipment and testing methods we were introduced to include:
  • Shadowgraph: used for observing a flow exhibiting variations of a fluid density. The process uses only light source and a recording plane onto which to project the shadow of the varying density field. In the case of a potentially damaged blade, you would find a silhouette against the background light, pointing out where the potential damage might be.
Shadowgraph in operation
The principle behind a shadowgraph
  • CMM: Coordinate Measuring Machine, a device used to measure the physical geometrical characteristics of an object.
  • Surface Trace: scans the surface of a component to detect scratches or dents and other defects not visible to the naked eye. You might be able to see a scratch, but the surface trace can let you know how deep the scratch is.
  • Leica Eye:  a scanner used to render large components such as aircraft wings, for further analysis and other parts not small enough to be analysed with other smaller equipment.
  • Laser Microscope: a tool used to perform measurement of roughness profile, or the visibility of irregularities on the surface of a material.
  • DPI: Dye Penetrant Inspection, a process of using a colourful dye to trace and make visible the path of cracks or defects in a material.

The DPI process involves, first cleaning the component, applying a dye penetrant to cover the surfaces, at this point due to the viscosity of the fluid, it will spread through and seep into cracks and small crevices in the component. Leaving to dry for a considerable time (in this case 15 minutes), the excess dye is wiped off. A “developer” is applied to the component  to make the path of the dye more apparent and enables the person conducting the test to make a better decision about what step to take.

After a visual inspection, we concluded that DPI should be the most appropriate first step for this task, considering the nature of the part involved. Then based on our DPI findings, we use the shadow graph to determine the extent of the damage or crack.

The CMM machine was next to be used to gauge the blade’s physical properties and give us more information about if the damages on the blade are within tolerance as per maintenance guidelines.

We sadly did not make it this far due to lab constraints. We encountered an issue with the CMM machine and its calibration. This somewhat put a damper on the direction of the activity. Nevertheless, our findings from the first two steps were sufficient to help us make a decision to deem the parts serviceable.

 

Some learning points

It is ill advised in industry to conduct NDT on a sealed unit. A good example would be a sealed hydraulic unit, if this is opened any time after initial manufacturing, it is no longer functional. This causes a lot of issues. The whole unit is voided because its serviceability is now rendered unreliable. This also paves the way for potential for further damage.

As someone in technical environment, it is important to always record findings no matter how small or insignificant they may seem. This makes it easier to track SOP/SMP changes. Also an added bonus, easy to provide evidence when legal issues arise.

 

Conclusion & Evaluation

The decision to deem the parts safe was met with the argument that we don’t full know their status. i.e. the only thing worse than a faulty component, is a component with an unknown status (whether it is safe or not). How could we clear a part that we can’t 100% certify its reliability, faulty CMM or not? We however came to the conclusion that if we were to fail these parts, we would be incurring more time and resources spent on reinstalling a new assembly.

The “bottom third”, showing the region of interest

Another reason that swayed our decision was the “bottom third” rule. This stipulates that if any significant cracks or damages in the bottom third of the blade deems the blade immediately unserviceable. The reason for this is because that region is where most of the stress is acting. Any cracks in this region compromises the structural integrity of the blade.

Any attempts to remedy a fault in this region would be futile and time consuming. As per our findings, the blades all had this region of interest free from any cracks. We made a note of where cracks were found, using fine measurement tools to denote their precise location on each blade. This was for record keeping and to establish some sort of accountability.

This task has helped me better appreciate the methodology in a repair process of any component in an aerospace environment. Anyone engaging in this would need to be meticulous and be objective at all times. There are repercussions to sloppy workmanship. Upon my introduction to these NDT equipment, I have developed a “just because you can’t see it doesn’t mean it’s not there” mindset. With this wide range of tools at my disposal, and my understanding of their applications and drawbacks, I can begin to factor them into my future technical diagnostic tasks.

There is also the ongoing internal debate between saving time and money for a company vs. upholding 100% reliability at all times for all components. Through this task, I’ve observed it is not always clear cut. There are other underlying factors that would make an engineer make technical decisions like these, be it looking through a business lens or making a data based move, it’s not always so straightforward (explains why engineers are always stressed, some of these decisions are never easy).

Aerospace companies will continue to push the envelope with  how far they can operate within a margin of error, while mitigating risk to their customers and investors. In this day and age, with research and design being more thorough, there shouldn’t be any cause for alarm. Flying is statistically still the safest way to travel after all.

 

References

 

 

 

 


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