Typical example of fatigue crack growth testing in aluminumWhere do test requirements originate? Testing requirements are often established by government agencies. This is especially true for the aerospace industry, and more specifically as related to defense. Many military specifications that have been developed over the past decades are frequently utilized for test methods. Other organizations, such as ASTM and ISO, are also frequently referenced. Required test methodologies are typically specified on part drawings, which means testing responsibility falls to the contracted manufacturer of the part.
Of course, testing requirements are not always dictated by governing bodies and endusers. Strong design and manufacturing teams understand the value of testing to validate models, improve production, and increase the overall quality of parts. The time and expense of testing might seem like an easy place to reduce costs, but unforeseen failures after parts are already in service are much more difficult to bear.
What are common types of testing?
Some organizations have in-house testing capability. If not, the manufacturer will need to contract an accredited lab to perform the work. The process begins with a review of the drawing or part specification to identify applicable test methods such as MIL, ASTM, and ISO. Typical mechanical tests include, but are certainly not limited to:
- Tensile testing of stock materials to ensure acceptance;
- Hardness testing of final parts to verify heat treatments
- Fatigue testing to establish long term durability
- Model validation to streamline the design process
- Fracture mechanics tests to aid in future maintenance schedules
Tensile testing is most often performed per ASTM E8. Raw material lots are susceptible to variability. Tensile testing of each lot will ensure that minimum strength requirements are met. Material suppliers are often equipped to handle these tests, though third party test labs should be used as an additional check. Confirming raw material is within acceptable limits is one of the easiest and most cost-effective insurance policies against future problems.
When heat treating is required, the production part will need to be tested to verify conformance. This is a quick and inexpensive option. Plus, hardness testing is one of the easiest approaches and does not usually require destructive testing of the final part.
Fatigue testing can be performed at a various levels of development. Standard methods can be utilized when the raw material is tested, since specimens can be machined to standard configurations. In most applications, the goal is to develop a fatigue strength curve that characterizes the material’s ability to resist cyclic loading over varying stress levels. Once the curve is established, the results are compared to the part design criteria to ensure that the material will enable the component to survive its intended life expectancy.
Final part geometry can make it difficult to apply raw material fatigue data. In this case, fatigue testing of the final part is recommended. This may require a more elaborate test setup and the destruction of some production parts. The bonus with this approach? Not only can a fatigue strength curve be generated, but failure modes will be determined. Knowing failure modes is valuable for design improvements and maintenance schedules.
Solid modeling of components and assemblies, along with Finite Element Analysis (FEA), are great tools for engineers to streamline the design process. However, FEA models do not guarantee correct results. The most prudent 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, this can be used to then go back and revise the FEA model. Once this step is taken, it is much easier and more reliable to make design modifications in the model without having to endure the time and expense of multiple prototype and test iterations.
Fracture mechanics testing provides empirical data about a material’s ability to withstand cracking and crack growth. Fracture mechanics testing is becoming an increasingly popular tool in developing maintenance schedules and predicting service life of components. It is inevitable that some machine components will develop cracks. Replacing parts can be costly and – interestingly enough – may not always be necessary. The key is to determine the severity of the flaw and to know when it becomes a critical matter.
Fracture mechanics consists of many different types of static and dynamic tests to characterize a material’s fracture toughness and resistance to crack growth once a flaw has initiated. Knowing how a material will react under such conditions enables engineers to estimate the remaining life of the component. In some cases, costly repairs can be postponed until it is more convenient to schedule such work. Knowing which materials have higher fracture toughness can improve designs and extend component life expectancy.
Forego Testing Requirements?
Product testing is an investment. It often requires specialized test equipment, custom-fabricated fixtures, experienced test engineers, and extra product/material devoted to testing. In today’s competitive marketplace, this type of investment can be difficult to include in already tight budgets. Failure to perform testing, however, can have devastating consequences far more costly than the testing itself. Some of the costs of foregoing testing requirements follow.
Internal Audit, Quality Check
The least costly consequence to foregoing testing requirements is an internal discovery of the nonconformance, whether during internal audit or internal quality check. Since the product has not gone out the door yet, the damage is limited. Situations like this can be handled by segregating the affected products, evaluating for conformance, and, if necessary, making reworks or repairs.
The costs: Rework/repair costs, production downtime, schedule delays, expedite fees, scrapped product.
Customer Findings or Fail Audit
A more severe consequence to foregoing testing requirements is being subjected with findings from an audit or first article inspection. If the error is exposed at this stage, there may be a chance to make amends without irreparable harm. Affected products can be recalled and evaluated for conformance. If necessary, reworks or repairs can be made and the products can be re-released.
The costs: All of the above, plus losing customer’s trust, losing contracts, or losing customers.
The most dreadful and costly consequence to foregoing testing requirements is catastrophic product failure in the field. We have seen it in the news – groundings, emergency landings, or worst of all, downed aircraft. The results can be devastating: product damage, personal injury, property damage, mission failure and loss of life.
The Costs: The consequences can be permanent: brand devaluation, legal prosecution, loss of professional license, physical injuries, and casualties.
Thanks to the redundant quality checks in aerospace, it seldom comes to this. However, this should be no comfort to the quality engineer. The engineer’s responsibility is to ensure absolute product conformance at every stage of the project and product lifecycle. Ensuring strong in-house testing capabilities or partnering with an accredited laboratory can help ensure continuity and quality. In the aerospace industry, there is far too much at stake to cut corners on testing.
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