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The Importance of Mechanical Testing Throughout the Aerospace Product Lifecycle

By Engaged Expert Ryan Castells Advanced Materials

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Mechanical testing is a standard and essential part of any design and manufacturing process and absolutely critical in the field of Aerospace. Whether it is characterizing the properties of materials or providing validation for final products, ensuring safety is the principal mission of all mechanical testing. Testing also plays a key role in ensuring a cost effective design as well as technological evolution and superiority.  

Origins of test requirements 

The requirements for testing are often put in place by government agencies, and this is especially true when it comes to the Aerospace industry, more specifically in relation to defense. In fact, many military specifications that have been developed in recent decades are often used for test methods. The test methodologies required are characteristically specified on drawings of the parts, which means that testing responsibility falls to the manufacturer contracted for that part. 

Testing requirements are not always set by governing bodies, with experienced design and manufacturing teams understanding the value of testing to validate models, make improvements, and increase the overall quality of parts. When faced with the cost and time implications of testing it may seem like this process is in fact a good place to start when it comes to reducing costs, however unanticipated failures when the parts are in service have greater cost and time implications. 

Common types of testing 

A number of organizations possess the in-house capabilities to do their own testing. Those that do not need to contract an accredited lab to carry out this work. The start of the process sees a review of the drawing or part specification in order to identify appropriate test methods, which can include: 

Tensile testing

Raw material lots are liable to variability, so tensile testing of each lot ensures that it meets minimum strength requirements. Although material suppliers are often equipped to handle these tests, it is recommended that they use a third party lab test as an additional check. Confirming that raw material is within acceptable limits is one of the easiest and most cost-effective assurances against future problems. 

Hardness

Testing is required for any production parts where heat treating is required in order to verify conformance. Like tensile testing, this is a very quick and inexpensive testing option and does not usually require the final part to undergo destructive testing. 

Fatigue testing

In the majority of applications, the objective of fatigue testing is to develop a fatigue strength curve that characterizes the material’s ability to resist cyclic loading over fluctuating stress levels. Once this curve has been established, the results are compared to the criteria of the part design to make sure that the material will enable the component to fulfill its projected life expectancy. 

Final part geometry can make it difficult to apply raw material fatigue data. When this is the case, it is recommended that the final part undergoes fatigue testing. Going a step beyond the previous methods discussed, this may require a more complex test setup and include the destruction of some production parts. The upside to this approach is that not only can a fatigue strength curve be generated, but failure modes will be determined. This knowledge is valuable for design improvements and maintenance schedules. 

Model validation
When it comes to streamlining the design process, solid modeling of components and assemblies, along with Finite Element Analysis (FEA) are excellent tools for engineers. That being said, FEA models do not guarantee accurate results so the most practical approach is to: 
  1. Develop a solid model
  2. Perform FEA to determine theoretical stress levels and locations, and
  3. Use mechanical testing to validate the FEA model.

Since mechanical testing provides empirical data, it can be used to then go back and revise the FEA model. Once this step has been achieved, making design modifications in the model is much easier and more reliable without the added time and cost of numerous prototype and test iterations. 

Fracture mechanics 

As the name suggests, fracture mechanics provides data about a material’s capability to withstand cracking and crack growth. When it comes to developing maintenance programs and predicting the service life of components, fracture mechanics testing is an extremely popular tool. It is inevitable that some machine components will develop cracks in their lifetime. Replacing parts can be extremely costly and in a number of cases completely unnecessary; the key is to determine the severity of the flaw and to recognize when it crosses the line to become a critical matter. 

Fracture mechanics is made up of many different types of static and dynamic tests that characterize a material’s fracture toughness and resistance to crack growth once a flaw has originated. This knowledge of how a material will react under such conditions enables engineers to estimate the life of the component. This means that, in some cases, costly repairs can be postponed to a time convenient to schedule the work. Knowing which materials have higher fracture toughness can improve designs and prolong component life expectancy.  

Is testing worth the investment?

As product testing often involves specialized test equipment, custom-fabricated fixtures, skilled test engineers, and extra products and materials devoted to testing, it requires investment from organizations. When today’s competitive marketplace is taken into account, this investment can be difficult to justify at the start of a project when budgets are already squeezed. However, it cannot be underestimated that the failure to perform appropriate testing can have devastating consequences that can cost significantly more than the testing itself. 

Here, we take a look at some of the true costs that organizations face if they decide to forego testing: 

Internal audit

The first and least costly consequence of foregoing testing is the internal discovery of nonconformance through an internal audit or quality check. In terms of scenarios, this is not the worst case as product has not yet left the premises, meaning the damage is limited. This situation can be alleviated by separating the affected products, evaluating for conformance, and, if necessary, making the required reworks. 

Failed audit

Being subjected to findings from an audit or inspection is one of the more severe consequences of foregoing testing requirements. Any errors discovered at this stage could be rectified as the affected products can be recalled and evaluated, and reworks and repairs carried out for the products to be re-released. However, as well as the time and cost implications, organizations risk losing a customer’s trust and potentially valuable contracts. 

Product failure 

The most costly consequence of foregoing testing requirements is catastrophic product failure in the Aerospace field. We are familiar with this from news channels reporting on groundings, emergency landings, or downed aircraft. The result of this can be devastating, ranging from product damage to loss of life, and the consequences permanent with organizations exposed to legal prosecution. 

Rest assured that, thanks to the redundant quality checks in Aerospace, it rarely comes to this. However, this should be no comfort to the quality engineer whose responsibility is to ensure absolute product conformance at every stage of a project and product lifecycle. By ensuring strong in-house testing capabilities and partnering with an accredited laboratory, organizations can help to ensure continuity and quality every step of the way. For the Aerospace industry, there is far too much at stake to consider cutting corners when it comes to mechanical testing.

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